Throughout its long history, Earth has warmed and cooled time and again. Climate has changed when the planet received more or less sunlight due to subtle shifts in its orbit, as the atmosphere or surface changed, or when the Sunâ€™s energy varied. But in the past century, another force has started to influence Earthâ€™s climate: humanity
How does this warming compare to previous changes in Earthâ€™s climate? How can we be certain that human-released greenhouse gases are causing the warming? How much more will the Earth warm? How will Earth respond? Answering these questions is perhaps the most significant scientific challenge of our time.
What is Global Warming?
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When they absorb the energy radiating from Earthâ€™s surface, microscopic water or greenhouse gas molecules turn into tiny heatersâ€” like the bricks in a fireplace, they radiate heat even after the fire goes out. They radiate in all directions. The energy that radiates back toward Earth heats both the lower atmosphere and the surface, enhancing the heating they get from direct sunlight.
This absorption and radiation of heat by the atmosphereâ€”the natural greenhouse effectâ€”is beneficial for life on Earth. If there were no greenhouse effect, the Earthâ€™s average surface temperature would be a very chilly -18Â°C (0Â°F) instead of the comfortable 15Â°C (59Â°F) that it is today.
The enhanced greenhouse effect
What has scientists concerned now is that over the past 250 years, humans have been artificially raising the concentration of greenhouse gases in the atmosphere at an ever-increasing rate, mostly by burning fossil fuels, but also from cutting down carbon-absorbing forests. Since the Industrial Revolution began in about 1750, carbon dioxide levels have increased nearly 38 percent as of 2009 and methane levels have increased 148 percent.
Increases in concentrations of carbon dioxide (top) and methane (bottom) coincided with the start of the Industrial Revolution in about 1750.
The atmosphere today contains more greenhouse gas molecules, so more of the infrared energy emitted by the surface ends up being absorbed by the atmosphere. Since some of the extra energy from a warmer atmosphere radiates back down to the surface, Earthâ€™s surface temperature rises. By increasing the concentration of greenhouse gases, we are making Earthâ€™s atmosphere a more efficient greenhouse.
How is Todayâ€™s Warming Different from the Past?
Earth has experienced climate change in the past without help from humanity. We know about past climates because of evidence left in tree rings, layers of ice in glaciers, ocean sediments, coral reefs, and layers of sedimentary rocks. For example, bubbles of air in glacial ice trap tiny samples of Earthâ€™s atmosphere, giving scientists a history of greenhouse gases that stretches back more than 800,000 years. The chemical make-up of the ice provides clues to the average global temperature.
Using this ancient evidence, scientists have built a record of Earthâ€™s past climates, or â€œpaleoclimates.â€ The paleoclimate record combined with global models shows past ice ages as well as periods even warmer than today. But the paleoclimate record also reveals that the current climatic warming is occurring much more rapidly than past warming events.
As the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 years. In the past century alone, the temperature has climbed 0.7 degrees Celsius, roughly ten times faster than the average rate of ice-age-recovery warming.
Models predict that Earth will warm...