New Findings Point Towards Higher Climate Sensitivity

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by Eric Ellman

For the second time in less than a month, researchers at Yale University have produced new evidence that Intergovernmental Panel on Climate Change (IPCC) scientists underestimate how quickly greenhouse gases warm the planet. Associate Professor Trude Storelvmo is principal investigator for two studies that independently support new, higher estimates of climate sensitivity, one of the most enduring and critical values for understanding climate change.

“Climate sensitivity” is the term coined by MIT’s Jules Charney to describe the relationship of global temperature to changing levels of atmospheric carbon dioxide.  In 1979, Charney estimated that a doubling of CO2 would cause a temperature rise of 3.0 degrees Centigrade, “plus or minus 1.5 degrees Centigrade.”  Nearly four decades later, the combined efforts of thousands of scientists, running increasingly sophisticated numerical models on ever more powerful computers, have hardly budged those numbers.  The 5th Assessment Report of the Intergovernmental Panel on Climate Change, released September 2014, still reports that equilibrium climate sensitivity (ECS) lies between 2.0 and 4.6 degrees.

Storelvmo’s work suggests that scientists are closer to resolving that range of values into a single number, in a direction that does not bode well.  Says Michael Mann, Distinguished Professor of Atmospheric Science at Penn State University, the latest studies “provide sobering evidence that Earth’s climate sensitivity may lie in the upper end of the current uncertainty range.”

The research published today in Science describes how the middle- and upper troposphere contains more super-cooled water in proportion to ice crystals than previously believed. That ratio is critical because tiny water droplets are highly reflective and ice crystals are not.  A higher liquid fraction causes greater reflectivity. More reflectivity (the goal of many geo-engineering projects) reduces global warming.  But while the models that researchers examined correctly replace ice with water as the world warms due to increased CO2 concentrations, they overestimate the availability of ice.  Less ice means the atmosphere cannot brighten as much as the models assume. Much of the energy they anticipate to be reflected back to space (as ice converts to water) will be retained instead, and heat the Earth.

To calculate how much additional heating occurs, the authors configured the National Center for Atmospheric Research’s Community Atmosphere (CAM 5.1) Model to calculate equilibrium climate sensitivity using a range of atmospheric water-to-ice ratios based on the latest satellite estimations. The newly constrained CESM1 model predicted ECS of between 5.0 and 5.3 degrees, compared to a 4.0 degree estimate prior to correction.  Storelvmo expects that when other researchers adjust their respective models to reflect appropriate proportions of ice and water in mixed-phase clouds, all of their ECS values will shift upward.

The implications of higher ECS are profound, and might already be making themselves felt were it not for a phenomenon reported on by Storelvmo’s group earlier this month in Nature Geoscience.  That study used ground-based measurements of incoming solar radiation to determine that highly reflective sulfate aerosols (similar to the chemicals proposed for use in geo-engineering) “mask” the full warming effect of carbon dioxide. Using econometric mathematical techniques, Storelvmo disentangled a record of temperature change from the competing influences of carbon dioxide and sulfates.  Her analysis determined that existing levels of sulfate aerosols counter-acted about ½ a degree Centigrade worth of global warming.  As reported in numerous publications (see here, here and here), continued efforts to reduce sulfate pollution will have the unintended consequence of accelerating global warming.

Higher estimates of climate sensitivity would help resolve historic disagreement with estimates obtained from geologic records says Yale professor Mark Pagani. “Evaluations of ancient Earth’s sensitivity to CO2 are difficult to make, but equilibrium climate sensitivities are often higher than model estimates for the modern climate system.”

Economic Models. Misinterpreting Climate Sensitivity Too?

Just as scientists use General Circulation Models (GCMs) to identify how CO2 impacts average global temperature, economists use Integrated Assessment Models (IAMs) to estimate how those rising temperatures impact society.  And just as Trude Storelvmo has argued that scientists have gotten the GCMs wrong, Bob Litterman, former director of risk management for Goldman Sachs, who now chairs posts such as the World Wildlife Fund’s Investment Committee, suspects that economists have gotten the IAMs wrong too. 

IAMs are where the rubber meets the road in climate change modeling efforts. Their output attempts to estimate the cumulative damages to society reflected in the “social cost of carbon.” The U.S. government is bound by law to use the social cost of carbon when promulgating rule-makings. Thus climate sensitivity – which sets future temperature, which helps establish future damages – is much more than an academic exercise. Establishing credible values for climate sensitivity and the social cost of carbon are necessary to set the sort of fee system which most experts agree is the ultimate solution for curbing carbon emissions.

As someone who sees climate change as a risk management problem, Litterman feels that the IAMs don’t incorporate risk aversion adequately.  Nor does he feel that they take the climate science component seriously. In an effort to reconcile both failings, he recently helped to organize a conference at Arizona State University that convened ten climate scientists and ten economists.  Storelvmo participated in a similar forum with climate scientists and economists organized by the Yale Climate and Energy Institute two years ago at Yale, and had similarly concluded that IAMs were deficient, but for different reasons.

“IAM’s calculate damages based solely on anticipated increases in global annual mean surface temperature,” rather than a more full suite of parameters, including sea-level rise and precipitation changes, she says, all of which profoundly impact economic output.  Equally problematic is the fact that economists’ models typically utilize equilibrium climate sensitivity, which estimates long-term attenuated warming, rather than transient climate sensitivity, which is intuitively more appropriate for calculating short-term economic effects.

What the climate science world needs now, both experts appear to agree, is a serious effort involving both scientists and economists to take advantage of new computational resources to couple an IAM to a full-scale global climate model and resolve such shortcomings.

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