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Greenhouse gases other than carbon dioxide are shorter-lived: methane is removed from the atmosphere in around a decade, and chlorofluorocarbons break down within a century. Individual water molecules spend only a few days at a time in the atmosphere, but unlike the other greenhouse gases, the total amount of water vapor in the atmosphere remains constant. Water evaporated from the oceans replaces water lost by condensation and precipitation.
Changing the composition of the Earth's atmosphere also changes the climate. Do you remember the Snowball Earth — how increasing ice cover also increased the Earth's albedo, eventually covering the entire planet in ice and snow? Today's climate is temperate—so we must have escaped this frozen trap. But how? The leading hypothesis is that the composition of the Earth's atmosphere changed, with volcanoes slowly adding more and more carbon dioxide to it. Without access to the oceans, plants, or surface rocks, this carbon dioxide was not removed from the atmosphere and so continued to build up over millions of years. Eventually, the additional warming caused by the increase in greenhouse gases overcame the cooling caused by the snow's high albedo, and temperatures rose enough to melt the ice, freeing the Earth.
For most of Earth's history, carbon dioxide concentrations have been higher than they are today. As a consequence, past climates have often been very warm. During the late stage of the dinosaur era (the Cretaceous , a period that lasted between 65 and 145 million years ago), carbon dioxide levels were about 5 times higher than they are today, and the average global temperatures were more than 10 o C higher than today's. There were no large ice sheets, and dinosaur fossils from this period have been found as far north as Alaska. These animals would not survive the cold conditions found in the arctic today. Further south, fossil crocodiles from 60 million years ago have been found in North Dakota. The modern average winter temperature in North Dakota is around -10 o C –but being cold-blooded, crocodiles are most at home when the air temperature is around 30 o C! The climate was warmer in the past when the amount of carbon dioxide was higher.
The text describes how the high albedo of snow acts as a positive feedback—if the Earth is made cooler, the highly reflective snow can act to further cool the Earth. Today, part of the Earth is covered with snow and ice. Can you describe a mechanism by which warmer temperatures would also produce a positive feedback—this time heating the Earth further—through a similar albedo mechanism?
Mars is colder than the Earth. Venus, on the other hand, is much hotter, with average surface temperatures of around 450 o C. Venus is closer to the Sun than the Earth is, and so receives about twice as much solar radiation. Venus's atmosphere is also different than Earth's, as it is much thicker and mainly consists of carbon dioxide. Using the terms insolation and greenhouse gases , can you suggest reasons why Venus is so hot?
Oxygen makes up over 20% of Earth's atmosphere, while carbon dioxide makes up less than 0.04%. Oxygen is largely transparent to both visible and infrared light. Explain why carbon dioxide is a more important greenhouse gas in the Earth's atmosphere than oxygen, even though there is much more oxygen than carbon dioxide.
Figure Insolation shows the insolation at the surface of the Earth. The Earth is spherical, so we would expect the values to be the same for places of the same latitude. But notice that this is not true – compare, for example, central Africa with the Atlantic Ocean at the same latitude. What feature of the atmosphere might explain this variation, and why?
The National Aeronautical and Space Administration (NASA) Earth Observatory website has an array of climate resources. For a more in-depth discussion of Earth's energy budget, go to (External Link)
Are you interested in finding more about the controversial Snowball Earth hypothesis? The National Science foundation and Harvard University have set up a website with more about the hypothesis and the evidence. Go to (External Link)
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