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Climate Sensitivity

Climate sensitivity is a term used by the Intergovernmental Panel on Climate Change (IPCC) to describe to what extent rising levels of greenhouse gases affect the Earth’s temperature. Specifically, it describes how much warmer the planet will get if the amount of greenhouse gases in the atmosphere doubles.

Although scientists don’t know exactly what Earth’s climate sensitivity is, it’s a useful concept for thinking about the range of risks we face from climate change: from slower and more manageable consequences if our climate sensitivity is low, to faster and more dire changes if it’s high. According to the most recent IPCC report, our climate sensitivity is somewhere between 1.5 and 4.5 ℃ (between 2.7 and 8 ℉).

Feedback Effects

If that range sounds pretty wide, that’s because there are many factors that can speed up or slow down the rise in atmospheric temperatures. The main ones are clouds, sea ice, and water vapor. Scientists call these factors “feedback effects,” and they can make predicting the planet’s future climate more complicated.

Climate scientists agree that with no feedback effects at all, our climate sensitivity would be just 1 ℃. Many controversies in climate science hinge on just how strong the various feedbacks are, and whether scientists have accounted for all of them. Clouds are a good example. Clouds can warm or cool the planet, depending on how high they are and the size of their water droplets. Overall, most scientists expect changes in clouds to mostly warm the planet, but some say it’s hard to know.

An Uncertain Future

While climate sensitivity can help us understand our climate risks today, we shouldn’t get too hung up on an exact number. The definition of climate sensitivity is based on doubling carbon dioxide and other greenhouse gases, compared with their levels in the atmosphere before humans started burning massive amounts of fossil fuels in the 1800s. If greenhouse gases more than double in the long run, the feedback effects could change and the Earth’s temperature could become more or less sensitive to extra emissions, causing global warming to speed up or slow down. Climate scientists also agree that it will take some time for the world’s climate to stabilize after emissions level off. So in the short run, even if we stop adding greenhouse gases to the atmosphere today, we should expect global warming to take a few years to catch up to our climate sensitivity.

 

Infographic: Risk scenarios. Climate scientists’ best estimate is that our climate sensitivity is somewhere between 1.5 and 4.5o C. But what do those numbers mean for us? Here are some effects of climate change that the Intergovernmental Panel on Climate Change reports we will be at “high risk” of experiencing at different levels of warming. Sources: The IPCC Special Report on Climate Change and Land, and the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate.
Click here to see data from the infographic above in a table.
Temperature rise Sample risks
1.5 ℃ (2.7 ℉)

All warm-water coral reefs shrink significantly, resulting in local extinctions.

Permafrost thaw degrades land quality in arctic regions.

The food supply becomes more unstable, with periodic food shocks across regions.

2 ℃ (3.6 ℉)

Wildfires become much more widespread as fire weather season grows longer.

Desertification results in shortages of water in dry regions of the world.

3 ℃ (5.4 ℉)

Ocean life suffers widespread losses in kelp and seagrass meadows and the upper ocean.

Crops yields decline in tropical regions, especially staple crops like wheat and corn.

4.5 ℃ (8.1 ℉)

The food supply suffers sustained disruptions worldwide as farmland is lost.

Ocean life losses become widespread in all ocean environments.

Wildfire risks are more severe, affect 100 million more people, and may be irreversible.

 

 

This Explainer was adapted from “Explained: Climate Sensitivity” by David Chandler, which originally appeared in MIT News.

Photo credit: Mickael Tournier on Unsplash

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