The Antarctic Peninsula has been warming for many decades, but an analysis now reveals that it has cooled since the late 1990s. Inspection of the factors involved suggests that this is consistent with natural variability. See Letter p.411

The Antarctic Peninsula is a triangular, mountainous land with a coastline of dramatic, calving glaciers and rich wildlife, and it exemplifies the popular image of Antarctica (Fig. 1). Over the past half century, it has been one of the most rapidly warming places on Earth. This warming is associated with major physical and biological changes, including a decline in the Adélie penguin population1 and the disintegration in 2002 of a large portion of the Larsen B ice shelf, a major geographical feature that had existed for millennia2. It is natural to view the changes at this northernmost point in Antarctica as part of the inexorable southward march of anthropogenic climate change. It may thus seem remarkable that Turner and colleagues3 now report (page 411) that the Antarctic Peninsula has actually cooled in the past two decades. Figure 1: Glacier ice at James Ross Island, Antarctic Peninsula. Tom Tobin Turner et al.3 report that the peninsula has cooled during the past 20 years. Full size image

Turner et al. show that the average temperature on the peninsula has decreased by about 0.5 °C per decade since the late 1990s, about the same rate at which it warmed in the preceding five decades. To understand this apparent reversal, one has to recognize that the average warming trend observed since the middle of the twentieth century reflects several aspects of Antarctic Peninsula climate. Which aspect dominates varies by both location and season. For example, the oft-heard claim that the peninsula is the fastest-warming place on Earth is accurate only during the winter, as recorded at the Faraday/Vernadsky station on the west coast. Winter warming at this station — more than 6 °C since records began in 1947 — is unambiguously associated with a rapid decrease in sea ice4.

By contrast, significant summer warming has been largely restricted to the peninsula's east coast and is related to poleward migration of the circumpolar westerly winds, commonly described by an increase in the Southern Annular Mode (SAM) index5. The SAM is a mode of natural atmospheric variability, but changes in the SAM index have been associated with increased greenhouse gases and the decline of stratospheric ozone6. Autumn is the only season during which both coasts of the Antarctic Peninsula have warmed together over multiple decades7. The autumn temperature trend has been associated with features of large-scale atmospheric circulation known as Rossby waves, which link changes in tropical convection (associated with El Niño ocean-warming events, for example) with temperatures on the Antarctic Peninsula7.

Turner et al. show that most of the factors that determine the peninsula's temperatures have changed in such a way as to favour regional cooling, rather than warming, since the end of the twentieth century. They find that there has been no increase in the SAM index since the late 1990s, but that there have been stronger easterly and southeasterly near-surface flows towards the peninsula, countering the long-term trend of increasing westerlies during the summer. Because southeasterlies tend to push sea ice towards the peninsula's east coast, there has been greater sea-ice cover there during the past two decades, reducing the flux of heat from the ocean and further amplifying the cooling effects of the changes in atmospheric circulation.

Moreover, the authors show that the average state of the tropical Pacific Ocean has changed, with the dominance of strong El Niño events during the 1990s giving way to colder, predominantly La Niña conditions during the 2000s. Although the relationship between the state of the tropics and Antarctic Peninsula temperatures is complex, the net result during autumn and winter has been increased easterlies and southeasterlies, similar to those that occur during summer. The changing state of the tropical Pacific similarly explains why temperatures over central West Antarctica — south of the peninsula — increased markedly through the 1990s, but have since remained relatively stable8.

A crucial point made by Turner et al. is that both the warming since the 1950s and the cooling since the late 1990s are entirely consistent with natural climate variability. This is to be expected: none of the known drivers of Antarctic Peninsula temperature change can be clearly associated with anthropogenic effects, apart from the summertime trend in the SAM index. Although recent changes in the peninsula's climate have been large, the natural decadal-scale variability is also large, making short-term fluctuations inherently unpredictable even in the presence of strong forcing.

Even before Turner and colleagues' analysis, there was little evidence that the rapid warming in Antarctica falls outside the range of natural variability. Indeed, the palaeoclimate record from ice cores has strongly suggested that it does not8,9 (with the possible exception of summer warming on the northernmost part of the peninsula, at James Ross Island10). In short, Turner and co-workers' findings should not be surprising.

A potentially controversial aspect of the new findings is the authors' conclusion that the observed peninsula cooling is independent of the recent period of relatively modest global warming, sometimes dubbed the hiatus. An objective choice for the start of the cooling period is 1998, the year commonly cited for the start of the hiatus. Although several explanations for the hiatus have been proposed, there is little doubt that exchange of heat between the atmosphere and the tropical ocean has a major role. This heat exchange is sometimes discussed in terms of a negative phase of a phenomenon called the Interdecadal Pacific Oscillation (IPO, detected as warm or cool surface waters in the Pacific)11. Turner et al. argue that negative phases of the IPO index tend to increase temperatures on the Antarctic Peninsula, which implies that the observed cooling is not connected to the hiatus. But relationships between tropical dynamics, global temperatures and Antarctic climate aren't likely to be captured by a single index.

Climate models12 suggest that the signal of anthropogenic climate change is damped by the tendency of mean ocean transport to carry heat northward. This, combined with the large magnitude of natural variability, suggests that anthropogenic climate change may not be unambiguously detectable in Antarctica for several more decades. Many decades of data are required to meaningfully characterize decadal climate variability. Looking at surface-air temperatures13 collected for more than 60 years, the long-term trend is one of warming. Importantly, the authors do not claim that this long-term warming has ended, and it would indeed be premature to come to such a conclusion.Footnote 1

Notes

References 1 Lynch, H. J., Naveen, R., Trathan, P. N. & Fagan, W. F. Ecology 93, 1367–1377 (2012). 2 Domack, E. et al. Nature 436, 681–685 (2005). 3 Turner, J. et al. Nature 535, 411–415 (2016). 4 Turner, J. et al. Int. J. Climatol. 25, 279–294 (2005). 5 Marshall, G. J. Int. J. Climatol. 27, 373–383 (2007). 6 Thompson, D. W. J. et al. Nature Geosci. 4, 741–749 (2011). 7 Ding, Q. & Steig, E. J. J. Clim. 26, 7570–7585 (2013). 8 Steig, E. J. et al. Nature Geosci. 6, 372–375 (2013). 9 Mulvaney, R. et al. Nature 489, 141–144 (2012). 10 Abram, N. J. et al. Nature Geosci. 6, 404–411 (2013). 11 Fyfe, J. C. et al. Nature Clim. Change 6, 224–228 (2016). 12 Armour, K. C., Marshall, J., Scott, J. R., Donohoe, A. & Newsom, E. R. Nature Geosci. 9, 549–554 (2016). 13 Nicolas, J. P. & Bromwich, D. H. J. Clim. 27, 8070–8093 (2014). Download references

Author information Affiliations Eric J. Steig is in the Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Eric J. Steig Authors Eric J. Steig View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Eric J. Steig.

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