Antarctic Peninsula: Warming pauses but glaciers go on melting
21 July 2016 by Barry Hague
A temporary respite in rising temperatures in one of the world's fastest-warming places doesn't mean climate change has stopped, says new British Antarctic Survey (BAS) research.
When it comes to climate change, few places on the planet have been such a focus of attention as the Antarctic Peninsula. Over the second half of the 20th century, this snaking, 800-mile strip of rock, snow and ice, reaching across the stormy Southern Ocean towards the tip of South America, experienced some of the most rapid temperature increases recorded anywhere on Earth - a phenomenon whose causes and implications have been hotly debated.
To bring the story right up to date and shed fresh light on the forces shaping the climate of this remote, mountainous landmass, a team of scientists from BAS - part of NERC - has just published some intriguing new insights in the prestigious science journal Nature. Their most striking finding is that the period of rapid warming, which helped trigger dramatic ice-shelf collapses across the peninsula and caused many glaciers to retreat, stopped in 1997. Indeed, since then, in the northern part of the region, the process has actually gone into reverse.
Just as significant, however, is their conclusion that this pause is due to stabilisation of the hole in the ozone layer above Antarctica, an increase in cold easterly winds and natural climate variability - factors masking long-term temperature trends. But if greenhouse gases in the atmosphere go on growing at the current rate, it's predicted that temperatures in the peninsula will rise by several degrees Celsius by the year 2100. In fact, despite the current 'lull', temperatures there have still been higher over the last 20 years than they were in the mid-20th century. The glaciers are still retreating.
Professor John Turner is the study's lead author. "The peninsula's climate system shows large natural variations that can overwhelm the signals of human-induced global warming," he explains. "But our models predict that greenhouse gases will lead to an increase in temperatures by the end of this century."
The current pause in warming in the Antarctic Peninsula represents just the blink of an eye.
It's also crucial to note that the NERC-funded study covered just one per cent of Antarctica (for example an area the size of England) and that past trends in the peninsula have not always been mirrored across the continent. Nor might they be in the future. The research's real significance has been in aiding understanding of climate's complex workings in a specific place over a specific period. Put simply, the pause in warming detected in the peninsula doesn't mean climate change has stopped.
The team analysed data gathered at six research stations in the region. Stretching back to the 1950s, the data spans weather conditions, levels of sea ice, atmospheric circulation and ocean surface temperatures. The team also used ice cores - long cylinders of ice drilled out from the 350m-thick ice sheet blanketing James Ross Island - to build up a 2000-year reconstruction of the peninsula's climate and to place their analysis in its longer-term context.
"The ice core record provides a 'memory' of past climate", co-author Dr Robert Mulvaney explains. "By analysing tiny air bubbles trapped in the layers of ice, we can detect the levels of greenhouse gases in the atmosphere at different points in time. We can also look for heavy isotopes of water, as their level is an accurate indicator of the temperature at the time the ice was deposited."
The study revealed that, from the early 1950s to the late 1990s, temperatures in the Peninsula rose by up to 0·5°C per decade. 1998 was the turning point, however, with a cooling of two-thirds of a degree recorded since then.
"Cold easterly winds, resulting from increased storminess in the Drake Passage caused by stabilisation of the ozone hole, have played a key role in this cooling," says John Turner. "They've contributed to an increase in sea-ice, particularly in the north-east of the peninsula, preventing ocean heat from entering the atmosphere." Caused primarily by heavy use of chlorofluorocarbons (CFCs) in aerosols and fridges, the ozone hole over Antarctica was first discovered by BAS scientists in 1985. Recent evidence indicates that efforts to limit CFCs are now having an effect and the hole has begun to heal.
The ice cores, meanwhile, revealed that the 20th century warming phase actually began in the 1920s, before the research stations began to generate data. They also indicated large natural temperature variability in the peninsula over many centuries, with several warming and cooling periods. The 20th century warming, then, was unusual but not unprecedented.
Looking to the future, a key development within the last month has seen the level of atmospheric carbon dioxide above Antarctica breaking through the 400 parts per million mark. Pre-industrial concentrations stood at just 280 parts per million. Drawing on work by the Intergovernmental Panel on Climate Change (IPCC), the BAS team believe continued increases at current rates will simply overwhelm natural climate variability and the cooling associated with recovering ozone levels. If so, news of the current pause in warming in the Peninsula could prove cold comfort in the longer term.
"In the drive to strengthen understanding of the complexity of climate change and the interplay between natural and human-induced factors, this new study provides just one piece of the overall jigsaw," comments Professor David Vaughan, BAS director of science. "But studies like this are vital as polar regions have seen some of the biggest climatic changes in recent decades. The research shows that climate change is never constant, in terms of where and when it occurs."
John Turner sums it up like this: "From a climate change perspective, the current pause in warming in the Antarctic Peninsula represents just the blink of an eye."
The paper "Absence of 21st century warming on Antarctic Peninsula consistent with natural variability" was published in Nature on 21 July 2016.