Climate Change Information Sheet 2

The greenhouse effect

• The earth's climate is driven by a continuous flow of energy from the sun. This energy arrives mainly in the form of visible light. About 30% is immediately scattered back into space, but most of the 70% which is absorbed passes down through the atmosphere to warm the earth's surface.
• The earth must send this energy back out into space in the form of infrared radiation. Being much cooler than the sun, the earth does not emit energy as visible light. Instead, it emits infrared, or thermal radiation. This is the heat thrown off by an electric fire or grill before the bars begin to glow red.
• "Greenhouse gases" in the atmosphere block infrared radiation from escaping directly from the surface to space. Infrared radiation cannot pass straight through the air like visible light. Instead, most departing energy is carried away from the surface by air currents and clouds, eventually escaping to space from altitudes above the thickest layers of the greenhouse gas blanket.
• The main greenhouse gases are water vapour, carbon dioxide, ozone, methane, nitrous oxide, and the chlorofluorocarbons (CFCs). Apart from CFCs all of these gases occur naturally. Together, they make up less than 1% of the atmosphere. This is enough to produce a "natural greenhouse effect" that keeps the planet some 30^oC warmer than it would otherwise be - essential for life as we know it.
• Levels of all key greenhouse gases (with the possible exception of water vapour) are rising as a direct result of human activity. Emissions of carbon dioxide (mainly from burning coal, oil, and natural gas), methane and nitrous oxide (due to agriculture and changes in land use), ozone (generated by the fumes in automobile exhausts) and CFCs (manufactured by industry) are changing how the atmosphere absorbs energy. Water vapour levels may also be rising because of a "positive feedback". This is all happening at an unprecedented speed. The result is known as the "enhanced greenhouse effect".
• The climate system must adjust to rising greenhouse gas levels to keep the global "energy budget" in balance. In the long term, the earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate must change somehow to restore the balance between incoming and outgoing energy.
• This adjustment will include a "global warming" of the earth's surface and lower atmosphere. But this is only part of the story. Warming up is the simplest way for the climate to get rid of the extra energy. But even a small rise in temperature will be accompanied by many other changes: in cloud cover and wind patterns, for example. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).
• Meanwhile, industrially-generated "sulphate aerosols" may have a local cooling effect. Sulphur emissions from coal– and oil–fired power stations produce clouds of microscopic particles that reflect sunlight back out into space. This partly compensates for greenhouse warming. These sulphate aerosols, however, remain in the atmosphere for a relatively short time compared to the long-lived greenhouse gases. They also cause problems, such as acid rain. This means we should not rely on sulphate aerosols to keep the climate cool indefinitely.
• Climate models predict that the global average temperature will rise by about 2^oC (3.6^oF) by the year 2100 if current emission trends continue. This projection uses 1990 as a baseline. It also takes into account climate feedbacks and the effects of sulphate aerosols as they are presently understood. Because there are still many uncertainties, current estimates of how much it will warm during the 21st century range from 1 to 3.5^oC.
• Past emissions have already committed us to some climate change. The climate does not respond immediately to emissions. It will therefore continue to change for many years even if greenhouse gas emissions are reduced and atmospheric levels stop rising. Some important impacts of climate change, such as a predicted rise in sea level, will take even longer to be fully realized.
• There is evidence that climate change has already begun. The climate varies naturally, making it difficult to identify the effects of rising greenhouse gases. But the pattern of temperature trends over the past few decades does resemble the pattern of greenhouse warming predicted by models. These trends are unlikely to be due entirely to known sources of natural variability. While many uncertainties remain, scientists believe that "the balance of the evidence suggests a discernible human influence on global climate."
• It is still too early to predict the size and timing of climate change in specific regions. Current climate models are only able to predict patterns of change for the continental scale. Predicting how climate change will affect the weather in a particular region is much more difficult. Thus the practical consequences of "global warming" for individual countries or regions remain very uncertain.