Hansen stated that the Earth's population has less than a decade to stop global warming from changing the world forever, and noted that the effects on the climate were already observable.
"We just cannot burn all the fossil fuels in the ground," he said. "If we do, we will end up with a different planet. I mean a planet with no ice in the Arctic, and a planet where warming is so large that it's going to have a large effect in terms of sea level rises and the extinction of species."
Hansen indicated that global warming is causing dangerous "positive feedbacks" in the high latitudes of the northern hemisphere, which can accelerate the rate of climate change. One such feedback is the receding sea ice, which subsequently increases Arctic temperatures because the amount of sunlight and heat reflected back into space is reduced. The melting ice also releases methane, which is 20 times more dangerous as a greenhouse gas than carbon dioxide, Hansen said.
"If we go another 10 years, by 2015, at the current rate of growth of CO2 emissions, which is about 2 percent per year, the emissions in 2015 will be 35 percent larger than they were in 2000," he said. "But if we want to get on a scenario that keeps global temperature in the range that it's been in for the last million years, we would need to decrease the emissions by something of the order of 25 percent by the middle of the century, and by something like 75 percent by the end of the century."
According to Hansen, the current trend will eventually cause the collapse of ice sheets in Greenland and the west Antarctic, which would cause massive increases in sea levels over the next couple of decades. This is especially worrying, Hansen pointed out, since about 50 percent of the world's population lives within 15 miles of a coastline
"The last time it was 3 degrees (Celsius) warmer, sea levels were 25 meters higher, plus or minus 10 meters. You'd not get that in one century, but you could get several meters in one century," he said. "The problem is that once you get the process started and well on the way, it's impossible to prevent it. That's why we need to address the issue before it gets out of control."
Human beings are not the only species in jeopardy, Hansen noted, as some plant and animal species could be "pushed off the planet" by rising temperatures. If the temperatures continue to rise, isotherms -- lines of equal temperature -- will begin to travel toward the poles at about 31 miles per decade, while the average rate of species migration is only about 3.7 miles per decade.
"Those species at high latitudes have no place to go to," Hansen said. "Many of them will be in trouble."
Hansen was one of the first scientists to warn of the effects of climate change in testimony to Congress in 1988, and made headlines last year when he complained of Bush-appointed NASA public relations officers limiting his access to the media in order to prevent him from discussing global warming. White House officials denied the allegations, but while Hansen said he now feels able to speak out, he reported that other scientists still feel gagged.