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What's the greenhouse effect?
Earth's greenhouse effect is a natural phenomenon that helps regulate the temperature of our planet. Simply put, the sun heats the Earth and some of this heat, rather than escaping back to space, is trapped in the atmosphere by clouds and greenhouse gases, such as water vapor and carbon dioxide. If all of these greenhouse gases were to suddenly disappear, our planet would be 60°F colder and uninhabitable.
The global temperature record shows an average warming of about 1.1°F over the past century . This warming has been recorded in both the northern and southern hemispheres, and over the oceans, with some areas substantially warmer and others actually cooler.
The ten warmest years have occurred since 1983, with seven of them since 1990. Recent evidence shows the 20th century was the warmest in the last 1,000 years. The 1990s were the warmest decade and 1998 was the single warmest year of the past millennium.
Are human activities responsible for the warming?
Separating out the impact of human activity from natural climate variation is extremely difficult. Nonetheless, the IPCC concluded there is a "discernible human influence" on climate. This means the observed global warming is unlikely to be the result of natural variability alone and that human activities are at least partially responsible.
What are the most important greenhouse gases?
Where are they coming from?
Many greenhouse gases occur naturally, but human activities are adding gases to the natural mix at an unprecedented rate. Water vapor is the most abundant greenhouse gas; it occurs naturally and makes up about two thirds of the natural greenhouse effect. Fuel burning and other human activities, however, are adding large amounts of greenhouse gases to the atmosphere – the most important ones being carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons
(HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). For an overview of greenhouse gases, see the following brochure:
In Brief--The U.S. Greenhouse Gas Inventory.
Because human emissions of CO2 and other greenhouse gases continue to climb, and because they remain in the atmosphere for decades to centuries (depending on which gas), we're committing ourselves to a warmer climate in the future. The IPCC projects an average global temperature increase of 1-4.5°F (0.6-2.5°C) in the next fifty years, and 2.5-10.4°F (1.4-5.8°C) in the next century. Temperatures in some parts of the globe (e.g., the polar regions) are expected to rise even faster. Even the low end of the IPCC's projected range represents a rate of climate change unprecedented in the past 10,000 years.
Our health, agriculture, water resources, forests, wildlife and coastal areas are vulnerable to global warming and the climatic changes it will bring. The IPCC concluded that "climate change is likely to have wide-ranging and mostly adverse impacts on human health, with significant loss of life."
Source: US EPA, Office of Air and Radiation
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