Searing summer temperatures are shattering records across much of the northern hemisphere. Some European nuclear power plants have cut output because river water used to cool reactors is too warm. Forest fires are breaking out in Europe and the United States. Are these signs of global warming?
Scientists say no single weather event can be attributed to warming. But they say those incidents are consistent with it and may worsen unless humans stop pumping greenhouse gases into the atmosphere. Skeptics argue that global warming is part of the natural climate cycle. They say whatever humans contribute to it will not cause it to be irreversible. VOA's David McAlary examines the issues.
In the past year, several scientific reports have alerted the world to increasing glacier melting in Alaska, Greenland, and Antarctica, reducing habitat for polar bears and other forms of life.
The habitat for beetles that ravage trees has expanded from the normally warm U.S. southwest into the evergreen forests of British Columbia.
Warmer tropical waters seem to be bleaching coral reefs.
The general scientific view is that these changes are caused by a heat-trapping blanket of carbon dioxide and other gases in the atmosphere emitted by coal, natural gas, and gasoline burning.
Richard Somerville of the Scripps Institution of Oceanography in San Diego says the current warming trend is different from ones that have occurred earlier in Earth's history. "We know enough now to be able to say that the current warming, the warming that we've seen in the last decades of the 20th century, is primarily due to human causes."
The United Nations Intergovernmental Panel on Climate Change says the atmosphere has 30 percent more carbon dioxide than a century ago and Earth's average surface temperature has risen nearly one degree Celsius in that time. The group warns that it can be expected to go up much more in the next 100 years -- between one-and-a-half and nearly six degrees.
The panel says this could mean a sea level rise of up to one meter by the end of this century, possibly engulfing coastal regions and island countries.
U.S. space agency climate expert James Hansen was one of the first scientists to warn of global warming in the 1980s. He says the world is nearing the time when it cannot be reversed. "We're getting very close to a tipping point in the climate system. If we don't get out of our business-as-usual scenarios and begin to reduce our greenhouse gas emissions, we are going to get big climate change."
But scientists say arresting global warming is a daunting challenge. For one thing, carbon dioxide has a lifetime of 50 to 100 years in the atmosphere. Rutgers University climate researcher Anthony Broccoli says ocean warming compounds the problem. "Heat is going into the ocean and gradually the effect of that heat going into the ocean would be to make the climate warmer, even if we stopped raising atmospheric CO-2 levels today."
The 1997 Kyoto Protocol commits more than 120 signing nations to limiting greenhouse gas emissions to 1990 levels. The United States is not part of the agreement because President Bush withdrew the country from it soon after taking office in 2001.
This was the correct move, according to Myron Ebell of the Competitive Enterprise Institute, a Washington public policy research group promoting government deregulation. "There is just so much exaggeration involved in these claims about the impacts of climate change."
Ebell does not believe global warming is a serious threat. But he says even if it were, the Kyoto Protocol is bad politics. He believes restricting energy use to reduce greenhouse gas emissions will hurt national economies. "All of this effort is going for nothing. The reason I believe that is because the world cannot afford to go on the kind of energy diet that the Kyoto Protocol is the first step of."
Richard Somerville at the Scripps Institution of Oceanography agrees that the Kyoto Protocol is flawed. But he believes the flaw is its insufficient limits on greenhouse gas emissions. He says they will make only a negligible difference, but argues that the accord is better than nothing. "Kyoto keeps the issue alive. One of the advantages of signing Kyoto is it gets you to the point where you can look past Kyoto, where the nations of the world can come together with the experience of Kyoto, which involves large industries, and decide what does it make sense to try next?"
But opponents of the Kyoto accord say the next step should be nature's. Myron Ebell says glaciers have been melting since the end of the last ice age about 10,000 years ago, yet people have adapted. He argues that global warming has benefits, such as a longer growing season and hardier crops.
"Carbon dioxide is necessary for plants to photosynthesize, so if there is more carbon dioxide in the atmosphere, plants should grow more quickly, more vigorously and they should be more resistant to things like drought," says Ebell.
Rutgers University's Anthony Broccoli disagrees that global warming will bring about an overall benefit. Yet he is also not willing to say the world will become uninhabitable -- just not the same. "Based on our best projections, we would find it to be a very different world."