Audrey Resutek and Erwan Monier
MIT Joint Program on the Science and Policy of Global Change
World Meteorological Organization Bulletin, October 17, 2014

The US National Climate Assessment, released this spring by the White House, describes a troubling array of climate woes, from intense droughts and heat waves to more extreme precipitation and floods, all caused by climate change. The report also describes how climate change is expected to impact regions across the United States in the future, yet it notes that exact regional forecasts are difficult to pin down. At the larger scale, it is clear that climate is changing, but local predictions can disagree on the extent to which temperatures will increase, and what regions will be hit the hardest by precipitation changes.

Researchers at the MIT Joint Program on the Science and Policy of Global Change examined four major factors that contribute to wide-ranging estimates of future regional climate change in the United States, with an eye toward understanding which factors introduced the most uncertainty into simulations of future climate. They find that the biggest source of uncertainty in climate modelling is also the only one that humans have control over – policies that limit greenhouse gas emissions.

In this context, the term “uncertainty” does not mean that there is a lack of scientific consensus that climate is changing. Instead, uncertainty refers to the fact that using different assumptions for the variables that go into a climate model – for example, the amount of greenhouse gases emitted over the next century, or how sensitive the climate is to changes in carbon dioxide levels – will produce a range of estimates. Overall, these estimates indicate that the Earth will be a warmer and wetter place over the coming century, but there is no single niversally agreed on amount of climate change that will take place.

In fact, estimates that point to a single number for changes in temperatures and precipitation may be misleading, precisely because they do not capture this uncertainty. It is more useful to think of estimates of future climate change as a range of possible effects. The range of potential warming, for example, follows a bell curve, with the most likely change in temperature falling at the highest point of the curve. The farther you travel from the curve’s peak, toward the tails, the more unlikely the temperature change. While the extreme temperature increases at the curve’s tails are unlikely, they still fall within the realm of possibility, and are worth considering because they represent-worst case scenarios.

Read the full article in the World Meterological Organization Bulletin