3 Questions: Refining climate warming projections by studying the past
Research led by EAPS Houghton Postdoctoral Fellow Vincent Cooper limits the upper boundary of modern climate sensitivity to 4 degrees Celsius, one degree lower than previously estimated. Image credit: Ralf-Adobe Stock Images
Climate sensitivity is the metric used by scientists to gauge how much the global average temperature will increase if CO2 levels double. Using historical data from previous periods of the Earth that were both hotter and cooler than today’s climate, they can set limits on the minimum and maximum global temperature changes used for future projections. Common estimates for our current climate sensitivity range from 2 to 5 degrees Celsius, a wide range that emphasizes how tricky it is to understand the contributions from different feedbacks, especially clouds.
Vincent Cooper, a Houghton Postdoctoral Fellow in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS), published a paper in PNAS that examined the impacts of non-CO2 climate changes, such as ice sheets, topography, and vegetation, on climate sensitivity during a previous period of Earth’s history known as the Pliocene. By accounting for these changes, he and his fellow authors were able to limit the upper bound of the range for modern climate sensitivity to 4 degrees Celsius, one degree lower than previously estimated. He took a moment to explain how we study past climates to understand our future one, why regional Earth processes matter on global scales, and what this means for climate change mitigation.
Q: What is the Pliocene, and why do we use it to study climate change?
A: The Pliocene is a period about 3 million years ago when CO2 levels were about as high as they are today and the continents and the ice sheets were in a relatively similar configuration. It is often viewed as the best analog to near-term modern warming; however, there are large uncertainties. A big focus of my work is trying to account for how different the climate changes were during the Pliocene from the changes that we would expect during the next 150 years or so. Those differences end up being very important because it turns out that not only were the changes from CO2 being higher important, but also changes from ocean gateways, smaller ice sheets and vegetation had large effects on the global mean temperature and the global mean energy balance.
Q: Those differences seem like small things, but why do they matter in comparing the two periods?
A: There’s a direct response in the energy budget of the earth when you make these changes. In ice sheets, for example, either removing or growing an ice sheet changes the amount of sunlight that’s reflected by the Earth as a whole. The new piece is that now we know changes in ice sheets or ocean gateways or vegetation also have follow-on effects, which is that they change the pattern of sea surface temperatures. So if you have a change in an ice sheet it makes sense, with the benefit of hindsight, that you would have a different pattern of temperature change – say more warming in the North Atlantic if the Greenland ice sheet changes a lot.
It turns out that those changes in the ocean temperature pattern have a huge impact on the energy budget. The reason for that is they end up changing clouds, and clouds are the dominant uncertainty but also the dominant control on climate sensitivity. Accounting for those small changes, or changes that seem small at face value, ends up having a very big impact on the climate sensitivity that we infer based on changes during the Pliocene.
Q: What were the results that you found in this paper, and what does it mean for projections for future warming?
A: When we constrain climate sensitivity, we’re trying to isolate the modern response purely from CO2 increasing in the atmosphere. When we look at the past, we find that more of the temperature changes in the Pliocene were from non-CO2 factors like those changes in ice sheets, vegetation and ocean gateways. Because [of this], the upper bound of warming estimates for modern equilibrium climate sensitivity is about one degree C lower than it was previously. That gives us more confidence in both the benefits of mitigation and the expected adaptations that would potentially need to be made.
Equilibrium climate sensitivity intentionally excludes changes in ice sheets, so a way to interpret our findings is that those have even more of an effect than we had previously thought. If we don’t stop warming in time to prevent major losses of the West Antarctic Ice Sheet, for example, then we’re all of a sudden kicking into a level of warming that could have been averted if we had stopped CO2 concentrations from rising earlier.The other piece is that it emphasizes that reducing CO2 emissions does have a material impact on our future. An alternative finding that could have come from our study is actually equilibrium climate sensitivity is six degrees Celsius, for example, and it could seem like those changes are almost inevitable. We’re finding the opposite of that: there’s a confident, relatively tight range on expected future warming, and actions that we take today do have a big impact on what warming will be over the next, say, 50 to 100 years. I think this is a hopeful message for the future.
