The View from the Top -Searching for responses to a rapidly-changing Arctic
In the fragile Arctic region the extent of sea ice was at a record low in September 2012. Land ice is also retreating, while snow is disappearing and permafrost is thawing. Rapid environmental change in the Arctic, as a result of climate change, is providing new development opportunities including easier access to oil and gas, minerals and fisheries. It is also threatening ecosystems – with ice-associated animals especially at risk. Changes in the Arctic will have consequences far beyond this region, including a global rise in sea levels and probably more extreme weather across much of the northern hemisphere. These current and future consequences of climate change require urgent responses. Arctic and non-Arctic countries share responsibility for protecting this region, in particular by limiting their greenhouse gas emissions.
Accelerated summer meltdown
Arctic sea ice extent is rapidly diminishing. The minimum sea ice cover in 2012, at 3.4 million km2, was 18 per cent below the previous recorded minimum in 2007 and 50 per cent below the
average in the 1980s and 1990s (Figure 1). Every year from 2007 the minimum has been lower than in any year before 2007 (NASA 2012a, NSIDC 2012). Floating ice has covered much of the Arctic Ocean for most of the past three million years (Polyak et al. 2010). But how much longer will this be the case? The retreat of sea ice has been much more rapid than projected in the Intergovernmental Panel on Climate Change’s latest report (IPCC 2007, Polyak et al. 2010, Stroeve et al. 2012) More recent modelling studies have come closer, but none has yet reproduced the observed trend (Stroeve et al. 2012). Nor have these studies been able to project precisely when ice-free conditions will first be observed during the Arctic summer. The IPCC report warned that this could happen around 2100 (IPCC 2007). One extrapolation of recent trends suggested that
September could be ice-free before the end of this decade (Wadhams 2012). However, the most common prediction is that this will take place around 2035 (Wang and Overland 2012).
Sea ice is frozen seawater that floats on the ocean surface. It forms in the winter and partly retreats in the summer. The extent and thickness of sea ice covering the Arctic Ocean is significantly decreasing.
Each winter new sea ice forms and covers the Arctic Ocean. The amount of old ice that survives from year to year is diminishing, as shown by the ice age distribution in March 1988 and March 2010. Because of a continuous counterclockwise rotation the older multi-year sea ice is extruded
into the North Atlantic and replaced during the winter by new sea ice, which is much younger. Historically, these processes were slower and less ice was extruded, hence remaining in the Arctic for longer periods of time. Every winter the Arctic sea ice reforms. While this will probably
continue to happen, the amount of thick, old ice surviving from one year to the next is diminishing. The multi-layer ice is at its maximum extent in March. However, it made up only 45 per cent of the total in 2010 compared to 75 per cent in 1988. The Arctic’s thinned winter ice is thus primed for destruction in summer.
Loss of sea ice has been accompanied by melting of the Greenland ice cap, thawing of permafrost on the tundra (the region where tree growth is hindered by low temperatures and short growing
seasons), less snow on land due to earlier snow melt, and melting of some snow cover on glaciers The average snow cover remaining in the northern hemisphere in June – virtually all of which is within the Arctic – has declined by more than 50 per cent in the past three decades, to less than 4 million km2. The five lowest values were all recorded since 2007 (Derksen and Brown 2012).
A changing energy balance The world is warming, and with it the Arctic (Figure 4). However,
the Arctic has been warming at least twice as fast as the global average (ACIA 2005, Arndt et al. 2012). One reason is that more heat is brought into the Arctic through the atmosphere and with
ocean currents. Several local factors are also increasing warming by changing the region’s energy balance.
The greatest local amplification is due to the melting itself, which reduces the reflection of incoming sunlight. White ice and snow act as a mirror, reflecting about 85 per cent of solar radiation back
to the sky. Dark ice-free areas of the ocean reflect only about 10 per cent and absorb the rest, while bare tundra reflects about 20 per cent (Climate Data Information 2012). As ice and snow melt, the exposed ocean and land absorb about 80 per cent of incoming radiation from the sun, increasing local surface warming. Heat in the ice-free ocean also directly warms the air above.
The role of black carbon (soot) Carbon dioxide (CO2) is the main anthropogenic greenhouse gas
responsible for warming the atmosphere. However, short-lived climate pollutants in the Arctic, such as organic carbon, methane and ozone, also increase warming. Soot, which scientists often refer to as black carbon, is produced by burning many things from diesel to dung. In the air, black carbon absorbs solar radiation and radiates it out again as heat, warming the surrounding air. But the tiny particles only stay aloft for a few days before falling to the ground.
In most places that is effectively the end of black carbon's climatic impact. In the Arctic, however, where particles may fall onto white snow and ice, its warming effect continues. As the particles accumulate, they darken the snow and ice, increasing heat absorption, warming the air and accelerating snow melt.
Typical levels of black carbon in Arctic ice and snow are around 5-10 parts per billion. This “dirtying of the mirror” increases the amount of heat absorbed by an estimated 1-4 per cent, raising
local temperatures and melting snow and ice. As a result, the black carbon’s warming effect
remains greater in the Arctic than in most of the rest of the world. The fallout of black carbon in the Arctic is at its maximum in late winter. Modelling studies suggest that this fallout may be increasing spring snow melt rates by between 20 and 30 per cent .
Although some black carbon is generated within the Arctic and adjacent areas – from diesel-powered electric generators and ship engines, flaring during oil and gas exploration, agricultural burning, forest fires, and use of wood stoves – much originates outside the region Better controls on air pollution in northern countries outside the Arctic have recently reduced black carbon fallout. As the Arctic is opened to increased shipping and industrial activity, that trend could be reversed. One modelling study has suggested a possible five-fold increase in black carbon emissions from Arctic shipping by 2030 (Corbett et al. 2010). Whether this will happen greatly depends on future emission controls.
Global controls on black carbon could slow global warming by about a decade, according to some estimates, while saving the lives of up to 2 million people in the world killed annually as a result of inhaling indoor smoke from cooking stoves (Streets 2006, Kandlikar et al. 2010, UNEP/WMO 2011). A number of concrete measures have been identified that could produce this slowdown of the warming in the Arctic (UNEP/WMO 2011) (Chapter 1, Box 2). Reducing black carbon
emissions is therefore important, but clearly is not a substitute for reducing emissions of CO2 and other greenhouse gases. Black carbon (or soot), a short-lived climate pollutant, darkens snow and ice and may also contribute to warming in the Arctic (AMAP 2011b) (Box 1). Dust and volcanic ash contribute to cooling while in the atmosphere, but have the same effect as black carbon
when they fall on snow or ice. Furthermore, shrubs and trees moving into the tundra increases absorption of sunlight by making the land surface darker (Chapin et al. 2005).
Other accelerators of atmospheric warming involve water vapour (Callaghan et al. 2011a). With more open sea in the Arctic, more water will evaporate, increasing the amount of water vapour in the air. Water vapour is a powerful, locally acting greenhouse gas. It traps heat, further escalating warming. On the other hand, more water vapour may increase cloud cover, which generally has a
cooling effect during daylight hours. The overall balance of these different feed backs has not been established, but strong warming suggests that positive feed backs dominate (AMAP 2011a).
22 UNEP Year Book 2013
SOURCE:UNEP
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