POPULATION AND ATMOSPHERE
he world is warming up. Average temperatures are half a degree centigrade higher than a century ago. The nine warmest years this century have all occurred since 1980, and the 1990s were probably the warmest decade of the second millennium . Pollution from "greenhouse gases" such as carbon dioxide (CO2) and methane is at least partly to blame.
The UN's Intergovernmental Panel on Climate Change (IPCC) concluded in 1995 that "the balance of evidence suggests a discernible human influence on global climate"; and that the accumulations of greenhouse gases are behind the marked global warming trend of the past 20 years. Its case was based on two pillars: the known physical heat-capturing properties of the greenhouse gases that are accumulating in the atmosphere, and the detailed patterns of average temperature change in the atmosphere, which mirrored that predicted by global climate models. [Add]
Emission rates for the most important anthropogenic greenhouse gas, CO2, have risen 120-fold in the past 150 years . Whereas in the 19th century emissions were overwhelmingly from deforestation and other landuse changes, they are now predominantly from burning fossil fuels. A direct product of industrialization, emissions now amount to 6 billion tons of carbon a year, or around 1 ton of carbon per head of the world's population. But emissions are very uneven. Per-capita North American emissions are 18 times those of Africa, nine times those of Asia and 2.3 times those of Europe . Low gasoline prices and the pervasive automobile culture in the United States ensure that its CO2 output, already the highest in the world, is continuing to rise, while levels in much of Europe are stable or falling.
Total emissions of greenhouse gases, including CO2 from deforestation and agricultural emissions of methane, are more evenly distributed. For instance, Germany emits three times more CO2 than Brazil from burning fossil fuels. But Brazil's total emissions of greenhouse gases now probably exceed Germany's, thanks largely to emissions from deforestation . < [Add]/p>
Unless the world curbs growing CO2 output, concentrations in the air are likely to double from pre-industrial levels by 2080, and may warm the world by 3 degrees C. Climate models predict that land areas will warm twice as much as the oceans; high latitudes will warm more quickly in winter; and there will be substantial changes in precipitation, especially in the tropics .
There is a high risk of extreme weather, including intense El Niño events in the Pacific Ocean, hurricanes in coastal areas and droughts in continental interiors. Rising sea levels as glaciers and ice sheets melt, and thermal expansion of the oceans, may inundate heavily populated coastal regions, such as large parts of Bangladesh and eastern China and some island nations, such as the Maldives . Sea levels are already committed to a substantial rise, probably of 1 or 2 meters over the next 500 years, as a result of warming to date, which will slowly penetrate to the ocean depths, causing thermal expansion as it goes .
Possible ecological impacts include the destruction of most of the Amazon rainforest (from warming and drying) by the end of the 21st century. The loss of forests globally will further accelerate the emissions of CO2 into the air, exacerbating climate change. Recent modelling studies suggest that changes in rainfall and evaporation rates are likely to cause a decline in runoff of 25 percent or more in much of Southern Africa, South and Central America, India, Australia and the Mediterranean basin. But runoff could increase by similar amounts in the United States, China and the catchment of the Aral Sea in Central Asia .
Warming is also likely to spread pests and diseases to new regions. Nearly two thirds of the world's population could be living in malaria transmission zones within a century . Declining rainfall and a low technical capacity to adapt are likely to cause falling crop yields in much of Africa and India.
There is also increasing concern about the risk of major climatic "surprises". Warming might reduce the strength of the North Atlantic ocean circulation through the 21st century, with possible collapse of the Gulf Stream and cooling of Western Europe in the 22nd century  [Add].
In general, fossil-fuel emissions of greenhouse gases and consumption by consumers go hand-in-hand: the richest nations have the highest emissions. But there is growing evidence that fossil-fuel emissions can be "delinked" from population size and economic activity. One route is a change in energy-generating technology. France generates most of its electricity from nuclear power and has per-capita emissions of CO2 less than two thirds those of its neighbor, the United Kingdom. Another is more efficient use of energy. China has halved its energy consumption per unit of economic output since 1980 by cutting subsidies to coal, the most polluting fuel  [Add].
The first serious effort to curb emissions of CO2 and other greenhouse gases was behind the Kyoto Protocol of 1997, with most industrialized nations agreeing to cut emissions of six greenhouse gases by around 5 percent by 2012. Flexibility mechanisms will allow them to meet the targets by investing in emissions reduction or carbon-sink enhancing projects (such as planting forests) in other countries. Recent research has suggested that the inclusion of projects to cut the atmospheric build-up of greenhouse gases other than CO2, notably methane, could cut the cost of meeting the Kyoto Protocol by 60 percent .
The European Union has proposed that, in the longer run, the world should aim to prevent greenhouse gas emissions rising above twice pre-industrial levels. There is no single route to achieve this. Scenarios suggested by the IPCC involve a rise in global emissions of about 25 percent in the next half-century (compared to a doubling likely if conditions at the time of the Kyoto Protocol persisted unaltered) before falling back to less than half current emissions.
Signs that this may be possible without dramatic damage to economic development emerged during the late 1990s, when global CO2 emissions did not rise in line with growing economic activity. This was due largely to increased fuel efficiency and a declining use of coal . A new IPCC assessment on future emission scenarios foresees a long-term "delinking" of CO2 emissions from wealth and population: "Technology is at least as important a driving force of future greenhouse gas emissions as population and economic development." Some scenarios with a world population of 15 billion had lower emissions than others with a population of 7 billion . [Add]