Lennert B. Stap

Lessons on Climate Sensitivity From Past Climate Changes

Over the last decade, our understanding of climate sensitivity has improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equilibrium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geological records as feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved differently. Do these insights constrain the climate sensitivity relevant for the present day? In this paper, we review the progress made in theoretical understanding of climate sensitivity and on the estimation of climate sensitivity from proxy records. Particular focus lies on the background state dependence of feedback processes and on the impact of tipping points on the climate system. We suggest how to further use palaeo data to advance our understanding of the currently ongoing climate change.

Modeled Influence of Land Ice and CO2 on Polar Amplification and Paleoclimate Sensitivity During the Past 5 Million Years

Polar amplification and paleoclimate sensitivity (S) have been the subject of many paleoclimate studies. While earlier studies inferred them as single constant parameters of the climate system, there are now indications that both are conditioned by the type of forcing. Moreover, they might be affected by fast feedback mechanisms that have different strengths depending on the background climate. Here we use the intermediate complexity climate model CLIMBER-2 to study the influence of land ice and CO2 on polar amplification and S. We perform transient 5-Myr simulations, forced by different combinations of insolation, land ice, and CO2. Our results provide evidence that land ice and CO2 changes have different effects on temperature, both on the global mean and the meridional distribution. Land ice changes are mainly manifested in the high latitudes of the Northern Hemisphere. They lead to higher northern polar amplification, lower southern polar amplification, and lower S than more homogeneously distributed CO2 forcing in CLIMBER-2. Furthermore, toward colder climates northern polar amplification increases and consequently southern polar amplification decreases, due to the albedo-temperature feedback. As an effect, a global average temperature change calculated from high-latitude temperatures by using a constant polar amplification would lead to substantial errors in our model setup. We conclude that to constrain feedback strengths and climate sensitivity by paleoclimate data, the underlying forcing mechanisms and background climate states have to be taken into consideration.

The effect of obliquity-driven changes on paleoclimate sensitivity during the late Pleistocene

Some studies suggest that specific equilibrium climate sensitivity S might be state-dependent. Reanalyzing existing paleodata of global mean surface temperature ∆Tg and radiative forcing ∆R of CO2 and land ice albedo for the last 800,000 years we show that this state-dependency of S is only found if ∆Tg is based on reconstructions, and not when ∆Tg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land-ice sheet growth and sea level fall) the multi-millennial component of reconstructed ∆Tg is diverging from atmospheric CO2, while in simulations both variables vary more synchronously. For a reconstruction-based extrapolation of S to the future we eliminate these periods due to an expected sea level rise. Consequently, S determined from proxy-based reconstructions without these data with strong ∆Tg-CO2 divergence is less state-dependent or even constant (state-independent), and yields into an equilibrium warming for 2 × CO2 of 1.9–3.8 K.