Greenhouse Gases
What is most responsible for changing our climate is a substance you can’t normally see, taste, or smell. It’s a gas called carbon dioxide, and we have been putting more and more of it in the air every year.
In pre-industrial times, carbon dioxide made up about 280 parts per million atmospheric molecules. In 2008, its concentration had risen to 385 parts per million—30% more. So what’s so important about a few hundred parts per million?
Not only is the present atmospheric carbon dioxide concentration higher than it has been for at least 2.1 million years, the rate of change is accelerating. Previously it never exceeded 30 parts per million per thousand years, but now, carbon dioxide has risen by 30 parts per million in the last 17 years.
Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 10.8, Cambridge University Press
That increasing concentration of carbon dioxide in the atmosphere has a big effect—on our current path, we could see Earth’s temperature rise 4.0°C by the end of the century. Why? Warmer temperatures increase evaporation. And that additional water vapor is also a greenhouse gas. This is called a positive feedback—in this case, one in which increases in temperature cause changes in the climate system that themselves lead to more warming.
Worldwide, burning fossil fuels is the major source of CO2, but CO2 can also come from cutting trees, burning forests or grasslands, agriculture, and making cement.
Climate Change 2007: Mitigation of Climate Change. Working Group III Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 1.2, Cambridge University Press.
Fossil fuels—like natural gas, coal, and oil—are burned chiefly in power plants; automobiles, airplanes, and other forms of transportation; and by industry. Agriculture produces its own set of emissions, as plowing soil releases carbon stored there and farm animals belch methane. Notice that emissions from deforestation rival those from the residential and service sectors and increased sharply in the mid-1990s.
What do you think?
Which country had the highest total CO2 emissions from fossil fuels and land use changes (including tropical deforestation) in 2000? (Choose the best answer.)
The correct answer is a).
According to data from the World Resources Institute, the U.S. had the highest total CO2 emissions, while Brazil ranked 5th. Brazil ranks high on the list because of land use activities like cutting forest and converting it to crop or pastureland. The other countries of South America don’t rank in the top 20.
Data from the World Resources Institute.
If we only look at CO2 emissions from land-use changes and forestry, in 2000, Brazil ranked second in the world after Indonesia. Peru, Columbia, Mexico, Bolivia, and Ecuador were in the top 20 in this category.
Carbon dioxide isn’t the only greenhouse gas we are releasing to the air. For example, the atmosphere now contains about 1,780 parts per billion of methane (CH4), which is a more powerful greenhouse gas than CO2.
Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 2.3, Geneva, Switzerland.
During the last 650,000 years, it never exceeded 790 parts per billion. Methane is produced during oil and gas extraction and transport, rice cultivation, and in the digestive systems of cattle and other farm animals.
And some industrial chemicals called halocarbons—used as cleaners, refrigerants, and in making modern conveniences like computer monitors and flat panel TVs—trap heat extremely well and stay in the atmosphere a long time once released.