A cutting-edge global climate model links atmospheric aerosol emissions to temperature variability in the North Atlantic Ocean, suggesting that human activity influences extreme weather events. See Letter p.228

Over the past century, the surface of the North Atlantic Ocean has gone through warm and cool periods that are not observed in other ocean basins. This Atlantic multidecadal oscillation (AMO)1 is thought to affect climate processes2 ranging from the current high levels of Atlantic hurricane activity to the devastating sub-Saharan droughts of the early 1980s. Although the influence of the AMO on extreme weather events has long been recognized, the physical processes underlying these temperature changes are not understood. In a paper published on Nature's website today, Booth et al.3 report their use of a state-of-the-art model of Earth's climate to demonstrate that, at least over the past century, the AMO is largely the response of the upper ocean to changes in the concentration of pollution aerosols in the atmosphere. If correct, their results imply that the influence of human activity on the Atlantic regional climate is more pervasive than previously thoughtFootnote 1.

The AMO is best depicted as the difference between average ocean surface temperatures over the North Atlantic and those over the global oceans4 (Fig. 1). It therefore reflects the deviation of the North Atlantic Ocean from global mean temperatures, which are dominated by the long-term warming that is forced by greenhouse gases. Conventional wisdom has held that the AMO is the natural result of internal processes in the Atlantic Ocean — most notably, fluctuations in deep-ocean circulation, as supported by multi-century climate-model studies5. Figure 1: The Atlantic multidecadal oscillation. The difference between average ocean surface temperatures over the North Atlantic and those over the global oceans has oscillated between cool and warm phases over the past hundred years or so, as depicted here. Booth et al.3 report that simulations of global climate link this temperature variability to atmospheric aerosol emissions. Full size image

Booth et al.3 simulated the climate of the past 150 years using a version of a well-known climate model6 that includes up-to-date parameterizations of aerosol emissions, aerosol chemistry and interactions of aerosols and clouds. Nearly all of the observed decadal variability in North Atlantic surface temperatures was reproduced in their simulation, including the AMO and the long-term warming associated with increasing amounts of greenhouse gases. This is the first time that changes in sea surface temperatures have been reproduced to this degree of accuracy by a climate model. The authors' analysis of the model's output reveals that this variability about the upward temperature trend results from cooling associated with periodic volcanic eruptions, and from the build-up of polluting aerosols in the atmosphere that occurred from pre-industrial times until the late 1960s and early 1970s, when clean-air legislation in the United States and Europe was implemented.

So how do aerosols affect sea surface temperatures? When suspended over water, aerosols tend to cool the surface by increasing the local albedo (the ability to reflect sunlight), a phenomenon known as the aerosol direct effect. Aerosols caused by human activity may also act as nuclei around which water vapour in the atmosphere can condense. When more of these nuclei are available for condensation within a cloud, the number of water droplets in the cloud goes up and the average size of the water droplets goes down, making the cloud brighter so that it reflects more sunlight back out to space. This process is known as the cloud albedo effect, or the first aerosol indirect effect. It is these aerosol–cloud interactions that have the most influence over the AMO in Booth and colleagues' model.

The idea that variations in aerosol concentration have caused decadal-scale changes in surface temperatures in the North Atlantic is not new. A body of work has emerged suggesting that elements of the AMO are externally forced by aerosols through direct and first indirect effects, and has implicated aerosols from volcanic eruptions7, West African dust storms8 and human activity4,9. However, before Booth and colleagues' work, no study had incorporated the direct and indirect forcings from these various aerosol types to paint a coherent picture of temperature changes in the Atlantic Ocean that was consistent with both the observed temporal variability and the dominant spatial structure of the changes.

Booth and colleagues' evidence3 that the AMO is caused by changes in the regional abundance of aerosols is compelling, but their results are sensitive to model parameterizations of microphysical processes, particularly the interaction between cloud water droplets and aerosols, that are not well constrained by observations. In addition, their model was unable to reproduce observed multidecadal variability in outbreaks of African dust storms10, which alter the temperature of the tropical Atlantic8; this may explain why the model does a poorer job of simulating temperatures in the tropical North Atlantic Ocean than it does in the extratropical regions. Furthermore, the authors' conclusion that internal variability of the Atlantic Ocean had a negligible role in shaping the AMO during the twentieth century is at odds with the findings of several previous studies5,11. The reason for this discrepancy is not clear.

If Booth and colleagues' results3 can be corroborated, then they suggest that multidecadal temperature fluctuations of the North Atlantic are dominated by human activity, with natural variability taking a secondary role. This has many implications. Foremost among them is that the AMO does not exist, in the sense that the temperature variations concerned are neither intrinsically oscillatory nor purely multidecadal.

Another implication concerns hurricanes. As noted earlier, quiescent and active periods of Atlantic hurricane activity have been linked2 to the AMO. These swings in hurricane frequency and intensity might therefore be the regional response to variations in the concentration of pollutant aerosols against a background of global warming, and thus completely man-made. Similarly, human activity might have caused periods of drought within the Sudano-Sahel region of Africa and in northeastern Brazil.

As we try to predict the climate in a warming world, an increasing body of work suggests that aerosols may have regional effects as great as those caused by the global increase in atmospheric carbon dioxide. Booth and colleagues' work3 underscores the importance of understanding the diverse pathways by which humans alter the climate.

Notes

References 1 Kerr, R. A. Science 288, 1984–1985 (2000). 2 Knight, J. R., Folland, C. K. & Scaife, A. Geophys. Res. Lett. 33, L17706 (2006). 3 Booth, B. B. B., Dunstone, N. J., Halloran, P. R., Andrews, T. & Bellouin, N. Nature 484, 228–232 (2012). 4 Mann, M. E. & Emanuel, K. A. Eos 87, 233–244 (2006). 5 Delworth, T. L. & Mann, M. E. Clim. Dyn. 16, 661–676 (2000). 6 Collins, W. J. et al. Geosci. Model Dev. 4, 1051–1075 (2011). 7 Otterå, O. H., Bentsen, M., Drange, M. & Suo, L. Nature Geosci. 3, 688–694 (2010). 8 Evan, A. T., Vimont, D. J., Heidinger, A. K., Kossin, J. P. & Bennartz, R. Science 324, 778–781 (2009). 9 Chang, C.-Y., Chiang, J. C. H., Wehner, M. F., Friedman, A. R. & Ruedy, R. J. Clim. 24, 2540–2555 (2011). 10 Prospero, J. M. & Lamb, P. J. Science 302, 1024–1027 (2003). 11 Deser, C., Alexander, M. A., Xie, S.-P. & Phillips, A. S. Annu. Rev. Mar. Sci. 2, 115–143 (2010). Download references

Author information Affiliations Amato Evan is in the Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22904, USA. Amato Evan Authors Amato Evan View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Amato Evan.

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