Climate Change

Understanding Climate Change

GREENHOUSE EFFECT | GREENHOUSE GASES AND AEROSOLS | TRENDS IN EMISSIONS

Greenhouse Effect

The earth's climate is driven by a continuous flow of energy from the sun. 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 to maintain a temperature balance. Being much cooler than the sun, the earth does not emit energy as visible light. Instead, it emits infrared, or thermal radiation.
Greenhouse gases in the atmosphere block some of the 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 30oC warmer than it would otherwise be - essential for life as we know it.
Levels of all key greenhouse gases are rising as a 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 are also 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 on average 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 can have a local cooling effect. Sulphur emissions from coal and oilfired power stations produce clouds of microscopic particles that reflect sunlight back out into space. This partly compensates for greenhouse warming in some regions. 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.

Greenhouse Gases and Aerosols

Greenhouse gases (GHGs) control energy flows in the atmosphere by absorbing infrared radiation. Their levels are determined by a balance between sources and sinks. Sources are processes that generate greenhouse gases; sinks are processes that destroy or remove them. Humans affect greenhouse gas levels by introducing new sources or by interfering with natural sinks.
The largest contributor to the natural greenhouse effect is water vapour. Its presence in the atmosphere is only marginally affected by human activity. Nevertheless, water vapour matters for climate change because of an important positive feedback. Warmer air can hold more moisture, and models predict that a small global warming would lead to a rise in global water vapour levels, further adding to the enhanced greenhouse effect. On the other hand, it is possible that some regions may become drier. Because modelling climate processes involving clouds and rainfall is particularly difficult, the exact size of this crucial feedback has some uncertainty.
Carbon dioxide is currently responsible for over 60% of the enhanced greenhouse effect, which is responsible for climate change. This gas occurs naturally in the atmosphere, but burning coal, oil, and natural gas is releasing the carbon stored in these fossil fuels at an unprecedented rate. Likewise, deforestation releases carbon stored in trees. Current annual emissions amount to over 7 billion tonnes of carbon, or almost 1% of the total mass of carbon dioxide in the atmosphere.
Carbon dioxide produced by human activity enters the carbon cycle. Many billions of tonnes of carbon are exchanged naturally each year between the atmosphere, the oceans, and land vegetation. The exchanges in this massive and complex natural system are precisely balanced; carbon dioxide levels appear to have varied by less than 10% during the 10,000 years before industrialization. In the 200 years since 1800, however, levels have risen by almost 30%. Even with half of humanity's carbon dioxide emissions being absorbed by the oceans and land vegetation, atmospheric levels continue to rise by over 10% every 20 years.
A second important human influence on climate is aerosols. These clouds of microscopic particles are not a greenhouse gas. In addition to various natural sources, they are produced from sulphur dioxide emitted mainly by power stations, and by the smoke from deforestation and the burning of crop wastes. Aerosols settle out of the air after only a few days, but they are emitted in such massive quantities that they have a substantial impact on climate in some regions.
Aerosols cool the climate locally by scattering sunlight back into space. Aerosol particles block sunlight directly and also provide seeds for clouds to form, and often these clouds also have a cooling effect. Over heavily industrialized regions, aerosol cooling may counteract nearly all of the warming effect of greenhouse gas increases to date.
Methane is a powerful greenhouse gas whose levels have already doubled. The main new sources of methane are agricultural, notably flooded rice paddies and expanding herds of cattle. Emissions from waste dumps and leaks from coal mining and natural gas production also contribute, especially in Canada. The main sink for methane is chemical reactions in the atmosphere.
Methane from past emissions currently contributes 1520% of the enhanced greenhouse effect. The rapid rise in methane started more recently than the rise in carbon dioxide, but methane's contribution has been catching up fast. However, methane has an effective atmospheric lifetime of only 12 years, whereas carbon dioxide survives much longer. This means that the relative importance of methane versus carbon dioxide emissions depends on the time horizon. For example, methane emitted during the 1980s is expected to have about 80% of the impact of that decade's carbon dioxide emissions over the 20year period 19902010, but only 30% over the 100year period 19902090.
Nitrous oxide, chlorofluorocarbons (CFCs), and low level ozone contribute the remaining 20% of the enhanced greenhouse effect. Nitrous oxide levels have risen by 15%, mainly due to more intensive agriculture. CFCs increased rapidly until the early 1990s, but levels of key CFCs have since stabilised due to tough emission controls introduced under the Montreal Protocol to protect the stratospheric ozone layer. Ozone is another naturally-occurring greenhouse gas whose levels are rising in some regions in the lower atmosphere due to air pollution, even as they decline in the stratosphere.

Trends in Emissions

Future greenhouse gas emissions will depend on global population, economic, technological and social trends. The link to population is clearest: the more people there are, the higher emissions are likely to be. The link to economic development is less clear. Rich countries generally emit more per person than do poor countries. However, countries of similar wealth can have very different CO2 emission rates depending on their geographical circumstances, their sources of energy, and the efficiency with which they use energy and other natural resources.
As a guide to policymakers, economists produce scenarios of future emissions. A scenario is not a prediction. Rather it is a way of investigating the implications of particular assumptions about future trends, including policies on greenhouse gases. Depending on the assumptions, a scenario can project growing, stable, or declining emissions.
Most scenarios suggest that future growth in emission rates will be dominated by what happens in developing countries. The bulk of emissions to date have come from industrialized countries. However, most future growth is likely to come from emerging economies where economic and population growth is fastest - and for which projections are most uncertain.
In a typical nonintervention scenario, carbon dioxide emissions rise from 7 billion tonnes of carbon per year in 1990 to 20 billion in 2100. Nonintervention means that no new policies are adopted to reduce emissions in response to the threat of climate change. It does not mean that nothing else changes: in this particular IPCC scenario (known as IS92a), world population doubles by 2100 while economic growth continues at 23% per year. (Remember that scenarios are based on assumptions, which are simply that, assumed futures of the world)
This scenario leads to the equivalent of a doubling of preindustrial CO2 concentrations by 2030, and a trebling by 2100. This includes the effects of other greenhouse gas emissions, translated into their carbon-dioxide equivalents. Even the present 30% of pre-industrial carbon dioxide takes levels of this long-lived greenhouse gas higher than they have been for at least 160,000 years.
Different assumptions about sources and sinks give very different results. Future emissions are uncertain, and they have to be translated into future atmospheric concentrations using models of the carbon cycle and atmospheric chemistry. This introduces more uncertainty, since it is unclear how key sinks (processes that absorb greenhouse gases) will respond to a changing climate. Rising carbon dioxide levels, for example, cause plants to grow faster (the CO2fertilisation effect) and absorb more carbon dioxide through photosynthesis. CO2 fertilisation, together with forest re-growth in northern countries, may be absorbing up to 25% of the carbon dioxide currently produced by human activity. No-one knows how this sink will behave in the future: if more land is required for food production or more forest fires occur, the trend may reverse.
Intervention scenarios are designed to examine the impact of efforts to reduce greenhouse gas emissions. They depend not only on assumptions about population and economic growth, but also about how future societies will respond to the introduction of policies such as taxes on carbon- rich fossil fuels or regulation of greenhouse gas emitting products.
Existing international commitments could slightly reduce the rate of growth in emissions through the 21st century. Under the Climate Change Convention, developed countries are aiming to return their greenhouse gas emissions to 1990 levels by the year 2000. If they were to succeed, the date of CO2 doubling would be postponed by less than five years. A goal of making more substantial reductions in atmospheric concentrations would clearly require all countries to make stronger cuts in their emissions.
Freezing global emissions at current levels would postpone CO2 doubling to 2100. This still would not be enough to prevent greenhouse gas concentrations from continuing to rise far beyond the year 2100. Stabilising carbon dioxide at double its pre-industrial concentration sometime in the future would require emissions to fall eventually to less than 50% of their current levels, despite growing populations and an expanding world economy.
Reducing uncertainties about climate change impacts and the costs of various response options is vital for policymakers. Stabilising or reducing emissions world-wide would have an impact on almost every human activity. To decide if it is worthwhile, we need to know how much it would cost, and how bad things will get if we let emissions grow. There are tough moral questions too: how much are we prepared to pay for the climate of the future, which only our children and their children will see?

Figure 1: CO2 concentration (ppm) / year, from 800 - 2000
(Source:Climate Cange 1994, Reports of Working Groups 1 and III of the IPCC)

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