When confronted with the power of a hurricane, a blizzard or the intensity of a heat wave, the idea that humans could control or even influence the elements seems ridiculous, even arrogant. However, thanks to the development of industrial societies and the accompanying use of fossil fuels, that farfetched idea is becoming a reality.
Concentrations of carbon dioxide (CO2), methane (CH4) and other gases (all of which add to the greenhouse effect that creates a warming blanket over the Earth) have increased sharply over the last century -- by 35 percent for CO2, and by more than 100 percent for CH4. This is overwhelmingly due to human industrial and agricultural activity.
The conclusions from theoretical studies, direct observations and models are all quite clear and show that these rising concentrations have led to a warming of the planet over the last century. This warming can be seen in the weather station records, ocean temperatures, disappearing mountain glaciers, melting permafrost and retreating Arctic ice, and is beginning to be felt in the statistics of extreme events.
There are other factors in climate change -- such as increases in particulate pollution, changes in the sun or volcanic eruptions. However, even when you take all these into account the temperature rise of the last few decades can only be understood if the warming influence of rising greenhouse gases is factored in. It is this unfortunate fact that is at the heart of the global warming crisis.
It is true that the changes we have seen so far (thankfully) have not been globally disastrous. But three factors mean that our expectation for the future is worse. First, CO2 put into the atmosphere now will continue to have a greenhouse effect for decades and centuries to come. Second, emissions of CO2, which are so closely linked to energy production, continue to rise at a rapid rate (around 2 percent a year). Finally, at concentrations which we are very likely to attain in the coming decades, it will become increasingly difficult to avoid temperature rises of 5 or more degrees Fahrenheit in the century to come.
Gavin Schmidt is a climate scientist at the NASA Goddard Institute for Space Studies. Scientific American magazine cited him as one of the 50 Research Leaders of 2004. For more information about the debate series, go to www.iq2us.org
What real difference do a few degrees of global warming make? That is a fair question, but to put it into perspective the global difference between the ice age and today -- when a mile or so of ice covered most of New York -- was only 9 degrees Fahrenheit. In addition, the last time the planet was likely more than a degree or so warmer than today, sea levels were around 20 feet higher due to significant melting of the Greenland and Antarctic ice sheets.
Regardless of future actions, some continued warming is inevitable since the planet is still playing catch-up with what has already been added to the atmosphere. Changing emission patterns and technology will take decades and so we are going to be adding significantly to the warming "in the pipeline" over coming decades. We don't know exactly what will happen. We don't know exactly where the thresholds are. But the more warming there is, the more chances for something surprising and possibly dangerous to occur.