Description: The transient ice-sheet predictions are forced by multiple climate snapshots derived from a climate model set up with Late Pliocene boundary conditions, forced with different orbital forcing scenarios appropriate to two Marine Isotope Stages (MIS), MIS KM5c and K1.

Description: Model output of the intermediate complexity climate model CLIMBER-2 over the past 5 million years. The simulations were forced with insolation data (O), insolation and land ice data (OI), insolation and carbon dioxide data (OC) and with insolation, land ice and carbon dioxide data (OIC). Sheet 1 contains the main results: northern hemispheric (30-90 deg N), southern hemispheric (30-90 deg S) and global temperatures. Sheet 2 contains the land ice and carbon dioxide forcing in terms of globally averaged radiative forcing. Details are given in the publication. More information or data can be obtained by contacting L.B. Stap (lennert.stap@awi.de).

Description: It is still an open question how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO2 proxy data together with a 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 has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of ice-sheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO2 and of the land ice albedo (LI) is combined, we find a state dependence 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 prevent 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 2.82 Myr BP), 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 widespread 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 palaeodata-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 details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large.

Description: Marine sediment records from the Oligocene and Miocene reveal clear 400,000-year (400-kyr) climate cycles related to variations in orbital eccentricity. These cycles are also observed in the Plio-Pleistocene records of the global carbon cycle. However they are absent in the Late Pleistocene ice-age record over the past 1.5 million years. Here, we present a simulation of global ice volume over the past 5 million years with a coupled system of four 3-D ice-sheet models. Our simulation shows that the 400-kyr long eccentricity cycles of Antarctica vary coherently with d13C records during the Pleistocene suggesting that they drive the long-term carbon cycle changes throughout the past 35 million years. The 400-kyr response of Antarctica is eventually suppressed by the dominant 100-kyr glacial cycles of the large ice sheets in the Northern Hemisphere (NH).

Description: Model output of a coupled ice sheet-climate model, inversely forced by benthic d18O over the past 38 million years. Sheet 1 contains the main results from the reference simulation: benthic d18O, CO2, ice-volume-equivalent sea level and global temperature. Sheet 2 contains global, Northern Hemisphere (40-80 deg N), and Antarctic (60-90 deg S) temperatures, from the reference run and the run with ice uncoupled, only albedo coupled, and only surface height coupled. Sheet 3 contains global temperature, from the reference run, and the runs with fixed PD ice, fixed LGM ice, and no ice. Details are given in the publication. More information or data can be obtained by contacting L.B. Stap (lennert.stap@awi.de).

Description: Variations in global ice volume and temperature over the Cenozoic era have been investigated with a set of one-dimensional (1-D) ice-sheet models. Simulations include three ice sheets representing glaciation in the Northern Hemisphere, i.e. in Eurasia, North America and Greenland, and two separate ice sheets for Antarctic glaciation. The continental mean Northern Hemisphere surface-air temperature has been derived through an inverse procedure from observed benthic d18O records. These data have yielded a mutually consistent and continuous record of temperature, global ice volume and benthic d18O over the past 35 Ma. The simple 1-D model shows good agreement with a comprehensive 3-D ice-sheet model for the past 3 Ma. On average, differences are only 1.0°C for temperature and 6.2 m for sea level. Most notably, over the 35 Ma period, the reconstructed ice volume–temperature sensitivity shows a transition from a climate controlled by Southern Hemisphere ice sheets to one controlled by Northern Hemisphere ice sheets. Although the transient behaviour is important, equilibrium experiments show that the relationship between temperature and sea level is linear and symmetric, providing limited evidence for hysteresis. Furthermore, the results show a good comparison with other simulations of Antarctic ice volume and observed sea level.

Description: Indian Ocean surface circulation is an important part of the global ocean conveyor belt, and is connected via two important gateways including the Indonesian Throughflow, and the Agulha Leakage. Changes in the surface hydrography of the Indian Ocean may therefore impact on the global overturning circulation. Using planktonic foraminifera-based reconstructions of Indian Ocean surface salinity and temperature, we find that Indian Ocean surface water salinify during glacial intensification. Here we present reconstructions of global mean sea level using the ice sheet model ANICE and benthic foraminifera oxygen isotope data from U1476 to analyse changes in mean global sea level and calculate whole ocean changes in seawater oxygen isotopes. We use this information to show that the Indian Ocean surface hydrography changed in different ways from other ocean basins during Late Pleistocene glacial-interglacial cycles.

Description: Indian Ocean surface circulation is an important part of the global ocean conveyor belt, and is connected via two important gateways including the Indonesian Throughflow, and the Agulhas Leakage. Changes in the surface hydrography of the Indian Ocean may therefore impact on the global overturning circulation. Using planktonic foraminifera-based reconstructions of Indian Ocean surface salinity and temperature, we find that Indian Ocean surface water became saliter during glacial intensification. Here we present bathymetrical change data for the Indonesian Archipelago which shows that the Indonesian Throughflow was likely impacted due to changes in global mean sea level and possibly drove the changes in salinity during a glacial cycle.

Description: Indian Ocean surface circulation is an important part of the global ocean conveyor belt, and is connected via two important gateways including the Indonesian Throughflow, and the Agulha Leakage. Changes in the surface hydrography of the Indian Ocean may therefore impact on the global overturning circulation. Here we present planktonic foraminifera-based stack of oxygen isotopes as proxy for surface salinity from the surface Indian Ocean. We find that Indian Ocean surface salinity (along with temperature) increases during glacial intensification. We link this phenomenon to dynamics in the Indonesian Archipelago.