The majority of climate scientists agree that human activity, especially the burning of fossil fuels, increases the levels of greenhouse gases like carbon dioxide, or CO2, in the earth's atmosphere.
Greenhouse gases trap the sun's energy, causing temperatures to rise. Scientists have recorded an average global temperature increase of about one degree Celsius during the past century. If greenhouse gas levels continue to rise, many scientists estimate that global temperatures could increase by as much as six degrees this century.
Skeptics of global warming contend that there is no way to measure the impact of human activity on climate. Some argue that no one knows how much warming will occur and how it might affect the earth. Others complain that they don't receive enough grant money to support their research because the issue has become highly politicized. But many scientists accuse energy corporations of funding research by global warming skeptics to argue against curbing fossil fuel use.
Some experts suggest that global warming may be part of natural climate cycles that humans can do little about. Bill Gray of Colorado State University's Atmospheric Science Department says humans are not to blame for global warming.
"This is nature operating on a very large scale with the oceans' circulations. And I don't think humans can do much about climate change. From everything I've known in over 50 years of study, I am convinced that the global temperature changes we've seen over the past 30 years are primarily natural and primarily due to the ocean circulation variations and they're not due to human-induced greenhouse gases," says Grayand adds that throughout its history, the earth has experienced warming and cooling cycles that typically last 30-to-35 years.
These short-lived cycles, argues Fred Singer, a University of Virginia environmental scientist, have come and gone without incident and should not be cause for alarm.
"We had an extremely strong warming between 1920 and 1940. So we can use that as a kind of a guide to see what would happen if the climate warms very strongly. And the answer is: not much. We also had a warm period lasting for a much longer time about a 1,000 years ago, when the climate was warm enough so that the Vikings could establish farms in Greenland and actually grow grapes in the north of England. And as best as we can tell, it didn't cause any calamities."
The Pace of Climate Change
But many scientists are alarmed by the pace of the current global warming trend. Among them is George Woodwell, Director Emeritus of the Massachusetts-based Woods Hole Research Center, who dismisses historical climate parallels.
"This is totally new to civilization and it's new to the earth. The changes have probably never happened at this speed in the past. This is the greatest environmental change in all of human history. Although there have been big environmental changes in the past, [they have] never [occurred] at this rate and never involving 6.5 billion people," says Woodwell.
According to the United Nations, carbon dioxide in the atmosphere has increased nearly 30 percent since 1750, more than at any time during the last 650,000 years.
Many experts say the acceleration of global warming is due to increased levels of human-produced greenhouse gasses in the atmosphere. Skeptics say it is difficult to distinguish between natural CO2 levels and industrial emissions.
But Greenpeace U.S.A. Research Director Kurt Davis argues there is enough evidence to show that human development is causing greenhouse gases to accumulate in the atmosphere.
"We know from measurements that have been taken for the last 50 years from the top of the volcanoes in Hawaii, the cleanest and most remote air on earth, that the atmosphere is increasing in its concentration of carbon dioxide. This is indisputable. The evidence has been gathered from so many different sources on the rising levels [of CO2] and the history of the planet vis-à-vis the levels that existed thousands of years ago in ice cores, in tree rings, in coral reef fossils," says Davis.
The Two Faces of CO2
Many scientists expect CO2 concentrations in the atmosphere to double during this century. But how that will affect the earth's climate remains to be seen, says Roy Spencer, principle research scientist at the University of Alabama in Huntsville.
"Increasing the CO2 content of the atmosphere has both advantages and disadvantages. Fertilization of vegetation is one of the advantages. And science can't really say anything about whether we are putting too much CO2 into the atmosphere. That's a judgment that needs economic and societal input in order to determine whether it's a good thing or not," says Spencer.
While plants may thrive on more carbon dioxide, some scientists say the effect would be short-lived because plants would not get enough nutrients from the soil to sustain long-term growth or consume enough CO2 to lower global temperatures.
George Woodwell of the Woods Hole Research Center says global warming will make continental farmland hot and dry, thereby altering established agricultural patterns. It would melt polar ice and raise sea levels, cause more forest fires, and increase the frequency and intensity of devastating storms.
"The increased temperature in high latitudes is increasing the rate of decay of organic matter in the soil, dumping more carbon into the atmosphere and raising the possibility that the warming of the earth will quickly swing out of control and change the earth in ways that will really reduce the human population," says Woodwell.
Regardless of whether the two sides of the global warming debate arrive at a consensus on the dangers of climate change and the extent to which humans are the cause, governments, environmental groups and some energy corporations around the world are already taking action to curb greenhouse gas emissions in an effort to stabilize the world climate.
This story was first broadcast on the English news program,VOA News Now. For other Focus reports click here.