Global CO2 emission

How close is the world to devastating climate upheaval? Eduardo Porter’ column argued that the natural gas boom is helping the United States reduce its emissions of heat-trapping carbon into the air, encouraging power generators to switch to gas from the much dirtier coal. American CO2 emissions last year totaled about 5.25 billion tons — almost 13 percent less than in 2007.

But as some readers argued, the decline is so modest that it is almost irrelevant. To stop the world’s temperature from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit) above the pre-industrial era — considered by most climate scientists as the prudent limit — emissions must fall much faster.

In fact, keeping to the target probably requires that a good share of the world’s fossil fuel remains untapped.

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• Carbon dioxide (CO2) - Fossil fuel use is the primary source of CO2. The way in which people use land is also an important source of CO2, especially when it involves deforestation. CO2 can also be emitted from direct human-induced impacts on forestry and other land use, such as through deforestation, land clearing for agriculture, and degradation of soils. Likewise, land can also remove CO2 from the atmosphere through reforestation, improvement of soils, and other activities.
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