State-dependent climate sensitivity in past warm climates and its implications for future climate projections

Abstract

Projections of future climate depend critically on refined estimates of climate sensitivity. Recent progress in temperature proxies dramatically increases the magnitude of warming reconstructed from early Paleogene greenhouse climates and demands a close examination of the forcing and feedback mechanisms that maintained this warmth and the broad dynamic range that these paleoclimate records attest to. Here, we show that several complementary resolutions to these questions are possible in the context of model simulations using modern and early Paleogene configurations. We find that (i) changes in boundary conditions representative of slow "Earth system" feedbacks play an important role in maintaining elevated early Paleogene temperatures, (ii) radiative forcing by carbon dioxide deviates significantly from pure logarithmic behavior at concentrations relevant for simulation of the early Paleogene, and (iii) fast or "Charney" climate sensitivity in this model increases sharply as the climate warms. Thus, increased forcing and increased slow and fast sensitivity can all play a substantial role in maintaining early Paleogene warmth. This poses an equifinality problem: The same climate can be maintained by a different mix of these ingredients; however, at present, the mix cannot be constrained directly from climate proxy data. The implications of strongly state-dependent fast sensitivity reach far beyond the early Paleogene. The study of past warm climates may not narrow uncertainty in future climate projections in coming centuries because fast climate sensitivity may itself be state-dependent, but proxies and models are both consistent with significant increases in fast sensitivity with increasing temperature.

Keywords: hyperthermal; superrotation.

Fig. 1.

Global mean surface temperature and MAT in simulations with modern (black) and Paleogene (red) boundary conditions. Dots show results from fully coupled simulations, and lines show results from slab-ocean simulations. The red box-and-whisker plot shows a proxy data-based estimate for the early Eocene (Methods). The vertical limits on the box and whisker are 1 and 2 SEs on the mean MAT value, respectively. The horizontal extent of the box spans a range of CO₂ values encompassing our best guess of the likely range of values based on the references cited in the main text.

Fig. 2.

Forcing analysis for the early Paleogene simulations. (A) Radiative forcing due to a doubling of CO₂ as a function of global mean surface temperature and MAT (black line) and contribution to the forcing due to ultrafast cloud adjustment (gray line). Error bars represent 95% confidence intervals. (B) Offline radiative forcing calculations using simulated climatological temperature, humidity, and cloud fields (Methods). Lines show results when all fields, as well as baseline CO₂, vary (blue); when only baseline CO₂ varies (red); when only temperature varies (magenta); and when only humidity and clouds vary (cyan). Gray shading shows total forcing minus the ultrafast cloud adjustment component (i.e., the difference between the black and gray lines in A). WV, water vapor.

Fig. 3.

Feedback analysis for the early Paleogene simulations. (A) Specific climate sensitivity as a function of global mean surface temperature and MAT computed using the method of Gregory et al. (46) (solid line) and the PRP method (dashed line). Error bars represent 95% confidence intervals. (B) Strengths of individual feedback mechanisms as a function of global mean surface temperature and MAT: surface albedo (Alb) feedback (green line); WV feedback (dashed magenta line), lapse rate feedback (dotted magenta line), and their sum (solid magenta line); cloud short-wave (SW) feedback (dashed red line); cloud long-wave (LW) feedback (dotted red line); total cloud feedback (solid red line); and Planck feedback (blue line).