Global warming is the increase in the average temperature of the Earth's near-surface air and the oceans since the mid-twentieth century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes that anthropogenic (human-sourced) greenhouse gases are responsible for most of the observed temperature increase since the middle of the twentieth century, and natural phenomena such as solar variation and volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward. These basic conclusions have been endorsed by more than 40 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries.
Climate model projections summarized in the latest IPCC report indicate that global surface temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century. The uncertainty in this estimate arises from the use of models with differing climate sensitivity, and the use of differing estimates of future greenhouse gas emissions. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Although most studies focus on the period up to 2100, warming is expected to continue beyond 2100, even if emissions have stopped, because of the large heat capacity of the oceans and the lifespan of carbon dioxide in the atmosphere.
• Pictured left: 1999-2008 Meantemperatures: This figure shows the difference in instrumentally determined surface temperatures between the period January 1999 through December 2008 and "normal" temperatures at the same locations, defined to be the average over the interval January 1940 to December 1980. The average increase on this graph is 0.48 °C, and the widespread temperature increases are considered to be an aspect of global warming. Source: NASA
The effective rate of change in glacier thickness, also known as the glaciological mass balance, is a measure of the average change in a glacier's thickness after correcting for changes in density associated with the compaction of snow and conversion to ice. The map shows the average annual rate of thinning since 1970 for the 173 glaciers that have been measured at least 5 times between 1970 and 2004. Larger changes are plotted as larger circles and towards the back.
All survey regions except Scandinavia show a net thinning. This widespread glacier retreat is generally regarded as a sign of global warming.
During this period, 83% of surveyed glaciers showed thinning with an average loss across all glaciers of 0.31 m/yr. The most rapidly growing glacier in the sample is Engabreen glacier in Norway with a thickening of 0.64 m/yr. The most rapidly shrinking was Ivory glacier in New Zealand which was thinning at 2.4 m/yr. Ivory glacier had totally disintegrated by circa 1988. 
Carbon taxes offer a potentially cost-effective means of reducing greenhouse gas emissions. From an economic perspective, carbon taxes are a type of Pigovian tax. They help to address the problem of emitters of greenhouse gases not facing the full (social) costs of their actions. Carbon taxes are a regressive tax, in that they disproportionately affect low-income groups. The regressive nature of carbon taxes can be addressed by using tax revenues to favour low-income groups.
A number of countries have implemented carbon taxes or energy taxes that are related to carbon content. Most environmentally related taxes with implications for greenhouse gas emissions in OECD countries are levied on energy products and motor vehicles, rather than on CO
2 emissions directly.
In his senior year at Harvard University, he took a class with oceanographer and global warming theorist Roger Revelle, who sparked Gore's interest in global warming and other environmental issues. After joining the U.S. House of Representatives, Gore held the first congressional hearings on the climate change, and co-sponsor[ed] hearings on toxic waste and global warming. Gore was known as one of the Atari Democrats, later called the "Democrats' Greens: politicians who see issues like clean air, clean water and global warming as the key to future victories for their party.
The basic function of a space sunshade to mitigate global warming. A 1000 kilometre diameter lens is sufficient, and much smaller than what is shown in this simplified image. As a Fresnel lens it would be only a few millimeters thick.