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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 extinctions are predicted as habitats change faster than species can adapt, and the World Health Organization has warned that the health of millions could be threatened by increases in malaria, water-borne disease and malnutrition.
What Don’t We Know
We don’t know exactly what proportion of the observed warming is caused by human activities or what the knock-on effects of the warming will be.
The precise relationship between concentrations of carbon dioxide (and other greenhouse gases) and temperature rise is not known, which is one reason why there is such uncertainty in projections Of temperature increase.
Global warming will cause some changes which will speed up further warming, such as the release of large quantities of the greenhouse gas methane as permafrost(永久冻结带) melts.
Other factors may mitigate(减轻) warming. It is possible that plants may take more carbon dioxide from the atmosphere as their growth speeds up in warmer conditions, though this remains in doubt.
Scientists are not sure how the complex balance between these positive and negative feedback effects will play out.
What about the Skeptics
Global warming "skeptics" fall into three broad camps:
· those who maintain temperatures are not rising;
· those who accept the climate is changing but suspect it is largely down to natural variation;
· those who accept the theory of human-induced warming but say it is not worth tackling as other global problems are more pressing.
Nevertheless, there is a growing scientific consensus (舆论) that, even on top of the natural variability of the climate, something out of the ordinary is happening and humans are to blame.
A scientific report commissioned by the U.S. government has concluded there is "clear evidence" of climate change caused by human activities. The report, from the federal Climate Change Science Program, said trends seen over the last 50 years "cannot be explained by natural processes alone".
It found that temperatures have increased in the lower atmosphere as well as at the Earth’s surface. However, scientists involved in the report say better data is badly needed.
Observations down the years have suggested that the troposphere, the lower atmosphere, is not warming up, despite evidence that temperatures at the Earth’s surface are rising. This goes against generally accepted tenets (原则) of atmospheric physics, and has been used by "climate skeptics" as proof that there is no real warming.
The new report, Temperature Trends in the Lower Atmosphere, re-analyses the atmospheric data and concludes that tropospheric temperatures are rising. This means, it says, that the impact of human activities upon the global climate is clear. "The observed patterns of change over the past 50 years cannot be explained by natural processes alone, nor by the effect of short-lived atmospheric constituents (such as aerosols and tropospheric ozone ) alone," it says.
Holes in the Data
But there are some big uncertainties which still need resolving. Globally, the report concludes, tropospheric temperatures have risen by 0.10 and 0.20℃ per decade since 1979, when satellite data became generally available. The wide gap between the two figures means, says the report, that "...it is not clear whether the troposphere has warmed more or less than the surface."
Peter Thorne, of the U.K. Meteorological Office, who contributed to the report, ascribes this uncertainty to poor data "Basically, we’ve not been observing the atmosphere with climate in mind," he told the BBC News website. "We’re looking for very small signals in data that are very noisy. From one day to the next, the temperature can change by 10℃, but we’re looking for a signal in the order of 0.1℃ per decade."
The report shows up a particular discrepancy concerning the tropics, where it concludes that temperatures are rising by between 0.02 and 0.19℃ per decade, a big margin of error.
Additionally, the majority of the available datasets show more warming at the surface than in the troposphere, whereas most models predict the opposite.
For Fred Singer, of the Science and Environmental Policy Project, a prominent climate skeptic, this suggests that the report’s support for the concept of human-induced climate change is spin rather than substance. "The basic data in the report is quite OK," he said, "but the interpretation that’s been given is different from what the data says."
No Inconsistency
Measuring tropospheric temperatures is far from a simple business. Satellites sense the "average" temperature of the air between themselves and the Earth, largely blind to what is happening at different altitudes.
To compound matters, instruments on board satellites degrade over time, orbits subtly drift, and calibration (校准) between different satellites may be poor.
Weather balloons (or radiosondes) take real-time measurements as they ascend, but scientists can never assess instruments afterwards; they are "fire-and-forget" equipment.
Correcting for all these potential sources of error is a sensitive and time-consuming process.
The report makes clear recommendations for the kind of infrastructure needed to produce higher-quality data and resolve remaining uncertainties. Key recommendations include:
· establishing reference sites for radiosonde measurements which would increase consistency between datasets;
· making sure the operating periods of satellites overlap so instruments can be cross-calibrated;
· observing factors such as wind, clouds, and humidity in the troposphere to make sure they are consistent with temperature data.
Such observations could produce an unambiguous picture of tropospheric warming, removing discrepancies over the scientific picture and providing better data which can be used to improve computer models. It may take large bodies of water and ice decades to respond to changes in temperature.