It is well-known that large changes in clouds often occur in response to increasing CO2 in climate models. What happens, however, if you then start to reverse the CO2 change? In a new paper by Tim Andrews and myself, just published online in the Journal of Climate, we outline a general method to separate rapid tropospheric adjustments and feedbacks in transient climate change experiments. We find that feedbacks – including cloud feedbacks – are almost entirely reversible under the idealised mitigation scenario we use, with no evidence for so-called “hysteresis” type behaviour. Our simple conceptual framework also appears to apply to regional feedback patterns. We do, however, stress the importance of interpreting the regional feedbacks carefully, especially in response to time-varying warming patterns.
The abstract and citation are below, the full paper is here. A recent talk I gave on this work is also available on the CFMIP website.
The HadGEM2-ES Earth system climate model is forced by a 1% per year compound increase in atmospheric CO2 for 140 years, followed by a 1% per year CO2 decrease back to the starting level. Analogous atmosphere-only simulations are performed to diagnose the component of change associated with the effective radiative forcing and rapid adjustments. The residual change is associated with radiative feedbacks that are shown to be linearly related to changes in global-mean surface-air-temperature and are found to be reversible under this experimental design, even for regional cloud feedback changes. The cloud adjustment is related to changes in cloud amount, with little indication of any large-scale changes in cloud optical depth. Plant physiological forcing plays a significant role in determining the cloud adjustment in this model and is the dominant contribution to the low-level cloud changes over land. Low-level cloud adjustments are associated with changes in surface turbulent fluxes and lower tropospheric stability, with significant adjustments in boundary layer cloud types and in the depth of the boundary layer itself. The linearity of simple forcing-response frameworks are examined and found to be generally applicable. Small regional departures from linearity occur during the early part of the ramp down phase, where the Southern Ocean and eastern tropical Pacific continue to warm for a few decades, despite the reversal in radiative forcing and global temperatures. We highlight the importance of considering time-varying patterns of warming and regional phenomena when diagnosing and understanding feedbacks in a coupled atmosphere-ocean framework.
Andrews, T. and M.A. Ringer, 2013, Cloud feedbacks, rapid adjustments and the forcing-response relationship in a transient CO2 reversibility scenario, J. Climate, in press. doi:http://dx.doi.org/10.1175/JCLI-D-13-00421.1
Mark Ringer 