Relative roles of aerosols and natural variability on the 20th century global mean temperature change: analysis from IPCC/AR4 GCMs and an energy balance model

The global mean temperature changes during the 20th century, retrieved from three different records (GISS, CRU, NCDC), all feature a warming trend before 1940s, and a cooling period during 1940s-1970s, followed by the rapid warming in recent three decades. The last warming trend is most significant on Northern Hemispheric (NH) land. The present study uses land-ocean and north-south asymmetry of warming or cooling rate to evaluate relative roles of GHGs, aerosols and natural variability on determining the global mean temperature change. Comparison between observation records and simulation outputs from global climate models used in the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report (AR4) show that, the GCMs reproduce the overall 20th century warming well, but most of them underestimate the decadal variations of global mean temperature change and particularly the large warming rate of NH land in the last three decades of the 20th century. An energy-balanced model with separate land and ocean component is then used to understand relative contribution to the observed anthropogenic global warming from different agencies, namely GHGs, cooling aerosols, energy-absorbing aerosols, and natural variability inside climate system. The results indicate that it is necessary to include energy-absorbing aerosol in the simulation to reproduce both the overall 20th century warming and the land/ocean warming asymmetry. Energy-balanced-model simulation, with input from newly developed aerosol emission historical dataset for next generation of climate models in IPCC AR5 demonstrate: 1) the cooling trend during 1940-1970 is attributed to combining effect of internal climate variability and large emission of cooling aerosol; 2) the rapid warming in the recent three decades can be only explained when the increased emission of energy-absorbing aerosol and reduced emission of cooling aerosol from atmosphere both are included in the model.