NEW YORK — Prominent climate scientist James Hansen has been warning that humans have brought the planet to a tipping point, after which changes, such as melting ice, can pick up momentum with potentially devastating effects.

At a discussion today (Oct. 11), Hansen suggested society has reached its own tipping point.

"We are at a fork in the road. We can either continue with business as usual and addiction to fossil fuels, or we can put an honest price on carbon that makes fossil fuels pay their cost to society," Hansen told an audience at the State of the Planet Conference held here at Columbia University.

The cost of fossil fuels is artificially low because it does not include the price they incur on society, through air and water pollution and the effects of global warming, such as more extreme weather, Hansen said. His solution: a gradually rising fee attached to all fossil fuels at their mine or port of entry. The fee would then be redistributed to the public.

A rising cost on greenhouse gas-producing fossil fuels would spur investment in clean alternatives, he said.

Hansen, director of NASA's Goddard Institute for Space Studies, and Lisa Goddard, director of the International Research Institute for Climate and Society, offered a climate science perspective during the conference, which focused on global sustainable development.

Goddard focused on drought, and the need to anticipate climate extremes and manage their effects.

She focused on a severe drought in southern Sudan, which, after two failed rainy seasons, has brought famine. It's difficult to attribute this drought directly to global climate change, since there has been no long-term trend in the precipitation in the region. However, decade-scale fluctuations in rainfall seem to have contributed, as did the long-ranging effects of cooler ocean temperatures over the equatorial Pacific (known as a La Niña event), she said. [8 Ways Global Warming Is Already Changing the World]

A lack of ability to anticipate extremes, such as droughts, can impair development because people make cautious decisions, such as refraining from taking out a loan to buy seeds for fear of drought. As a result, people may miss important opportunities, she said.

Both Hansen and Goddard weighed in on the presidential election.

Goddard faulted President Barack Obama's administration for not supplying adequate funding for climate research and making little explicit mention of climate.

"Meanwhile, the U.S. has been experiencing not only climate change but climate variability of quite a severe nature," Goddard said, referring to the recent unusual weather, including unprecedented warmth across the lower 48 states.

Hansen, too, expressed frustration. Obama should have had a Franklin Roosevelt-style fireside chat with the nation shortly after his election, when his popularity was high, and explained the need to put a price on carbon, the primary ingredient in greenhouse gases, Hansen said.

Obama did back a failed cap-and-trade proposal, which would have allowed clean companies to sell their leftover pollution credits to not-so-clean ones. Hansen opposed this system.

When asked the implications of a victory for Republican challenger Mitt Romney, Hansen noted that he found the sway held by climate-change deniers over Republican candidates discouraging. "Nevertheless, the realities can be made clear to educated, intelligent people," he said.

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