Global surface temperatures have been recorded since 1850. According to the
2007 report from the Intergovernmental Panel on Climate Change, how many of the
years between 1995 and 2006 (a 12-year period) are one of the hottest 12 years
recorded? 

What is the percentage change in the atmospheric levels of methane (a greenhouse gas) since 1750? 

What is the change in percentage of the world’s ocean ice cover since the 1960s?

According to observation data collected at Mauna Loa Observatory in Hawaii,
what is the percent change in atmospheric CO₂ levels from 1959 (when
observation began) to 2009?

A 2010 article examined the 908 active researchers with at least 20 climate publications on Google Scholar. What percentage of them have stated that it is “very likely” that human-caused emissions are responsible for “most” of the “unequivocal” warming?

From 1850 to 2004, what is the percent change of volume of glaciers in the European Alps?

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The mechanism of the greenhouse effect

Scientists tell us that human activities are changing Earth’s atmosphere and increasing Earth’s average temperature. What causes these climate changes?

First, let’s understand Earth’s “normal” temperature: When Earth absorbs sunlight, which is mostly visible light, it heats up. Like the sun, Earth emits energy––but because it is cooler than the sun, Earth emits lower- energy infrared wavelengths. Greenhouse gases in the atmosphere (methane, carbon dioxide, etc.) let visible light pass through, but absorb infrared light––causing the atmosphere to heat up. The warmer atmosphere emits more infrared light, which tends to be re-absorbed––perhaps many times––before the energy eventually returns to space. The extra time this energy hangs around has helped keep Earth warm enough to support life as we know it. (In contrast, the moon has no atmosphere, and it is colder than Earth, on average.)

Since the industrial age began around the year 1750, atmospheric carbon dioxide has increased by 40% and methane has increased by 150%. Such increases cause extra infrared light absorption, further heating Earth above its typical temperature range (even as energy from the sun stays basically the same). In other words, energy that gets to Earth has an even harder time leaving it, causing Earth’s average temperature to increase–– producing global climate change.

In molecular detail, greenhouse gases absorb infrared light because their molecules can vibrate to produce asymmetric distributions of electric charge, which match the energy levels of various infrared wavelengths. In contrast, non-greenhouse gases (such as oxygen and nitrogen––that is, O2 and N2) don’t absorb infrared light, because they have symmetric charge distributions even when vibrating.

Summary: (a) Earth absorbs most of the sunlight it receives; (b) Earth then emits the absorbed light’s energy as infrared light; (c) greenhouse gases absorb a lot of the infrared light before it can leave our atmosphere; (d) being absorbed slows the rate at which energy escapes to space; and (e) the slower passage of energy heats up the atmosphere, water, and ground. By increasing the amount of greenhouse gases in the atmosphere, humans are increasing the atmosphere’s absorption of infrared light, thereby warming Earth and disrupting global climate patterns.

Shorter summary: Earth transforms sunlight’s visible light energy into infrared light energy, which leaves Earth slowly because it is absorbed by greenhouse gases. When people produce greenhouse gases, energy leaves Earth even more slowly––raising Earth’s temperature.