Blowing hot and cold

Climate change made 2014 the hottest year on record, and delivered record snowfalls too.

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According to United States climatologists at the National Oceanic and Atmospheric Administration (NOAA), 2014 was the hottest year on record—0.69ºC above the 20th-century average. Independent studies agreed.

The previous three warmest years were all boosted by El Niño events, which typically raise global average temperatures about half a degree. An El Niño threatened to develop during 2014, but didn’t eventuate, making the record more extraordinary.

Although the world was far warmer on average, there were a few areas that were colder. Ironically, however, even these conditions may have been caused by global warming.

One area of colder temperatures surrounds the edge of Antarctica. Thin sea-ice forms over the ocean around the continent every winter, and satellite measurements have shown that the amount of sea-ice has increased in recent years. When sea-ice extends further away from the continent, cold air sliding off the Antarctic continent travels further north before it reaches liquid water. The air is heated much more slowly over sea-ice than over liquid water, so colder air temperatures can spread further away from the continent.

The reason for the increase in sea-ice is not certain, but several explanations have been put forward. One is that the strengthening of the westerly winds around Antarctica observed over the past few decades is probably caused by a combination of the ozone hole and global warming. Stronger winds move cold ocean water away from Antarctica more efficiently, thereby allowing sea-ice to grow further north. The sea-ice may also be increasing because of an increased supply of cold fresh water on the sea surface caused by increased melting of the Antarctic Ice Sheet.

The stronger winds have changed ocean currents so that comparatively warm sub-surface water has been reaching the front of the glaciers that drain the West Antarctic Ice Sheet. It melts the glaciers faster, and increases melting under the great ice shelves that extend into the sea too. The buoyant fresh meltwater then runs up the sloping underside of the ice shelf to the sea surface, where it freezes more readily than salty water, increasing the production of sea-ice.

The acceleration in the melting of the ice shelves has been measured in an ingenious way. Where the advancing glacier separates from the seafloor, it begins to float. Consequently, the ice shelf rises and falls with the tide, even though still attached to the parent glacier. Satellite altimeters can detect where the hinge point occurs between floating and grounded ice, at what is known as the grounding line. The grounding line of the Pine Island Glacier retreated 31 kilometres in the ten years to 2011. Moreover, the bedrock it is resting on shelves downwards towards the interior of the continent. Consequently, the more the glacier retreats, the greater the depth of ice in contact with sea-water and so the faster the melting progresses.

Eric Rignot, an NOAA scientist studying Antarctica, fears that the West Antarctic Ice Sheet has entered an irreversible phase of disintegration that will take between several hundred and a thousand years to complete, adding four metres to sea-level. All we can do by cutting down carbon emissions is slow the speed at which the ice melts.

Rapid changes to glaciers grounded below sea-level are also occurring at the other end of the Earth. In the archipelago of Svalbard, between continental Norway and the North Pole, lies Austfonna, the largest ice cap in the Eurasian Arctic. It contains 2500 cubic kilometres of ice and has experienced a 45-fold increase in ice loss in the past decade. The flow of the glacier has increased from 150 metres to 3800 metres a year, and its surface has been reducing in elevation by 25 metres a year in places.

More than 90 per cent of the extra heat entering the Earth’s atmosphere and ocean due to the increase in greenhouse gases is going into the ocean. While warming can be detected at a depth of two kilometres in the Earth’s oceans, the greatest warming has occurred at the sea surface.

Much of the western North Atlantic is 2ºC warmer than average, while parts of the Gulf Stream are 4ºC warmer. The warm ocean feeds large amounts of extra heat into the sky, causing storms to intensify as well as providing more water vapour to turn into snow.

The effect is exacerbated by warming in the far north. As the Arctic warms under the influence of climate change, the polar jet stream weakens and develops large bends, allowing cold air to spill into mid-latitudes. These ‘polar blasts’ are also more likely to repeat in the same areas. lasting weeks at a time.

This contributed to the recent record snowfalls over the eastern United States, including the 2.3 metres that fell on Boston in the 30 days to February 17. Snowploughs cumulatively worked for 136,652 hours to clear 392,783 kilometres of road.

While repeated streams of cold air chill the eastern United States, records for heat are being set further west—in Yellowstone National Park, Wyoming, grizzly bears are now waking from hibernation a month early.

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