and releases all the CO 2 stored in rainforest trees. With more people to feed, the destruction
of rain forests has increased.
CO 2 is the most important greenhouse gas that human activities affect. And, after water va-
por, CO 2 is the most abundant. But other greenhouse gases are increasing as well. Methane
is released from raising livestock, rice production, and the incomplete burning of rainforest
plants. Chlorofluorocarbons (CFC’s) are human-made chemicals that were invented and
used widely in the 20th century. Tropospheric ozone, mostly from vehicle exhaust, has more
than doubled since 1976. All of these gases act as greenhouse gases as well as CO 2.
Global Warming
With more greenhouse gases trapping heat, average annual global temperatures are rising.
This is known asglobal warming.There is now nearly 40% more CO 2 in the atmosphere
than there was 200 years ago, before the Industrial Revolution. About 65% of that increase
has occurred since the first CO 2 measurements were made on Mauna Loa Volcano, Hawaii
in 1958 (Figure17.34).
Figure 17.34: The Keeling Curve shows the upward trend in atmospheric CO 2 on Mauna
Loa volcano since measurements began in 1958. The blue line shows yearly averaged CO 2.
The red line shows seasonal variations in CO 2. ( 22 )
The United States has been the largest emitter of greenhouse gases, with about 20% of
total emissions in 2004 (Figure17.35). China has been the second highest emitter (18.4%),
followed by the European Union (11.4%). As a result of China’s rapid economic growth, in
early 2008 its CO 2 emissions probably surpassed those of the United States. However, it’s
also important to keep in mind that the US has only about 1/5 the population of China.
Therefore, the average US citizen produces far more greenhouse gases than the average