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Is global warming already happening?
Yes. The IPCC concluded in its Third Assessment Report, "An increasing body of observations gives a collective picture of a warming world and other changes in the climate system." The kinds of changes already observed that create this consistent picture include the following:
Examples of observed climatic changes
Increase in global average surface temperature of about 1°F in the 20th century
Decrease of snow cover and sea ice extent and the retreat of mountain glaciers in the latter half of the 20th century
Rise in global average sea level and the increase in ocean water temperatures
Likely increase in average precipitation over the middle and high latitudes of the Northern Hemisphere, and over tropical land areas
Increase in the frequency of extreme precipitation events in some regions of the world
Examples of observed physical and ecological changes
Thawing of permafrost
Lengthening of the growing season in middle and high latitudes
Poleward and upward shift of plant and animal ranges
Decline of some plant and animal species
Earlier flowering of trees
Earlier emergence of insects
Earlier egg-laying in birds
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koita. den nomizw oti ta posts moy aksizoyn tetoias fthhnhs eirwneias. to ksekatharhsa oti den einai h anodos ths thermokrasias poy arneitai kaneis (ayth einai eykolo na thn metrhsei kaneis - kai ta liwsimata twn "pagwn" opws les ktl.), alla oi aities pisw apo ayta ta fainomena. o planhths den exei isorrophsei se mia statherh thermokrasia kai to klima toy pernaei fysikotates ayksomoiwseis se diafores xronikes klimakes.
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What is causing global warming?
Scientists have concluded that human activities are contributing to global warming by adding large amounts of heat-trapping gases to the atmosphere. Our fossil fuel use is the main source of these gases. Every time we drive a car, use electricity from coal-fired power plants, or heat our homes with oil or natural gas, we release carbon dioxide and other heat-trapping gases into the air. The second most important source of greenhouse gases is deforestation, mainly in the tropics, and other land-use changes.
Since pre-industrial times, the atmospheric concentration of carbon dioxide has increased by 31 percent. Over the same period, atmospheric methane has risen by 151 percent, mostly from agricultural activities like growing rice and raising cattle.
As the concentration of these gases grows, more heat is trapped by the atmosphere and less escapes back into space. This increase in trapped heat changes the climate, causing altered weather patterns that can bring unusually intense precipitation or dry spells and more severe storms.
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