Laura Wilcox and colleagues of the University of Reading and the Met Office used the most recent climate models to confirm that aerosols played a large role in the temperature decrease in that timeframe. They have, for the first time, quantified the size of the contribution made by anthropogenic aerosols, as opposed to natural aerosols.

“The idea that aerosols cause cooling is not new,” Wilcox told environmentalresearchweb. “Previous work with single models has suggested that it is the indirect effect of aerosols – their interaction with clouds – that accounts for most of their cooling influence. We have shown this to be the case for multiple models. Without the indirect effect, the models are unable to reproduce the cooling in the 1950s and 1960s. By running the models with all climate-forcing agents fixed, except anthropogenic aerosols, we have for the first time been able to quantify the contribution of anthropogenic aerosols.”

The results, said Wilcox, are striking. “Even in such a short timeframe, and taking into account the uncertainties in the models we used, the evidence is clear – anthropogenic aerosols were the main contributor to the global mean temperature drop between 1950–1965,” she said. “Aerosols could have a more significant effect on global mean temperature than greenhouse gases, and with anthropogenic aerosols expected to decrease in the coming years, our findings have implications for future policy.”

The research also showed that the cooling effect of anthropogenic aerosols on temperature is so great that it cancelled out the warming effects due to greenhouse gases to give an overall global cooling between 1950 and 1965.

Compared with greenhouse gases, aerosols are short-lived and tend to result in more local effects, both directly by modification of radiative transfer and indirectly, through aerosol-cloud interactions. The effect is largest in the Northern Hemisphere, where the majority of anthropogenic aerosols occur. This gradient between the Northern and Southern Hemispheres has major consequences for global climate.

Because aerosol effects tend to be felt more locally than those of greenhouse gases, Wilcox and her colleagues are now concentrating on modelling the regional effects of aerosols.

“Only five of the models we used performed simulations of the past with anthropogenic aerosols alone, and with natural forcing agents alone,” added Wilcox. “This allowed us to see the response to each of these in isolation and quantify their contribution to the overall trends. We hope that more models will do this in the future. Our research has shown that models that include the aerosol-cloud interactions are likely to produce more reliable predictions of near future climate change.”

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