Hansen’s Number and What It Tells Us If 350 parts per million is the healthy limit for CO2, we need to shift into reverse, right now.

By Chuck Kutscher


In the world of climate scientists, one name invariably stands out. That name is James Hansen, director of NASA’s Goddard Institute for Space Studies. Ralph Cicerone, president of the National Academy of Sciences, has said, “I can’t think of anybody who I would say is better than Hansen. You might argue that there’s two or three others as good, but nobody better.” (It was reported that when he was asked to name the two or three others who were as good, none immediately came to mind.) When Time Magazine named Hansen one of the world’s 100 most influential people, Al Gore penned his bio.

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Data from earlier warming periods show that slow feedbacks melt the ice sheets if the CO2 level remains above 350 ppm for a sufficient period of time. Image: Kutscher/Thompson

Hansen first established his reputation in climate science back in the 1970s as a pioneering modeler of the Earth’s climate. He became a public figure after testifying to Congress in 1988 that he was 99 percent certain that fossil fuel emissions had put the Earth in a long-term warming trend. It was his group that accurately predicted the short-term global cooling that would result from the 1991 eruption of Mt. Pinatubo. When it comes to global warming, Hansen has a track record for being ahead of his time. It is understandable, then, that when Hansen talks, people listen.

In recent years, Hansen has grown impatient with the limitations of computer models. He has turned his attention instead to studying the climate history of the earth — the paleoclimate — to obtain a better understanding of how the Earth actually responds to changes like the greenhouse gases we are dumping into the atmosphere. Perhaps none of Hansen’s findings have caused as much of a stir as the one he announced to a packed room in San Francisco on Dec. 13, at the annual American Geophysical Union meeting. Hansen had arrived at an astonishing new upper limit for atmospheric carbon dioxide beyond which dangerous climate change would occur.

To understand its significance, a brief review is in order.

Studies of trapped air bubbles in deep ice cores drilled in the Antarctic have shown that for the past 800,000 years the amount of CO2 in the Earth’s atmosphere has varied between about 170 and 300 parts per million in approximately 100,000-year cycles.

Since mankind began burning fossil fuels, that number has risen dramatically from a value of 280 ppm in pre-industrial times to today’s value of 387 ppm. For many years, scientists considered a level of 550 ppm, or about twice the preindustrial value, as the danger limit. In recent years, with rapid ice melting and other CO2– induced changes apparent, many scientists, including Hansen, have considered 450 ppm to be the upper limit.

So you can imagine the reaction when Hansen announced that his latest study of the paleoclimate data had led him to conclude that unless CO2 is stabilized under 350 ppm (37 ppm below today’s level!), Earth’s permanent ice sheets will melt completely, with a catastrophic rise in sea level. Reactions ran the gamut. Some observers rejected Hansen’s number, arguing that because we are already above that number, accepting it would amount to admitting it’s too late to do anything. Others seized upon the number as a new goal, with noted environmental writer Bill McKibben going so far as to set up the website 350.org. (Hansen is a bit uncomfortable when too much emphasis is placed on the number itself, because it’s approximate.)

Why 350, and what does it imply?

When CO2 is added to the atmosphere, not only does it directly raise the Earth’s temperature by trapping outgoing infrared radiation, but it also generates a number of feedback mechanisms that can amplify the warming. The target of 450 ppm took into account “fast” feedback mechanisms like the additional water vapor that the warmer atmosphere can hold (water vapor is a strong greenhouse gas) and the melting of sea ice. But there are also “slow” feedback effects. As land-based ice sheets shrink, they reduce the reflectivity of the planet and also allow the release of long-trapped carbon from the melting permafrost. When Hansen studied data from earlier warming periods on the Earth, he concluded that these slow feed-backs would melt the ice sheets if the CO2 level remains above 350 ppm for a sufficient period of time.

But what’s a sufficient period of time to melt the ice sheets? Hansen doesn’t know, but the rapid loss of ice now being observed in Greenland and the Antarctic suggests that “slow” feedbacks may work faster than previously thought. Hansen believes that if we can limit the peak CO2 level at 400 – 425 ppm in the near future and then get back below 350 ppm within this century, then the thermal inertia of the oceans and the ice sheets may buy us enough time to withstand the temporary overshoot. But when you consider that atmospheric CO2 is rising at a rate of 2 ppm per year (which would put us at 400 ppm in less than seven years), it is clear that drastic action must be taken immediately.

What’s Hansen’s prescription for action?

He argues that we must place an immediate moratorium on the construction of all new coal plants (unless they capture and sequester their carbon), ban coal-to-liquids and other non-conventional fossil fuels, phase out existing coal plants by 2030, switch to carbon-free energy sources and adopt improved agricultural practices and reforestation.

Of course, Hansen could be wrong. We may not need to get back to 350 ppm. But history has shown that betting against Hansen is a risky proposition. And the stakes couldn’t be higher.

About the Author:
Chuck Kutscher is a principal engineer and manager of the Thermal Systems Group at the National Renewable Energy Laboratory. He is a past ASES chair and was chair of the SOLAR 2006 conference, which resulted in the ASES report, “Tackling Climate Change in the U.S.” He is teaching a course at the University of Colorado entitled “Climate Change Solutions.” This is the first installment of a regular column.