Climate change is a uniquely long-term problem, which can lead to potentially irreversible changes in the Earth’s climate system.

The long-term climate impacts of unmitigated greenhouse gases (GHG) emissions are well-understood—thanks in a large part to the efforts by the UN IPCC and the rest of the scientific community—and the confidence level in these long-term effects has risen over time.

The most devastating potential impacts of climate change—such as rising sea levels, ocean acidification, or the melting of ice sheets—play out over multiple decades or even centuries.

The risk of abrupt, non-linear changes in the climate system also likely increases with rising global temperature levels. However, using only a very short timeframe for estimating climate impacts, like 20 years, would excessively weigh near-term impacts, understating the effect of long-term GHG accumulation on total warming by the end of the century, and thereafter.

The use of GWP factor of 100 years yields a balanced approach to an effective outcome for climate policy, as MIT researchers put it: A 20-year GWP would emphasize the near-term impact of methane but ignore serious longer-term risks of climate change from GHG’s that will remain in the atmosphere for hundreds to thousands of years, and the 500-year value would miss important effects over the current century. Methane is a more powerful GHG than CO2, and its combination of potency and short life yields the 100- year GWP used in this study.