CO2

Atmospheric CO2 at Mauna Loa Observatory. Credit: NOAA.

From The Guardian:

The amount of heat-trapping carbon dioxide in the atmosphere reached a record 390.9 parts per million (ppm) in 2011, according to a report released Tuesday by the UN's World Meteorological Organization (WMO). That's a 40 percent increase over levels in 1750, before humans began burning fossil fuels in earnest.
Although CO2 is still the most significant long-lived greenhouse gas, levels of other heat-trapping gases have also climbed to record levels, according to the report. Methane, for example hit 1813 parts per billion (ppb) in 2011, and nitrous oxide rose to 324.2 ppb. All told, the amount of excess heat prevented from escaping into outer space was 30 percent higher in 2011 than it was as recently as 1990.

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The CO2 that remains in the atmosphere, meanwhile, takes centuries to dissipate, which is why the numbers continue to climb. As a result of all the extra CO2 pumped into the air, worldwide average temperatures have already risen by 1.8°F since 1900.
Yet despite all of this knowledge, the world has largely failed to act on reducing emissions. The best they could do at a UN-sponsored climate meeting in Copenhagen in 2009 was to agree to a non-binding target of limiting the world's greenhouse-gas-triggered temperature increase to no more than 2°C (3.6°F) above preindustrial levels to limit the potential damage. Just a year later, it was already clear that they wouldn't come close to making it.
Frustrated with this global inaction, the World Bank released a report on Sunday saying that without significant emissions reductions, the world's average temperature could climb by 4°C (7.2°F) by as early as 2060. The report highlighted the dire consequences for human health and safety — including dangerous sea level rise, heat waves, and other extreme weather events.