Human activities, such as burning fossil fuels, add carbon dioxide to the atmosphere. Photo courtesy of lamazone/Flickr.
There was once a great debate about whether or not the Earth is warming, but the evidence now has scientists convinced. Since the start of the Industrial Revolution in 1880, the average temperature of the planet has increased by just over one degree Fahrenheit. While this may not seem like a big deal, it does show a rapid warming trend. Scientists estimate that temperatures will increase by 3-10 degrees by the end of the 21st century. Think about this: the temperature difference between now and the last ice age was 9 degrees F. Natural cycles of warming and cooling usually take tens of thousands of years; they usually don't happen this quickly! So what's going on?
To understand global warming, you must first understand why our planet is warm in the first place. The Earth's surface warmth is due to an atmospheric process called the greenhouse effect. The gases that make up Earth's atmosphere act like a blanket, trapping in sunlight and preventing heat from escaping. These gases act much like the glass of a greenhouse, and for this reason they are called greenhouse gases. The most important (and naturally occurring) greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).
For most of the Earth's history, greenhouse gas concentrations (and temperatures) have naturally gone up and down, as the Earth cycled through warm periods and ice ages. But since the Industrial Revolution, human activities have sent more and more greenhouse gases into the atmosphere. Burning wood and fossil fuel (gas, coal, and oil) adds carbon dioxide (which alters the carbon cycle); livestock and coal production add methane; and agricultural and industrial processes add nitrous oxide.
Oceans and land plants can absorb only half of the extra greenhouse gases; the rest accumulates in the atmosphere, where it strengthens the greenhouse effect leads to warming of Earth's climate. Scientists can't predict all the consequences of increasing greenhouse gas concentrations in the atmosphere, but they've got their eyes on the sky and will be watching closely for changes.
The next pages describe how biogeochemical cycles (specifically, the carbon and water cycles) are related to the greenhouse effect and global warming. Specifically, they explain how human activities influence the greenhouse effect, and they describe the relationship between carbon dioxide levels in the atmosphere and global temperatures.