The most common of the gases making up the earth’s atmosphere are nitrogen (78 percent) and oxygen (21 percent). Combined, then these two account for 99 percent of the dry atmosphere, and because of the peculiarities of molecular structure, heat passes through them easily. The largest part of the remaining 1 percent is the inert gas argon. But while even less abundant, some of the other gases—most significantly water vapor, carbon dioxide, methane, nitrous oxide, and ozone—intercept, on average, about 83 percent of the heat emitted by the earth’s surface.“ So the earth does indeed emit energy equivalent to what it absorbs from the sun, but instead of directly flowing off into space, cooling our planet to a chilly average of 0°F, much of that energy is intercepted by the atmosphere blanketing us.
Water vapor is the most important of the greenhouse gases. Of course, the amount in the atmosphere at any given place and time varies greatly (the humidity changes a lot with the weather). But on average, water vapor amounts to only about 0.4 percent of the molecules in the atmosphere. Even so, it accounts for more than 90 percent of the atmosphere’s ability to intercept heat. John Tyndall, the Irish physicist who was the first to study the infrared properties of gases, eloquently expressed its importance in an 1863 public lecture:
Aqueous [water] vapor is a blanket, more necessary to the vegetable life of England than clothing is to man. Remove for a single summer night the aqueous vapor from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequited into space, and the sun would rise upon an island held fast in the iron grip of frost.
The next most significant greenhouse gas, carbon dioxide (CO), is different from water vapor in that its concentration in the atmosphere is much the same all over the globe. CO2 currently accounts for about 7 percent of the atmosphere’s ability to intercept heat. It’s also different in that human activities have affected its concentration (that is, the fraction of air molecules that are CO2). Since 1750, the concentration has increased from 0.000280 (280 parts per million or ppm) to 0.000410 (410 ppm) in 2019, and it continues to go up 2.3 ppm every year. Although most of today’s CO2 is natural, there is no doubt that this rise is, and has been, due to human activities, primarily the burning of fossil fuels.
The CO2 that humans have added to the atmosphere over the past 250 years increases the atmosphere’s ability to impede heat (it’s like making the insulation thicker), and is exerting a growing warming influence on the climate. The exact increase in insulation at any place and time depends upon temperature, humidity, cloudiness, and so on. Taking average clear sky (no clouds) conditions as an example, the CO2 added from 1750 until today increases the fraction of heat intercepted from 82.1 percent to 82.7 percent. And as the amount of CO2 continues to increase, the atmosphere’s heat-intercepting ability (and hence its warming influence) will also increase; doubling the CO2 concentration from the 1750 value of 180 ppm to 560 ppm would increase it to 83.2 percent under clear sky conditions. Such an increase in concentration would amount to an increase of just 2.8 molecules per 10,000—in other words, an increase of fewer than three molecules of CO2 out of every 10,000 molecules of air would increase the amount of heat intercepted from 82.1 percent to 83.2 percent, or by about 1 percent.
If you’ve followed this far, you might be puzzled by two things. First, how could changing fewer than three molecules out of 10,000, a 0.03 percent change, increase the atmosphere’s heat intercepting ability by about thirty times that amount (1 percent)? And second, how could a mere 1 percent increase in heat-intercepting ability be such a big deal?
The answer to the first question depends upon the details of the infra-red (heat) radiation the planet emits to keep cool. While we’ve talked about how the overall amount of that radiation has to balance the warming sunlight, the radiation is actually spread over a spectrum of different wavelengths. Think of those like “colors,” although not visible to our eyes. Water vapor, the most significant greenhouse gas, intercepts only some colors, but because it blocks almost 100 percent of those it does, adding more water vapor to the atmosphere won’t make the insulation much thicker—it would be like putting another layer of black paint on an already black window. But that’s not true for carbon dioxide. That molecule intercepts some colors that water vapor misses, meaning a few molecules of CO2 can have a much bigger effect (like the first layer of black paint on a clear window). So the greater potency of a CO2 molecule depends upon relatively obscure aspects of how it, and water vapor, intercept heat radiation – another example of why the details are important when attempting to understand human influences on the climate.
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Originally Posted by Foundation_Of_Liberty
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