Читать книгу Climate Change For Dummies - Elizabeth May - Страница 40

If you want to understand the greenhouse effect, the best place to start is with the object that provided this analogy in the first place, the greenhouse. A greenhouse works by letting in sunlight, which plants and soil absorb, thus heating up the greenhouse. The panes of glass ensure that the warmer air doesn’t escape the greenhouse, or does so very slowly. If you’ve ever parked your car with the windows rolled up on a sunny day, you’ve experienced this effect. When you open your car door, you’re hit with a blast of hot air. The windows of your car have acted like the panes in a greenhouse, letting sunlight in, which heats the car, and then trapping that heat.

Certain gases in the atmosphere trap the sun’s heat in a similar way. These particular gases are called greenhouse gases because they cause this greenhouse effect. The Earth is bombarded by radiation from the sun. Some of this radiation can be seen (think visible light), and some of it can’t be (ultraviolet light, for example).

Very hot bodies give off different amounts of energy than cold ones do. A basic law of physics says that everything gives off radiative (mostly heat or light) energy, and how much energy it emits depends on its temperature. The sun, for example, is a toasty 10,300 degrees Fahrenheit (5,700 degrees Celsius) — a little bit hotter than people are used to here on Earth. So, the sun gives off a lot of radiative energy, and the Earth gives off very little. Earth is warm mostly because of the heat it gets from the sun — most of the sun’s radiative energy actually zooms right through the atmosphere to the Earth’s surface. (The helpful high level ozone layer protects us by absorbing a lot of the harmful ultraviolet rays.)

A portion of this radiation, about 30 percent on average, bounces off clouds, ice, snow, deserts, and other bright surfaces, which reflect the sun’s rays back into outer space. The other 70 percent is absorbed by land or water, which then heats up. And the Earth emits some of that heat — in the form of infrared radiation (electromagnetic waves most commonly known as heat). The unique qualities of the GHGs come into play: The GHGs absorb some of the escaping infrared radiation in the lower atmosphere, and re-radiate part of that back down. So, less of the radiation from the Earth’s surface gets to outer space than it would have without those gases, and that energy remains in the atmosphere and returns to the Earth’s surface — making both the atmosphere and Earth itself warmer than they would be otherwise.

To see the greenhouse effect in action, look at Figure 2-1.

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FIGURE 2-1: The greenhouse effect in action.

If the planet had no atmosphere or GHGs, humanity would be left out in the cold. The Earth wouldn’t be able to keep any of the heat that it gets from sun. Thanks to GHGs, humanity is kept reasonably warm, enjoying an average temperature of 59 degrees Fahrenheit (15 degrees Celsius), some 62.6 to 64.4 degrees Fahrenheit (17 to 18 degrees Celsius) warmer than without GHGs.

This natural greenhouse effect and the ozone layer allow life to exist on Earth. Without the greenhouse effect, the Earth would be too cold. And without the ozone layer, life couldn’t survive the sun’s ultraviolet radiation.

Too much GHG turns the heat up beyond that to which societies and ecosystems have become adjusted. The atmosphere on the planet Venus is 96 percent carbon dioxide (the key GHG that we talk about in the following section). Because of Venus’s concentration of GHGs and its relatively close proximity to the sun, it’s extremely hot — surface temperatures of up to 500 degrees Celsius. Meanwhile, the atmosphere on Mars has 95 percent carbon dioxide, but it’s very thin, and the planet’s position is farther away from the sun than Earth, so it’s extremely cold — a chilly –80 degrees Fahrenheit (–60 degrees Celsius).