Arctic sea ice is disappearing. This we know, but the exact cause of the disappearance — and the effect it has on global climate — is proving a lot harder to understand. While a proliferation of new research into sea ice might be exciting for both climatologists and environmental scientists, it also means that game-changing data is generated on a regular basis.
Julienne Stroeve, Senior Research Scientist at the National Snow and Ice Data Center (NSIDC), has spent years investigating the decline of Arctic ice cover and the implications for both local and global climates.
“For a really long time, people have been saying that we would first see climate change in regions like the Arctic,” says Stroeve. “Even climate models from back in the seventies said so. Recent simulations suggest 50 to 60 per cent of the current loss of sea ice is a result of warming from greenhouse gases. What’s interesting is that the old climate models are basically starting to come true.”
Efforts to understand global warming are centred around something called albedo. In simple terms, this is a measurement for the amount of incoming solar radiation that is reflected by the surface of an object. New snow has a high albedo, meaning it reflects most of the sun’s energy back out to space. But as snow and ice melt, they expose the sea or land beneath — much darker surfaces that absorb more of the sun’s energy. This absorbed energy causes these materials to heat up, which in turn melts more ice and snow. It’s this — a positive feedback loop — that seems to be driving the rapid loss of Arctic ice.
“Over the past century, temperatures across the Arctic have risen at nearly twice the global average rate.”
And as Stroeve explains, this effect is combining with some of the Arctic’s natural mechanisms, particularly the wind-driven patterns that move sea ice around the Arctic Ocean. “In the nineties, there was a change in the circulation pattern that was removing the older and thicker ice — a natural climate variability,” she says. “But if this takes place at the same time as background warming, they act together and you lose a lot of ice.”
The significance of Arctic ice to the global climate has recently come under scrutiny. Studies led by Alaska’s International Arctic Research Centre have suggested a link between the loss of sea ice and the extreme weather events that we’ve recently seen in mid-latitude countries. At the start of 2014, a record deep freeze brought Chicago to a snow-covered standstill while recent years have seen severe floods in Pakistan, tropical storms in Asia, and heat waves across Russia and the US.
The latest hypothesis argues that when the Arctic is warming disproportionately faster than the rest of the northern hemisphere, it reduces the thermal gradient between the planet’s poles and the mid- latitudes. It’s this thermal gradient — the interaction between the cold air at the top and bottom of the planet, and the warmer tropical air in the middle — that drives the jet stream, the air current that in turn governs our weather.
The jet stream flows around the planet from west to east in an undulating wave pattern driven by high and low areas of atmospheric pressure. According to the hypothesis, the shift in the thermal gradient is weakening the waves in the jet stream.The latter move slower and become more ‘wavy’, with more amplitude — peaks and troughs. These bigger peaks and troughs means more extremes in the world’s weather.
David Robinson is professor of geography at Rutgers University and specialises in polar climate research. He explains how a warmer arctic with less ice might affect the Gulf Stream, a powerful, Atlantic ocean current from the tip of Florida that follows the US eastern seaboard before crossing the Atlantic Ocean.
“Right now it’s a hypothesis and not a theory,” he says. “But one of the parts of this hypothesis is that as the ice disappears, you’re going to have more blocking — that’s when you have a ridge or a bulge in the jet stream that just parks itself for a period of time. When you’ve got a ridge in place nothing can transgress — or, for that matter, regress — in the atmosphere wave pattern. For example, this year in North America there’s been a very persistent ridge in the west and a trough in the East. That’s why California is having one of their hottest, driest years on record.”
“I don’t want to give the impression we can wait on this,” he says. “Our climate system is changing, and we are going to have to adapt to new means and extremes.”
So far, none of this has been proven, which makes scientists like David Robinson and Julienne Stroeve so cautious about assigning the cause of extreme weather events to climate change.
“The problem is cause and effect,” explains Stroeve. “Is it the loss of sea ice that is causing these changes that we’re really starting to see in the atmosphere, or are there other things that are also contributing to that? For example, in the 1950s, when there was much more Arctic ice, there are still records of slowing zonal winds in the jet stream.”
Nevertheless, if there is a link between melting sea ice and extreme weather events, the consequences could be very serious — and almost all current climate models agree that the ice will vanish eventually if current trends continue. Over the past century, temperatures across the Arctic have risen at nearly twice the global average rate.
There’s a lot of work still to be done before the link can be proved. Researchers are currently looking at the empirical data being generated in the Arctic and in the mid- latitudes alike, and then modelling different scenarios over different spatial scales. But this will take time, and as Robinson points out, time isn’t a commodity that we have in abundance.
“I don’t want to give the impression we can wait on this,” he says. “Our climate system is changing, and we are going to have to adapt to new means and extremes. We need to know why they’re changing and how they’re going to continue to change.
“The Arctic ice is going, and if this hypothesis is correct, the implications for mid-latitude populations are enormous.”