Climate Sensitivity

Climate Sensitivity on Instrumental Time Scales

Climate sensitivity is perhaps the most prominent parameter associated with global warming. By virtue of being a change in the globally averaged temporal mean temperature, it should be easier to constrain than any metric associated with regional change or higher moments of the temperature distribution. However, the latest IPCC report (AR5) is no closer to constraining its uncertainty range than the Charney report was 35 years prior. The question arises to what degree can modern instrumental records constrain equilibrium climate sensitivity?

We use conceptual models based on a linear feedback framework to go beyond equilibrium sensitivity and try to understand the time-scale structure of the climate response. We find that instrumental records inadequately sample the slow modes of the response function.

Illustration of the degree to which constraining the fast response constrains  equilibrium sensitivity:  Response functions for CMIP 5 models, computed based on the linear fit of Caldeira and Myhrvold [2013]. Left: Unconstrainted. Right: Constrained such that all models have equal Transient Climate Response (TCR). The remaining spread is associated with inter-model differences in the slow modes of response 

Palaeo-sensitivity

[Presentation on the topic] The palaeoclimate record offers the opportunity for constraining the full range of the climate system's response. An extensive body of work exists attempting to constrain various formulations of the equilibrium  response [see Rohling et al. 2012]. 
We are developing a frequency-domain formalism to estimate the full spectrum of the climate system's response to radiative forcing perturbations. 
Empirical estimate of a time-scale-dependent climate sensitivity from Paleoclimate records [Proistosescu and Huybers, forthcoming]
 
References:
  • Caldeira, K., and N. P. Myhrvold, 2013: Projections of the pace of warming following an abrupt increase in atmospheric carbon dioxide concentration. Environmental Research Letters.
  • Rohling, E., and Coauthors, 2012: Making sense of palaeoclimate sensitivity. Nature