Natural Variability Plays Bigger Role in Antarctic Than Thought

The Antarctic ice sheet is one of the tipping elements in the climate system and hence of vital importance for our planet’s future under man-made climate change. Even a partial melting of the enormous ice masses of Antarctica would raise sea-levels substantially. Therefore it is of utmost importance to provide sound knowledge on the extent of anthropogenic warming of the ice-covered continent. A new analysis by German physicists shows that the uncertainties in the temperature trends over Antarctica are larger than previously estimated.

“So far it seemed there were hardly any major natural temperature fluctuations in Antarctica, so almost every rise in temperature was attributed to human influence,” says Armin Bunde of Justus-Liebig-Universität Gießen (JLU). “Global warming as a result of our greenhouse gas emissions from burning fossil fuels is a fact. However, the human influence on the warming of West Antarctica is much smaller than previously thought. The warming of East Antarctica up to now can even be explained by natural variability alone.” The results of their study are now published in the journal Climate Dynamics.

The melting of Antarctic ice shelves is not only influenced by warming air but also by warming oceans, causing ice loss at the coast. However, as there are no sufficient long-term records for Antarctic ocean warming yet, the study focuses on air temperature trends. In collaboration with Hans Joachim Schellnhuber of the Potsdam Institute for Climate Impact Research and Christian Franzke of the Cluster of Excellence “Integrated Climate System Analysis and Prediction” (CliSAP) of Hamburg University, the physicists of JLU Armin Bunde and Josef Luderer were able to show that there are major and very persistent temperature fluctuations in Antarctica.

“The climate in Antarctica, just like the global climate, tends to be distinctly persistent by nature – it remains in certain temperature ranges for a long time before it changes. This creates a temporal temperature structure of highs and lows,” explains Christian Franzke. “A low, i.e. a longer cold period, will be followed by a longer warm period, and this natural warming has to be differentiated from the superimposed anthropogenic warming,” adds Armin Bunde. The scientists did not only analyze data from individual measuring stations but also generated regional averages. The results show a human influence on the warming of West Antarctica, while this influence is weaker than previously thought. However, the warming of Antarctica altogether will likely increase more strongly soon.

For several years temperatures in Antarctica, but also globally, have been increasing less rapidly than in the 1990s. There are a number of reasons for this, e.g. the oceans buffering warmth. The study now published by the German team of scientists shows that man-made global warming has not been pausing – it was temporarily superimposed and therefore hidden by long-term natural climate fluctuations like in Antarctica.

“Our estimates show that we are currently facing a natural cooling period – while temperatures nonetheless rise slowly but inexorably, due to our heating up the atmosphere by emitting greenhouse gas emissions,” explains Hans Joachim Schellnhuber. “At the end of this natural cold spell temperatures will rise even more fiercely. Globally, but also in Antarctica which therefore is in danger of tipping.”  In fact, in March 2015 two Antarctic measuring stations registered high-temperature records.

Previous estimates of the strength and the uncertainty of the observed Antarctic temperature trends assumed that the natural annual temperature fluctuations can be represented by an auto-regressive process of first order [AR(1)]. Here we find that this hypothesis is inadequate. We consider the longest observational temperature records in Antarctica and show that their variability is better represented by a long-term persistent process that has a propensity of large and enduring natural excursions from the mean.

As a consequence, the statistical significance of the recent (presumably anthropogenic) Antarctic warming trend is lower than hitherto reported, while the uncertainty about its magnitude is enhanced. Indeed, all records except for one (Faraday/Vernadsky) fail to show a significant trend.

When increasing the signal-to-noise ratio by considering appropriate averages of the local temperature series, we find that the warming trend is still not significant in East Antarctica and the Antarctic Peninsula. In West Antarctica, however, the significance of the trend is above TeX , and its magnitude is between 0.08 and 0.96 °C per decade. We argue that the persistent temperature fluctuations not only have a larger impact on regional warming uncertainties than previously thought but also may provide a potential mechanism for understanding the transient weakening (“hiatus”) of the regional and global temperature trends.

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