Under the Microscope: The Earth is essentially the same distance from the Sun as the Moon. However, the average surface temperature of the Earth is 15 degrees C while the Moon's average temperature is minus 18 degrees C. This is because the Earth has an atmosphere and the moon has none. The greenhouse atmospheric effect keeps our planet comfortably warm and suitable for life, writes Prof William Reville
There is scientific consensus that our world is gradually warming up because of human additions of several gases to our atmosphere, thereby enhancing the greenhouse effect. International efforts are under way to halt the artificial warming of the Earth but there is an underlying pessimism as to the prospects for success. James Hansen, director of the NASA Goddard Institute for Space Studies, argues persuasively (www.sciam.com) that the actions necessary to arrest global warming are feasible.
The Sun, the Earth's only external source of energy, radiates energy mainly as visible light. Most of this light passes unaltered through the atmosphere and warms the surface of the Earth. The warmed Earth radiates energy back up into the atmosphere but in the form of infrared light not visible light. The atmosphere is not readily transparent to infrared radiation and some of it is trapped close to the ground. Water vapour and carbon dioxide strongly absorb infrared radiation. The trapped radiation warms the lower atmosphere and keeps the earth hotter than it would otherwise be - the greenhouse effect.
Relatively minor components of the atmosphere are responsible for the greenhouse effect - water vapour, carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, and ozone. The two principal gases are carbon dioxide and water vapour. Man-made aerosols, principally soot, absorb sunlight and also heat the atmosphere.
Since the late 1800s average global surface temperature has increased by 0.75 degrees Celsius. About 0.5 degrees Celsius of this warming occurred after 1950. This warming correlates positively with increasing atmospheric concentrations of carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and ozone. These human-made gaseous additions are heating the earth's surface at a rate of about two watts per square metre, the equivalent of burning two Christmas tree light bulbs day and night over every square metre of the earth's surface.
The Earth's climate has repeatedly swung between ice ages and warm interglacial periods over the past few million years. We are presently in the Holocene interglacial period, which is about 12,000 years old. The Antarctic ice sheet has not melted for the past 400,000 years and preserves a record of the earth's temperature and atmospheric gaseous conditions.
These natural large-scale climate swings are associated with slow changes of Earth's orbit caused by the gravity of other planets. These changes alter the geographical and seasonal distribution of incoming solar energy and, over long periods, these changes affect the building and melting of ice sheets.
Changes of the earth's orbit initiate climate change, but they operate by altering atmospheric and surface properties of the earth and thereby the planetary energy balance. Up to about 1850, changes in the gaseous composition of the atmosphere were entirely determined by natural causes but today the composition of the atmosphere is influenced more by humans thaby our planet's orbital variations.
Hansen calculates that the highest level of additional global warming we can afford is not more than one degree Celsius, corresponding to an additional climate-forcing of one watt per square metre. To exceed this limit would destabilise the Antarctic and Greenland ice sheets beyond the point of no return causing sea levels to rise, flooding coastal areas worldwide.
Additional climate-forcing over the next 50 years can be kept at about one watt per square metre if a two-component policy is implemented. First we must halt or reverse growth of air pollutants, specifically soot, and also reduce atmospheric ozone and methane. Secondly, we must maintain carbon dioxide emissions over the next 50 years to about the same level as today. These are feasible targets.
Methane emissions can be reduced by capping methane at landfills and waste management facilities and during the mining of fossil fuels. Diesel fuels and bio-fuels (wood and cow dung for example) are prime sources of soot. Improved technologies will burn diesel more cleanly and in time diesel may be replaced altogether by hydrogen fuel.
Increasing energy efficiency and the use of renewable energies could level off carbon dioxide emissions in the near term. Longer-term reduction of emissions is more difficult because energy use will continue to rise. New technologies will have to be introduced which either produce little carbon dioxide or capture that which is produced. A new generation of nuclear power, acceptable to the public, could greatly reduce carbon dioxide emissions.
The achievement of Hansen's objectives will require international cooperation on an unprecedented scale. The preventive medicine he prescribes may be somewhat unpalatable but the disease that will surely follow if we do not take this medicine could be devastating.
William Reville is associate professor of biochemistry and director of microscopy at University College Cork