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What is the "greenhouse effect"?

A greenhouse allows sunlight in through its transparent glass roof and walls. The Sun’s energy heats the plants and other solid objects in the greenhouse. These in turn heat the air. This same process goes on outside as well, but the heated air rises and is replaced by cooler air. In the greenhouse, because it is an enclosed space, the heated air cannot escape. It gets warmer inside.

A somewhat similar process is going on with the Earth’s atmosphere. Sunlight passes through the transparent atmosphere and heats the land and seas. The lower atmosphere is heated by being in contact with the warm ground and oceans. The air itself is not heated by the Sun.

Some of the heat given off by the Earth is radiated back into space. If this were the entire story the average temperature of the Earth would be -18°C (-0.4°F), which is about 33 C degrees (59 F degrees) colder than it actually is. This is much colder than it was during the depths of the ice ages.

So why aren't we in a deep freeze? Because there are "greenhouse gases"— carbon dioxide, methane, water vapor, and others—that keep us warm. Here’s how it works.

The molecules of these greenhouse gases absorb some of the heat. They then release it back into the atmosphere, keeping the climate warmer than it otherwise would be.

The greenhouse effect itself is not a problem. In fact, it is essential for life on Earth as we know it. But since the late 1800s, there has been a dramatic increase in CO₂ levels. Over hundreds of thousands of years there has been a close correspondence between atmospheric CO₂ levels and average global temperature.

Look very closely at the right edge of the chart below. The level of CO₂ in the atmosphere is suddenly way up. It rose gradually from the end of the last ice age 12,000 years ago to about 280 parts per million (ppm) in the 1880s. This is at the high end of the historical range, but is consistent with the pattern revealed by the Antarctic ice cores. In fact, recent research based on such cores has taken the record back even further than that shown in the chart below. During the past 600,000 years the concentration of CO₂ has never been greater than 300 ppm...until now. In 2004 it was 379 ppm.

What has caused this sudden increase? The reason is the increased burning of fossil fuels—coal, oil, and gas—since the Industrial Revolution.

The future of the Earth’s climate is hard to predict because it is so complicated. But the relationship between CO₂ levels in the atmosphere and temperature is known. So there is concern that we may be heading into a much warmer era.

Yes, there have been fluctuations in global temperature in the past. At first glance it may look like we are just in a warm phase of a cyclical pattern. But there is an important difference: The concentration of CO₂ in the atmosphere is dramatically higher than in any other warm period on record. We also know that the effect of increased CO₂ concentrations in the atmosphere is felt over time. This means that there will be further global warming even if there is no further increase in CO₂ level. This time it is different.