Observed Changes in Arctic Climate
Records of increasing temperatures, melting glaciers, reductions in extent and thickness of sea ice, thawing permafrost, and rising sea level all provide strong evidence of recent warming in the Arctic. There are regional variations due to atmospheric winds and ocean currents, with some areas showing more warming than others and a few areas even showing a slight cooling; but for the Arctic as a whole, there is a clear warming trend. There are also patterns within this overall trend; for example, in most places, temperatures in winter are rising more rapidly than in summer. In Alaska and western Canada, winter temperatures have increased as much as 3-4°C in the past 50 years. Observations suggest that precipitation has increased by roughly 8% across the Arctic over the past 100 years, although uncertainties in measuring precipitation in the cold arctic environment and the sparseness of data in parts of the region limit confidence in these results. There are regional variations in precipitation across the Arctic, and there will be regional variations in the changes in precipitation as well. In addition to the overall increase, changes in the characteristics of precipitation have also been observed. Much of the precipitation increase appears to be coming as rain, mostly in winter, and to a lesser extent in autumn and spring. The increasing winter rains, which fall on top of existing snow, cause faster snowmelt and, when the rainfall is intense, can result in flash flooding in some areas. Rain-on-snow events have increased significantly across much of the Arctic, for example, by 50% over the past 50 years in western Russia. In order to assess whether recent changes in arctic climate are unusual, that is, outside the range of natural variability, it is helpful to compare recent observations to records of how climate has behaved in the past. Data on past climate come from ice cores and other sources that provide reasonable representations of what climate was like in the distant past. Examining the record of past climatic conditions indicates that the amount, speed, and pattern of warming experienced in recent decades are indeed unusual and are characteristic of the human-caused increase in greenhouse gases. Both natural and human-caused factors can influence the climate. Among the natural factors that can have significant effects lasting years to decades are variations in solar output, major volcanic eruptions, and natural, sometimes cyclic, interactions between the atmosphere and oceans. Several important natural modes of variability that especially affect the Arctic have been identified, including the Arctic Oscillation, the Pacific Decadal Oscillation, and the North Atlantic Oscillation. Each of these can affect the regional patterns of such features as the intensity and tracks of storm systems, the direction of the prevailing winds, the amount of snow, and the extent of sea ice. In addition to changing long-term average climatic conditions, human-induced changes in the climate may also affect the intensity, patterns, and features of these natural variations.
Changes in Sea Ice: A Key Climate Change Indicator
“Climate" refers to much more than just temperature and precipitation. In addition to long-term average weather conditions, climate also includes extreme events, as well as aspects of the system such as snow, ice, and circulation patterns in the atmosphere and oceans. In the Arctic, sea ice is one of the most important climatic variables. It is a key indicator and agent of climate change, affecting surface reflectivity, cloudiness, humidity, exchanges of heat and moisture at the ocean surface, and ocean currents. And as illustrated later in this report, changes in sea ice have enormous environmental, economic, and societal implications. Just as miners once had canaries to warn of rising concentrations of noxious gases, researchers working on climate change rely on arctic sea ice as an early warning system. The sea ice presently covering the Arctic Ocean and neighboring seas is highly sensitive to temperature changes in the air above and the ocean below. Over recent decades, Arctic watchers detected a slow shrinkage of the ice pack, suggestive of the initial influences of global warming. In recent years, the rate of retreat has accelerated, "Ice is a supporter of indicating that the “canary" is in trouble.
Over the past 30 years, the annual average sea-ice extent has decreased by about 8%, or nearly one million square kilometers, an area larger than all of Norway, Sweden, and Denmark combined, and the melting trend is accelerating. Sea-ice extent in summer has declined more dramatically than the annual average, with a loss of 15-20% of the late-summer ice coverage. There is also significant variability from year to year. September 2002 had the smallest extent of arctic sea-ice cover on record, and September 2003 was very nearly as low. Sea ice has also become thinner in recent decades, with arctic-wide average thickness reductions estimated at 10-15%, and with particular areas showing reductions of up to 40% between the 1960s and late 1990s. Impacts of a decline in sea ice are discussed throughout this report and include increased air temperature, decreased salinity of the ocean’s surface layer, and increased coastal erosion.