The discount rate is the interest rate that links a future cash flow received a time [math]\displaystyle{ t }[/math] to the same cash flow received now, at [math]\displaystyle{ t=0 }[/math]. It takes in account the length of the time period (the more time it is, the higher it should be) and the risk related to the cash flow (the more incertain it is, the higher the discount rate is).
Assume I have $80, and I buy a government bond that pays me $100 in a year's time. The discount rate represents the discount on the future cash flow:
[math]\displaystyle{ \frac{(100)}{80}-1= 25% }[/math]
One of the major issues in economics is what is an appropriate discount rate to use under various circumstances. For example, in assessing the impact of very long-term phenomena such as climate change, use of any discount rate much more than 1% per annum renders long-term damage (occurring in, say, 200 years time) of negligible importance now, and therefore entails (implausibly) that there is no need to take preventative action.
Conversely, governments often take a short-term view of things, effectively applying discount rates of perhaps 20% p.a. or higher, on the grounds that anything they do or fail to do which has detrimental effects in (say) 10 or more years' time won't prevent their re-election sooner than that.
In practice, discount rates such as 2%, 3%, 5% and 10% are widely used in economics. However there is little consensus on what value is appropriate in any given circumstance, and it often makes a significant difference.
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