Although greenhouse gas emissions are primarily associated with the burning of fossil fuels (chiefly, coal, oil and natural gas), they come from many sources. As a result, any effort to reduce the human impact on the climate will need to engage all sectors of society.

The largest contributors to total U.S. emissions are the electricity generation and transportation sectors; significant emissions also come from other commercial and agricultural activity and from residential and industrial buildings.

Most emissions come from a relatively small number of countries. The seven largest emitters—the United States, the European Union (EU), China, Russia, Japan, India and Canada—accounted for more than 70% of energy-related CO2 emissions in 2004. An effective strategy to avert dangerous climate change requires commitments and action by all the world’s major economies.

The United States, with 5% of the world’s population, is responsible for 25% of global GHG emissions, more than any other country. On an intensity basis (emissions per gross domestic product or GDP), U.S. emissions are roughly 50% higher than the European Union’s or Japan’s. On a per capita basis, U.S. emissions are roughly twice as high as those of the EU and Japan (and five times the world average). U.S. emissions are projected to rise 8% above 2004 levels by 2010 (and 28% by 2025). By comparison, emissions are projected to hold steady in the EU, and decline 5% in Japan, by 2010.

Emissions are rising fastest in developing countries. China’s emissions are projected to nearly double, and India’s to increase an estimated 80%, by 2025. Annual emissions from all developing countries are projected to surpass those of developed countries between 2013 and 2018. Their per capita emissions, however, will remain much lower than those of developed countries. In 2025, per capita emissions in China are expected to be one-fourth—and in India, one-fourteenth — those of the United States.

The main sources of CO2 are:
Solid fuels (e.g., coal): 35%
Liquid fuels (e.g., gasoline, fuel oil): 36%
Gaseous fuels (e.g., natural gas): 20%
Currently there is an increase in greenhouse effect because of an increase in the
concentrations of greenhouse gases due to human activities. This causes “global
warming” [a rise in mean earth temperatures] plus other damaging climate changes; for example, changes in precipitation, storm patterns, and rise in the level of the oceans.
Basically, greenhouse effect can be summarized thus:
1. Solar radiation passes through the clear atmosphere.
2. Most radiation is absorbed by the Earth’s surface and warms it.
3. Some solar radiation is reflected by the Earth and the atmosphere.
4. Infrared radiation is emitted from the Earth’s surface.
5. Some of the infrared radiation is absorbed and re-emitted back to earth by the greenhouse gases.