Climate change is a reality. Today, our world is hotter than it has been in two thousand years. By the end of the century, if current trends continue, the global temperature will likely climb higher than at any time in the past two million years.
Temperature change 1765-2100: Graph excerpted from the IPCC's Third Assessment Report showing past and predicted changes to global temperature.
While the end of the 20th century may not necessarily be the warmest time in Earth's history, what is unique is that the warmth is global and cannot be explained by the natural mechanisms that explain previous warm periods. There is a broad scientific consensus that humanity is in large part responsible for this change, and that choices we make today will decide the climate of the future.
How we are changing the climate
For more than a century, people have relied on fossil fuels such as oil, coal and gas for their energy needs. Burning these fossil fuels releases the greenhouse gas (GHG) carbon dioxide into the atmosphere. Other, even more potent greenhouse gases are also playing a role, as well as massive deforestation.
The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action.
Joint statement by 11 national science academies to world leaders (full text)
What we know
While there are still uncertainties, particularly related to the timing, extent and regional variations of climate change, there is mainstream scientific agreement on the key facts:
Certain gasses in the atmosphere, such as carbon dioxide, create a "greenhouse effect", trapping heat and keeping the Earth warm enough to sustain life as we know it.
Burning fossil fuels (coal, oil, etc.) releases more carbon dioxide into the atmosphere. Although not the most potent greenhouse gas, carbon dioxide is the most significant in terms of human effects because of the large quantities emitted.
Carbon dioxide concentrations in the atmosphere are now the highest in 150,000 years.
The 1990's was most likely the warmest decade in history, and 1998 the warmest year.
There is also widespread agreement that:
A certain amount of additional warming - about 1.3 degrees Celsius (2.3 degrees Fahrenheit) compared to pre-industrial levels - is probably inevitable because of emissions so far. Limiting warming to under 2 degrees Celsius (3.6 degrees Fahrenheit) is considered vital to preventing the worst effects of climate change.
If our greenhouse gas emissions are not brought under control, the speed of climate change over the next hundred years will be faster than anything known since before the dawn of civilization.
There is a very real possibility that climate feedback mechanisms will result in a sudden and irreversible climate shift. No one knows how much global warming it would take to trigger such a "doomsday scenario."
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