Here comes a very long answer summarised for a much longer report that I wrote...
EFFECTS ON WEATHER
An increase in the amount of precipitation. Although rainfall overall will increase there are some parts of the world that are already receiving less rainfall and this trend is likely to continue.
Hurricanes and storms will become more frequent, widespread and severe, in the last 30 years the proportion of category 4 and 5 hurricanes has almost doubled. Once exclusively confined to the northern hemisphere there have recently been hurricanes in the southern hemisphere, most notably in Brazil.
Shifting weather patterns mean some areas receive less rainfall; the ground becomes barren and unable to sustain crops. In many parts of the world the layer of topsoil is both very thin and very poor. The dry, dusty soil is readily blown away and the area becomes desert. African and Asian countries are particularly hard hit.
For the reasons mentioned above there will be a major impact on agriculture. Rising sea levels will result in the loss of large areas of agricultural land, the consequences of which will be population migration, famine and / or the need to import food from elsewhere. Again, it will be the African and Asian countries that are hardest hit with crop production falling by up to a third.
RISING SEA LEVELS
Sea levels are rising faster now than at any time since the melting of the glaciers that marked the end of the last ice age. As the glaciers melted sea levels rose by some 120 metres (400 feet) but for the last few thousand years sea levels have been almost constant rising only 10 centimetres (4 inches) per thousand years.
Since the onset of global warming the seas have risen much faster. A hundred years ago they were rising by 1mm a year, today they are rising by 3mm a year and indications are that they will rise much quicker in the future. Sea levels are rising 30 times faster now than they were before industrialisation and the onset of global warming.
The Sundarman Delta has seen levels rising by more than 30mm a year, millions have already been forced to leave their homes and the Carteret Islands in the Pacific Ocean are currently being evacuated.
In years to come rising sea levels could swamp cities including London, New York and Miami. Low lying areas such as Bangladesh, much of the European and American coasts and island groups including the Seychelles and Maldives would be submerged.
MELTING OF ICE CAPS
The Arctic ice cap is floating; if it were to melt entirely there would be no change in sea levels. However, Antarctica is a continent, a land mass, it isn’t floating and any melting here adds to the sea level as does melting of ice sheets and glaciers in places such as Greenland.
The edges of the Antarctic Ice Sheet are breaking off and collapsing into the sea. One of the biggest breaks occurred in 2002 when the Larsen B Ice Shelf broke off. This huge mass of ice covered 3250 square kilometres and weighed half a trillion tons.
For the reasons mentioned above, large numbers of people are going to find it increasingly difficult to remain in their present locations and in many cases it will be impossible. In Bangladesh some 13 million people will be forced to move if the sea level rises by just one metre and in the US tens of millions will be affected particularly along the north east coast, Florida, Louisiana and California. Globally it is expected that hundreds of millions of people will need to relocate or emigrate.
The world’s oceans absorb carbon dioxide, some of which dissolves to form carbonic acid and this is affecting the alkalinity of the oceans. Many forms of marine life are highly sensitive to the level of alkalinity. The formation of corals is being affected and plankton, which forms the basis of the marine food chain, is also very sensitive. The increasing levels of carbon dioxide make it harder for many species of fish and shellfish to breathe and reproduce. Changes in the ecology and chemistry of the seas and oceans reduce their ability to absorb CO2, which consequently increases the rate of global warming.
Parts of Antarctica are now covered in grass and there is a massive migration of animals towards the polar regions, these migrations extend an average of 6.1km further from the equator each decade. Butterflies have extended their territory by some 200km further north in Europe and North America.
In the Arctic the habitat of polar bears and emperor penguins is being threatened. The waters of the Hudson Bay for example, are now ice free for three weeks more each year than they were 30 years ago. Polar bears are starving because they need to venture onto the frozen ice to hunt food. In another 10 or 20 years there may no longer be polar bears in this region and within a hundred years they could be extinct.
The most extensive report into global warming and climate change predicts that up to 40% of animal species could become extinct due to global warming.
HEALTH AND DISEASE
Global warming will be both beneficial and detrimental to people's health. There will be fewer deaths from cold related conditions but more deaths from heat related conditions. Globally there are more heat related deaths than cold related deaths so the net impact will be a loss of life.
Warmer temperatures lead to an increase in air and water pollution thus increasing the risk to humans from infection and respiratory conditions such as asthma. Warmer temperatures are conducive to the breeding and spread of rats, mice, other rodents, ticks, mosquitoes and other vectors for disease. This will lead to an increase in the number of people affected and an expansion into previously unaffected areas.
Rising sea levels lead to the contamination of groundwater rendering it undrinkable whilst at the same time higher temperatures lead to greater evaporation of fresh water from reservoirs. This will impact on the human population as well as plants, animals and agriculture.
Higher temperatures and changing climate patterns mean that some areas will receive less rainfall, in other areas when the rain does fall it is likely to do so on hard, sun-baked ground which is unable to absorb it. Instead of replenishing groundwater supplies the water will run off causing flash flooding and a lowering of the water table.
Glaciers are melting faster than has been known before - up to 40 metres per day in some places. In the last 100 years 50% of the world’s glaciers outside the polar regions have melted. The effects of glacial melt include flooding, landslides, avalanches and loss of habitat. In some mountainous regions melt water provides a year round water supply and without the glaciers there will be near drought conditions.
Increased adverse weather in itself could cost 1% of the worldwide GDP and a temperature rise of 2 or 3°C would reduce global economic output by 3%. In percentage terms these aren’t very big numbers but in terms of dollars the cost runs into trillions.
Insurance premiums are rising in line with the increased number of claims. The Association of British Insurers notes a 100% rise in weather related claims in recent years and states that climate change is already seriously impacting on the insurance industry.
A possible effect of global warming is the failure or slowing of the ocean conveyor belts or meridional overturning circulation. Research is continuing and it's unclear at this time what effect, if any, global warming will have on thermohaline circulation. One theory is that the melting of polar ice could reduce salinity and introduce cold water into the oceans which could trigger a slowing or shutdown of thermohaline circulation. It's thought that the Gulf Stream which conveys warm Caribbean water to the coasts of Ireland, Britain and northern Europe is the most likely to be affected. Should this occur temperatures in these regions would drop be several degrees.
A rise in sea levels would need to be countered with extensive flood defence schemes. Costly engineering schemes would need to be constructed in order to protect coastal cities. Many countries would not be able to afford such schemes and low lying communities may need to be evacuated.
Temperature fluctuations cause damage to road surfaces, pipelines including oil, gas, water and sewerage pipes, railway lines and airport runways. Greater contraction and expansion resulting from fluctuating temperatures and greater temperature ranges weakens the structure.
The developing nations are expected to be the worst affected by global warming. Decreasing rainfall, drought, insect infestation and severe weather events will seriously impact on crop production leading to famine and starvation. Economic development is likely to be restricted whilst at the same time poverty increases. The poorer countries of the world are the ones that have the least resources to mitigate the effects of global warming.
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The most extensive report into climate change and global warming was conducted by the IPCC and concluded that temperature rises of between 1.8°C and 4.0° should be expected by the end of the 21st century.
In 2007 one person is dying every three minutes as a result of global warming, this figure is expected to double by 2020. The World Health Organisation predicts that rising temperatures will result in an additional 300,000 deaths and 10 million illnesses a year by 2030. This is in addition to the tens of millions of additional cases of malaria that the WHO expects to see.
A quarter of the species of animals and plants could be extinct by 2050. More than three million square kilometres of Arctic sea ice has melted threatening the habitat of polar bears, experts predict that their numbers will ‘plummet’; the survival of grizzly bears is also threatened. Other species that are threatened include penguin, turtle, caribou, walrus, plankton, krill, whale, crab and seal.
In recent years there has been an 80% increase in the number of category 4 and 5 hurricanes. Along with cyclones, tornados and other storms the frequency looks set to keep increasing. The UK Met Office predicts a nine-fold increase in the extent of flooding within the next 50 years.
In 2003 a heatwave in Europe, one of many in recent years, claimed up to 50,000 lives, predictions are that the annual number of heat related deaths will increase significantly.
DROUGHT AND FAMINE
The heat is likely to greatly increase the number of forest fires, reduce timber production, impact on agricultural land and lead to more insect infestation which in turn impacts on crop production and human health.
Rising temperatures have a huge impact on food production, a 1°C rise equates to a 10% loss in grain production, areas already facing food shortages will be hardest hit. Warmer temperatures create severe drought conditions; billions of people will be affected by contaminated water, a lack of water or both, in China and south east Asia alone hundreds of millions will be affected.
RISING SEA LEVELS
Rising sea levels have already impacted hard on many island and coastal communities including the forced evacuation of their populations. Areas affected include Kenya, Kiribati, Marshall Islands, Tuvalu, Bangladesh, Papua New Guinea, Micronesia, Maldives, Antigua and Bermuda. Sea levels have been rising for some time and predictions indicate further rises of between 200 and 600mm by the end of the century. In the US this would affect many coastal regions especially the coastlines of Florida and the Gulf of Mexico, cities affected would include Boston, New York, Charleston, Miami and New Orleans.
Some of the other impacts of global warming include: arctic warming, increased power outages, marine food chain disruption, changes to bird migration patterns, more wildfires, erosion, desertification, disappearing beaches, damage to tundra regions, impact on ozone layer, insect infestation, threat to boreal forests, coastal erosion, threat to mountain environments, loss of wetland and marshland habitats, decline in bird population, thawing of permafrost, increased acidity of oceans, release of further greenhouse gases, increased allergens (asthma etc), coral destruction and bleaching, loss of ocean conveyor belts (Gulf Stream) etc, loss of Arctic sea ice, impact on winter sports.