The Earth's Changing Climate
Warming more and faster
— The rate of warming of the Earth’s average surface temperature over the last 50 years was nearly twice the rate for the last 100 years. Over the past 100 years, the Earth’s average surface temperature rose by around 0.74°C. If atmospheric concentrations of the dominant greenhouse gas, carbon dioxide, were to double compared to pre-industrial level, this would lead to an average warming of around 3°C. The late 1990s and early 21st century had the warmest years since modern records began. Arctic sea ice has been shrinking on average by 2.7 per cent per decade.
Evidence of warming — The changes that scientists can measure in the atmosphere, oceans, ice caps and glaciers conclusively indicate that the world is already warming in response to past greenhouse gas emissions. These changes are part of a consistent pattern and evidence for a warmer world of greater heat waves, new wind patterns, worsening drought in some regions, heavier precipitation in others, melting glaciers and Arctic ice and rising sea levels.
Warmest years on record — The IPCC found that, over the past 100 years (1906-2005), the Earth’s average surface temperature has risen by around 0.74°C, with the warming greater over land than over the oceans. The rate of warming averaged over the last 50 years is nearly twice the rate for the last 100 years. The late 1990s and early 21st century have featured the warmest years since modern records began. A further warming of about 0.2°C per decade is projected for each of the next two decades decades for a range of scenarios that do not include deliberate reductions in greenhouse gas emissions. The amount of warming that would occur afterwards will depend on the amount of greenhouse gases emitted to the atmosphere.
Atmospheric concentrations of greenhouse gasses
— If atmospheric concentrations of the dominant greenhouse gas, carbon dioxide, were to double compared to its pre-industrial level, this would "likely" lead to an average warming of 2-4.5°C (3.6-8.1°F). Other greenhouse gases also contribute to such warming and according to a number of scenarios, their combined effect would be equivalent to such a doubling in the second half of this century. Atmospheric concentrations of carbon dioxide, according to measurements on ancient air trapped in ice cores, are greater now than at any time over the last 650,000 years.
Less glaciers, snow and ice — One of the most dramatic consequences of global warming is sea-level rise. Sea levels rose by around 17 centimetres during the course of the 20th century. Geological observations indicate that they rose far less over the previous 2,000 years. In temperate regions, many mountain glaciers have shrunk, and snow cover has generally declined, especially in the spring. The maximum extent of frozen ground in winter decreased by about 7 per cent in the Northern Hemisphere over the 20th century. The average freezing date for rivers and lakes is quite variable but on average over the past 150 years has become later by some 5.8 days per century, while the average break-up date has become earlier by 6.5 days per century.
