Climate change - the science
The Greenhouse Effect
Heat comes to the Earth from the sun, warms Earth's surface, and makes life possible. Heat also leaves the Earth and radiates back out into space. As long as the amount of heat that enters the planet's atmosphere is equal to the amount that leaves, the average temperature of the Earth's surface will remain constant.
The light that comes to the Earth from the sun is of a shorter wavelength than the heat energy that is radiated from the Earth's atmosphere back out into space. Greenhouse gases, such as carbon dioxide, water vapour, methane, nitrous oxide, ozone, and sulfur hexafluoride, have little effect on the radiation coming from the sun, but block the longer-wave radiation which is emitted by the planet Earth. Essentially greenhouse gases are transparent to radiation from the sun, but they are opaque to radiation emitted by the Earth.
The Earth has had a greenhouse effect for millions of years. Without it the Earth's average temperature would be below freezing, and life would not be possible. The most important naturally occuring greenhouse gases are water vapour and carbon dioxide.
However for the past 200 years or so, since the Industrial Revolution, humans have been adding extra carbon dioxide into the atmosphere, on top of the carbon dioxide that would be there as a result of natural processes. In the past 50-100 years or so the pace at which carbon dioxide and other greenhouse gases are being added has increased dramatically. This has caused extra heat to be trapped inside the Earth's atmosphere, and thus the Earth has been getting warmer.
Over the past 200 years the concentration of carbon dioxide in the Earth's atmosphere has increased by about 30%, from 270 parts per billion to 370 parts per billion. This is the highest it has been in 150,000 years.
Effects of global warming
As a result of greenhouse gas emissions, the Earth has been getting hotter. Strong global warming has been observed since 1975. Globally 1995 is the hottest year on record. The second hottest was 2005, in which there was a record US hurricane season, and accelerated melting of Arctic sea ice and Siberian permafrost. The third hottest year on record was 2003, in which Europe experienced the hottest summer for at least 500 years, with an estimated 30,000 casualties as a result. Later researchers concluded that this hear wave was the first extreme weather event almost certainly attributable to climate change caused by human activities. The IPCC (Intergovernmental Panel on Climate Change) has predicted that the temperatures in Europe in the summer of 2003 may be normal by 2040, and cool by 2060.
The sea level is rising both because glaciers are melting, and because warm water takes up more space and is less dense than cold water. Global warmer means that the sea gets warmer just as the Earth's land surface does. It is expected that in the coming century many coastal regions will be flooded due to the sea level rise.
Globally glaciers have retreated. Arctic sea ice has decreased by about 2.5% since 1970. Biologists predict that polar bears will become extinct due to the loss of sea ice which allows them to hunt for seals. The Greenland ice sheet contains enough water to increase the global sea level by 7m. It is clear from satellite photos that the surface area of this ice sheet has been reduced significantly in the past ten years. The West Antarctic ice sheet is also melting and could contribute 6m to the global sea level.
Carbon dioxide can stimulate plants to grow more rapidly. Over the past century this enhanced growth has taken place and plants have acted as an important carbon sink. However experts believe that the Earth's forests are now reaching the limit of their ability to absorb carbon dioxide. Furthermore the world's remaining forests are being destroyed to clear new agricultural lands, and for the timber industry. When an area of forest is cleared the carbon dioxide stored within it, over a period of decades, is released back into the atmosphere. Thus the world's forests, which have up until now acted as an important carbon sink, are on the verge of becoming a source of carbon dioxide.
Most forms of life on Earth are adapted for a particular climatic zone. For instance, palm trees only thrive in tropical climates, while polar bears can only live in an arctic climate. Climate change will cause climatic zones to shift outward from the equators. So an area of woodland may begin to die off because the local climate is suddenly a degree or two too hot. Some forms of life, such as birds and insects, will simply migrate north to an appropriate climatic zone. But trees migrate very slowly. It would take centuries for seeds carried by birds or by the wind to allow a woodland to shift its location. The pace of climate change is much too fast for trees to keep up with. In addition some species that do manage to migrate may die off anyway because other species which they depend on food food or shelter have failed to migrate. We are already living in a time of mass extinctions and climate change will bring about the loss of many more species.
One category of organisms which does find it easy to migrate quickly is those that cause disease, such as malaria and cholera. The IPCC predicts that in the coming decades tropical diseases will spread to regions where they were previously know, and will cause significant loss of life.
Climate change doesn't just mean a hotter world overall, it means greater extremes of both hot and cold, and more extreme weather such as storms. This is because energy is being added to a complex dynamic system - the global climate system. Some of the negative feedback systems which would normally have a stabilizing influence on the climate have been disrupted, which means that extreme weather events will become far more frequent and severe in the coming decades.
The scariest effects of climate change are those we don't know. The climate is a complex system and it is impossible to predict all the effects of global warming. In recent years several climate experts have pointed out that computer-generated models of the climate such as those used by the IPCC are good at predicting slow, gradual shifts, but are not able to predict rapid changes that might take place. Some climatologists believe that a near-total shutdown of the thermohaline circulation could happen within the next 20 years, bringing dramatic changes to the climate of Northern Europe. Other experts claim that this shift, if it happens at all, will be gradual and will not occur within the next 100 years.
It takes time for greenhouse gas emissions into the atmosphere to be translated into changes in the climate. Even if all greenhouse gas emissions ceased today the planet would continue heating up for at least the next 50 years. We have already caused a far greater disruption to the climate than we realise.
Is there any chance left to prevent the Earth from becoming an uninhabitable nightmare world? If we stopped all greenhouse emissions now the global average temperature would continue to rise for several decades to come, and would then stabilize (at a higher temperature than before). Some species would face extinction as a result of these shifts, but most would adapt to the new climate.
Various campaign groups have claimed that we need to cut our present emissions by 70% - 95% within the next couple of decades, in order to avoid a truly catastrophic situation. What is clear is that small cuts to emissions won't have much effect - drastic cuts to our greenhouse emissions are needed now, if we are to have any hope of providing our children and grandchildren with a world they might want to live in.
References and sources:
http://www.gci.org.uk/contconv/protweb.html#214
http://www.meto.gov.uk/research/hadleycentre/pubs/brochures/2005/climate_greenhouse.pdf
http://www.climate.org