For one leading official, the verdict was beyond doubt: "This day marks the removal from the debate over whether human action has anything to do with climate change," said Achim Steiner, head of the U.N. Environment Program.

For years, alongside the discussion on what to do about global warming has been another, equally vigorous debate -- is it caused by humanity or is it just a natural cycle for the Earth?

According to the most authoritative international scientific body studying the issue, the answer is very much the former.

According to the first report since 1991 by the Intergovernmental Panel on Climate Change (IPCC), released this month, humanity is "very likely" the cause of climate change.

In a 21-page document for policymakers -- the full report will follow later -- the panel of 600 climate experts from 40 countries said it was at least 90% likely that the increase of average global temperatures since the mid-20th century is due to the increase of manmade greenhouse gases in the atmosphere.

The study labeled global warming an "unequivocal" fact, warning that it is likely to continue for centuries with a likely rise in world average temperatures of up to 4C.

In comparison, the world is currently around 5C warmer than during the last ice age.

It predicted global average temperatures will rise by between 1.1C and 6.4C (2-11.5F) by 2100, with a best estimate of between 1.8C and 4C.

The report also forecast a rise of between 18cm and 58cm in sea levels by the end of this century, a figure that could increase by as much as 20cm if the recent melting of polar ice sheets continues.

According to the scientists, greenhouse gases are already responsible for a series of existing phenomena, including fewer cold days, hotter nights, intense heatwaves, floods and heavy rains, droughts and an increase in the strength of hurricanes and tropical storms.

The United States has faced criticism over its climate change policy, notably a refusal to sign up to the Kyoto Protocol on greenhouse gas emissions. President George W. Bush has labeled the protocol ineffective and likely to damage the U.S. economy.

Following the publication of the IPCC report, the White House issued a statement defending the president's record, saying his administration had devoted almost $30 billion to related science, technology, international assistance and incentive programs, "more money than any other country".

Some nations at the frontline of the effects of global warming said that the IPCC report showed such efforts might already be too late.

"The question is, what can we do now? There's very little we can do about arresting the process," said Anote Tong, the president of Kiribati, a group of 33 Pacific coral atolls threatened by rising seas.

For their part, environmental groups argued that the report means the main arguments on global warming are now effectively over.

"If the last IPCC report was a wakeup call, this one is a screaming siren," said Stephanie Tunmore, of Greenpeace.

The IPCC was established in 1988 to study climate change information. The group doesn't do independent research but instead reviews scientific literature from around the world.

The United Nations-sanctioned group was formed by the World Meteorological Organization and U.N. Environment Program.

Is there any doubt about the causes of global warming? What is your opinion?

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Climate change is happening, but I'm not convinced that human activity is its sole cause. I think the fervor around this explanation gets in the way of competent, practical, human response to climatic conditions. So do some of our regulations designed with best intention to protect against possible health risk. Use of grey-water on backyard gardens comes to mind as a frequently prohibited sensible behavior. Our ancestors, my grandparents, and even my family on the farm when I was a child, used many means to maximize available resources. People tended to use more muscle power, stayed more physically fit, and traveled less. There are all behaviors that could have positive effect with climate challenges. Granted, folks maybe lived a decade or two less, but perhaps with less lengthy lingering in weak health over those last years. Where is our natural intelligent and sensible response to what is developing?

Important science was never done by committee, it was done by the courage, commitment and perseverance of individual scientists and small teams. There are dissenting voices from very significant scientists as to the role of man in global warming and just so long as there is one there should be dialogue.

Instead there are witch hunts with a religious fervor that have hijacked good science. Opinions are out there but I've yet to see a true debate. Please let us put aside the issues of globalization and anti-US rhetoric, the agendas of some environmental movements and the 'hollywoodizing' of global warming that appear to cloud the main issue and let's concentrate on real science.

It is very important to understand global warming and then take the steps to solve this problem. We need to use renewal energy sources for industries and vehicles.

Global warming caused a serious rise in sea levels at the height of the Roman Empire. Then it was the change in the albedo (the diffusion of electromagnetic radiation) at the Antarctic that caused most of the melting. Today we are actually still seeing the effect of the 1991 eruption of Mount Pinatubu's ash in the ice.

The reduced vegetation cover and increased dependency on fossil fuels is doing more harm than good.

What we see in the last 100 years is only a part of a second in the Earth's evolution, thus conclusions taken from such observations lack statistical significance.

Climate changes have happened along the Earth's natural history and will do again, no matter what humans have done or will do. To blame industrial activity and the greenhouse effect gases on the present warming cycle is a simplistic approach to a very complex issue, fueled more by human ignorance of all variables involved in climate change and political or philosophical bias.

Hot, cold, hot, cold. In the 70s it was cold here in Phoenix, We had snow in the 60s, 122F in the 90s. Millions of years ago it was hot and cold. Seems the scientists and some of the Democrats are scaring the public half to death.

Right now in Boston we have -16 C (+3 F), near the record low temperatures for this day from 1927 when they suffered -17 C (0 F). The wind chill is -28 C (-18 F). It will be neither too pleasant nor too easy to walk outside, even for the last of the real men.

It's going to be the second coldest day in this century so far in Boston: the coldest day was 1/15/2004 when Al Gore gave a major speech about the global warming in New York City (this frequently observed type of correlation is known as the Gore effect).

According to the Wunderground.com weather Web site, other places have already breeched their daily record lows. For example, -11 C seems enough in Philadelphia, while -13 C (+9 F) in Stamford, Connecticut seems to tie the record from 1982, and they may have already broken it. The flag at Buffalo airport, New York froze in full-wave mode. Recall that the freezing point for flags is rather low. ;-)

The winds from north and northwest seem to go on. They're the main reason why the high on this day will be a record low for January 26th in much of the East Coast.

Cold weather is also moving to North Dakota and is already creating headaches in Utah. Alaska saw record low temperatures today, too. Frigid weather is expected to go on throughout February in Colorado. Denver's January has so far been the seventh coldest January on record.

Western and Central Europe has experienced some snowstorms. For example, 100 flights were cancelled in Prague, 100,000 families in France were cut out of electricity. Snow arrived in Spain, too.

When the winter was balmy, every other journalist would find a climate scientist who would say that this could really be a result of the "climate change". Once the weather becomes frigid, you won't find any articles about the climate. Suddenly it's only the cold facts themselves - cold weather - that is reported. Isn't it interesting that warm weather is related to the climate but cold weather is not?