C. Mitchell and B. Geerts (1)
The greenhouse effect
About 100 years ago (1896) a Swedish scientist made the first calculation of global warming that could be expected due to a doubling of carbon dioxide in the atmosphere.
As our understanding of climate expanded through the decades of the 20th century, it became clear that a number of other gases in a planet's atmosphere act to trap net radiation. These gases, known as greenhouse gases, are more transparent to solar radiation than to the infrared radiation emitted by the Earth's surface. Since a planet's temperature is determined by the balance between incoming solar and out-going planetary radiation, the overall effect of gases that trap net radiation near the surface of a planet will be to elevate the surface temperature to a level above that which would otherwise be the case (Section 2.8).
By careful observation on planet Earth, it also became clear that the comfortable temperature of the planet - averaging about 15�C - is largely due to the effect of these greenhouse gases. In the case of the Earth the major naturally-occurring greenhouse gases are: water vapour, carbon dioxide, methane, nitrous oxide, and ozone where it occurs near the Earth's surface.
In1957, the International Geophysical Year, high on a volcano in Hawaii, Dr Dave Keeling started continuous measurement of carbon dioxide at the Mauna Loa Observatory. The idea behind the observatory was to situate it far away from sources of city pollution, so that scientists could determine the composition of 'unpolluted' or baseline air. Data from Mauna Loa and elsewhere have shown, without doubt, that major greenhouse gases (carbon dioxide, nitrous oxide and methane) have been, and are, continuing to increase in the global atmosphere.
In parallel with these developments, scientists realised that it is possible to recover and analyse air from bubbles trapped in polar ice. Ice core measurements have enabled us to place recent changes in the atmosphere in an historic context. Very precise records are available for the last thousand years, with other records reaching back 200,000 years. The most surprising discovery was that during the last 200,000 years there is an excellent correlation between the atmospheric carbon dioxide concentration and the average temperature. During cold periods there was less CO2. The data do not, of course, explain this correlation.
During this century a number of artificial compounds: chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs), which are also powerful greenhouse gases, have been introduced into the atmosphere.
Furthermore, scientific analysis clearly shows that the combustion of fossil fuels and large-scale deforestation have driven the increase in carbon dioxide. Increases in methane and nitrous oxide are also linked to human activity.
5 . Find and Replace 'so' with 'green'. Also 'air' with 'yellow Change the color of the word to match the color - green & yellow