Climate_change

Climate Change, in its broadest sense, is any change in the state of the climate or climate system that persists for an extended time period (decades or longer), whether caused by natural (solar changes, etc.) or anthropogenic processes. An example would be the Younger Dryas, a cold-climate period from roughly 12,900 to 11,500 BP.
In popular, current usage, the term is typically employed in the narrow sense of contemporary changes in the climate or climate system in which humans are regarded as a major or the dominant contributor, and specifically the perceived warming of the Earth’s air and ocean temperature via anthropogenic changes in the atmospheric composition of greenhouse gases (GHGs). As such, it is also known as global warming or the theory of anthropogenic global warming. Such warming is linked to various, deleterious global trends, such as rising sea levels. Some modern definitions consider climate change as the phenomena that includes both global warming and the consequences of global warming.
There are three basic components of the consensus view of anthropogenic climate change. The first is that, all things being equal, an increase in GHG concentrations should translate to increase in global mean temperature due to the greenhouse effect. Second, there has been an observed increase in GHG concentrations since the industrial revolution, considered to be attributed to human activities (burning of fossil fuels, deforestation, etc.). Third, there has been an increase in the average temperature of the Earth’s near surface air and oceans since the mid-20th century, and this is most likely a result of the observed increase in anthropogenic GHG concentrations. Addressing the human impacts on atmospheric concentrations of GHGs would thus mitigate the various harms linked to the warming of the planet.
The first two components are well established. Controversy arises over the third point in terms of the extent of human contribution to the perceived warming and the ratio of benefit versus harm from the various mitigation actions proposed. The analysis is complicated by the complexity in the climate system, the reliance on modeling, and political and economic challenges.

Contents

1 Definitions
2 Consensus view: three components

2.1 An increase in GHGs should result in an increase in temperature: the greenhouse effect
2.2 GHG concentrations have increased significantly since the industrial revolution
2.3 Global temperature considered to have increased and as a result of anthropogenic GHG concentrations


3 Evidences/Consequences

3.1 Global temperature rise
3.2 Warming of oceans
3.3 Ocean acidification
3.4 Sea level rise
3.5 Shrinking ice sheets and Arctic sea ice, retreating glaciers, and decreased snow cover
3.6 Extreme weather


4 Climate change regime and mitigation
5 Challenges to the Consensus View

5.1 Natural variability
5.2 Issue of consensus
5.3 Modeling inaccuracy
5.4 Politicization, bias, and credibility
5.5 Mitigation challenges


6 Alternative Views

6.1 Warming, but not anthropogenic
6.2 Warming minimal or not harmful
6.3 Potential for global cooling


7 Politicization, bias, and suppression of opposing views
8 See also
9 References
10 Credits



This is a highly charged issue. Concern over negative impacts—rising sea levels, increased drought and severity of storms, impact on fresh water, and so forth—have led to passionate appeals and the advance of international and national mitigation strategies. The backing of prominent international, governmental, and scientific bodies, such as the International Panel on Climate Change (IPCC), have led to the issue in terms of human activity being the dominate cause of contemporary climate change and the need for mitigation often being treated as settled science. Equally passionate are those who hold that the human contribution or the threats are overstated or that mitigations have a poor cost/benefit ratio in terms of human well-being, including potential negative impacts of an overly aggressive rejection of fossil fuels as a mitigation strategy.

Definitions
The general and narrow definitions can be seen in those presented by the Intergovernmental Panel on Climate Change (IPCC) and the United Nations Framework Convention on Climate Change (UNFCCC).
The IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess climate changes (scientific basis, impacts and future risk, mitigation and adaptation strategies). It published major Assessment Reports in 1990, 1995, 2001, 2007, 2014, and 2022, with the most recent being the Sixth Assessment Report. A glossary published by the IPCC in 2021 defined climate change as follows (IPCC 2021b):


Climate change. A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use.


The IPCC further defines climate, climate system, and global warming as follows (IPCC 2021b):

Climate. Climate in a narrow sense is usually defined as the average weather, or more rigorously as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization (WMO). The relevant quantities are most often surface variables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a statistical description, of the climate system. 
Climate system. The global system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere and the interactions between them. The climate system changes in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations, orbital forcing, and anthropogenic forcings such as the changing composition of the atmosphere and land-use change.
Global warming. Global warming refers to the increase in global surface temperature relative to a baseline reference period, averaging over a period sufficient to remove interannual variations (e.g., 20 or 30 years). A common choice for the baseline is 1850–1900 (the earliest period of reliable observations with sufficient geographic coverage), with more modern baselines used depending upon the application.
The United Nations Framework Convention on Climate Change (UNFCCC) was signed by 154 states in June 1992 at the Earth Summit in Rio de Janeiro and entered into force in March 1994. In its Article 1 (Definitions), it defines climate change as anthropogenic in nature:

2. “Climate change” means a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.
Some definitions, such as provided by NOAA Climate.gov, include the consequences of global warming as part of the definition of climate change (Kennedy and Lindsey 2015):

Global warming refers only to the Earth's rising surface temperature, while climate change includes warming and the "side effects" of warming—like melting glaciers, heavier rainstorms, or more frequent drought. Said another way, global warming is one symptom of the much larger problem of human-caused climate change.
Consensus view: three components
In discussing the consensus view of climate change, the term "consensus" is used not in the sense of unanimity, that everyone agrees, but in the sense of a general or broad agreement. It is the view held by a large swath of scientists and international and governmental bodies.
There are three basic components of the consensus view of anthropogenic climate change:

An increase in GHGs should result in an increase in temperature: the greenhouse effect.
There has been an observed increase in GHG concentrations since the industrial revolution, considered to be attributed to human activities.
There has been an increase in the average temperature of the Earth’s near surface air and oceans since the mid-20th century, and this is most likely a result of the observed increase in anthropogenic GHG concentrations.
From these there is also consensus on impacts from the warming of the planet.

An increase in GHGs should result in an increase in temperature: the greenhouse effect
 Greenhouse effect
Because of the greenhouse effect, a significant increase in greenhouse gases should translate to increase in global mean temperature.
The greenhouse effect is a heat-trapping process by which a planet's atmosphere allows a portion of radiation from its host star to pass through to warm the planet, but partially captures the heat radiated back toward space from the planet. This process results in a planet that is warmer than would occur without this effect.
In terms of the Earth, electromagnetic radiation from the Sun is received in the form of visible, infrared, and ultraviolet radiation (about 90% is within the visible and infrared ranges). About 26% of the incoming solar energy is reflected back to space by the atmosphere and clouds, and 19% is absorbed by the atmosphere and clouds. Most of the remaining energy is absorbed by the land and water surfaces of Earth. The warmed surface re-radiates thermal radiation back toward space with longer wavelengths (long-wave infrared radiation). Some of this thermal radiation is absorbed by the atmosphere, and re-radiated both upwards and downwards; that radiated downwards is absorbed by the Earth's surface. Thus, the presence of the atmosphere results in the surface receiving more radiation than it would were the atmosphere absent; and it is thus warmer than it would otherwise be.

 Global emissions based on global emissions from 2010
The key to this effect is the infrared-absorbing constituents of the atmosphere and most particularly the greenhouse gases. Most components in the atmosphere are transparent to the infrared radiation emitted from the surface toward space, but greenhouse gases are effective in capturing this radiation. Indeed, the presence of greenhouse gases, despite being only a small component of the atmosphere, makes the atmosphere largely opaque to the longer wavelengths emitted by the surface. The greenhouse gas molecules absorb the radiation and then emit radiant heat in all directions: some out to space, some warming other greenhouse gas molecules and passing heat on to the surrounding air, and some radiating back downwards to the Earth's surface, increasing the surface temperature. Without Earth's natural greenhouse effect the Earth would be more than 30°C (54°F) colder than its current average surface temperature of about 14°C to 15°C (57-59°F).
The IPCC Sixth Assessment Report working group 1 defines the greenhouse effect and greenhouse gases as follows (IPCC 2021b):


Greenhouse effect. The infrared radiative effect of all infrared-absorbing constituents in the atmosphere. Greenhouse gases (GHGs), clouds, and some aerosols absorb terrestrial radiation emitted by the Earth's surface and elsewhere in the atmosphere. These substances emit infrared radiation in all directions, but, everything else being equal, the net amount emitted to space is normally less than would have been emitted in the absence of these absorbers because of the decline of temperature with altitude in the troposphere and the consequent weakening of emission. An increase in the concentration of GHGs increases the magnitude of this effect; the difference is sometimes called the enhanced greenhouse effect. The change in a GHG concentration because of anthropogenic emissions contributes to an instantaneous radiative forcing. Earth's surface temperature and troposphere warm in response to this forcing, gradually restoring the radiative balance at the top of the atmosphere.
Greenhouse gases (GHGs). Gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary GHGs in the Earth’s atmosphere. Humanmade GHGs include sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs); several of these are also O3-depleting (and are regulated under the Montreal Protocol)
 Carbon emissions from fossil fuels
The major atmospheric gases causing such greenhouse effects (those that can absorb and emit infrared radiation) are:

Water vapor (H2O). Most potent greenhouse gas owing to the high concentration in the atmosphere (0.4% overall) and the presence of the hydroxyl bond, which strongly absorbs in the infrared region of the light spectrum
Carbon dioxide (CO2). About 0.04% of the atmosphere, it is the most potent greenhouse gases in terms of human contribution. Produced by combustion (coal, oil, natural gas, etc.), respiration, volcanoes, hot springs, geysers, and so forth. It absorbs strongly in the infrared and near-infrared.
Methane (CH4). Only about 0.0002% of the atmosphere, but it is more efficient at trapping radiation than carbon dioxide. Pound for pound, the comparative impact of methane on warming is about 21 times that of carbon dioxide over a 100-year period. It is the second most prevalent greenhouse gas from human activities. Methane also comes from coal, rice farms, biofuel consumption, and is produced when methanogenic bacteria utilize organic matter in moist places that lack oxygen. The most favorable sites of methane productions are swamps and other natural wetlands, paddy fields, landfills, as well as the intestines of ruminants, termites, and so forth.
Nitrous oxide (N2O). Nitrous oxide is released in the atmosphere from burning biomass, nitrogen-rich fossil fuel (especially coal), nylon production, etc.
Chlorofluorocarbons (CFCs). CFCs are being phased out by the Montreal Protocol because they contribute to ozone depletion, but the previously introduced enormous amount is still active.
Ozone (O3). Comprises about 0.000004% of the atmosphere.
Perfluorocarbons (PFCs)
Although molecules of CFCs, methane, and nitrous oxide absorb and radiate much more infrared per molecule than carbon dioxide, the much larger input of carbon dioxide makes it the most important greenhouse gas produced by human activities.
Because of the greenhouse effect, all other things being equal, a significant increase in greenhouse gases should translate to increase in global mean temperature. The heat-trapping nature of carbon dioxide and other greenhouse gases was demonstrated in the mid-19th century and is not in dispute.

GHG concentrations have increased significantly since the industrial revolution
 Carbon dioxide levels over last 800,000 years
 Recent Carbon dioxide levels measured at Mauna Loa Observatory
There has been an observed increase in greenhouse gas concentrations since the industrial revolution, considered to be attributed to human activities, such as from burning of fossil fuels and deforestation.
The levels of carbon dioxide in the atmosphere can actually be calculated for the distant past via air bubbles trapped in ice. Over the past 400,000 years, the levels of CO2 have generally ranged from around 200 parts per million (ppm) in colder periods, such as ice ages, to around 280 ppm in warmer periods. However, the levels greatly increased since the industrial revolution and surpassed 400 ppm in 2013. Emissions from fossil fuels show a similar increase.
The 2014 Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC 2014) noted the following:

Anthropogenic greenhouse gas (GHG) emissions since the pre-industrial era have driven large increases in the atmospheric concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). . . .
Anthropogenic greenhouse gas emissions have increased since the pre-industrial era, driven largely by economic and population growth... This has led to atmospheric concentrations of carbon dioxide, methane, and nitrous oxide that are unprecedented in at least the last 800,000 years.
 Recent global methane levels
The most recent Sixth Assessment Report (AR6) reported the following (IPCC 2021a):

Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities. Since 2011 (measurements report in AR5), concentrations have continued to increase in the atmosphere, reaching annual averages of 410 parts per million (ppm) for carbon dioxide (CO2), 1866 parts per billion (ppb) for methane (CH4), and 332 ppb for nitrous oxide (N2O) in 2019.
In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years (high confidence), and concentrations of CH4 and N2O were higher than at any time in at least 800,000 years (very high confidence). Since 1750, increases in CO2 (47%) and CH4 (156%) concentrations far exceed—and increases in N2O (23%) are similar to—the natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years (very high confidence).
Note that for the Sixth Assessment Report, the level of confidence for a finding is expressed using the five qualifiers of very low, low, medium, high and very high, while the likelihood of an outcome or result is expressed as virtually certain (99–100% probability), very likely (90–100%), likely (66–100%), about as likely as not (33–66%), unlikely (0–33%), very unlikely (0–10%), and exceptionally unlikely (0–1%). The terms extremely likely (95–100%), more likely than not (50–100%), and extremely unlikely (0–5%) are also employed. Square brackets [x to y] are used to provide the assessed very likely range, or 90% interval.

Global temperature considered to have increased and as a result of anthropogenic GHG concentrations
 Yearly global temperatures from 1880-2021 compared to the twentieth-century average
A substantial increase in global temperatures (land and ocean surface temperatures and that of the atmosphere) has been observed in recent decades and this has been attributed to human-caused increases in GHG concentrations.
According to the 2014 Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC 2014):

The globally averaged combined land and ocean surface temperature data . . . show a warming of 0.85 [0.65 to 1.06]°C over the period 1880 to 2012. [90% confidence of range in brackets]
Warming of the climate system is unequivocal.
Increase in anthropogenic greenhouse gas emissions since the pre-industrial era has led to high atmospheric concentrations of carbon dioxide, methane, and nitrous oxide ... Their effects, together with those of other anthropogenic drivers . . . are extremely likely to have been the dominant cause of the observed warming since the mid-20th century.
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together.
According to the 2021 Contribution of Working Group 1 to the Sixth Assessment Report (AR6) of the IPCC (IPCC 2021a):

It is unequivocal that human influence has warmed the atmosphere, ocean, and land.
Each of the last four decades has been successively warmer than any decade that preceded it since 1850. Global surface temperature in the first two decades of the 21st century (2001-2020) was 0.99 [0.84 to 1.10] °C higher than 1850-1900. Global surface temperature was 1.09 [0.95 to 1.20] °C higher in 2011-2020 than 1850-1900, with larger increases over land (1.59 [1.34 to 1.83] °C) than over the ocean (0.88 [0.68 to 1.01] °C).
The likely range of total human-caused global surface temperature increase from 1850–1900 to 2010–201911 is 0.8°C to 1.3°C, with a best estimate of 1.07°C. It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by –0.1°C to +0.1°C, and internal variability changed it by –0.2°C to +0.2°C. It is very likely that well-mixed GHGs were the main drivers of tropospheric warming since 1979 and extremely likely that human-caused stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s.
Human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years.
It is virtually certain that the global upper ocean (0–700 m) has warmed since the 1970s and extremely likely that human influence is the main driver. It is virtually certain that human-caused CO2 emissions are the main driver of current global acidification of the surface open ocean.
Observed warming is driven by emissions from human activities, with greenhouse gas warming partly masked by aerosol cooling.
Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence). Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago [0.2°C to 1°C relative to 1850–1900] (medium confidence). Prior to that, the next most recent warm period was about 125,000 years ago, when the multi-century temperature [0.5°C to 1.5°C relative to 1850–1900] overlaps the observations of the most recent decade (medium confidence).
Evidences/Consequences
Direct support for the consensus view comes from observed increases in global land and ocean temperatures, combined with expectations from the greenhouse effect and the documented increases in GHGs in the atmosphere, which in turn correlate with increased emissions from fossil fuels. Ocean acidification also correlates with the increase in carbon dioxide in the atmosphere.
Additional indirect support for the consensus view comes from predicted consequences of global warming that have been reported, such as rising sea levels, shrinking ice sheets, and increase in extreme weather.

Global temperature rise
Since the late nineteenth century, the average, global surface temperature has risen about 1 degrees Celsius (about 2 degrees Fahrenheit). The IPCC (2021a) reports that global surface temperature from 2001–2020 was 0.99 °C higher than 1850–1900, and from 2011-2020 was 1.09°C higher than 1850–1900, with an increase of 1.59°C over land and 0.88°C over the ocean. It estimates that the human-caused global surface temperature increase from 1850-1900 to 2010-2019 is 1.07°C. The IPCC further reports that each of the last four decades was warmer than the previous (and than any since 1850) and NASA (2022) notes that the years 2016 and 2020 tied for the warmest years on record during this time. The IPCC further concludes (with high confidence) that global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years.

 Average Global Sea Surface Temperature, 1880–2020
The IPCC concludes that it is likely that increased GHGs contributed a warming of 1.0°C to 2.0°C during the time period from 1850-1900 to 2010-2019, that natural drivers had an impact of by –0.1°C to +0.1°C, and that there were also cooling factors (mainly aerosols) that reduced temperatures from 0.0°C to 0.8°C.

Warming of oceans
 Heat Content in the Top 700 Meters of the World’s Oceans, 1955–2020
Much of the global surface temperature increase has been stored in the ocean, with the IPCC’s Fifth Assessment Report (AR5) noting that more than 90% of the energy increase between 1971 and 2010 was stored via ocean warming. NASA (2022) reports that the top 100 meters of the ocean have warmed more than 0.33 degrees Celsius (0.6 degrees Fahrenheit) since 1969. The U.S. Environmental Protection Agency (EPA 2022b) reports that from 1901 through 2020, the sea surface temperature rose at an average rate of 0.14°F per decade. This ocean warming is greatest nearest the surface of the ocean (IPCC 2014). Factors that can impact year-to-year changes in ocean heat include the greenhouse effect, volcanic eruptions, and ocean-atmosphere patters, such as El Niño (EPA 2022a). The EPA (2022b) notes that since reliable observations began in 1880, seas surface temperature has been higher during the past three decades; the EPA also notes, however, that there was also an increase between 1910 and 1940, relative stasis from 1940 to 1970, a cooling between 1880 and 1910, and a present-day warming between 1970 and the present. The EPA further notes that changes in temperature vary regionally and there has been a cooling in some areas recently, notably the North Atlantic.
The IPCC (2021a) reports, with medium confidence, that the warming has been faster in the past century than since at least 11,000 years ago. It also concludes that "It is virtually certain that the global upper ocean (0-700 m) has warmed since the 1970s and extremely likely that human influence is the main driver."

Ocean acidification
Increased carbon dioxide in the atmosphere has resulted in increased ocean carbon dioxide levels and thus an increase in acidity (decrease in pH). NASA (2022) notes that “since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30%.”

Sea level rise
 Global Average Absolute Sea Level Change, 1880–2019
 Malé, the capital of Maldives
The IPCC (2021a) reports that the global mean sea level has risen by 20 centimeters (about 8 inches) between 1901 and 2018, with the average rate being 1.3 mm per year (0.05 inches/year) between 1901 and 1971, then 1.9 mm (0.07 inches) per year between 1971 and 2006, and subsequently by 3.7 mm (0.15 inches) per year.
The IPCC reports high confidence that the rate since 1900 is faster than any preceding century over the past 3000 years.
The EPA (2022d) reports that regional and local changes in land movement, combined with coastal circulation patterns results in the relative sea level increase not being uniform: in some places along the US coastline (notably areas of Mid-Atlantic and Gulf coasts) it increased more than 8 inches between 1960 and 2020, while in some areas relative sea levels fell (such as Alaska and Pacific Northwest), where the elevation of land exceeded the rise in absolute sea level.

Shrinking ice sheets and Arctic sea ice, retreating glaciers, and decreased snow cover
An ice sheet (or continental glacier) is a large body of ice on land. Currently there are two; the Greenland ice sheet and the Antarctic ice sheet. The Greenland ice sheet averages more than a mile thick in the interior and has an estimated 700,000 cubic miles of ice; the Antarctic ice sheet has a volume of about 6 million cubic miles and in places is nearly 3 miles thick. These two ice sheets hold about 70% of the world's fresh water (EPA 2022e).
Sea ice is frozen sea water that floats on water and covers about 12% of the Earth's oceans, with much in the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. In the winter months, essentially the entire Arctic Ocean is covered by sea ice (EPA 2022g).
A glacier is a large mass of ice and snow that is present year round and naturally flows like a river, only much more slowly. Examples can be found in the Rocky Mountains and throughout Alaska (EPA 2022f).
According to the NASA (2022), based on their Gravity Recovery and Climate Experiment, the Greenland ice sheet have decreased by an average of 279 billion tons of ice per year from 1993 to 2019, while Antarctic ice sheet lost about 148 billion tons per year during that time period. The IPCC (2021a) reports that it is very likely that human influence has contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades, but there is only limited evidence, with medium agreement, of human influence on the Antarctic Ice Sheet mass loss.
Regarding sea ice, the IPCC (2021a) reports with high confidence that "in 2011–2020, annual average Arctic sea ice area reached its lowest level since at least 1850," and with medium confidence that the "late summer Arctic sea ice was smaller than at any time in at least the past 1000 years." The EPA (2022g) reports that "September 2012 had the lowest sea ice extent ever recorded." The IPCC (2021a) reports that "human influence is very likely the main driver of . . . the decrease in Arctic sea ice area between 1979–1988 and 2010–2019."
Glaciers are reported to be "retreating almost everywhere around the world—including in the Alps, Himalayas, Andes, Rockies, Alaska, and Africa" (NASA 2022). The EPA (2022f) reports that worldwide glaciers "have been losing mass since at least the 1970s," with suggestions of shrinking since the 1950s. The IPCC reports that "human influence is very likely the main driver of the global retreat of glaciers since the 1990s.
Regarding snow cover, NASA (2022) reports that "satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and the snow is melting earlier." The IPCC (2021a) reports that "human influence very likely contributed to the decrease in Northern Hemisphere spring snow cover since 1950."

Extreme weather
The IPCC (2022a), in their Sixth Assessment Report (AR6), states that "human-induced climate change is already affecting many weather and climate extremes in every region across the globe," noting such impacts as changes in heavy precipitation events, droughts, heatwaves, and tropical cyclones. It further notes that "it is virtually certain that hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe." It records that the heavy precipitation events have increased in frequency and intensity since the 1950s over most land areas with sufficient observation data for trend analysis and that it is likely that the occurrence of major tropical cyclones (categories 3-5) has increased in proportion over the last four decades.
Regarding the United States, NASA (2022) reports that "The number of record high temperature events in the United States has been increasing, while the number of record low temperature events has been decreasing, since 1950. The U.S. has also witnessed increasing numbers of intense rainfall events."

Climate change regime and mitigation
 Global Carbon dioxide emissions by country
Climate change is an issue that cuts across national borders and has become an important focus of international relations. In international relations, the term ‘’regime’’ can refer to those systems (policy, regulatory, administrative) that are formed to coordinate and regulation actions in particular issues. A narrow definition is provided by Axelrod and VanDeveer (2019):

A system of principles, norms, rules, operating procedures, and institutions that actors create or accept to regulate and coordinate action on a particular area of international relations.
For the climate change regime, one key institution is the Intergovernmental Panel on Climate Change (IPCC). Organized in 1988 by the United National Environment Programme (UNEP) and the World Meteorological Organization (WMO), the IPCC is the United Nations body that prepares reports on the scientific, technical and socio-economic knowledge, impacts, and future risks related to climate change and options for mitigation. The IPCC has completed six major assessment reports, as well as other special reports.
Another key institution of the climate change regime is the United National Framework Convention on Climate Change (UNFCCC), its secretariat, and its annual Conference of the Parties (COP). The UNFCCC was signed by 154 countries at the Rio Conference (Earth Summit, United Nations Conference on	Environment and Development) in 1992 and entered into force in 1994. As a framework convention, it does not provide for legally binding limits on emissions, but gave rise to agreements that did have binding targets and timetables (Kyoto Protocol, Paris Agreement) and the Green Climate Fund (established in 2010 at UN Climate Change Conference in Cancun)
The climate change regime includes principles, norms, rules, and procedures of the UNFCCC, Kyoto Protocol, and Paris Climate Agreement. One such principle is that of "common but differentiated responsibilities," with the parties agreeing that all States have a common responsibility, but that those that have a greater capability to address the issue, and which have the largest share of the historical and current global emissions of GHGs, should take more responsibility. Another principle is the "precautionary principle," that, in essence, the absence of full scientific certainty is not a reason to avoid taking action to mitigate the threat or impact of climate change.
The climate change regime centered on the consensus view has had a major focus on mitigation in recent years.
The IPCC (2022b) refers to climate change mitigation as:

Climate change mitigation refers to actions or activities that limit emissions of GHGs from entering the atmosphere and/or reduce their levels in the atmosphere. Mitigation includes reducing the GHGs emitted from energy production and use (eg. that reduces use of fossil fuels) and land use, and methods to mitigate warming eg. by carbon sinks which remove emissions from the atmosphere through land use or other (including artificial) mechanisms.
Working Group III's contribution to the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change, which dealt with "Mitigation of Climate Change," emphasizes the importance of mitigation (IPCC 2022a): "Without urgent, effective and equitable mitigation actions, climate change increasingly threatens the health and livelihoods of people around the globe, ecosystem health and biodiversity." It notes that without strong mitigation efforts, it is "likely that warming will exceed 1.5°C during the 21st century. Likely limiting warming to below 2°C would then rely on a rapid acceleration of mitigation efforts after 2030." Emphasized in the report is the importance of transitioning from fossils fuels and, where fossil fuels are used, the development and widespread employment of new technologies for carbon capture and storage (CCS), where carbon dioxide is captured and stored before it can escape to the atmosphere, such as turning it into a liquid and injecting it underground. From this report:


Reducing GHG emissions across the full energy sector requires major transitions, including a substantial reduction in overall fossil fuel use, the deployment of low-emission energy sources, switching to alternative energy carriers, and energy efficiency and conservation. The continued installation of unabated fossil fuel infrastructure will 'lock-in' GHG emissions.
All global modelled pathways that limit warming to 1.5°C (>50%) with no or limited overshoot, and those that limit warming to 2°C (>67%) involve rapid and deep and in most cases immediate GHG emission reductions in all sectors. Modelled mitigation strategies to achieve these reductions include transitioning from fossil fuels without CCS to very low- or zero-carbon sources such as renewables or fossils fuels with CCS . . .
Net-zero CO2 energy systems entail: a substantial reduction in overall fossil fuel use, minimal use of unabated fossil fuels, and use of CCS in the remaining fossil fuel system; electricity systems that emit no net CO2; widespread electrification of the energy system including end uses; energy carriers such as sustainable biofuels….."


The report emphasizes a transition to electric vehicles and the importance of equity (IPCC 2022a):


Electric vehicles powered by low-GHG emissions electricity have large potential to reduce land-based GHG emissions on a life cycle basis (high confidence).
Equity remains a central element in the UN climate regime, notwithstanding shifts in differentiation between states over time and challenges in assessing fair shares.


Johnsson et al. (2019) conclude that achieving the Paris Agreement target of keeping the global average temperature increase to below 2°C above pre-industrial limits will require “a disruptive change in the use of fossil fuels” and specifically “emissions from fossil use must indeed approach zero in the early second half of this century.” They state that this “can only be achieved” via two fossil fuel options:

There are only two main options on the table to significantly limit their contribution to global warming: To leave the fossils fuels in the ground or to apply carbon capture and storage (CCS) technology (and, of course, a combination of these).

Challenges to the Consensus View
Natural variability
Natural variability in global climate, including decades-and-longer trends of warming and cooling temperatures, have been a pervasive part of geologic history.
For example, the Younger Dryas was a period of cooling that interrupted a long, warming transition from a cold glacial world to a warmer interglacial state (NOAA 2022). This near-glacial state began about 12,900 years ago and ended about 11,500 years ago. The United States' National Oceanic and Atmospheric Administration (NOAA) states regarding this cooling period, “The Younger Dryas is clearly observable in paleoclimate records from many parts of the world. In the Cariaco Basin north of Venezuela, for example, temperatures decreased about 3°C (5.5°F) (NOAA 2022). NOAA notes the abrupt increase in temperature that occurred at the end of this cooling period:

The end of the Younger Dryas, about 11,500 years ago, was particularly abrupt. In Greenland, temperatures rose 10°C (18°F) in a decade (Alley 2000). Other proxy records, including varved lake sediments in Europe, also display these abrupt shifts (Brauer et al. 2008).
Another example is the Last Glacial Maximum, or Late Glacial Maximum (LGM). Occurring about 29,000 to 19,000 years ago, the United States Geological Survey (USGS 2022) notes that glaciers during the LGM covered approximately 8% of the Earths surface and 25% of Earth’s land area. According to a study by Tierney et al. (2020), also reported by the University of Arizona (2020), the average global temperature around 20,000 years ago was about 6.1 °C (11°F) colder than today, being about 7.8°C (46°F) versus the contemporary average of about 14°C (57°C). Tierney et al. (2020) also reported that high latitudes, such as the Arctic, experienced the biggest cooling, being about 14°C (25°F) colder than today.
The natural variation of temperature over time makes it challenging to determine the amount of warming that can be attributed to natural versus anthropogenic causes. The IPCC reports the global surface temperature has risen about 1°C (about 2°F) since the late 19th century. However, the Younger Dryas showed in Greenland a temperature rise of 10°C in a decade, and about 20,000 years ago during the LGM the temperature was about 6°C colder than today. The Nongovernmental International Panel on Climate Change (NIPCC) claims the present warming falls within the natural variation (Idso et al. 2016):


Over recent geological time, Earth’s temperature has fluctuated naturally between about +4°C and -6°C with respect to twentieth century temperature. A warming of 2°C above today, should it occur, falls within the bounds of natural variability.

The overall warming since about 1860 corresponds to a recovery from the Little Ice Age modulated by natural multidecadal cycles driven by ocean-atmosphere oscillations, or by solar variations at the de Bries (~208 year) and Gleissberg (~80 year) and shorter periodicities.
The NIPCC further concludes (Idso et al. 2016):

Neither the rate nor the magnitude of the reported late twentieth century surface warming (1979-2000) lay outside normal natural variability.
Melting of Arctic sea ice and polar icecaps is not occurring at “unnatural” rates and does not constitute evidence of a human impact on the climate.

No convincing relationship has been established between warming over the past 100 years and increases in extreme weather events. Meteorological science suggests just the opposite: A warmer world will see milder weather patterns. 
On the other hand, the IPCC (2021a) concludes that recent changes in temperature are unprecedented over the past 2000 years and assess that natural drivers had at most an impact of +0.1°C in the 1°C rise in temperature:

The scale of recent changes across the climate system as a whole—and the present state of many aspects of the climate system—are unprecedented over many centuries to many thousands of years.
Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence). Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago [0.2°C to 1°C relative to 1850–1900] (medium confidence).

It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by –0.1°C to +0.1°C, and internal variability changed it by –0.2°C to +0.2°C. It is very likely that well-mixed GHGs were the main driver of tropospheric warming since 1979 and extremely likely that human-caused stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s.
Issue of consensus
 Bar chart representation of scientific consensus on human-caused global warming
The term consensus is often used to indicate strong scientific support for the view that contemporary climate change/global warming is caused by human activity. For example, a 2016 Forbes article notes that the "claim that there is a 97% consensus among scientists that humans are the cause of global warming is widely made in climate change literature and by political figures," and "has been heavily publicized" (Ritchie 2016). An example of a study on consensus would be the 2021 one by Lynas et al. titled "Greater than 99% consensus on human caused climate change in the peer-reviewed scientific literature." Ramanujan (2021) summarizes and elaborates on this study as follows:


More than 99.9% of peer-reviewed scientific papers agree that climate change is mainly caused by humans, according to a new survey of 88,125 climate-related studies.
“We are virtually certain that the consensus is well over 99% now and that it’s pretty much case closed for any meaningful public conversation about the reality of human-caused climate change,” said Mark Lynas, a visiting fellow at the Alliance for Science and the paper’s first author.


Critics of the view that humans are the dominant cause of climate change assert that the claim on scientific consensus does not reflect reality, and that the studies claiming such consensus have fatal flaws. Among the flaws cited are selection bias in terms of the scientists surveyed or published articles included, and the inclusion of opinions and articles by nonscientists and non-experts who write about climate. Climate science is a very complex subject matter involving many thousands of scientists spanning a multitude of disciplines——physics, biology, chemistry, geology, geochemistry, biogeochemistry, cosmology, meteorology, oceanography, paleontology, climatology, paleoclimatology, paleotempestology, and so forth—who critics claim are incompletely or selectively surveyed. Another flaw advanced is numbering among those scientists and articles included as supporting human-caused climate change those which merely assert humans have some impact on warming rather than humans being the dominant cause. There is also the criticism that some of the consensus claims are based only on abstracts of publications, rather than what is actually in the published article, or inclusion of articles where the cause of climate change is only tangentially addressed.
Idso et al. (2016), in their report Why Scientists Disagree About Global Warming: The NIPCC Report on Scientific Consensus, note the following:


Probably the most widely repeated claim in the debate over global warming is that "97 percent of scientists agree" that climate change is man-made and dangerous. This claim is not only false, but its presence in the debate is an insult to science.
The articles and surveys most commonly cited as showing support for a "scientific consensus" in favor of the catastrophic man-made global warming hypothesis are without exception methodologically flawed and often deliberately misleading.
Many prominent experts and probably most working scientists disagree with the claims made by the United Nations' Intergovernmental Panel on Climate Change (IPCC).
In contrast to the studies described above, which try but fail to find a consensus in support of the claim that global warming is man-made and dangerous, many authors and surveys have found widespread disagreement or even that a majority of scientists oppose the alleged consensus. These surveys and studies generally suffer the same methodological errors as afflict the ones described above, but they suggest that even playing by the alarmists' rules, the results demonstrate disagreement rather than consensus.
There is no scientific consensus on global warming.


Idso et al. (2016) further cite the Global Warming Petition Project, a 2015 statement signed by 31,478 American scientists, including 9,012 with Ph.D.s, that states, in part, "There is not convincing scientific evidence that human release of carbon dioxide, methane, or other greenhouse gases is causing or will , in the foreseeable future, cause catastrophic heating of the Earth's atmosphere and disruption of the Earth's climate." They note that this is "more than ten times as many scientists" as have participated in the IPCC's Fourth Assessment Report; that they "actually endorse the statement"; and, "by contrast, fewer than 100 of the scientists (and nonscientists) who are listed in the appendices to IPCC reports actually participated in writing of the all-important Summary for Policymakers or the editing of the final report."
Other criticisms of the promotion of consensus is that it is used as an effort to shut down opposing views, a charge reflected in the above-noted comment by Lynas on his survey claiming consensus, "it's pretty much case closed for any meaningful public conversation about the reality of human-caused climate change." It is noted that consensus is a term more appropriate for politics than science. In this respect, author and filmmaker Michael Crichton weighed in on a particular flaw of consensus science in general:


I want to pause here and talk about this notion of consensus, and the rise of what has been called consensus science. I regard consensus science as an extremely pernicious development that ought to be stopped cold in its tracks. Historically, the claim of consensus has been the first refuge of scoundrels; it is a way to avoid debate by claiming that the matter is already settled. Whenever you hear the consensus of scientists agrees on something or other, reach for your wallet, because you're being had.

Let's be clear: The work of science has nothing whatever to do with consensus. Consensus is the business of politics. Science, on the contrary, requires only one investigator who happens to be right. . . . The greatest scientists in history are great precisely because they broke with the consensus. . . . In addition, let me remind you that the track record of the consensus is nothing to be proud of.
Modeling inaccuracy
Global climate models (GCMs) are integral to governments and intergovernmental organizations forecasting the effects of changes in GHG emissions and atmospheric concentration on temperature and climate in general. These have played a central role in IPCC reports and as a foundation for mitigation strategies.
However, despite the models' pivotal importance in policy making and the expenditure of vast sums and use of ever-more powerful computers, the models have often proven inadequate in their forecasts. The models need to incorporate complex processes related to the atmosphere, land, oceans, ice, and biosphere, of which many of the dynamics are not completely understood, requiring input of assumptions.
Steven E. Koonin, who was undersecretary of science in the Energy Department under U.S. President Barack Obama, stated parts of the models "involve technically informed estimation" and concluded that "computer modeling of complex systems is as much an art as a science" (Koonin 2014). The NIPCC was more pointed, calling GCMs "speculative thought experiments by modelers who often lack a detailed understanding of underlying processes" (Idso et al. 2016).
Koonin (2014) further noted that "as far as the computer models go, there isn't a useful consensus at the level of detail relevant to assessing human influences." Discussing the IPCC's Fifth Assessment Report, he presented several shortcomings in terms of models, including "they differ in their descriptions of the past century's global average surface temperature by more than three times the entire warming recorded during that time," and failed to predict the growth of Antarctic sea ice.
Richard S. Linzen, an atmospheric physicist at MIT, who has developed models for Earth's climate, including sensitivity to increases in carbon dioxide, has stated "the direct impact of doubling carbon dioxide on the Earth's temperature is rather small: on the order of .3 degrees C. Larger predictions depend on positive feedbacks…. [T]hose factors arise from models with errors in those factors" (Idso et al. 2016). The NIPCC states that "GCMs systematically over-estimate the sensitivity of climate to carbon dioxide (CO2), many known forcings and feedbacks are poorly modeled, and modelers exclude forcings and feedbacks that run counter to their mission to find a human influence on climate" (Idso et al. 2016).
The IPCC's Fifth Assessment Report itself notes: “For the period from 1998 to 2012, 111 of the 114 available climate-model simulations show a surface warming trend larger than the observations.”
Regarding this later assessment, part of the problem may be the unanticipated slowdown in warming experienced from 1998 to 2012. Sometimes called the "global warming hiatus" or "global warming pause," it was a period of relatively stable temperatures or at least temperatures rising at a substantially slower rate that those immediately prior. The Fifth Assessment Report (AR5) noted that from 1998 to 2012 the global surface temperatures did not show a statistically significant increase, despite a significant increase in carbon dioxide in the atmosphere.
Many explanations have been presented to account for this relatively flat period, including the buildup of sub-surface ocean temperatures, natural variability, longer than usual solar minimum cycle, low amounts of water vapor in the stratosphere, volcanic eruptions, the exceptionally warm 1998 El Niño year, and cherry-picking the time period. Such factors could explain why models show a warming trend greater than what really occurred.
Nonetheless, the fact that the models failed to capture this showdown exhibit the difficulty of modeling such complex phenomena as climate. Koonin (2014 )noted, regarding the hiatus:


[T]he models famously fail to capture this slowing in the temperature rise. Several dozen different explanations for the failure have been offered, with ocean variability most likely playing a major role. But the whole episode continues to highlight the limits of our modeling.


Politicization, bias, and credibility
See Politicization, bias, and suppression of opposing views below for an overview of this issue relative to climate change in general. This section summarizes a few of the criticisms lobbied by the Nongovernmental International Panel on Climate Change (NIPCC) regarding the IPCC and its reports.
Idso et al. (2016), authors of the second edition of the NIPCC publication Why Scientists Disagree About Global Warming, critique the IPCC assessment reports as political (politicized selection of contributors, political interference), biased (exclusion of opposing views), and lacking scientific credibility (use of "fake confidence intervals," "reliance on unpublished and non-peer-reviewed sources," "use of secret data" unwilling to share with critics).
Idso et al. (2016) further state:


IPCC, created to find and disseminate research finding a human impact on global climate, is not a credible source. It is agenda-driven, a political rather than scientific body, and some allege it is corrupt.
IPCC's reports have been subjected to withering criticism by scientists and authors almost too numerous to count, including even high-profile editors and contributors to its reports… and no fewer than six rigorously research books by one climate scientist, Patrick Michaels, former president of the American Association of State Climatologists…"
IPCC misrepresents its findings and does not properly peer review its reports. The selection of scientists who participate is politicized.
Politicians, environmental activists, and rent-seeking corporations in the renewable energy industry began to routinely quote IPCC's alarming claims and predictions shorn of the important qualifying statements expressing deep doubts and reservations. Rather than protest this mishandling of its work, IPCC encouraged it by producing Summaries for Policymakers that edit away or attempt to hide qualifying statements.


Mitigation challenges
The climate change consensus view, as reflected in the reports of the IPCC and the Paris Agreement, requires mitigation efforts that deal with the emissions from burning of fossil fuels.
This is a fuel source that has powered major advancements in transportation, industry, agriculture, health, and so forth, and is central to modern civilization. As summarized by Gross (2020), the use of coal, then oil and natural gas:


allowed rapid growth in industrial processes, agriculture, and transportation. The world today is unrecognizable from that of the early 19th century, before fossil fuels came into wide use. Human health and welfare have improved markedly. . . . The fossil fuel energy system is the lifeblood of the modern economy. Fossil fuels powered the industrial revolution, pulled millions out of poverty, and shaped the modern world. . . . We haven’t found a good substitute for oil, in terms of availability and fitness for purpose. Although the supply is finite, oil is plentiful and the technology to extract it continues to improve, making it ever-more economic to produce and use. The same is also largely true for natural gas


But Gross also notes that the use of fossil fuels “comes with a devastating downside” in terms of global warming.
The challenge to the mitigation component of the consensus view is a rapid transition from fuels that "are the lifeblood of the modern economy" and "for which we haven't found a good substitute" (Gross 2020). Johnsson et al. (2019) see a problem in the sheer abundance of this energy source: “the main obstacle to compliance with any reasonable warming target is the abundance of fossils fuels”; this “implies a tremendous threat to climate change mitigation.”
To reach emission goals, the consensus view of climate change requires tackling this central source of energy. As worded by Johnsson et al. (2019), to approach emission goals there needs to be a “highly disruptive reduction in the use of fossil fuels," with the two options being to leave the fossil fuels in the ground or apply CCS technologies. They note a number of avenues that can help foster this transition away from fossil fuels, including fossil fuel taxation, high carbon emissions pricing, emission performance standards “stringent enough to prevent the use of certain fossil fuels, such as coal as fuel in power plants,” phasing out of subsidies, and so forth. However, any rapid transition from fossil fuels has the likelihood of causing major disruptions in the quality of life: impacting jobs, inflation, transportation, and so forth.
The NIPCC (2019) sees transition from fossil fuels as part of a mitigation strategy as an unnecessary burden and a process being undertaken without a true cost/benefit analysis:


Economists have explained how proposals to force a transition away from fossil fuels advanced without an understanding of the true costs and implications of alternative fuels can lead to unnecessary expenses and minimal or even no net reduction in greenhouse gas emissions.
The IPCC and its national counterparts have not conducted proper cost-benefit analyses of fossil fuels, global warming, or regulations designed to force a transition away from fossil fuels. The global war on fossil fuels, which commences in earnest in the 1980s and reached a fever pitch in the second decade of the twenty-first century, was never founded on sound science or economics.
Rather than invest scarce world resources in a quixotic campaign based on politicized and unreliable science, world leaders would do well to turn their attention to the real problems their people and their planet face.


Alternative Views
Three of the common alternative viewpoints to the consensus view are: (1) Global average temperatures are increasing, but more due to natural causes than anthropogenic; (2) Any increases in temperatures and carbon dioxide are minimal and/or actually more beneficial than harmful; and (3) there is a risk of global cooling in the near future.

Warming, but not anthropogenic
A popular alternative view is that the present warming period is primarily a result of natural factors rather than the dominant cause being human activity. In other words, the observed trend in rising temperatures is part of a natural cycle of temperature fluctuations seen throughout geologic history.
Proponents of this view recognize the anthropogenic increases in carbon dioxide emissions and GHGs in the atmosphere, but claim the observed increases are not enough to make a significant temperature difference or that the carbon dioxide increases actually follow rather than precede the temperature increases.
Many of these points underlying this view can be seen in the “natural variability” subsection above. Following are some additional quotes that fit within this perspective, as offered by the Nongovernmental International Panel on Climate Change (NIPCC):


The human impact on global climate is small, and any warming that may occur as a result of human carbon dioxide (CO2) and other greenhouse gas emissions is likely to have little effect on global temperatures, the cryosphere (ice-covered areas), hydrosphere (oceans, lakes, and rivers), or weather (NIPCC 2014).
The late twentieth century warm peak was of no greater magnitude than previous peaks caused entirely by natural forcings and feedbacks (Idso et al. 2016).
Historically, increases in atmospheric CO2 followed increases in temperature, they did not precede them. Therefore, CO2 levels could not have forced temperatures to rise (Idso et al. 2016).
Solar forcings are not too small to explain twentieth century warming. In fact, their effect could be equal to or greater than the effect of CO2 in the atmosphere (Idso et al. 2016).
The Sun may have contributed as much as 66 percent of the observed twentieth century warming, and perhaps more (Idso et al. 2016).


Among scientists who have cautioned skepticism of the prevailing view that warming is decidedly anthropogenic in favor of openness to the warming being natural include S. Fred Singer, Craig D. Idso, Robert Carter, Richard S. Lindzen, Roy Spencer, and Ian Clark. (These same individuals have also expressed support for the second alternative viewpoint presented below, that the temperature increase has not been high enough to be catastrophic, and warming temperatures and increased carbon dioxide can be beneficial.)
S. Fred Singer, a former atmospheric and space physicist who founded the Science and Environmental Policy Project (SEPP) and the Nongovernmental International Panel on Climate Change (NIPCC), has stated the following (Singer 2016):


Climate change has been going on for millions of years—long before humans existed on this planet. Obviously, the causes were all of natural origin and not anthropogenic. There is no reason to think that these natural causes have suddenly stopped.
There are no sufficient conditions to establish the existence of any significant ACC [anthropogenic climate change] from existing data.


Ian Clark, a professor with the Department of Earth Sciences at the University of Ottawa, has stated that "natural forces have in the past and continue today to dominate the climate signal " (NIPCC 2014). In a testimony at a Canadian Senate Hearing, he stated, "What is driving global warming? Well, we have to look at the sun. . . . We have a reason that our climate is warming. It’s not carbon dioxide” (Clark 2011).
Lindzen, former Alfred P. Sloan Professor of Meteorology at the Massachusetts Institute of Technology, has stated:

Doubling CO2 involves a 2% perturbation to this budget. So do minor changes in clouds and other features, and such changes are common. In this complex multifactor system, what is the likelihood of the climate (which, itself, consists in many variables and not just globally averaged temperature anomaly) is controlled by this 2% perturbation in a single variable? Believing this is pretty close to believing in magic.

Warming minimal or not harmful
Another popular alternative view is that the observed temperature increase is not catastrophic, but relatively minor, and the rising temperatures and increased carbon dioxide levels may have a net positive impact. For example, warmer climates and increased carbon dioxide may offer more advantages than colder temperatures in terms of the ability to grow crops.
S. Fred Singer, mentioned above, has stated, "any warming observed during the past century appears to be trivially small and most likely economically beneficial overall," and "policies to limit CO2 emissions are wasting resources that could better be used for genuine societal problems like public health. They are also counter-productive since CO2 promotes plant growth and crop yields, as shown by dozens of agricultural publications (Singer 2016).
Richard S. Lindzen, also mentioned above, has stated (Lindzen 2017):


Nothing can alter the fact that the changes one is speaking about are small.
I have presented evidence as to why the issue is not a catastrophe and may likely be beneficial.
Despite this, climate change has been the alleged motivation for numerous policies, which, for the most part, seem to have done more harm than the purported climate change, and have the obvious capacity to do much more. Perhaps the best that can be said for these efforts is that they are acknowledged to have little impact on either CO2 levels or temperatures despite their immense cost. This is relatively good news since there is ample evidence that both changes are likely to be beneficial although the immense waste of money is not.


The Nongovernmental International Panel on Climate Change (NIPCC) has weighed in extensively in support of this view in diverse publications:


Higher levels of carbon dioxide and warmer temperatures benefit nearly all plants, leading to more leaves, more fruit, more vigorous growth and greater resistance to pests, drought and other forms of “stress.” Wildlife benefits as their habitats grow and expand. Even polar bears, the poster child of anti-global warming activist groups such as the Natural Resources Defense Council (NRDC), are benefiting from warmer temperatures (NIPCC 2014).
Terrestrial ecosystems have thrived throughout the world as a result of warming temperatures and rising levels of atmospheric CO2 (Idso et al. 2016).
Higher atmospheric CO2 levels are causing plants to grow better and require less water (NIPCC 2019).
Melting of Arctic sea ice and polar icecaps is not occurring at "unnatural" rates and does not constitute evidence of a human impact on climate. . . The link between warming and drought is weak, and by some measures drought decreased over the twentieth century. . . . No convincing relationship has been established between warming over the past 100 years and increases in extreme weather events (Idso et al. 2016).
There has been no increase in the frequency or intensity of drought in the modern era. Rising CO2 lets plants use water more efficiently, helping them overcome stressful conditions imposed by drought (NIPCC 2019).
A warming of 2°C or more during the twenty-first century would probably not be harmful, on balance, because many areas of the world would benefit from or adjust to climate change (Idso et al. 2016).
Atmospheric carbon dioxide (CO2) is a mild greenhouse gas that exerts a diminishing warming effect as its concentration increases. Doubling the concentration of atmospheric CO2 from its pre-industrial level, in the absence of other forcings and feedbacks, would likely cause a warming of -0.3°C to 1.1°C, almost 50 percent of which must already have occurred (Idso et al. 2016).
A few tenths of a degree of additional warming, should it occur, would not represent a climate crisis (Idso et al. 2016).


The U.S. Environmental Protection Agency has also noted there is some advantage of warming for the United States in terms of agriculture (EPA 2022h):


The average length of the growing season in the contiguous 48 states has increased by more than two weeks since the beginning of the 20th century. . . . The length of the growing season has increased more rapidly in the West than in the East. . . . The length of the growing season has increased in almost every state. States in the Southwest (e.g., Arizona and California) have seen the most dramatic increase. In contrast, the growing season has actually become shorter in two states, both in the Southeast: Alabama and Georgia . . . In recent years, the final spring frost has been occurring earlier than at any point since 1895, and the first fall frost has been arriving later.


Potential for global cooling
There has been some concern voiced that the greater concern for human beings is the potential for global cooling.
Of course, this is not a new concern, having been expressed during the 1970s. For example, an April 8, 1977, Time magazine cover page was titled "How to Survive the Coming Ice Age," and an April 28, 1975, Newsweek magazine cover page was labeled "The Cooling World." This later issue included such comments as, "The evidence in support of these predictions [of global cooling] has now begun to accumulate so massively that meteorologists are hard-pressed to keep up with it"; "resulting famines could be catastrophic"; "the most devastating outbreak of tornadoes ever recorded", "droughts, floods, extended dry spells, long freezes, delayed monsoons," "impossible for starving peoples to migrate," and "the present decline has taken the planet about a sixth of the way toward the Ice Age." The Washington Post, Times Herald had a July 9, 1971, article on "U.S. Scientist See New Ice Age Coming," and a January 29, 1974, The Guardian had an article titled "Space Satellites Show New Ice Age Coming Fast."
Some of this concern has resurfaced even in the face of a decades-long trend of increasing temperatures.
S. Fred Singer, who passed away in 2020, stated in 2016:


The real threat to humanity comes not from any (trivial) greenhouse warming but from cooling periods creating food shortages and famines.
I am much more concerned by a cooling climate—as predicted by many climate scientists—with its adverse effects on ecology and severe consequences for humanity.
Historical records identify the recent cycle of a (beneficial) Medieval Warm Period (MWP) and the (destructive) Little Ice Age (LIA) with its failed harvests, starvation, disease, and mass deaths. Many solar experts predict another LIA cooling within decades.


The NIPCC, an organization founded by S. Fred Singer, has stated, "Forward projections of solar cyclicity imply the next few decades may be marked by global cooling rather than warming, despite continuing CO2 emissions" (Idso et al. 2016).

Politicization, bias, and suppression of opposing views
The two main sides on climate change—those who claim the main driver of warming to be anthropogenic (the consensus view) and those who maintain the main driver to be natural—accuse the other of bias and placing political or economic concerns above science. The animosity can be seen in the terms the two sides use for the other. Those advocating the main driver to be natural often reference themselves as "climate change skeptics," while labeling the opposing side as "climate alarmists" (Lindzen 2010; IPCC 2016). On the other hand, they themselves are commonly labeled as "climate change deniers," a term that some take as creating linkage with the highly pejorative "holocaust deniers."
For example, Richard S. Lindzen, an atmospheric physicist who was a lead author for a chapter on the IPCC's Third Assessment Report but disputes the idea of scientific consensus, writing in his role as professor of meteorology at the Massachusetts Institute of Technology (MIT), used the terms "climate alarmists," "alarmist concerns," and "alarmist focus on carbon emissions" in his article in the Wall Street Journal (Lindzen 2010). On the other hand, Michael E. Mann, a climatologist and geophysicist, who is director of the Earth System Science Center at Pennsylvania State University and author of the book, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines, is an example of someone who references the "climate change denial movement" and "deniers" (Feder 2012).
However, the rhetoric and aggressiveness between the two camps has proven much more intense than the use of the alarmists and deniers terminology.
In terms of those disputing humans as the dominant cause of contemporary warming, they have been labeled "climate criminals" by Greenpeace and other organizations, and Grist magazine advocated "Nuremberg-style war crime trials" (Driessen 2006). The hostile attitude included "Wanted posters" with pictures at the 2015 United Nations Climate Change Conference (COP21) in Paris ((Stothard and Clark 2015; Avaaz). Emails leaked from the University of East Anglia's Climatic Research Unit in 2009 showed efforts by scientists to prevent publication of opposing views in peer-reviewed journals and to have editors of peer-reviewed journals replaced if they allowed articles opposing the consensus view. In 2013, the Los Angeles Times letters' editor announced an editorial policy to not publish letters from "climate deniers"; admitting his lack of science credentials beyond high school biology, he stated that he would defer to experts, notably the IPCC, in not allowing views that denied humans as the cause of climate change (Thornton 2013; Byers 2013).
Idso et al. (2016) note that climate change "skeptics" have been "demonized by environmental advocacy groups, censored by the mainstream media, and threatened by politicians and their allies in government agencies," and that "some who've spoken freely now have fallen silent, fearing for their livelihoods and even their safety and that of their families."
The hostility has not been one sided: Those advocating humans as the dominant cause of contemporary climate change have likewise been targeted. Michael Mann has been quoted as saying: “Political intimidation, character attacks, what appear to be orchestrated phone and email campaigns, nasty and thinly veiled threats, not just to us but to our families, are what it means in modern American life to be a climate scientist" (Feder 2012). Gavin Schmidt, a climate researcher at NASA, was quoted as stating that fossil fuel advocates “have adopted a shoot-the-messenger approach. It’s been a very successful strategy. They have created a chilling effect, so other [scientists] won’t say what they think and the conversation in public stays bereft of anyone who knows what they are talking about" (Feder 2012).
Those denying humans as the dominant cause of climate change have been criticized for the funding received from the fossil fuel industry, a charge the NIPCC denies, staying that few of the "skeptics" have been paid by that industry and the most prominent organizations "supporting global warming skepticism get little if any money from the fossil fuel industry." On the other hand, they accuse the funding of the consensus view as coming from "government agencies, liberal foundations, environmental advocacy groups, and the alternative energy industry" and that such funding of "alarmists' exceeds "funding from the fossil fuel industry by two, three, or even four orders of magnitude." They claim ""massive transfers of wealth from the general public to a small politically connected cabal of climate profiteers." The NIPCC claims "because we do not work for any governments, we are not biased toward the assumption that greater government activity is necessary."
Steven E. Koonin, who served as undersecretary in the Energy Department under President Barack Obama, noted the problem of IPCC's simplified summaries and called for openness in evaluating the science behind climate change (Koonin 2014):


Yet a public official reading only the IPCC's "Summary for Policy Makers" would gain little sense of the extent or implications of these deficiencies. These are fundamental challenges to our understanding of human impacts on the climate, and they should not be dismissed with the mantra that "climate science is settled."
Policy makers and the public may wish for the comfort of certainty in their climate science. But I fear that rigidly promulgating the idea that climate science is "settled" (or is a "hoax") demeans and chills the scientific enterprise, retarding its progress in these important matters. Uncertainty is a prime mover and motivator of science and must be faced head-on. It should not be confined to hushed sidebar conversations at academic conferences.




See also
Alternative energy
Greenhouse effect
Greenhouse gas
Paleoclimatology
ReferencesISBN links support NWE through referral fees
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