Climate Sensitivity, Feedbacks, and Forcing: 4x/2 = 2x?
Abstract
Equilibrium climate sensitivity is often estimated using instantaneously quadrupled instead of doubled CO2 concentrations relative to the pre-industrial value ("abrupt4x" and "abrupt2x"), although ECS is defined as the global surface warming in response to a doubling of the CO2 concentration. There are several assumptions behind this method: that forcing depends logarithmical on the CO2 concentration, that feedbacks are not dependent on temperature, and that the fraction of realized warming and the SST pattern change is similar in the abrupt4x versus abrupt2x simulations. Unlike in CMIP5, the CMIP6 protocol encourages the submission of abrupt2x and abrupt4x simulations (as of July 2019, three model contributions are available). A recent effort to gather millennial-length simulations of CMIP5-type models (LongRunMIP) also includes this set of simulations (for currently six models). We expect the model sample to increase over the coming months. We quantify the difference between the abrupt2x and abrupt4x simulations in terms of their changing feedbacks (in time and with temperature) and different estimates of climate sensitivity and radiative forcing. In most models, the temperature of an abrupt4x simulation is larger than doubling the abrupt2x simulation. However, in more models the radiative imbalance at the top of the atmosphere scales well with the the logarithmic forcing as a function of CO2 concentration. Radiative feedbacks do not change necessarily on the same timescales in the abrupt2x and abrupt4x simulations. Equilibrium climate sensitivity estimated from abrupt4x simulations is larger (in the current sample 5-34%) than estimated from abrupt2x simulations in all but one model. The value of effective (or estimated equilibrium) climate sensitivity depends on the method to estimate it. These simulations offer the first assessment across models of the temperature versus time/pattern dependence of radiative feedbacks.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A33O3016R
- Keywords:
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- 1620 Climate dynamics;
- GLOBAL CHANGE;
- 1627 Coupled models of the climate system;
- GLOBAL CHANGE