Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability

A state-of-the-art climate model shows that radiative forcing due to anthropogenic and volcanic aerosols explains the variability in sea surface temperature of the North Atlantic between 1950 and 2005. Influence of anthropogenic aerosols on climate Changes in North Atlantic sea surface temperatures (SSTs) have profound impacts on the climate of much of the globe. Multidecadal variability in Atlantic SST has long been thought to be governed by internal ocean dynamics, but here Booth et al . present evidence that human-generated aerosols — predominantly from fossil-fuel and biomass burning — were a prime driver of twentieth-century North Atlantic climate variability. Using a sophisticated Earth system climate model, they show that from 1860 to 2005, anthropogenic aerosol emissions strongly influenced Atlantic multidecadal SST variability and therefore the climate processes and events linked to Atlantic SSTs, such as drought and tropical cyclones. Systematic climate shifts have been linked to multidecadal variability in observed sea surface temperatures in the North Atlantic Ocean 1 . These links are extensive, influencing a range of climate processes such as hurricane activity 2 and African Sahel 3 , 4 , 5 and Amazonian 5 droughts. The variability is distinct from historical global-mean temperature changes and is commonly attributed to natural ocean oscillations 6 , 7 , 8 , 9 , 10 . A number of studies have provided evidence that aerosols can influence long-term changes in sea surface temperatures 11 , 12 , but climate models have so far failed to reproduce these interactions 6 , 9 and the role of aerosols in decadal variability remains unclear. Here we use a state-of-the-art Earth system climate model to show that aerosol emissions and periods of volcanic activity explain 76 per cent of the simulated multidecadal variance in detrended 1860–2005 North Atlantic sea surface temperatures. After 1950, simulated variability is within observational estimates; our estimates for 1910–1940 capture twice the warming of previous generation models but do not explain the entire observed trend. Other processes, such as ocean circulation, may also have contributed to variability in the early twentieth century. Mechanistically, we find that inclusion of aerosol–cloud microphysical effects, which were included in few previous multimodel ensembles, dominates the magnitude (80 per cent) and the spatial pattern of the total surface aerosol forcing in the North Atlantic. Our fin