Jessica Marshall, Discovery News
June 27, 2008 -- Earlier this week, renowned NASA climate scientist James Hansen warned Congress of the dangers of climate change, exactly 20 years after he did so for the first time.
The message he delivered was almost the same as it was in 1988, but there was one key difference: "The difference is that now we have used up all slack in the schedule for actions needed to defuse the global warming time bomb," he said.
Hansen's message painted a stark and urgent picture of a world already past the point where significant damage would occur. Discovery News wanted to know if other scientists shared his view. Are we really in for it and at what point? What are our options for avoiding the worst?
Earth's Carbon Budget
Hansen argued this week that the "safe level of atmospheric carbon dioxide is no more than 350 ppm (parts per million), and it may be less." This recommended level is less than the amount currently in the atmosphere -- 385 ppm. It may also be less than the commonly discussed stabilization target of 3.6 degrees Fahrenheit (2 degrees Celsius) of temperature increase, which probably corresponds to an atmospheric CO2 concentration of about 350-400 ppm.
Already, he argued, arid lands are expanding, glaciers are receding, and Arctic sea ice is shrinking, driven by cycles of positive feedback, where melting leads to more warming of the exposed dark ocean water, which leads to more melting.
"As a result, without any additional greenhouse gases, the Arctic soon will be ice-free in the summer," Hansen said.
To forest ecologist Lee Frelich at the University of Minnesota, Twin Cities, Hansen's argument that a lower stabilization target is safer makes sense.
"If you look at the paleological record, in the last interglacial period 110,000 to 120,000 years ago, the world was thought to have a climate that was two degrees warmer than today," Frelich said. "The oceans were 20 to 25 feet higher, but CO2 was only 290 ppm. I've always thought that if a CO2 content of 290 could cause that, why won't it do it now? Maybe there's just a lag time."
"I'm sympathetic to a more aggressive goal," said glaciologist Jay Zwally of NASA Goddard Space Flight Center in Greenbelt, Md. "The goal that people have adopted of keeping it to a total of two degrees [Celsius] rise since the preindustrial is still going to allow enough warming that we'll have an even more significant impact than we've already seen," he said.
While other scientists agree that 350 ppm is a safer target that increases the likelihood we will avoid many of the negative effects of climate change, some also think it's unrealistic.
"Three hundred and fifty is impossible," said climatologist Stephen Schneider of Stanford University in Palo Alto, Calif. "We're going to overshoot 350 and 450 and probably 550, though I sure hope not."
Schneider's hope is that while it might still be 20 years before actions to reduce CO2 emissions really have an effect, innovations over the next two decades will make it possible to dramatically reduce emissions.
"My cynical scenario is that there will be more Katrinas, massive fires, melting of the Arctic, and people will say, 'Oh my God, what have we done. We'd better undo this,'" he said. Such catastrophes could finally spark the dramatic change that's needed, he suggests, if we don't take action sooner of our own accord.
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