We all agree. Climate change is real, and we humans are its chief cause. Yet even now, few people fully understand the gravity of the threat, or its immediacy.

Certainly I did not. It was only after I took a recent fact-finding "eco tour" of vulnerable regions that I realized the true magnitude of the danger. I have always considered global warming to be a matter of utmost urgency. Now I believe we are on the verge of a catastrophe if we do not act.

Last week, in Antarctica, I saw extraordinarily dramatic landscapes, rare and wonderful. It was the most vivid experience of my life. Yet it was deeply disturbing, as well, for I could see this world changing. The age-old ice is melting, far faster than we think.

You have heard how the famous Larsen ice shelf collapsed and disappeared five years ago. A giant slab of ice 87-kilometers long - the size of some small countries - vanished in less than three weeks. What if this "Larsen effect" were to repeat itself on a vastly greater scale?

At the Chilean research base on King George Island, scientists told me that the entire West Antarctic Ice Sheet is at risk. Like Larsen, it is a continuous sheath of floating ice, comprising nearly one-fifth of the continent.

If it broke up, sea levels could rise by six meters. Think of the effect on the coastlines and cities: New York, Mumbai and Shanghai, not to mention small island nations. It may not happen for 100 years - or it could happen in 10. We simply do not know. But when it happens, it could occur quickly, almost overnight.

It sounds like the script of a disaster movie. But this is science, not science-fiction.

Dr. Gino Casassa, a leading Chilean glaciologist with the Chilean Center for Scientific Studies and a member of the Intergovernmental Panel on Climate Change that recently shared a Nobel Prize, worries particularly about the Antarctic Peninsula - a finger of land on the northern coast that he designates as one of three global "hot spots," along with Central Asia and Greenland.

Temperatures there are rising 10 times faster than the global average, he has found. Glaciers are visibly retreating. Grasses are taking root in Antarctica's barren soil, including one used on American golf courses. In the summer, it rains rather than snows increasingly often. A decade ago, Dr. Casassa was a skeptic on climate change. Today, he fears a calamity.

I am not scare-mongering. But I believe we are nearing a tipping point. These are signs. I saw them everywhere I visited.

In Chile, researchers told me that roughly half of the 120 glaciers they monitor are shrinking, at rates twice as fast as a decade or two ago. These include the glaciers in the mountains outside the capital, Santiago, that provide fresh water for six million residents. To the north, increasing drought threatens the country's mining industry, a mainstay of the economy, as well as agriculture and hydroelectric power.

I spent a day in perhaps the world's most magnificent national park, Torres del Paine. Like Antarctica, it was beautiful, pristine and majestic - and equally troubling. The snows of the Andes are also melting faster than we think. I flew over Grey glacier, a virtual ice sea framed by towering alpine peaks. In 1985, it retreated a full three kilometers in little more than two weeks. Yet another demonstration of the abrupt, unpredictable and potentially devastating Larsen effect.

I ended my travels under a great Samaumeira tree on the island of Combu, not far from Belem in the Amazon river delta. This was the heart of the fabled "lungs of the earth," the tropical rain forest prey to the de-forestation and land degradation that accounts for an estimated 21 percent of global carbon emissions.

Scientists say that climate change could turn the eastern Amazon into savannah within decades. My own itinerary had to be changed at the last moment because a tributary of the Amazon I planned to visit, near the port of Santarem, had run dry from drought.

All this might have been discouraging. Yet I left Brazil immensely heartened. Largely unnoticed by the rest of the world, Brazil has transformed itself into a quiet green giant - a leader in the fight against global warming. Over the past two years, it has cut deforestation in the Amazon by half. Vast tracts of jungle have been placed under federal protection.

In Brasilia, President Luis Inácio Lula da Silva assured me that the Amazon and its immense treasure chest of biodiversity was the common heritage of mankind and would be preserved. Brazil leads the world in renewable energy. It is one of only a few nations to successfully produce biofuels on a large scale. Yes, controversy surrounds the program. Some fear that land currently used to grow food will be converted to fuel. Others worry that forests will be cut to make way for biomass plantations.

It is up to governments to balance social costs and benefits. But the important point is that Brazil is acting. Its efforts to combat global warming are worth watching, as lessons for us all.

For too long, we have underestimated the urgency of climate change. It is time to wake up. Last month, the UN Environment Program released its GEO-4 report, calling for "drastic steps" in the face of a challenge that "may threaten humanity's survival." This weekend in Valencia, Spain, I will present the latest synthesis report of the Intergovernmental Panel on Climate Change. It is sobering reading.

Yet its conclusions are encouraging. The over-arching message: we can beat this. There are real and affordable ways to deal with climate change.

A report last week from the International Energy Agency was also cautiously upbeat. Global energy demand is rising more quickly than most estimates suggest - increasing 57 percent by 2030, according to IEA projections. But the amount of power generated by renewable sources, excluding hydroelectric, is expected to grow five-fold or more. As we see almost daily in the financial news, global business is going "green" in a big way.

All this sets the stage for the critical UN Climate Change Summit in Bali two weeks from now. We need a break-through: an agreement to launch serious negotiations for a comprehensive climate change deal that all nations can embrace. The challenge will be to lay out an achievable agenda of issues, from transferring alternative energy technologies to helping developing nations finance their own programs for fighting and adapting to climate change.

We are all responsible for this. Climate change respects no borders; solutions must be global.