"Very likely," the IPCC said in a February 2007 report.
The report, based on the work of some 2,500 scientists in more than 130 countries, concluded that humans have caused all or most of the current planetary warming. Human-caused global warming is often called anthropogenic climate change.
Industrialization, deforestation, and pollution have greatly increased atmospheric concentrations of water vapor, carbon dioxide, methane, and nitrous oxide, all greenhouse gases that help trap heat near Earth's surface. (See an interactive feature on how global warming works.)
Humans are pouring carbon dioxide into the atmosphere much faster than plants and oceans can absorb it.
These gases persist in the atmosphere for years, meaning that even if such emissions were eliminated today, it would not immediately stop global warming.
Some experts point out that natural cycles in Earth's orbit can alter the planet's exposure to sunlight, which may explain the current trend. Earth has indeed experienced warming and cooling cycles roughly every hundred thousand years due to these orbital shifts, but such changes have occurred over the span of several centuries. Today's changes have taken place over the past hundred years or less.
Other recent research has suggested that the effects of variations in the sun's output are "negligible" as a factor in warming, but other, more complicated solar mechanisms could possibly play a role.
What's Going to Happen?
A follow-up report by the IPCC released in April 2007 warned that global warming could lead to large-scale food and water shortages and have catastrophic effects on wildlife.
Sea level could rise between 7 and 23 inches (18 to 59 centimeters) by century's end, the IPCC's February 2007 report projects. Rises of just 4 inches (10 centimeters) could flood many South Seas islands and swamp large parts of Southeast Asia.
Some hundred million people live within 3 feet (1 meter) of mean sea level, and much of the world's population is concentrated in vulnerable coastal cities. In the U.S., Louisiana and Florida are especially at risk.
Glaciers around the world could melt, causing sea levels to rise while creating water shortages in regions dependent on runoff for fresh water.
Strong hurricanes, droughts, heat waves, wildfires, and other natural disasters may become commonplace in many parts of the world. The growth of deserts may also cause food shortages in many places.
More than a million species face extinction from disappearing habitat, changing ecosystems, and acidifying oceans.
The ocean's circulation system, known as the ocean conveyor belt, could be permanently altered, causing a mini-ice age in Western Europe and other rapid changes.
At some point in the future, warming could become uncontrollable by creating a so-called positive feedback effect. Rising temperatures could release additional greenhouse gases by unlocking methane in permafrost and undersea deposits, freeing carbon trapped in sea ice, and causing increased evaporation of water.
What is Climategate?
In late November 2009, hackers unearthed hundreds of emails at the U.K.'s University of East Anglia that exposed private conversations among top-level British and U.S. climate scientists discussing whether certain data should be released to the public.
The email exchanges also refer to statistical tricks used to illustrate climate change? trends, and call climate skeptics idiots, according to the New York Times.
One such trick was used to create the well-known hockey-stick graph, which shows a sharp uptick in temperature increases during the 20th century. Former U.S vice president Al Gore relied heavily on the graph as evidence of human-caused climate change in the documentary An Inconvenient Truth.
The data used for this graph come from two sources: thermostat readings and tree-ring samples.
While thermostat readings have consistently shown a temperature rise over the past hundred years, tree-ring samples show temperature increases stalling around 1960.
On the hockey-stick graph, thermostat-only data is grafted onto data that incorporates both thermostat and tree-ring readings, essentially presenting a seamless picture of two different data sets, the hacked emails revealed.
But scientists argue that dropping the tree-ring data was no secret and has been written about in the scientific literature for years.
Climate change skeptics have heralded the emails as an attempt to fool the public, according to the Times.
Yet climate scientists maintain that these controversial points are small blips that are inevitable in scientific research, and that the evidence for human-induced climate change is much broader and still widely accepted.
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