Nothing but blue skies… This rare satellite image of a completely cloudless Alaska triggered media debate this week on whether climate change and melting Arctic sea ice are to blame. The true picture looks more nuanced.
Release of the image by NASA coincided with a heatwave that saw the mercury hit 36 °C in Anchorage on 17 June, and record-breaking temperatures in at least three other towns. Yet just weeks ago, the state saw unseasonably late snow and cold weather, prompting former Alaskan governor and climate sceptic, Sarah Palin, to mockingly dismiss global warming on her Facebook page: "Global warming my gluteus maximus".
Meteorologists at the US National Oceanic and Atmospheric Administration (NOAA) warned last week that it is far too early to link the freak Alaskan weather to climate change. "The jury is out on any individual events and their relationship to specific long term drivers," said Deke Arndt, chief of climate monitoring at NOAA's National Climatic Data Center in Asheville, North Carolina, during a routine monthly climate teleconference. "It takes time to connect the dots."
Unusual blockage
Earlier today, Eugene Petrescu, a regional NOAA scientist in Anchorage, told New Scientist that unusual blockages and deviations in the jet stream – the fast current of air that normally travels West to East across the northern hemisphere – are most likely to be responsible for both the cold and hot spells in Alaska.
When the jet stream turned sharply south, it trapped Alaska in currents of cold air from Siberia and the Arctic. More recently, it veered sharply north, bringing with it bubbles of warm tropical air from the south, resulting in the current Alaskan heatwave. "It's eased a bit, but it's going to happen again next week," says Petrescu.
As to whether global warming is what is causing the jet stream to deviate, Petrescu says there is too little data as yet. "Right now, we can't say yes or no," he says.
Ice shrinking
But Petrescu said that global warming and the rapid melting of Arctic ice is definitely having an impact on Alaska's ice cover, particularly in the north, where average annual temperatures rose by 1.7 degrees between 2000 and 2010.
"Each summer, the main Arctic ice pack has been melting back, and now by each autumn there's less than 50 per cent of what was there historically," he says.
The result in Alaska is that the ice that borders the north of the state for most of the year is absent for a month longer in summer, re-forming in November rather than October. The delayed freezing is blamed on higher sea-surface temperatures.
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