Figure 1: The greenhouse effect
Most people nowadays refer to the greenhouse effect as the cause of global climate change. According to the world scientific community, greenhouse gases, mostly carbon dioxide (CO2) that is released in the atmosphere by burning fossil fuels, are to blame for global warming. However, few people know that the greenhouse effect has not started just after the industrial revolution. There is consensus among scientists that the greenhouse effect did not originate from human activities but that it has always been a natural process.
About one-third of the solar energy that reaches the upper layers of Earth’s atmosphere is reflected back to space. The rest is warming the surface of the Earth, which emits infrared radiation back to the atmosphere. Gases forming the atmosphere such as carbon dioxide, methane, and nitrous oxide have the capacity to absorb part of this infrared radiation, which causes the greenhouse effect (Figure 1).
The phenomenon has been named after the analogous process that involves the glass walls in a greenhouse. Without the greenhouse effect, the Earth’s average temperature would be around -18°C. It is due to this effect that the average temperature on Earth is maintained at about 15oC. Therefore, the greenhouse effect is a physical process which provides an Earth’s surface temperature allowing emergence and support of life.
The greenhouse effect acts in about the same way as car windows on a sunny day (Figure 2). They do allow the sun to heat up the car, but prevent the heat from escaping. This is why the inside of a car is much warmer than the air outside.
Human activities, primarily the burning of fossil fuels and clearing of forests, have altered the carbon cycle and have greatly intensified the greenhouse effect, causing global warming. Since burning of fossil fuels after the industrial revolution accelerated the pace by which carbon dioxide and other greenhouse gases are concentrated in the atmosphere (Figure 3), the amount of infrared radiation captured by greenhouse gases is also increased, which leads to an overall increase of the Earth’s mean temperature (Figure 4). We are probably familiar with melting of polar ice caps, rising sea levels. Other such impacts refer to changes to weather patterns, which are predicted to reduce agricultural productivity. Scientists claim that even a small rise of the temperature can have detrimental environmental impacts. There are scientific reports claiming that if the Earth’s mean temperature increases more than 2°C, irreversible damage could occur to many ecosystems throughout the world.
The rate of global warming will depend on various feedback mechanisms. For example, as the atmosphere gets warmer, the concentration of water vapor increases, further intensifying the greenhouse effect. This in turn causes more warming, which eventuates in a further increase in water vapor. This self-reinforcing cycle may double the increase in the greenhouse effect due to the added CO2 alone.
Very often people confuse the greenhouse effect with ozone depletion. However, these are two different phenomena. Despite the fact that ozone (O3) and chlorofluorocarbons (CFCs) are greenhouse gases, the greenhouse effect and ozone depletion are distinct phenomena. Ozone in the upper atmosphere absorbs most harmful ultraviolet light in sun’s radiation. Chlorofluorocarbons destroy the molecule of ozone and make its layer thinner. There are international agreements in force, according to which all nations are obliged to reduce ozone-destroying chemicals to let the ozone layer eventually recover. Scientists monitor the ozone ‘hole’ over the Antarctic (Figure 5) as well as depletions occurring in the Arctic.
The ozone ‘hole’ is a region of exceptionally low concentration of ozone in the stratosphere over the Antarctic. This can be observed during the spring, namely from August to October for the Southern Hemisphere. Values of ozone concentration less than 220 Dobson Units are as the boundary of ozone ‘hole’ (outlined by the white line in Figure 5; http://ozonewatch.gsfc.nasa.gov/facts/hole.html). A Dobson Unit, named after the scientist G.M.B. Dobson, is the most basic measure used in ozone research. If all the ozone over a certain area was compressed down to 0oC and 1atm pressure and spread out evenly over the area, it would be expected to form a slab 3mm thick, corresponding to 300 Dobson Units (http://jwocky.gsfc.nasa.gov/dobson.html).