An explorer looks on in the Pastoruri glacier in Huaraz in November 2006. Ice atop the Cordillera Blanca, the largest glacier chain in the tropics, is melting quickly because of rising temperatures. (Karel Navarro/Associated Press)
Public interest in global warming tends to rise during unseasonably warm weather, or during flashpoint moments like droughts or the collapse of a piece of Antarctic ice shelf in 2006. But the everyday reality of the trend is perhaps even more startling: Eleven of the highest average global annual temperatures recorded since 1861 have come in the past 12 years.
Few issues have galvanized the scientific community like climate change has in the last decade. In 2007, scientists from the United Nations Intergovernmental Panel on Climate Change (IPCC) and the American Association for the Advancement of Science, to name two, have called on world leaders to take action to curb global warming.
But before action can be taken, it helps to have some understanding of the process itself and what part humans have had in its recent development.
How it works
Global warming is the increase over time of the Earth's average surface temperature. Although the term "global warming" may conjure up images of coal-fired plants and yuppies in SUVs, the climate of the Earth has always been in flux, including periods of sustained warming and cooling.
In general, it works like this: The sun shines on the Earth. Most of those rays pass through the Earth's atmosphere, although some are reflected back into space. The surface of the Earth absorbs the energy. Some of that heat energy is re-emitted. The heat reflected from the surface travels back up into the atmosphere.
While on its way back up, this heat can be absorbed by the gases like carbon dioxide and methane, commonly known as greenhouse gases. These are naturally occurring gases as well as those from burning fossil fuels; they trap the heat, warming up the Earth's surface even more. Without naturally occurring greenhouse gases, the Earth would be about 33 C colder than it is, a temperature hostile to human life.
Over thousands of years, changes in atmospheric conditions, such as gas concentrations, and singular events – volcanic eruptions, for instance – have caused climate change. Most of those changes have taken hundreds or thousands of years to play out.
But climatologists now agree that the world appears to be in a sustained, relatively rapid period of warming.
Links to extreme weather
Scientists also suggest global warming will increase the severity – though not the number – of extreme weather events such as El Nino and hurricanes. Many researchers note the increase in temperature coincides with the Industrial Revolution, and the resulting increase of carbon dioxide released into the atmosphere from the burning of fossil fuels and the cutting of forests.
According to research cited by the IPCC in 2007, the Earth's average surface temperature has gone up about 0.6 C since the start of the 20th century. That may not sound like much, and many climatologists will agree that it isn't. But some say it's a sign of things to come.
The IPCC projects an average global temperature increase between 1.8 and 4 C in the next 100 years, with sea levels rising between 18 and 59 cm over the same period.
Warming the Earth doesn't necessarily sound like a bad thing, but climate change, especially when it's more rapid than the Earth normally experiences, could have significant effects on animal, plant and human life.
Climate change will not happen uniformly. A global temperature increase of 1 C could mean some areas will warm by half a degree, some by three or four, and some may actually get cooler. Scientists tend to point to the Arctic and the Antarctic climates as the signs of things to come.
Climate change in those areas could rise as high as six degrees in the winter months, according to some projections from the IPCC. That could lead to glacier melts, rising sea levels and endangered Arctic wildlife.
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