1、 threats to endangered species and unique systems, damages from extreme climate events, effects that fall most heavily on developing countries and the poor within countries, global aggregate impacts （i.e., various measurements of total social, economic and ecological impacts）,34 and large-scale high
2、-impact events. The above burning embers diagram was produced by the IPCC in 2001. A later revision of the diagram, published in 2009, but not approved by the IPCC, shows increased risks.5This article is about the effects of global warming and climate change. The effects, or impacts, of climate chan
3、ge may be physical, ecological, social or economic. Evidence of observed climate change includes the instrumental temperature record, rising sea levels, and decreased snow cover in the Northern Hemisphere.6 According to the Intergovernmental Panel on Climate Change （IPCC, 2007a:10）, most of the obse
4、rved increase in global average temperatures since the mid-20th century is very likely due to the observed increase in human greenhouse gas concentrations. It is predicted that future climate changes will include further global warming （i.e., an upward trend in global mean temperature）, sea level ri
5、se, and a probable increase in the frequency of some extreme weather events. Signatories of the United Nations Framework Convention on Climate Change have agreed to implement policies designed to reduce their emissions of greenhouse gases.DefinitionsIn this article, the phrase “climate change” is us
6、ed to describe a change in the climate, measured in terms of its statistical properties, e.g., the global mean surface temperature.7 In this context, “climate” is taken to mean the average weather. Climate can change over period of time ranging from months to thousands or millions of years. The clas
7、sical time period is 30 years, as defined by the World Meteorological Organization. The climate change referred to may be due to natural causes, e.g., changes in the suns output, or due to human activities, e.g., changing the composition of the atmosphere.8 Any human-induced changes in climate will
8、occur against the “background” of natural climatic variations （see attribution of recent climate change for more information）.In this article, the phrase “global warming” refers to the change in the Earths global average surface temperature.9 Measurements show a global temperature increase of 1.4F （
9、0.78C） between the years 1900 and 2005. Global warming is closely associated with a broad spectrum of other climate changes, such as increases in the frequency of intense rainfall, decreases in snow cover and sea ice, more frequent and intense heat waves, rising sea levels, and widespread ocean acid
10、ification.10Temperature changesGlobal mean surface temperature difference from the average for 18802009This article breaks down some of the impacts of climate change according to different levels of future global warming. This way of describing impacts has, for instance, been used in the IPCCs Asses
11、sment Reports on climate change.11 The instrumental temperature record shows global warming of around 0.6 C over the entire 20th century.12 The future level of global warming is uncertain, but a wide range of estimates （projections） have been made.13 The IPCCs SRES scenarios have been frequently use
12、d to make projections of future climate change.14:22-24 Climate models using the six SRES marker scenarios suggest future warming of 1.1 to 6.4 C by the end of the 21st century （above average global temperatures over the 1980 to 1999 time period）.15 The range in temperature projections partly reflec
13、ts different projections of future social and economic development （e.g., economic growth, population level, energy policies）, which in turn affects projections of greenhouse gas （GHG） emissions. The range also reflects uncertainty in the response of the climate system to past and future GHG emissio
14、ns （measured by the climate sensitivity）.The projected rate of warming under these scenarios would very likely be without precedent during at least the last 10,000 years （see footnote 1）.16 The most recent warm period comparable to these projections was the mid-Pliocene, around 3 million years ago.1
15、7 At that time, models suggest that mean global temperatures were about 23 C warmer than pre-industrial temperatures.18Physical impactsMain article: Physical impacts of climate changeKey climate indicators that show global warming.19:2-3Working Group Is contribution to the IPCC Fourth Assessment Rep
16、ort, published in 2007, concluded that warming of the climate system was unequivocal.20 This was based on the consistency of evidence across a range of observed changes, including increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average se
17、a level.21Human activities have contributed to a number of the observed changes in climate.22 This contribution has principally been through the burning of fossil fuels, which has led to an increase in the concentration of GHGs in the atmosphere.23 Another human influence on the climate are sulfur d
18、ioxide emissions, which are a precursor to the formation of sulfate aerosols in the atmosphere.24Human-induced warming could potentially lead to some impacts that are abrupt or irreversible （see the section on Abrupt or irreversible changes）.1525 The probability of warming having unforeseen conseque
19、nces increases with the rate, magnitude, and duration of climate change.26Radiative forcingThe effect of human activities on the climate system can be measured by radiative forcing: Energy is constantly flowing into the atmosphere in the form of sunlight that always shines on half of the Earths surf
20、ace.27 Some of this sunlight is reflected back to space and the rest is absorbed by the planet. Some energy from the Earth is also radiated back out into space as invisible infrared light. Radiative forcing is a measure of the energy flowing into the Earth-atmosphere system, minus the energy flowing
21、 out. A positive radiative forcing will tend to warm the climate, while a negative forcing will tend to cool the climate.28 Anthopogenic forcing （i.e., the radiative forcing due to human activites） was estimated to have been positive in the year 2005.28 This is relative to the estimated forcings at
22、the start of the industrial era, taken as the year 1750. Anthropogenic forcing of the climate has likely to contributed to a number of observed changes, including sea level rise, changes in climate extremes （such as warm and cold days）, declines in Arctic sea ice extent, and to glacier retreat.22Eff
23、ects on weatherObservations show that there have been changes in weather.29 As climate changes, the probabilities of certain types of weather events are affected.Changes have been observed in the amount, intensity, frequency, and type of precipitation.14:18 Widespread increases in heavy precipitatio
24、n have occurred, even in places where total rain amounts have decreased. IPCC （2007d） concluded that human influences had, more likely than not （greater than 50% probability, based on expert judgement）,30 led to an increase in the frequency of heavy precipitation events.31 Projections of future chan
25、ges in precipitation show overall increases in the global average, but with substantial shifts in where and how precipitation falls.14:24 Climate models tend to project increasing precipitation at high latitudes and in the tropics （e.g., the south-east monsoon region and over the tropical Pacific） a
26、nd decreasing precipitation in the sub-tropics （e.g., over much of North Africa and the northern Sahara）.32Evidence suggests that, since the 1970s, there have been substantial increases in the intensity and duration of tropical storms and hurricanes.33 Models project a general tendency for more inte
27、nse but fewer storms outside the tropics.14:24Accumulated cyclone energy in the Atlantic Ocean and the sea surface temperature difference which influences such, measured by the U.S. NOAA.Extreme weatherSee also: Extreme weather, Tropical cyclone#Global warming,and List of Atlantic hurricane recordsS
28、ince the late 20th century, changes have been observed in the trends of some extreme weather and climate events, e.g., heat waves.34 Human activities have, with varying degrees of confidence, contributed to some of these observed trends. Projections for the 21st century suggest continuing changes in
29、 trends for some extreme events. Solomon et al. （2007）, for example, projected the following likely （greater than 66% probability, based on expert judgement）30 changes:34 an increase in the areas affected by drought; increased tropical cyclone activity; and increased incidence of extreme high sea le
30、vel （excluding tsunamis）.Projected changes in extreme events will have predominantly adverse impacts on ecosystems and human society.3536Glacier retreat and disappearance Retreat of glaciers since 1850A map of the change in thickness of mountain glaciers since 1970. Thinning in orange and red, thick
31、ening in blue.IPCC （2007a:5） found that, on average, mountain glaciers and snow cover had decreased in both the northern and southern hemispheres.20 This widespread decrease in glaciers and ice caps had contributed to observed sea level rise. With very high or high confidence （see footnote 2）, IPCC （2007b37
