When "global warming" became a household phrase, greenhouse gases, which trap heat in the Earth's atmosphere, got a bad reputation. After all, those gases are to blame for heating up the planet. But, as we discuss in Chapter 2, greenhouse gases in reasonable quantities aren't villains, they're heroes. They capture the sun's warmth and keep it around so that life is possible on Earth. The problem starts when the atmosphere contains too great an amount of greenhouse gases. (In Chapter 3, we look at how scientists have determined the correlation between carbon dioxide in the atmosphere and temperature.)
Human activities — primarily, the burning of fossil fuels (which we look at in the section "The Roots of Global Warming," later in this chapter) — have resulted in growing concentrations of carbon dioxide and other greenhouse gases in the atmosphere. As we explain in Chapter 2, these increasing quantities of greenhouse gases are retaining more and more of the sun's heat. The heat trapped by the carbon dioxide blanket is raising temperatures all over the world — hence, global warming.
So far, Earth has seen a 1.4-degree Fahrenheit (0.8-degree Celsius) increase in global average temperature because of increased greenhouse gases in the atmosphere. Unfortunately, the amount of greenhouse gases that human activities produce grows daily. So, humanity's current behavior is driving temperatures up at an alarming rate. Temperatures in polar regions, such as the Arctic, are experiencing temperature rises that are twice the global average.
Global warming is a very complex issue that you can't totally understand without looking at the ifs, ands, or buts. Scientists are certain that the rapid changes to climate systems are due to the build-up of greenhouse gases, and they can't explain the current rates of global warming without factoring in the impact of human greenhouse gas emissions. Other elements play a role in shaping the planet's climate, however, including the following:
I Cloud cover: Clouds are connected to humidity, temperature, and rainfall. When temperatures change, so can a climate's clouds — and vice versa.
I Long-term climate trends: The Earth has a history of going in and out of ice ages and warm periods. Scientific records of carbon dioxide levels in the atmosphere go back 800,000 years, but people can only give educated guesses about the climate earlier than that.
I Solar cycles: The sun goes through a cycle that brings it closer to or farther away from the Earth. This cycle ultimately affects the temperature of this planet, and thus the climate.
We go over these other issues in greater detail in Chapter 3.
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