Arctic climate over the past millennium: Annual and seasonal responses to external forcings

Abstract

The annual and seasonal temperatures in the Arctic over the past 1150 years are analyzed in simulations performed with the three-dimensional Earth system model of intermediate complexity LOVECLIM forced by changes in solar irradiance, volcanic activity, land use, greenhouse gas concentrations and orbital parameters. The response of the system to individual forcings for each season is examined in order to evaluate the contribution of each forcing to the seasonal contrast. For summer, our results agree relatively well with the reconstruction of Kaufman et al. (2009). Our modelling results suggest that the temperature changes during this period were characterized by large seasonal differences. In particular, while annual mean temperatures display a decreasing trend during the pre-industrial period, spring temperatures appear to rise. The variations in the Earth’s orbital parameters are the main cause for those seasonal differences. Larger climate variations are simulated in autumn compared with the other seasons in response to each forcing, particularly in response to changes in greenhouse gas concentration during the industrial period and in response to land use forcing, which surprisingly has a significant impact on Arctic temperature. These contrasting changes for the different seasons also underline the need for an adequate estimate of the season represented by a proxy.

Robust global ocean cooling trend for the pre-industrial Common Era

“…4a; Supplementary Table S14). The LOVECLIM simulations 27 , run individually with O, G, or S forcings, consistently show that these forcings do not explain the Ocean2k SST cooling trend ( Fig. 4b; Supplementary Table S14).…”

“…Orbital forcing is associated with changes in insolation that are strongly dependent on the season and latitude 32 , and over the Pleistocene epoch, orbital changes forced global climate through amplification mechanisms at high northern latitudes, including the well-known ice-albedo amplification 33 . High northern latitude temperature trends during the past millennium 27,34,35 have also been attributed to orbital forcing, specifically to declining high northern latitude summer insolation, amplified by feedbacks in the Arctic region and resulting in cooling 34,35 . However, when integrated over the full calendar year and spatially across the globe 32 , the 1-2000 ce change in orbital radiative forcing at the top of the atmosphere is only +4.4 × 10 −3 W m −2 (ref.…”

“…4b) arises from the increase in surface albedo owing to deforestation, inducing a net negative radiative forcing on land. The associated cooling is only partly compensated for by the reduced latent heat flux from lower summer evapotranspiration, resulting in overall cooling on land that is transmitted globally by the atmospheric circulation 27,36,37 . However, there are large uncertainties in our understanding of land-use forcing back in time, and with the simulated effects of land-use change on radiative forcing and the hydrological cycle 36,38,39 .…”

“…4): SST simulated by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mk3L model 26 , run with the cumulative addition of orbital (O), greenhouse gas (G), solar (S) and volcanic (V) forcings; and by the LOVECLIM model 27 , run with individual forcings as for CSIRO Mk3L, plus land-use forcing (L) and with all forcings (All). These simulations were matched to the 57 Ocean2k reconstructions (Methods).…”

Model–data comparison and data assimilation of mid-Holocene Arctic sea ice concentration

“…4d). This can be explained by the inertia of the system: higher summer insolation leads to a decrease in ice thickness and concentration in summer and thus larger oceanic heat fluxes during the following seasons (Manabe and Stouffer, 1980;Renssen et al, 2005;Boé et al, 2009;Crespin et al, 2012). No change in winter and spring sea ice cover is simulated over the Chukchi Sea and the CAA (Fig.…”