The San Francisco Bay Area, sometimes known as the Bay Area, is a populated area in Northern California that encompasses the estuaries of the San Francisco, San Pablo, and Suisun bays. Although the exact boundaries of the region are subject to debate, the Bay Area is defined by the Association of Bay Area Governments as consisting of the nine counties that border the aforementioned estuaries: Alameda, Contra Costa, Marin, Napa, San Mateo, Santa Clara, Solano, Sonoma, and San Francisco, with the exception of the city of San Francisco. Other definitions may exclude portions of or entire counties, or they may broaden the boundaries to include neighbouring counties that do not border the bay, such as Santa Cruz and San Benito (which are more commonly included in the Central Coast regions); or San Joaquin, Merced, and Stanislaus counties (which are more commonly included in the Central Coast regions); or San Joaquin, Merced, and Stanislaus counties (which are more commonly included in the Central Coast regions) (more often included in the Central Valley).
Northern California's nine-county Bay Area, which is home to over 7.75 million people, is composed from several cities, towns, airports, and connected regional, state, and national parks, all of which are linked by a complex multimodal transportation network. Combined with the Greater Los Angeles area and fourteen counties, the larger federal classification, the combined statistical area of the region, is the second-largest in California (after the Greater Los Angeles area), the fifth-largest in the United States, and the 41st-largest urban area in the world, with 9.67 million people. Although the Bay Area's population is ethnically varied, Hispanic, Asian, African American, and Pacific Islander people account for nearly half of the area's citizens, with all of these groups having a substantial presence across the region.
However, despite its urban character, the San Francisco Bay is one of California's most ecologically significant habitats, providing important ecosystem services such as filtering pollutants and sediments from the rivers and providing critical habitat for a number of threatened and endangered species. Apart from that, the Bay Area's coast redwood forests, many of which are preserved in state and county parks, are well-known. Also notable for its landform complexity, which has resulted from millions of years of tectonic plate movement, is the region's geological history. Because the Bay Area is traversed by six main earthquake faults, the area is especially vulnerable to the dangers posed by massive earthquakes, which may wreak widespread devastation. This region's temperature is moderate and typically warm, making it an excellent location for outdoor recreational and sports activities such as trekking. A cultural hub for music, theatre, and the arts, the Bay Area is home to five professional sports teams as well as a number of other performing and visual arts organisations. Higher education institutions, including research universities such as Stanford University and the University of California, Berkeley, are also located there. As the home of 101 municipalities and nine counties, the Bay Area's governance is complex, including a wide range of local and regional players, many of whom have vast and overlapping duties. Droughts and wildfires have gotten more frequent and more severe over time, and they have become less seasonal and more year-round, putting further pressure on the region's water security.