On the role of sulfates in recent global warming: a Granger causality analysis

Attribution studies clearly show that the recent global warming is mainly linked with changes in atmospheric composition induced by human activities, especially by emissions of greenhouse gases (GHGs). Even if the increasing trend in temperatures is well caught by global climate models (GCMs), some periods of quasi-constant temperature behaviour are not completely understood. In this article we focus on the role of sulfates and apply an inferential method (Granger causality analysis) in order to analyse in detail their role in changes of global temperature. Starting from the identification of two sub-periods (1943-1975 and 1976-2011) characterized by a quasi-constant plateau and a steep increase in temperature, respectively, we find that in the first period the cooling induced by sulfates is able to balance the warming effect of GHGs. This does not happen in the second period, when GHG forcing probably overwhelms the other causal factor here considered. In any case there exists a strong evidence of a joint driving of temperature by these two forcings. This allows us to identify sulfates as a major driver of the quasi-stationary period 1943-1975.