Climate Change For Dummies - Elizabeth May

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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.

Schematic illustration of the greenhouse effect in action.

      © John Wiley & Sons, Inc.

      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).

      

Water vapor, not carbon dioxide, is technically the GHG with the biggest impact. But human activities don’t directly affect in a significant way water vapor in the atmosphere.

      Given the important role that carbon dioxide plays in warming the Earth, you may be surprised by how little of it is in the atmosphere.

      

In fact, 99.95 percent of the air that humans breathe (not including water vapor) is made up of

       Nitrogen: 78 percent

       Oxygen: 21 percent

       Argon: 0.95 percent

      Carbon dioxide, by contrast, currently makes up only 0.0412 percent of all the air in the atmosphere. Human activities have helped increase that concentration from pre-industrial times, when it was about 0.0280 percent.

      When scientists talk about air quality and the chemistry of the atmosphere, they often use the term parts per million (ppm). So, currently out of every million parts of air, only 412 are carbon dioxide. That’s not much carbon dioxide, but what a difference it makes! Until recent changes in atmospheric chemistry caused by human activity, for around the last million years, carbon dioxide concentrations never exceeded 285 ppm. It’s like the hot pepper you put into a pot of chili — just right is just right, but if you have just a little too much, watch out.

      Looking at the carbon cycle

      Carbon dioxide occurs naturally — in fact, you, and every animal and insect on Earth that breathes in air, produce carbon dioxide every time you exhale. You inhale oxygen (and other gases), which your body uses as a nutrient, and you breathe out what your body doesn’t need, including carbon dioxide. You aren’t alone in using this process. But other organisms, mostly plants, suck carbon dioxide out of the air. Trees and grasses, for example, take in carbon dioxide and give out oxygen — the complete opposite of what people do.

      The carbon cycle is the natural system that, ideally, creates a balance between carbon emitters (such as humans) and carbon absorbers (such as trees), so the atmosphere doesn’t contain an increasing amount of carbon dioxide. It’s a huge process that involves oceans, land, and air. Life as humans know it — from microscopic bugs in the oceans to you and me, and every fern and plant in between — would disappear without this cycle. You can think of the carbon cycle almost as the Earth breathing in and out.

      

The carbon cycle is called in balance when roughly the same amount of carbon that’s being pumped into the air is being sucked out by something else. The atmospheric concentration of carbon dioxide was historically at a concentration of 280 ppm — carbon dioxide concentrations have fluctuated up and down through natural processes, but 280 ppm has been about the highest recorded concentration for the past 800,000 years — until recently when humans started to increase the concentration. (We look at how humans contribute carbon dioxide to the atmosphere in Part 2.)

      A RECIPE THAT GIVES YOU GAS

      Carbon dioxide is composed

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