To answer the question, we first need to understand exactly what is meant by greenhouse gases and how they cause the earth to warm up. The real questions are: Why does the earth have the temperature it has, and what causes it to change? Physics provides the answers. The earth gets its temperature from the energy bombarding it from the sun. The average amount of energy hitting the earth from the sun each second is about 342 watts per square meter (W/m2), but about 30% of this is reflected by clouds and water and ice on the earth’s surface, so about 235 W/ m2 is absorbed by the earth.1 A basic law of physics says that objects that get heated up also have to radiate energy to keep energy in balance, so about 235 W/m2 are also radiated by the earth. The Stefan-Boltzmann law says that the rate at which an object (known as a black body) radiates energy goes up as the fourth power of its temperature (T4). Using this law and the rate at which energy is absorbed and emitted by the earth, you can calculate that a black body earth should have a temperature of about -18°C or 0°F. But that is not what the temperature of the earth actually is—its average temperature is actually about 15°C (59°F) (10). It is lucky that the earth’s temperature is not what the Stefan-Boltzmann law predicts because the earth would be a ball of ice!

So is physics wrong? No, but we didn’t account for the earth’s atmosphere. The atmosphere consists mostly of nitrogen and oxygen but also contains gases known as greenhouse gases—principally water vapor and CO2 but also methane, nitrous oxide, ozone, and a few other minor gases. The energy from the sun is mostly vis­ible light, but when it is absorbed by the earth and radiated back into space, it is radiated as infrared light that has a longer wavelength and less energy than visible light. The earth’s atmosphere is largely transparent to the visible light from the sun, but the greenhouse gases absorb most of the infrared radiation. The nitro­gen and oxygen that form most of the atmosphere do not absorb the infrared radiation, so they do not contribute to warming the earth. So, the simple expla­nation of the greenhouse effect is that the earth is like a greenhouse where the sun’s energy passes through the glass panes but is trapped inside the greenhouse because the infrared radiation can’t pass through the glass—the glass being the greenhouse gases.

A more sophisticated and accurate view is that the infrared radiation is absorbed by the greenhouse gases, which then re-emit the radiation in all directions, with some of it coming back to the earth and some of it heating up the atmosphere and being radiated into space. The net result is that the earth heats up to a higher temperature than it would have if the greenhouse gases didn’t exist. It still has to be in balance energetically, so as the earth gets a higher temperature, it radiates energy at a higher rate—with some of it coming back to earth—until it is balanced to match the incoming solar radiation, according to the Stefan-Boltzmann law (10). The balancing act can take a very long time, however, and the big problem is that currently there is a mismatch. The earth is radiating 0.6 W/m2 less than it is absorbing, inevitably leading to global warming (10, 11)

All of this is non-controversial and natural—it is how the earth and its atmo­sphere work, though there are lots of details glossed over here. The real contro­versy is whether humans are changing the earth’s temperature by adding such large amounts of CO2 (and other greenhouse gases) to the atmosphere that its greenhouse effect is causing the earth to heat up to achieve a new energy balance. That is the question!