Climate of the last millennium: a sensitivity study

Abstract

Seventy-one sensitivity experiments have been performed using a two-dimensional sectoraveragedglobal climate model to assess the potential impact of six different factors on the lastmillennium climate and in particular on the surface air temperature evolution. Both natural(i.e, solar and volcanism) and anthropogenically-induced (i.e. deforestation, additional greenhousegases, and tropospheric aerosol burden) climate forcings have been considered.Comparisons of climate reconstructions with model results indicate that all the investigated forcings are needed to simulate the surface air temperature evolution. Due to uncertainties inhistorical climate forcings and temperature reconstructions, the relative importance of a particularforcing in the explanation of the recorded temperature variance is largely function of theforcing time series used. Nevertheless, our results indicate that whatever the historical solar andvolcanic reconstructions may be, these externally driven natural climate forcings are unable togive climate responses comparable in magnitude and time to the late—20th-century temperaturewarming while for earlier periods combination of solar and volcanic forcings can explain theLittle Ice Age and the Medieval Warm Period. Only the greenhouse gas forcing allows themodel to simulate an accelerated warming rate during the last three decades. The best guesssimulation (largest similarity with the reconstruction) for the period starting 1850 AD requireshowever to include anthropogenic sulphate forcing as well as the impact of deforestation toconstrain the magnitude of the greenhouse gas twentieth century warming to better fit theobservation. On the contrary, prior to 1850 AD mid-latitude land clearance tends to reinforcethe Little Ice age in our simulations.