ALBERT

Description: 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.

Description: The concept of a positive feedback between ice flow and enhanced melt rates in a warmer climate fuelled the debate regarding the temporal and spatial controls on seasonal ice acceleration. Here we combine melt, basal water pressure and ice velocity data. Using 20 years of data covering the whole ablation area, we show that there is not a strong positive correlation between annual ice velocities and melt rates. Annual velocities even slightly decreased with increasing melt. Results also indicate that melt variations are most important for velocity variations in the upper ablation zone up to the equilibrium line altitude. During the extreme melt in 2012, a large velocity response near the equilibrium line was observed, highlighting the possibility of meltwater to have an impact even high on the ice sheet. This may lead to an increase of the annual ice velocity in the region above S9 and requires further monitoring.

Description: A still open question is how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – is depending on background climate. We here present paleo-data based evidence on the state-dependency of S, by using CO2 proxy data together with 3-D ice-sheet model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land-ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This non-linearity was in similar approaches not accounted for due to previously more simplistic approximations of land-ice albedo radiative forcing being a linear function of sea level change. Important for the non-linearity between land-ice albedo and sea level is a latitudinal dependency in ice sheet area changes.In our setup, in which the radiative forcing of CO2 and of the land-ice albedo (LI) is combined, we find a state-dependency in the calculated specific equilibrium climate sensitivity S[CO2 ,LI] for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods S_[CO2,LI] is on average ∼45% larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 Myr BP) the CO2 data uncertainties prevents a well-supported calculation for S_[CO2,LI], but our analysis suggests that during times without a large land-ice area in the Northern Hemisphere (e.g. before 2.82MyrBP) the specific equilibrium climate sensitivity S_[CO2,LI] was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state-change in the climate system with the wide appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land-ice albedo radiative forcing, which is important for similar paleo data-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the necessary corrections for other slow feedbacks are in detail unknown and the still existing uncertainties in the ice sheet simulations and global temperature reconstructions are large.