Warning Signs

Among the thousands of warning signs of climate change, the array of extreme events that took place in 2010 stand out.

Current climate change is affecting all continents and most oceans. Thousands of case studies of physical changes (such as reduced snow cover and ice melt) and changes in biological systems (such as earlier flowering dates and altered species distributions) have correlated with observed climate changes over the past three decades and more. Scientists have high confidence that these environmental changes are part of the early warning signs of climate change.

Effects on social and economic activities are harder to attribute to climate impacts, although major events attract considerable attention. From prolonged drought in Africa and Australia to the dire flooding in Australia, China, and Pakistan, livelihoods, economies, and politics are at risk.

A single extreme weather event or change in the natural environment does not prove that humans are changing the climate. However, the proven physical science, the history of recent observations, and the consistency in model assessments all support only one explanation: the emission of greenhouse gases by human activity is causing profound changes to the climate system and to the world we live in.

The pace of change appears to be accelerating. Reports of sea levels rising faster than previously expected, of new temperature records, of an increasing toll of weather-related disasters, and anecdotal stories of impacts on livelihoods are accumulating. The year 2010 tied as the warmest year since records began in the 1850s, and threw up an astonishing series of extreme events.

Increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising sea levels led the Intergovernmental Panel on Climate Change (IPCC) to report in 2007 that "warming of the climate system is unequivocal". As evidence continues to mount, that statement is even truer today.

Polar Changes

Warming in the Antarctic Peninsula and Arctic is driving large-scale melting of ice that will have both local and global consequences.

The presence of a hole in the ozone layer over the southern polar region has altered weather circulation patterns on Antarctica. It has brought more warm, moist, maritime air over the Antarctic Peninsula, contributing to warming and melting there, but has created a cooling effect in other areas. As the ozone hole recovers, that cooling effect is expected to diminish.

In East Antarctica, the changes are much less dramatic than those on the Peninsula, with some melting and thinning on coastal edges and some thickening in the interior. In West Antarctica, however, a coastal section of the ice sheet is now thinning quite rapidly.

Floating Arctic ice has covered the North Pole for millions of years. Its extent fluctuates with the seasons, but eight of the ten lowest extents have occurred in the last decade. The remaining ice is also thinner, with approximately 50 percent of the maximum recorded thickness having been lost by 2008. Already, the North Pole is free of ice in some summers.

In September 2007, the Arctic ice cap shrank to its smallest recorded extent, opening up the possibility of commercial shipping routes operating for the first time along the northern coasts of Canada and Russia. Some projections suggest that sea ice will disappear completely in the summer months by 2080.

While an open Arctic sea would facilitate shorter trade routes, industrial-scale fishing and the exploitation of minerals, it would be at great cost to the environment and to traditional livelihoods. A delay in the formation of the winter ice, an earlier break-up of ice in the spring, and thinner ice year round makes it hard for indigenous people using largely traditional methods to make a living.

The permafrost around the Arctic is generally warming. In some areas, it is making a weakened coastline more prone to erosion, and causing subsidence, leading to the collapse of roads and buildings. It is also creating lakes of trapped melt water, which may increase carbon dioxide and methane emissions.

Each summer, parts of the Greenland ice sheet melt at the edges and on the surface. Although the melt area varies each year, the overall trend since 1979 has been upwards. Surface melt water finds its way through crevasses to the base of the ice, and forms a thin film between ice and bedrock. There are fears that this could increase the speed at which the ice sheet slides towards the sea.

Shrinking Glaciers

Around the world, glaciers are losing mass and are in retreat.

The changes in glaciers over time provide valuable evidence of long-term climate change. The mass and extent of glaciers respond to temperature and snowfall in the very local geography of mountains and polar regions. g Because the tell-tale signs of their expansion and retraction are clearly visible, scientists are able to draw conclusions about climate change from periods well before instrumental records became widespread.

Globally, glaciers have lost an average of more than half a meter (water equivalent) during the past decade or so. This is twice the rate of loss in the previous decade, and over four times the rate of loss in the late 1970s.

The front of most glaciers is receding to higher altitudes, and at such a rate that glaciologists, mountaineers, tourists and local residents are astonished by the changes that have occurred in...