Arctic ice is breaking up due to warmer temperatures.
The debates about global warming can be broken down into three issues. First, to what extent the climate is getting warmer. Second, whether this is a long-term trend. And third, whether any of the warming is caused by human activity. Proponents of all three propositions include many reputable scientists, but opponents have been able to pinpoint some mistakes in the data and have successfully attacked the credibility of some pro-global warming research institutes.
History
Temperature data gathered from weather stations around the globe started showing a warming trend in the 1990s. By 2000, scientists such as Dr. James Hansen and politicians such as Al Gore were sounding the alarm that immediate action was necessary to avoid imminent disaster. Steve McIntyre, a Canadian mathematician, studied the data used by Hansen and Gore and found inconsistencies. Subsequent revisions in the data showed that global warming was not going to rise rapidly as per the "hockey stick" graph. Pro-global warming scientists lost more credibility when leaked emails showed that they were preventing global warming opponents from publishing research papers and deleting data when it didn't agree with their views, the so-called "hide the decline" in global temperatures measured more recently.
Existence of Global Warming
The idea of global warming is based on the "greenhouse effect," which proposes that an increased level of carbon dioxide in the atmosphere will trap more of the sun's heat near the Earth's surface, raising temperatures. Data from weather stations in the U.S. and Europe show rising temperatures from the 1960s to 2007, and the level of carbon dioxide in the atmosphere has increased substantially. However, it was just as hot in the U.S. in the 1930s and the temperature data for the years since 2007 is inconclusive. The consensus that global warming exists is less strong than it was.
Ice Studies
The weather station data is not absolute because many stations were originally in rural areas and are now in cities, subject to local heating. Scientists such as Dr. Hansen therefore adjusted the data, but the adjustment method is subjective. Measurements of glacial, arctic, antarctic and Greenland ice tend to point toward substantial global warming, but that data is hard to quantify. If there is still some agreement on the existence of global warming, the amount at present and the predictions of future warming diverge widely. The fear of imminent disaster evident at the time of the Kyoto accord on climate change response is no longer present.
Human Activity and Climate Change
A key question for the climate change debate is whether any temperature rise is the result of human activity. This is important because, if humans are not responsible for global warming, changes in human behavior will not solve the problem. Alternative theories point to sun spots, changes in solar output or natural climate cycles. Some studies have demonstrated a good match between solar activity and global warming. Others show that the Earth was warmer in prehistoric times, before any human activity. These studies are ongoing, but none have yet proposed a comprehensive theory which can rival that of global warming as a result of the greenhouse effect from carbon dioxide produced by human industrialization.
