Turner, J., H. Lu, I. White, J. C. King, T. Phillips, J. S. Hosking, T. J. Bracegirdle, G. J. Marshall, R. Mulvaney, and P. Deb (2016), Absence of 21st century warming on Antarctic Peninsula consistent with natural variability, Nature, 535(7612), 411–415, doi:10.1038/nature18645.
The Antarctic Peninsula has been one of the poster children of climate change, claimed to be one of the fastest warming regions of the world. In the last few decades of the 20th century, air temperatures there were increasing at an alarming rate of over 0.3°C per decade. Since the turn of the century however, that rate has decreased, and even reversed. As John Turner and colleagues from the British Antarctic Survey describe in a July 2016 issue of Nature, the Antarctic Peninsula has actually experienced a period of cooling since 1999.
These results are really not that surprising though, according to the authors of the study. Air temperatures over the Antarctic Peninsula are extremely variable and, throughout the relatively short period of time we’ve been measuring temperatures there, the variability overpowers any overall trend. The natural rise and fall of air temperatures occur on timescales of decades. Similar patterns can be seen in temperature records all over the world. In the state of New York, for example, there have been decadal warming and cooling trends overlying a clear overall warming since the beginning of the record. But if we were to just focus on the period since 1979, when the Antarctic Peninsula record used in this study begins, it would be difficult to separate the fluctuations from the overall trend.
Over the last 40 years, air temperatures on the Antarctic Peninsula have been driven by several factors including ozone depletion, the direction and strength of the dominant winds, and the concentration of sea ice. These factors are interlinked and can work together to either amplify or diminish an overall trend.
During the end of the last century, ozone depletion was an important factor in the warming of the Antarctic Peninsula, particularly during summer when ozone concentrations in the stratosphere were at their lowest. Winds were predominantly from the northwest bringing warm air to the region. Around the turn of the century, the wind patterns shifted; they came more often from the east and southeast, bringing cold air to the peninsula.
The easterly winds also had the effect of pushing sea ice up against the eastern side of the peninsula creating a cap over the ocean. Ocean temperatures in this region tend to be warmer than the overlying air, but when the heat was trapped by a ceiling of ice, the atmospheric temperatures remain low.
These easterly winds and the climatic feedbacks they encourage have led to the observed cooling since 1999, but that doesn’t mean that the cooling will continue. All six stations used in this study had warmer temperatures at the end of the record than at the beginning. In fact, Vernadsky and Rothera, the two most southern stations, appear to have continued warming throughout the observation period. At the same time, ocean temperatures, especially in the south, have been warming consistently along the peninsula. This trapped ocean heat has been melting glaciers from below and is likely to eventually lead to warmer atmospheric temperatures.
Large scale atmospheric trends also point to continued warming. The Southern Annular Mode (SAM) is an atmospheric climate pattern similar to the El Nino Southern Oscillation, but with more direct influence over Antarctica. It is associated with warm northwesterly winds over the peninsula. The Southern Annular Mode has been at its strongest when ozone concentrations are low. During the beginning of the temperature record, the lowest ozone concentrations occurred during the summer, so that’s when SAM had its greatest influence. In recent years, however, SAM has mostly been positive. If that trend continues, we should expect warmer winds from the west to become more frequent.
The Antarctic Peninsula has experienced some of the fastest warming temperatures because it is one of the most intensely variable regions of the world. The recent cooling doesn’t mean that a long term warming trend isn’t there. It just means that the natural variability of the system is currently overpowering the long-term signal. We’ll have to continue monitoring the temperature for a long time to separate the long-term trends from the short term highs and lows.