Various factors drive the earth’s climate to change continuously over many different time scales. Processes linked to continental drift have influenced atmospheric circulation, ocean currents, and the composition of the atmosphere over millions of years. A global cooling over the past 60 million years has changed conditions in the Arctic from an ice-free surface year round, to a surface completely covered with ice. Variations in solar radiation over many millennia as a result of alterations in earth’s orbit around the sun have created warm and cool cycles, leading to changes about half as large as those caused by continental drift. On a shorter time scale, the current interglacial period (Holocene) has seen brief periods of cooling caused by volcanic eruptions, weak variations in solar radiation, and probably other factors as well.
Climate in the Arctic
The Arctic has a cold climate. This is mainly because the earth’s axis is tilted relative to the sun and less solar energy reaches the polar regions. In addition, the Arctic is covered with snow and ice much of the year. Snow and ice have high reflectivity, albedo, which helps keep the Arctic cool.
The degree of warming seen in the Arctic since 1980 is twice that in the rest of the world. Temperatures in most parts of the Arctic have increased substantially in the past few decades, particularly in winter. The winter increase in Alaska and western Canada has been around 3–4°C over the past half century. Meteorological stations in the Norwegian High Arctic have also noted increasing temperatures. In Longyearbyen, the yearly average temperature increased by about 0.25°C per decade from when measurements started in 1912 through 2011, a slightly larger increase than for the Arctic as a whole over the same time period. Examination of available knowledge and evidence reveals that the years 2005–2011 have been the warmest ever recorded in the Arctic. In recent years, the warming trend has been strongest in fall and spring.
Research conducted by the Norwegian Polar Institute
Research at the Norwegian Polar Institute has advanced scientific knowledge about temperature in ancient times, for example through studies of microfossils in sea sediments and of ice cores.
By examining the tiny fossilised shells of foraminifers – organisms that are exquisitely sensitive to temperature – the Institute has contributed toward charting the relationship between species distribution and temperature; this information can then be used to interpret and calculate temperature trends back over time.
The Norwegian Polar Institute also studies ice cores from Svalbard and Antarctica to help chart temperature trends in ancient times. In a broader context, the data gathered through these projects contribute toward making climate models better.
The geographic distribution of warming in the Arctic strongly suggests that the decrease in sea ice coverage has enhanced the warming. The largest temperature increases have been recorded in the lower atmosphere over the marginal ice zone in autumn. This can be an indication that the Arctic has reached a threshold where solar radiation absorbed in the summer limits formation of ice during the following autumn and winter. Such positive feedback mechanisms enhance the increase in air temperature over the Arctic Ocean. Larger temperature increases have also been registered over the central Arctic Ocean after the ice cover has thinned and/or retreated earlier in summer. This contrasts with the situation earlier in the warming period, when warming over land was the dominating effect. The increased warming in the spring appears to be related to snow melting away earlier in recent years.
Long-term change in summer Arctic air temperatures, as estimated from lake sediments, ice cores and tree rings (“proxy records”). Figure: SWIPA
Comparison between temperature observations in the Arctic over a fifty-year period (1957–2006) and model predictions for the same period shows that the models are able to describe both the observed temperature increase in the Arctic and the degree of temperature rise. On average, the temperature in the Arctic as a whole has risen between 0.5 and 1.0°C in this period, according to both the models and the actual observations.Temperatures in the distant past (before thermometers) can be calculated by use of proxies? such as lake-bottom sediments, growth rings in trees, and ice cores. From this type of data we know that summer temperatures in the Arctic are higher today than at any time in the past 2000 years. The last time the polar regions were significantly warmer than they are now over an extended period of time was 125 000 years ago.
Temperatures in the Arctic are determined by a range of factors – both natural and man-made – that interact in complex ways and result in the changes we are currently seeing. Changes in sea ice and its influence on heat transfer between ocean and atmosphere, heat transport in the atmosphere, cloud cover and water vapour that influence long-wave radiation, soot on snow and higher concentrations of soot aerosols in the atmosphere: all these are fundamental conditions for how temperatures evolve.
Total precipitation in the Arctic in recent years has been about 5% higher than the average in the 1950s. This trend is not statistically significant, because year-to-year variability is large. However, both the amount of precipitation and the flow rate in rivers evoke the impression that the Arctic has been wet since 2000. The five wettest years since 1950 have all been during the 2000s. Increased precipitation has also been observed in Svalbard, and at Longyearbyen, the annual precipitation has increased by 2% per decade since measurements started there in 1912.
Precipitation is important in evaluating recent climate changes in the Arctic. There are indications of increased cloud formation in the Arctic, particularly low clouds in the summer; this coincides with a longer summer season and reduced sea ice. At present, our knowledge about clouds and cloud formation is still fraught with uncertainty.
Climate at sea
The capacity to absorb and store heat is much greater in the ocean than in land masses. This means that oceans have a cooling effect during periods of global warming. In times when the temperature falls, the opposite will occur: the oceans will release heat to the atmosphere and retard cooling. A large part of the global rise in temperature in recent decades is stored in the ocean.
The Arctic Ocean is warmed by inflow of warm water from the North Atlantic and the northern Pacific. On the Pacific side of the Arctic Ocean, this appears to be one of the causes underlying reduction in sea ice. From the North Atlantic, warm water comes to the Arctic in intermittent surges. One such surge of warm water flowed into the Arctic Ocean along the Siberian continental shelf in the mid-2000s. The next surge is expected to come through Fram Strait. The relationship between warm Atlantic water and melting sea ice on our side of the Arctic Ocean is more complex than on the Pacific side, and much research is being dedicated to this field.
Extreme weather events
In some places on the North American side of the Arctic, extreme temperatures and storms have been recorded. However, no consistent increase in storm activity in the Arctic as a whole has been recorded over the past half century.
Evaluating the observations
Today’s climate models are able to recreate with good accuracy the changes seen in the Arctic – both qualitatively and on a broader scale: global warming driven by increased emission of greenhouse gases. Nevertheless, some major changes come “unannounced” or long before they were expected, as in the case of the shrinking sea ice cover. The gradual effect of human influence, combined with major events in the atmosphere or ocean, or deviations caused by natural variability and modified by arctic feedback mechanisms: this is what leads to irreversible changes in the Arctic.
When observed changes in temperature are subdivided according to region and season, we see an increase of 1°C above and beyond natural climate variations (in each region and each season) throughout the Arctic; in some places and in some seasons, the increase is even larger.
The driving force behind the rise in temperature is the human contribution. No consistent patterns similar to this can be found during earlier warm periods.