Is the climate more sensitive to CO2 than we thought?

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Is the climate more sensitive to CO2 than we thought?

Postby HBS Guy » 01 Aug 2018, 19:58

Climate sensitivity has been hard to pin down accurately. Climate models give a range of 1.5-4.5℃ per doubling of CO₂, whereas historical weather observations suggest a smaller range of 1.5-3.0℃ per doubling of CO₂.

In a new study published in Science Advances, Cristian Proistosescu and Peter J. Huybers of Harvard University resolve this discrepancy, by showing that the models are likely to be right.

According to their statistical analysis, historical weather observations reveal only a portion of the planet's full response to rising CO₂ levels. The true climate sensitivity will only become manifest on a time scale of centuries, due to effects that researchers call "slow climate feedbacks"

On the other hand, when greenhouse gas forcing rises at a rate as high as 2–3 parts per million (ppm) of CO₂ per year, as is the case during the past decade or so, the rate of slow feedback processes may be accelerated.

Measurements of atmosphere and marine changes made since the Industrial Revolution (when humans first began the mass release of greenhouse gases) capture mainly the direct warming effects of CO₂, as well as short-term feedbacks such as changes to water vapour and clouds.

A study led by climatologist James Hansen concluded that climate sensitivity is about 3℃ for a doubling of CO₂ when considering only short-term feedbacks. However, it's potentially as high as 6℃ when considering a final equilibrium involving much of the West and East Antarctic ice melting, if and when global greenhouse levels transcend the 500-700ppm CO₂ range.

Abstract of the study mentioned above:

The latest Intergovernmental Panel on Climate Change Assessment Report widened the equilibrium climate sensitivity (ECS) range from 2° to 4.5°C to an updated range of 1.5° to 4.5°C in order to account for the lack of consensus between estimates based on models and historical observations. The historical ECS estimates range from 1.5° to 3°C and are derived assuming a linear radiative response to warming. A Bayesian methodology applied to 24 models, however, documents curvature in the radiative response to warming from an evolving contribution of interannual to centennial modes of radiative response. Centennial modes display stronger amplifying feedbacks and ultimately contribute 28 to 68% (90% credible interval) of equilibrium warming, yet they comprise only 1 to 7% of current warming. Accounting for these unresolved centennial contributions brings historical records into agreement with model-derived ECS estimates.

So the CO2 we have dumped into the atmosphere willy nilly is going to warm the planet more than it already has. More heat, more humidity, humungous flooding as sea levels zoom!

The paper goes on:
A major challenge in inferring ECS from instrumental records is that the current climate system is not in energetic equilibrium. On average, Earth’s surface currently takes up between 0.1 and 0.9 W/m2 more heat than it loses (9), in which this rate of heating is denoted as H. To extrapolate to the temperature at which radiative equilibrium would be reestablished, a proportionality between changes in outgoing radiation, R, and global mean temperature, T, is generally assumed (3–8, 10), λ = (F − H)/T. F represents anomalies in downward radiative forcing, and the difference gives the upward radiative response, R = F − H. If λ is assumed constant over time, then ECS can be inferred by zeroing out H and assigning F equal to the forcing associated with a doubling of atmospheric CO2, giving an estimated ECS of F2×λ−1.

The validity of this extrapolation is questionable, however, because of the well-documented time dependence of λ, or the net radiative feedback, in climate simulations (11–18). This time dependence is a prime suspect for the systematic differences between GCMs and historical estimates of ECS (19). So far, efforts at resolving the discrepancy have been stymied by the lack of methodology to quantitatively account for changes in λ (8). We derive a generalized fit of the change in λ in 24 GCM simulations from phase 5 of the Climate Model Intercomparison Project (CMIP5) (20) and use the results to estimate the bias inherent in historical energy budget constraints on climate sensitivity.

Ha! From the PhysOrg paper above, sources of forcings of climate—Lees won’t like it one bit.
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