Due largely to the combustion of fossil fuels, atmospheric concentrations of carbon dioxide, the principal greenhouse gas, are at a level unequaled for more than 400,000 years. As a result, an enhanced greenhouse effect is trapping more of the sun’s heat near the earth’s surface and gradually pushing the planet’s climate system into uncharted territory .
Carbon dioxide (CO2) and other greenhouse gases always have been present in the atmosphere, keeping the earth hospitable to life by trapping heat. Yet, since the industrial revolution, emissions of these gases from human activity have accumulated steadily, trapping more heat and exacerbating the natural greenhouse effect.
As a result, global average temperatures have risen both on land and in the oceans, with observable impacts already occurring that foretell increasingly severe changes in the future. Polar ice is melting. Glaciers around the globe are in retreat. Storms are increasing in intensity. Ecosystems around the world already are reacting, as plant and animal species struggle to adapt to a shifting climate, and new climate-related threats emerge.
Scientists predict that if the increase in greenhouse gas emissions continues unabated, temperatures will rise by as much as 10 degrees Fahrenheit by the end of this century, potentially causing dramatic—and irreversible—changes to the climate.
The consequences, both anticipated and unforeseen, will have profound ramifications for humanity and the world as a whole. Water supplies in some critical areas will dwindle as snow and ice disappear. Sea levels will rise, threatening coastal populations. Droughts and floods will become more common. And hurricanes and other powerful storms will increase in intensity. Adding to the threat will be the impacts of climate change on agricultural production and the spread of disease. Human health will be jeopardized by all of these changes.
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.
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