Climate Change Information Sheet 4
How will greenhouse gas levels change in the future ?
- 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 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 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 may be quite wrong.)
- 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 a doubling of pre-industrial carbon dioxide would take levels of long-lived greenhouse gases higher than they have been for several million 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 or destroy 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, 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.
- Existing international commitments could slightly reduce the rate of growth in emissions through the 21st century. Under the Climate Change Convention, developed countries are trying 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 dramatically stronger cuts in their emissions.
- Freezing global emissions at current levels would postpone CO2doubling to 2100. While such a scenario is far beyond any proposals now being considered, it 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 22nd century would require emissions to fall eventually to less than 30% 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 22nd century, which only our children's children will see?
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