Global Warming "Tipping Points" Reached, Scientist Says

Mason Inman in San Francisco, California
for National Geographic News
December 14, 2007
Earth has already crossed a number of climate change "tipping points" at which today's levels of greenhouse gases will cause additional large and rapid changes, a leading climate scientist said yesterday.

But it's not too late to avoid much of the damage by curbing the use of fossil fuels such as oil and coal, climatologist James Hansen added during a presentation at the American Geophysical Union fall meeting in San Francisco.

Such fuels are responsible for most of human-caused emissions of carbon dioxide (CO2), which are widely believed to be driving global warming.

Today's level of CO2 in the atmophere is enough to cause Arctic sea ice cover and massive ice sheets such as in Greenland to eventually melt away, said Hansen, of the NASA Goddard Institute for Space Studies in New York City.

Climate zones such as the tropics and temperate regions will continue to shift, and the oceans will become more acidic, endangering much marine life, he added. (Related: "Climate Change Pushing Tropics Farther, Faster" [December 3, 2007].)

"I think in most of these cases, we have already reached the tipping point," Hansen said.

But there's still hope if people soon change how they use energy.

"In my opinion, we have not passed the point of no return, so that it's still possible to avoid the impacts," Hansen said

"The problem is that it's just been taken as a God-given fact that we're going to burn all of these fossil fuels and let the CO2 in the atmosphere," he added.

"You just can't do that if you're going to keep this planet resembling the one that we've had for the last 10,000 years."

Chilly News

Melting ice and subsequent increases in ocean levels were among the most cited examples of reaching or nearing a tipping point.

"The very small warming that's happened to date is having a large effect—pretty much everywhere we look—on the ice of the planet," said Richard Alley of Pennsylvania State University in University Park.

"Pretty much every glacier on the planet has been shrinking," he added.

Earlier research had also suggested that the Greenland and Antarctic ice sheets are melting and contributing to sea level rise.

New research presented at the San Francisco meeting this week "shows this really is right," Alley said. "Very recently there has been loss from these ice sheets."

The future has more in store, he said.

"The warming which will be coming, under various [projections], is very large compared to what we've seen so far," Alley said.

"We don't believe you can dump an ice sheet in decades. It would be at least centuries, if not longer," Alley said.

"But [it] is possible that we can reach that tipping point that commits us to [such rapid melting] within decades."

For the so-called perennial sea ice in the Arctic, which stays frozen through each summer, it may already be too late, according to another scientist. (Related: "Warming Oceans Contributed to Record Arctic Melt" [December 14, 2007].)

"I think the tipping point for perennial sea ice has already passed," said Josefino Comiso of the NASA Goddard Space Flight Center in Greenbelt, Maryland.

"It looks like the perennial sea ice will continue to decline and there's no hope for it to recover," Comiso added yesterday at the San Francisco meeting.

Time to Act

If people stop burning fossil fuels soon or are able to capture CO2 before it reaches the air, then we may be able to reduce the level of the gas in the atmosphere, Hansen said.

"It's going to be very interesting the next few decades," he added.

"We either begin to roll back not only the emissions [of carbon dioxide] but also the absolute amount in the atmosphere, or else we're going to get big impacts."

"We should set a target of CO2" that's low enough to avoid the point of no return, he said.

The CO2 tipping point for many parts of the climate is around 300 to 350 parts per million, Hansen estimated.

This is below today's level of about 380 parts per million and a much more ambitious target than even the greenest governments, such as those of Germany and Britain, have announced.

The oceans and land currently absorb roughly half of the CO2 people emit each
year, Hansen said.

So if humans stop emitting CO2 through fossil fuel use, the gas would continue to be soaked up and levels in the atmosphere would drop.

"We have to figure out how to live without fossil fuels someday," Hansen said. "Why not sooner?"

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