When comparing apples to apples, a new study finds energy budget climate sensitivity estimates consistent with climate models
July 6, 2016 — Scientists use a variety of approaches to estimate the Earth’s climate sensitivity – how much the planet will warm as a result of humans increasing greenhouse effect. For decades, the different methods were all in good general agreement that if we double the amount of carbon dioxide in the atmosphere, Earth’s surface temperatures will immediately warm by about 1–3°C (this is known as the ‘transient climate response’). Because it would take decades to centuries for the Earth to reach a new energy balance, climate scientists have estimated an eventual 2–4.5°C warming from doubled atmospheric carbon (this is ‘equilibrium climate sensitivity’).
However, a 2013 paper led by Alexander Otto disrupted the agreement between the various different approaches. Using a combination of recent climate measurements and a relatively simple climate model, the ‘energy budget’ approach used in Otto’s study yielded a best estimate for the immediate (transient) warming of 1.3°C and equilibrium warming of 2.0°C; within the agreed range, but less than climate model best estimates of 1.8°C and 3.2°C, respectively.
This new energy budget approach, which was replicated by several subsequent studies, seemed to indicate the Earth’s climate is a bit less sensitive to carbon pollution than previously thought. As a result, the IPCC adjusted its estimated range for equilibrium climate sensitivity from 2–4.5°C in its 2007 report to 1.5–4.5°C in its 2014 report. This suggested perhaps a slightly less dire climate situation.
New finding: disagreement due to apples-to-oranges comparison
Later in 2013, Kevin Cowtan and Robert Way published a paper finding that climate scientists had been underestimating global surface warming, largely because of a lack of measurements in the rapidly-warming Arctic. Additionally, while climate models simulate surface air temperatures (the temperature of the air a few meters above the Earth’s surface), over the oceans, climate scientists measure sea surface temperatures. It turns out that the water surface isn’t warming quite as fast as the air above it. Thus looking at modeled surface air temperatures versus measured global land-ocean surface temperatures is an apples-to-oranges comparison.
A new study in Nature Climate Change led by Mark Richardson in collaboration with Kevin Cowtan, Ed Hawkins, and Martin Stolpe accounts for these differences to make an apples-to-apples comparison. They find that the use of sea surface temperatures biases the Otto result low by about 9%, and the lack of Arctic observations by another 15%. When observations are adjusted to estimate surface air temperatures (red bars in the figure below), or when models are adjusted to estimate land-ocean surface temperatures (blue bars), the estimated transient climate response from climate models and the Otto approach are in close agreement.
Climate sensitivity may be higher yet
Previous studies have identified other flaws in the energy budget model approach. Several papers, most recently led by Kate Marvel at NASA, found that Otto’s and similar studies have not accounted for the different efficiencies of various factors that cause global energy imbalances. Other papers have noted that the energy balance model approach assumes that feedbacks amplifying or dampening global warming will stay constant over time, while in reality, some feedbacks kick in later than others. This suggests that energy budget climate sensitivity estimates, based only on today’s feedbacks, will be biased low.
The new Nature Climate Change paper didn’t address how much combining these various corrections would change our estimates of the Earth’s climate sensitivity. As the authors noted:
The results of Marvel et al are independent of [our] work. If both studies are correct, climate sensitivity from the historical record could be higher than climate sensitivity from the models. However we are not in a position to comment on that paper, and so draw the weaker conclusion that the historical record offers no reason to doubt the estimates of climate sensitivity from climate models.
According to lead author Mark Richardson:
Bad news for the “skeptic” case
In a recent court case, the world’s largest private sector coal company made its best case against the need to take action on climate change. Their witness testimony relied upon two primary claims – that climate models are wrong and Earth’s climate sensitivity is low, which led them to conclude that carbon pollution is lovely.
This new research shows that their arguments are on even weaker footing than previously thought. When comparing apples to apples, climate models accurately simulate global surface temperature changes, and the evidence for lower climate sensitivity failed to withstand scientific scrutiny and the test of time. The coal company lost its case, and we’re now at the point where we have no excuse to continue delaying action on climate change.
We’re also left with the question, when evaluating dangerous global warming, should we consider measured land-ocean temperatures, or faster-warming air temperatures? As Ed Hawkins notes, if we decide climate targets refer to the latter, it puts us about 24% closer to dangerous thresholds. Thus the climate situation may be even more, not less urgent than previously believed.