1.3.1 The Greenhouse Effect
Global warming is caused by an increase in the greenhouse effect. The greenhouse effect is generally a good phenomenon because it keeps the earth warm, which enables life to survive. When the sun’s energy comes into the earth’s atmosphere, about 70% of the energy gets absorbed by the atmosphere, oceans, and land surface while the remaining 30% is reflected into space by the clouds, atmosphere, and surface of the Earth .
The heat that stays on the planet eventually gets radiated out by the atmosphere, oceans, and land masses. Some of the heat gets absorbed into gases (such as carbon dioxide, methane gas and water vapor) in the atmosphere and this trapped heat is what keeps the planet warm. Figure 1-10 shows the cycle of solar energy and the greenhouse effect. If the earth did not have a greenhouse effect, it would probably look a lot like Mars. Some scientists have suggested that Mars used to have an atmosphere much like Earth’s, but that it disappeared long ago. The atmosphere on Mars is now so thin that its air pressure is only 0.6% of Earth’s and its average surface temperature is just -55°C. An image of Mars’ current atmosphere is shown in Figure 1-11.
Another planet in our Solar System shows us what too much greenhouse effect can do. Venus has a thick atmosphere that’s 90 times more massive than Earth’s and made up of mostly carbon dioxide. The surface temperature on Venus is 465°C, so hot that zinc, lead, and tin would all melt. Even though the planet Mercury is closer to the Sun, it only gets up to 427°C, quite cool compared to the hellish conditions on Venus.
The gases that cause the greenhouse effect have always been a part of Earth’s atmosphere. But since the Industrial Revolution, those gases have gone into the atmosphere at a much higher rate than they would naturally be produced. Some of these gases include :
Carbon dioxide (CO2): This is a colorless gas that is one of the byproducts of the combustion of fossil fuels. Most of the CO2 currently in the atmosphere was put there from volcanic eruptions millions of years ago. We have been helping to increase carbon dioxide concentration for many years. Carbon dioxide is the primary contributor to global warming because it absorbs infrared radiation. Global CO2 emissions have increased from 1 billion tons in 1900 to 8 billion tons in 2000 .
Nitrous oxide (NO2): The nitrous oxide levels that have been released are less than the CO2 levels, but the amount of energy that NO2 absorbs is about 270 times as much . NO2 is another byproduct of the combustion of fossil fuels.
Methane (CH4): Methane is the main component of natural gas. It is created from the burning of coal, the decomposition of garbage, and from large herds of livestock. It absorbs about 20 times more energy than CO2, and therefore also heats up the earth .
Water Vapor: The greenhouse effect is also exacerbated by water vapor but it is usually a result of climate change rather than man-made emissions. When the earth heats up, water vapor forms and rises, and the temperature of the lower air decreases. Eventually, the water vapor cools enough that it converts back into liquid water, and it falls again. As water vapor rises, more of it will condense into the clouds which will help to reflect incoming solar radiation, allowing less energy to enter the earth’s atmosphere. Figure 1-11 illustrates the water cycle of evaporation, condensation, and precipitation. Scientists are uncertain of the exact effect of the increased amounts of water vapor on the earth, but they believe that the concentration of water vapor is correlated with increased amounts of carbon dioxide.