Climate Change Scientists predict increasing droughts, floods and extreme weather and say there is growing evidence that human activities are to blame. What Is Climate Change? The planet's climate is constantly changing. The global average temperature is currently in the region of 15℃. Geological and other evidence suggests that, in the past, this average may have been as high as 27℃ and as low as 7℃. But scientists are concerned that the natural fluctuation (波动) has been overtaken by a rapid human-induced warming that has serious implications for the stability of the climate on which much life on the planet depends. What Is the 'Greenhouse Effect'? The greenhouse effect refers to the role played by gases which effectively trap energy from the Sun in the Earth's atmosphere. Without them, the planet would be too cold to sustain life as we know it. The most important of these gases in the natural greenhouse effect is water vapor, but concentrations of that are changing little and it plays almost no role in modern human-induced greenhouse warming. Other greenhouse gases include carbon dioxide, methane (甲烷) and nitrous (含氮的) oxide, which arc released by modern industry, agriculture and the burning of fossil fuels. Their concentration in the atmosphere is increasing-the concentration of carbon dioxide has risen by more than 30% since 1800. The majority of climate scientists accept the theory that an increase in these gases will cause a rise in the Earth's temperature. What Is the Evidence of Warming? Temperature records go back to the late 19th century and show that the global average temperature increased by about 0.6℃ in the 20th century. Sea levels have risen 10～20 cm-thought to be caused mainly by the expansion of warming oceans. Most glaciers in temperate regions of the world and along the Antarctic Peninsula are in retreat, and records show Arctic sea-ice has thinned by 40% in recent decades in summer and autumn. There are anomalies (异常) however-parts of the Antarctic appear to be getting colder, and there are discrepancies between trends in surface temperatures and those in the troposphere(对流层) (the lower portion of the atmosphere). How Much Will Temperatures Rise? If nothing is done to reduce emissions, current climate models predict a global temperature increase of 1.4～5.8℃ by 2100. Even if we cut greenhouse gas emissions dramatically now, scientists say the effects would continue because parts of the climate system, particularly large bodies of water and ice, can take hundreds of years to respond to changes in temperature. It also takes greenhouse gases in the atmosphere decades to break down. It is possible that we have already irrevocably(不可撤回地) committed the Greenland ice sheet to melting, which would cause an estimated 7m rise in sea level. There are also indications that the west Antarctic ice sheet may have begun to melt, though scientists caution further research is necessary. How Will the Weather Change? Globally, we can expect more extreme weather events, with heat waves becoming hotter and more frequent. Scientists predict more rainfall overall, but say the risk of drought in inland areas during hot summers will increase. More flooding is expected from storms and rising sea levels. There are, however, likely to be very strong regional variations in these patterns, and these are difficult to predict. What Will the Effects Be? The potential impact is huge, with predicted freshwater shortages, sweeping changes in food production conditions, and increases in deaths from floods, storms, heat waves and droughts. Poorer countries, which are least equipped to deal with rapid change, will suffer most. Plant and animal ex