There is a scientific consensus that the world is warming up and that this is at least partially explained by human activities that release warming gases into the atmosphere - the enhanced greenhouse effect. Brendan McWilliams starkly described our warming world in a recent Irish Times article by pointing out that 1998 was the warmest year, in the warmest decade, in the warmest century of the last millennium.
The recent floods were a great shock. The rising temperature is changing weather patterns in ways that are difficult to predict accurately, but it seems likely the negative effects of these changes will greatly outweigh the positive effects.
One consequence we don't hear much about of a warming world is that many diseases will surge as the atmosphere heats up. These health consequences are described graphically by Paul Epstein in the August 2000 edition of Scientific American.
One reason a warming world changes weather patterns is that it accelerates the water cycle, the process whereby water evaporates, mainly from the oceans, into the atmosphere and then condenses out as rainfall. A warmed atmosphere speeds up evaporation and it also holds more water vapour. When this extra water vapour condenses it frequently falls as larger downpours.
As land heats up it can become parched. This steepens air pressure gradients that cause winds to develop, leading to violent gales, tornadoes and storms. Altered pressure and temperature gradients can also change the geographical distribution of storms, droughts and floods.
It is predicted that diseases spread by mosquitoes - malaria, dengue fever, yellow fever and several kinds of encephalitis - will become much more widespread as the world warms up. Mosquitoes pick up pathogenic micro-organisms when they eat a blood meal from an infected animal. The pathogen reproduces inside the mosquito and is passed on when the mosquito next inflicts a bite.
Mosquitoes can only live in regions and seasons where temperatures stay above certain minima. As these regions and seasons enlarge in a warming world, so will the incidence of mosquito-borne diseases. This is already starting to happen. Malaria is already spreading north and south of the tropics. Since 1990 several outbreaks of malaria have occurred in the US. Malaria has also returned to the Korean peninsula and to parts of southern Europe.
Increased incidence of flooding and of drought in a warming world also encourages the mosquito, whose eggs hatch in stagnant water. As floods recede they leave puddles, and in droughts streams become stagnant pools and people put out pots to catch what little rain may fall. Climate change may also reduce the population of predators that normally cull mosquitoes.
Malaria and dengue fever are the two diseases most likely to rise sharply as increasing global temperatures offer more scope to mosquitoes. Malaria, characterised by chills, fever, aches and anaemia, kills 3,000 people (mostly children) every day. The disease can occur in any tropical or sub-tropical region, but is a particular problem in parts of Africa and south-east Asia.
Computer models project that by the end of the 21st century world warming will have enlarged the zone of malarial transmission from an area containing 45 per cent of the global population to an area containing 60 per cent. There is no vaccine against malaria, and the parasites that cause the disease are becoming resistant to standard drugs.
Dengue fever is a severe flu-like viral illness that can cause fatal internal bleeding. It affects between 50 and 100 million people today, mainly in tropical and sub-tropical urban areas. It has spread over the last 10 years, reaching Buenos Aires in South America and northern Australia. No vaccine or specific drug treatment is available.
The spread of mosquito-borne diseases as the world warms doesn't prove that warming is the cause of the spread. Other factors, such as a decline in public health mosquito control programmes, might explain the spread. However, the warming world almost certainly is the explanation.
The warming-world model also predicted that warm conditions would creep up many mountains, melting glaciers and allowing plants and insects to thrive at higher altitudes than heretofore.
This prediction has come true. Since 1970 the elevation at which temperatures are always below freezing has risen by 500 feet in the tropics. Hand in hand with this, mosquitoes and their diseases have also marched upwards in altitude in south and central America, Asia and Africa. Dengue fever has been reported one mile high in Mexico.
Apart from mosquito-borne illness, global warming will also spread waterborne disease, including cholera which causes severe diarrhoea. Warming in itself can contribute to the increase by encouraging growth of pathogens, as can increased incidence of droughts and floods. Droughts wipe out scarce supplies of clean drinking water. Floods wash sewage and other sources of pathogens into drinking water and wash fertiliser into general water supplies.
Fertiliser, sewage and warm water can trigger massive growth (blooms) of harmful algae. Some blooms emit toxic vapours, others contaminate fish which, when eaten, sicken the consumer. Larger blooms also support the proliferation of pathogens such as the agent that causes cholera.
Since 1976 the world has been warming at a rate equivalent to 2 per century. This rate is unprecedented in the last 1,000 years and can only be explained by taking account of human emissions of greenhouse gases. We must reduce these emissions by 60 per cent within 20 years. This will be difficult. However, if we don't take action the consequences will be even more difficult.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC