Few things these days are as heavily discussed and as easily misunderstood as Global Warming and Climate Change. This series of sessions will seek to present the fundamentals of the theory of manmade global warming and why action related to it is so important. Before getting into some of the details, itís worth reviewing the basics of the manmade global warming theory. We know that the Earth has been around for more than 4.5 billion years and that, at any given time, the Earthís climate is subjected to multiple natural forcings, from changes in our orbit around the Sun to volcanic eruptions. As a result, we know that the climate experiences many natural variation cycles, with both colder and warmer conditions. Examining Earthís history, we know that these cycles have periodically encountered tipping points, resulting in the global environment reaching extreme conditions ranging from a planet largely covered in ice one on end to having tropical temperatures at the planetís poles at the other. Global warming theory is based on the introduction of a new forcing based on human activities driving up the atmospheric levels of greenhouse gases.
These increased levels increase the Earthís greenhouse effect, impacting the global climate. Unmitigated, this new human forcing may cross one or more of the climateís tipping points making mitigation actions irrelevant. But how important is the greenhouse effect? And, specifically, how important is carbon dioxide to that effect? By this point youíve likely seen many depictions of how the greenhouse effect operates. Light from the Sun reaches the Earth and penetrates the Earthís atmosphere. Some of this light is reflected back into space. Some of it is radiated away from the Earthís surface as infrared heat. Certain gases in the atmosphere trap some of this heat near the Earthís surface, keeping the Earth temperate. But how important is this greenhouse effect? Looking at this diagram, itís easy to conclude that the Sun is the primary determination of Earthís climate. Our planet is dwarfed thousands of times over by our resident star. Well, if the Sun is the deciding factor of Earthís climate, it should be even more so for Mercury.
After all, Mercury is the planet closest to the Sun, and itís dwarfed even more than the Earth by the Sunís massive size. So, letís take a closer look at these two planets. As you can see Mercury is over 2 Ω times closer to the Sun than the Earth and is over 2 Ω times smaller than the Earth in diameter. Then consider the relative temperatures on the two planets. As the planet closest to the Sun, the average maximum temperature on Mercury reaches a scorching 800 deg F. However, with no notable atmosphere and greenhouse effect to moderate its climate, the average minimum temperature on Mercury can plummet to -280 deg F, more than a one thousand degree temperature swing and much colder than any place on Earth. Now letís do a similar comparison, but a little closer to home. Letís examine Earthís own moon. Similar to Mercury, our moon lacks its own atmosphere and greenhouse effect. Consequently, despite being about the same distance from the Sun, the moonís temperature swings 500 degrees reaching temperatures both much hotter and much colder than those experienced here on Earth.
Comparing Earth to Mercury and our own moon, we see the tremendous influential role the greenhouse effect plays in moderating our climate. To get a better appreciation of this powerful effect, letís consider the height of the Earthís atmosphere along with the gases that comprise it. Here is our planet, and this blue ring represents the thickness of the Earthís atmosphere relative to the planetís size. The troposphere, the lowest level of the atmosphere where almost all of Earthís weather occurs, extends only about 12 miles above the Earthís surface at its maximum near the equator. At its minimum near the poles, it only reaches about 4 miles above the surface. By comparison, our atmosphere, so essential to our climate, is thinner than the skin of an apple. Even then, not every gas in the atmosphere contributes to the planetís greenhouse effect.
The primary greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. But before we consider those, letís take a closer look at the non-greenhouse gases. Nitrogen, oxygen, and argon comprise over 99% of the Earthís atmosphere, and none of these gases is considered a greenhouse gas. So, the Earthís greenhouse effect, which is so influential in moderating the planetís temperature, is driven by less than 1% of the Earthís atmospheric gases, quite a responsibility for such a small portion of our atmosphere. Now the two primary greenhouse gases are water vapor and carbon dioxide, so letís take a few minutes to consider each of those gases individually. Water vapor is, by far, the most powerful greenhouse gas in the atmosphere, absorbing heat across many wavelengths in the infrared spectrum. However, the impact of a greenhouse gas must also consider how long that gas remains in the atmosphere and how much it varies from place to place.
From a humid rainforest to an arid desert, the amount of water vapor varies wildly around the world, making up anywhere between zero and four percent of the atmosphere. It also varies over time through seasonal changes and with height. The higher you get in the atmosphere, the drier it can become. Water vapor varies so much because of how little time it spends in the atmosphere. Once emitted, water vapor leaves the atmosphere again in only a few days, preventing it from being spread evenly. We could inject huge amounts of water vapor into the atmosphere at the same time, and it would have a negligible long-term effect. It would simply precipitate back out in about a week. This very short atmospheric lifetime prevents water vapor from driving long-term changes in the greenhouse effect on its own.
However, warmer air can hold more water vapor than can colder air. So, if the air is warmed by some other means, it can hold more water vapor at any given moment, even if the vapor itself is cycling through the atmosphere quickly. As the amount of water vapor in the air increases, itís greenhouse strength can result in additional warming. So, while water vapor is incapable of driving long-term changes in the greenhouse effect on its own, it can provide a very potent feedback mechanism to warming trends initiated elsewhere. But, increased amounts of water vapor can also result in more clouds, and clouds can trap more heat near the Earthís surface and reflect more sunlight back into space. This dual role of clouds is one of the most complex mechanisms in the climate system. The second greenhouse gas to discuss is carbon dioxide, a vital aspect of life here on Earth. CO2 enters the atmosphere in multiple ways through respiration, fossil fuel combustion, and deforestation.
But not all methods are equivalent. Whereas respiration can only return carbon that has been recently consumed in food, fossil fuel combustion releases carbon that has been stored for millions of years. Once emitted, CO2 is used by plants in photosynthesis and is absorbed by the Earthís oceans and plant life. When these plants and trees die and decay, the carbon they store is released. And ice around the globe can trap CO2 and other atmospheric gases as it forms and release those gases as it melts. While not nearly as powerful in its heat-absorbing abilities as water vapor, CO2 is a ìwell-mixedî atmospheric gas, which means that, unlike water vapor, if you measure the amount of CO2 in the atmosphere at any location on the planet or at varying heights from the Earthís surface, youíll get very nearly the same result. CO2 can become well-mixed because of how long it remains in the atmosphere, which can be upwards of 100 years.
This long lifetime enables a rapid build-up of CO2 to have very long-lasting and wide-ranging effects. So, the climatic impact of any greenhouse gas is determined by its ability to absorb infrared heat, its longevity in the atmosphere, and its dispersion throughout the atmosphere. Methane, another greenhouse gas, shares characteristics of both water vapor and carbon dioxide. It is a relatively strong greenhouse gas, 20 times stronger than CO2, and it can remain in the atmosphere for a decade or longer enabling it to get widely dispersed throughout the atmosphere. So, letís take a checkpoint of what weíve covered so far. Weíve seen how incredibly thin the Earthís atmosphere is, and, comparing Earth to Mercury and its own moon, weíve seen the vital role the greenhouse effect plays in determining the Earthís climate Looking at its composition, weíve seen that this vital greenhouse effect is driven by less than 1% of the Earthís atmosphere and that the impact of an individual greenhouse gas is determined by a combination of its characteristics Focusing on water vapor, we know that, while it is a very powerful greenhouse gas, its short atmospheric duration makes it incapable of initiating long-term changes in the climate CO2, on the other hand, can remain in the atmosphere for over a century, enabling it to accumulate and have a very significant long-term impact.
This long atmospheric lifetime of CO2, coupled with its widespread dispersion, results in worldwide effects. These characteristics of CO2 enable it to be a primary driver of long-term changes related to the greenhouse effect How humans impact this effect will be the subject of our next section..