The predicted effects of global warming on the environment and for human life are numerous and varied. It is generally difficult to attribute specific natural phenomena to long-term causes, but some effects of recent climate change may already be occurring. Raising sea levels, glacier retreat, Arctic shrinkage, and altered patterns of agriculture are cited as direct consequences, but predictions for secondary and regional effects include extreme 
weather events, an expansion of tropical diseases, changes in the timing of seasonal patterns in ecosystems, and drastic economic impact. Concerns have led to political activism advocating proposals to mitigate, eliminate, or adapt to it.
The 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) includes a summary of the expected effects.
Sea level rise
With increasing average global temperature, the water in the oceans expands in volume, and additional water enters them which had previously been locked up on land in glaciers, for example, the Greenland and the Antarctic ice sheets. An increase of 1.5 to 4.5 °C is estimated to lead to an increase of 15 to 95 cm (IPCC 2001).
The sea level has risen more than 120 metres since the peak of the last ice age about 18,000 years ago. The bulk of that occurred before 6000 years ago. From 3000 years ago to the start of the 19th century, sea level was almost constant, rising at 0.1 to 0.2 mm/yr; since 1900, the level has risen at an average of 1.7 mm/yr [51]; since 1993, satellite altimetry from TOPEX/Poseidon indicates a rate of about 3 mm/yr[51].
In a paper published 18th May 2007[52], the climatologist, James Hansen presented new evidence. George Monbiot, a British journalist, summarises his findings as follows:
"The IPCC predicts that sea levels could rise by as much as 59 cm this century. [53] Hansen’s paper argues that the slow melting of ice sheets the panel expects doesn’t fit the data. The geological record suggests that ice at the poles does not melt in a gradual and linear fashion, but flips suddenly from one state to another. When temperatures increased to 2-3 degrees above today’s level 3.5 million years ago, sea levels rose not by 59 centimetres but by 25 metres. The ice responded immediately to changes in temperature."[54]
Effects on agriculture
For some time it was
hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis, especially in preventing photorespiration, which is responsible for significant destruction of several crops. In Iceland, rising temperatures have made possible the widespread sowing of barley, which was untenable twenty years ago. Some of the warming is due to a local (possibly temporary) effect via ocean currents from the Caribbean, which has also affected fish stocks.[90]
While local benefits may be felt in some regions (such as Siberia), recent evidence is that global yields will be negatively affected. "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades, large-scale experiments have shown" [91].
Moreover, the region likely to be worst affected is Africa, both because its geography makes it particularly vulnerable, and because seventy per cent of the population rely on rain-fed agriculture for their livelihoods. Tanzania's official report on climate change suggests that the areas that usually get two rainfalls in the year will probably get more, and those that get only one rainy season will get far less. The net result is expected to be that 33% less maize—the country's staple crop—will be grown.[92]
Climate change may be one of the causes of the Darfur conflict. The combination of decades of drought, desertification and overpopulation are among the causes of the conflict, because the Arab Baggara nomads searching for water have to take their livestock further south, to land mainly occupied by farming peoples.[93]
"The scale of historical climate change, as recorded in Northern Darfur, is almost unprecedented: the reduction in rainfall has turned millions of hectares of already marginal semi-desert grazing land into desert. The impact of climate change is considered to be directly related to the conflict in the region, as desertification has added significantly to the stress on the livelihoods of pastoralist societies, forcing them to move south to find pasture," the UNEP report states.[94]
In 2007, higher incentives for farmers to grow non-food biofuel crops[95] combined with other factors (such as rising transportation costs, climate change, growing consumer demand in China and India, and population growth)[96] to cause food shortages in Asia, the Middle East, Africa, and Mexico, as well as rising food prices around the globe.[97][98] As of December 2007, 37 countries faced food crises, and 20 had imposed some sort of food-price controls. Some of these shortages resulted in food riots and even deadly stampedes.[99][100][101]
weather events, an expansion of tropical diseases, changes in the timing of seasonal patterns in ecosystems, and drastic economic impact. Concerns have led to political activism advocating proposals to mitigate, eliminate, or adapt to it.The 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) includes a summary of the expected effects.
Sea level rise
With increasing average global temperature, the water in the oceans expands in volume, and additional water enters them which had previously been locked up on land in glaciers, for example, the Greenland and the Antarctic ice sheets. An increase of 1.5 to 4.5 °C is estimated to lead to an increase of 15 to 95 cm (IPCC 2001).
The sea level has risen more than 120 metres since the peak of the last ice age about 18,000 years ago. The bulk of that occurred before 6000 years ago. From 3000 years ago to the start of the 19th century, sea level was almost constant, rising at 0.1 to 0.2 mm/yr; since 1900, the level has risen at an average of 1.7 mm/yr [51]; since 1993, satellite altimetry from TOPEX/Poseidon indicates a rate of about 3 mm/yr[51].In a paper published 18th May 2007[52], the climatologist, James Hansen presented new evidence. George Monbiot, a British journalist, summarises his findings as follows:
"The IPCC predicts that sea levels could rise by as much as 59 cm this century. [53] Hansen’s paper argues that the slow melting of ice sheets the panel expects doesn’t fit the data. The geological record suggests that ice at the poles does not melt in a gradual and linear fashion, but flips suddenly from one state to another. When temperatures increased to 2-3 degrees above today’s level 3.5 million years ago, sea levels rose not by 59 centimetres but by 25 metres. The ice responded immediately to changes in temperature."[54]
Effects on agriculture
For some time it was
hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis, especially in preventing photorespiration, which is responsible for significant destruction of several crops. In Iceland, rising temperatures have made possible the widespread sowing of barley, which was untenable twenty years ago. Some of the warming is due to a local (possibly temporary) effect via ocean currents from the Caribbean, which has also affected fish stocks.[90]While local benefits may be felt in some regions (such as Siberia), recent evidence is that global yields will be negatively affected. "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades, large-scale experiments have shown" [91].
Moreover, the region likely to be worst affected is Africa, both because its geography makes it particularly vulnerable, and because seventy per cent of the population rely on rain-fed agriculture for their livelihoods. Tanzania's official report on climate change suggests that the areas that usually get two rainfalls in the year will probably get more, and those that get only one rainy season will get far less. The net result is expected to be that 33% less maize—the country's staple crop—will be grown.[92]
Climate change may be one of the causes of the Darfur conflict. The combination of decades of drought, desertification and overpopulation are among the causes of the conflict, because the Arab Baggara nomads searching for water have to take their livestock further south, to land mainly occupied by farming peoples.[93]
"The scale of historical climate change, as recorded in Northern Darfur, is almost unprecedented: the reduction in rainfall has turned millions of hectares of already marginal semi-desert grazing land into desert. The impact of climate change is considered to be directly related to the conflict in the region, as desertification has added significantly to the stress on the livelihoods of pastoralist societies, forcing them to move south to find pasture," the UNEP report states.[94]
In 2007, higher incentives for farmers to grow non-food biofuel crops[95] combined with other factors (such as rising transportation costs, climate change, growing consumer demand in China and India, and population growth)[96] to cause food shortages in Asia, the Middle East, Africa, and Mexico, as well as rising food prices around the globe.[97][98] As of December 2007, 37 countries faced food crises, and 20 had imposed some sort of food-price controls. Some of these shortages resulted in food riots and even deadly stampedes.[99][100][101]
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