Overestimate of Committed Warming

Palaeoclimate variations are an essential component in constraining future projections of climate change as a function of increasing anthropogenic greenhouse gases. The Earth System Sensitivity (ESS) describes the multi-millennial response of Earth (in terms of global mean temperature) to a doubling of CO2 concentrations. A recent study used a correlation of inferred temperatures and radiative forcing from greenhouse gases over the past 800,000 years to estimate the ESS from present day CO2 is about 9 degrees C, and to imply a long-term commitment of 3-7 degrees C even if greenhouse gas levels remain at present-day concentrations. However, we demonstrate that the methodology of ref. 2 does not reliably estimate the ESS in the presence of orbital forcing of ice age cycles and therefore conclude that the inferred present-day committed warming is considerably overestimated

Interglacial climate variability during MIS 15 to Holocene Insight from Coupled climate modelling

Understanding the mechanisms and effects of natural long-term climate variability is essential for providing projections of possible climate change for the near future. This study examines the mechanisms of the climate variability over the time frame of the past 600 kyr using CCSM3-DGVM (Community Climate System Model version 3 with Dynamic Global Vegetation Model). A set of 13 interglacial time slice experiments was carried out to study global climate variability between and within the Quaternary interglacials of Marine Isotope Stages (MISs) 1, 5, 11, 13, and 15. Here, this study focuses on the effect of different roles of obliquity, precession and greenhouse gases (GHG) forcing on global surface temperature and precipitation patterns. Local insolation anomalies induced by the astronomical forcing play a role in most regions of seasonal surface temperature anomalies. Climate feedbacks, however, may modify the surface temperature response in specific regions, most pronounced in the monsoon domains and the polar oceans. Especially in high latitudes and early Brunhes interglacials (MIS 13 and 15) when GHG concentrations were much lower than during the later interglacials, GHG forcing may also play an important role for seasonal temperature anomalies. During boreal summer, high-versus-low obliquity climates are generally characterized by strong warming over the Northern Hemisphere extratropics and slight cooling in the tropics. A moderate cooling over large portions of the Northern Hemisphere continents and a strong warming at high southern latitudes during winter is found. Additionally, a significant role of obliquity in forcing the West African monsoon is identified. In this case, other regional monsoon systems are less sensitive or not sensitive at all to obliquity variations during interglacials. Based on two specific time slices (394 and 615 ka), the model results suggest that the West African and Indian monsoon systems do not always vary in concert, challenging the concept of a global monsoon system at orbital timescales. Furthermore, GHG forcing is positively correlated with surface temperature over most regions of the globe in the annual mean and GHG radiative forcing exhibits no clear response in annual and seasonal precipitation during the interglacials except for the high latitudes in both hemispheres during annual, for southern high latitudes during summer, and for northern high latitudes during winter where the hydrologic cycle accelerates with higher GHG concentrations. In order to disentangle the impact of dynamic vegetation on the early (9 ka) and mid- Holocene (6 ka) North African climate, experiments with the dynamic and fixed-vegetation were carried out. In this study, the coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to pre-industrial. With dynamic vegetation, the orbitally triggered summer precipitation anomaly is enhanced by approximately 20% in the Sahara/Sahel region (10a 25AdegreeN,20AdegreeWa 30Adegree E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation-rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African Easterly Jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Furthermore, this study constrains a three-dimensional thermomechanical-ice model Genie Land Ice Model with Multiple-Enabled Regions (GLIMMER) forced by CCSM3 climate model output for MIS 5 and MIS 11 time slices to simulate a sensitivity of Greenland ice sheet (GrIS). The GrIS is thought to have contributed substantially to high global sea levels during the interglacials of MIS 5 and MIS 11. Geological evidence suggests that the mass loss of the GrIS was similar or even greater during the interglacial of MIS 11 than MIS 5, despite a weaker insolation forcing. This study shows a stronger sensitivity of the GrIS to MIS 11 climate forcing than to MIS 5 forcing. The greater MIS 11 GrIS mass loss relative to MIS 5 is attributed to a larger heat transport towards high latitudes by a stronger Atlantic meridional ocean circulation in addition to a stronger GHG radiative forcing. The results, however, suggest a substantial modification of orbital insolation forcing by internal climate feedbacks, which add significant complexity to the traditional Milankovitch theory

Variability of the Southern Hemisphere Westerly Winds during the Holocene: Insights from coupled climate modelling

The Southern Hemisphere Westerly Winds (SWW) constitute an important zonal circulation that significantly influences the climate system. Variability of SWW during the Holocene under solar and orbital forcings is studied. In order to study the response of the SWW during the Holocene under the above mentioned forcings, numerical modelling is applied. Comparison with proxy records is also carried out to validate the model results. Taken together, the proxy and model results suggest that centennial-scale periods of lower (higher) solar activity caused equatorward (southward) shifts of the annual mean SWW. Under the influence of orbital forcing, the annual and seasonal mean SWW are subjected to an overall strengthening and poleward shifting trend during the course of the mid-to-late Holocene, except for the austral spring season, where the SWW exhibit an opposite trend of shifting towards the equator