The sustained rate of increase over the past century in the combined radiative forcing from the three well-mixed greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) is very likely unprecedented in at least the past 16 kyr. Pre-industrial variations of atmospheric greenhouse gas concentrations observed during the last 10 kyr were small compared to industrial era greenhouse gas increases, and were likely mostly due to natural processes. • It is very likely that the current atmospheric concentrations of CO2 (379 ppm) and CH4 (1,774 ppb) exceed by far the natural range of the last 650 kyr. Ice core data indicate that CO2 varied within a range of 180 to 300 ppm and CH4 within 320 to 790 ppb over this period. Over the same period, antarctic temperature and CO2 concentrations covary, indicating a close relationship between climate and the carbon cycle. • It is very likely that glacial-interglacial CO2 variations have strongly amplifi ed climate variations, but it ...

Authors: von der Heydt A. S., Dijkstra H. A., van de Wal R. S. W., Caballero R., Crucifix M., Foster G. L., Huber M., Kohler P., Rohling E., Valdes P. J., Ashwin P., Bathiany S., Berends T., van Bree L. G. J., Ditlevsen P., Ghil M., Haywood A., Katzav J., Lohmann G., Lohmann J., Lucarini V., Marzocchi A., Palike H., Ruvalcaba Baroni I., Simon D., Sluijs A., Stap L. B., Tantet A., Viebahn J., Ziegler M. 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.

Global mean surface temperatures are rising in response to anthropogenic greenhouse gas emissions. The magnitude of this warming at equilibrium for a given radiative forcing—referred to as specific equilibrium climate sensitivity (S)—is still subject to uncertainties. We estimate global mean temperature variations and S using a 784,000-year-long field reconstruction of sea surface temperatures and a transient paleoclimate model simulation. Our results reveal that S is strongly dependent on the climate background state, with significantly larger values attained during warm phases. Using the Representative Concentration Pathway 8.5 for future greenhouse radiative forcing, we find that the range of paleo-based estimates of Earth's future warming by 2100 CE overlaps with the upper range of climate simulations conducted as part of the Coupled Model Inter-comparison Project Phase 5 (CMIP5). Furthermore, we find that within the 21st century, global mean temperatures will very likely exceed maximum levels reconstructed for the last 784,000 years. On the basis of temperature data from eight glacial cycles, our results provide an independent validation of the magnitude of current CMIP5 warming projections.