Arguments about climate change
At the UN climate summit in Copenhagen, 192 governments are aiming for a new global agreement to constrain greenhouse gas emissions and curb human-induced climate change.
But some commentators are unconvinced that rising greenhouse gas emissions are the cause of modern-day warming. Either they say the world is not actually getting warmer or that a new treaty would hurt economic growth and well-being.
So what are their arguments, and how are they countered by scientists who assert that greenhouse gases, produced by human activity, are the cause of modern-day climate change?
1. IS TEMPERATURE OF EARTH GETTING WARMER?
Most long-term data comes from weather stations
Instruments show there has been some warming of the Earth's surface since 1979, but the actual value is subject to large errors. Most long-term data comes from surface weather stations. Many of these are in urban centres which have been expanding and using more energy. When these stations observe a temperature rise, they are simply measuring the "urban heat island effect". In addition, coverage is patchy, with some regions of the world almost devoid of instruments. Data going back further than a century or two is derived from "proxy" indicators such as tree-rings and stalactites which, again, are subject to large errors.
Warming is unequivocal. Ocean measurements, decreases in snow cover, reductions in Arctic sea ice, longer growing seasons, balloon measurements, boreholes and satellites all show results consistent with records from surface weather stations. The urban heat island effect is real but small; and it has been studied and corrected for. Analyses by Nasa , for example, use only rural stations to calculate trends. Research has shown that if you analyse long-term global temperature rise for windy days and calm days separately, there is no difference. If the urban heat island effect were large, you would expect to see more warming on calm days when more of the heat stays in the city. Furthermore, the pattern of warming globally doesn't resemble the pattern of urbanisation, with the greatest warming seen in the Arctic and northern high latitudes. Globally, there is a warming trend of about 0.8C since 1900, more than half of which has occurred since 1979.
2. THE AVERAGE TEMPERATURE HAS STOPPED RISING
Weather balloons or balloon sondes gather and transmit data
Since 1998 - more than a decade - the record, as determined by observations from satellites and balloon radiosondes, shows no discernible warming.
The year 1998 was exceptionally warm because of a strong El Nino event, while 2008 was unusually cold because of La Nina conditions. Variability from year to year is expected, and picking a specific warm year to start an analysis (or a cold one to end with) is "cherry-picking". If you start in 1997 or 1999 you will see a sharp rise. Furthermore, while the UK Met Office regards 1998 as the hottest year yet, Nasa thinks it was 2005 (they use the same data but interpret it differently). According to the Met Office, the 10 warmest years in the modern record have all occurred since 1997.
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