Hansen on 350ppm

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The most common position among climate change analysts is that we need to stabilize the atmospheric concentration of carbon dioxide somewhere between 450 and 550 parts per million (ppm). That is, for instance, the target range endorsed by Nicholas Stern. It is also thought by many to be compatible with the EU goal of generating less than two degrees Celsius of temperature increase, though that is only really plausible at the low end.

In recent Congressional testimony, James Hansen, director of the NASA Goddard Institute for Space Studies, argued that we actually need to cut concentrations from the present 385ppm to 350ppm or less. Basically, his argument is that even stabilization at the present level would have unacceptable consequences: both directly, in terms of impacts on physical and biological systems, and by kicking off feedback loops that will further worsen things. The distinction between the numbers may seem abstract to those not familiar with climate policy, but the practical differences between stabilizing between 550, 450, or 350ppm are massive. Each scenario requires that emissions peak at a different date, and that they fall more or less rapidly afterwards. Even staying below 450ppm requires that global emissions peak within 10-15 years, and that they fall to a small fraction of present levels by 2050.

If accurate, the 350ppm target invalidates a great deal of climate change planning. The general view is that we still have a cushion for additional emissions, to be split up between developed and developing countries. The former would lead the way, showing the latter how they can also do so once they reach a somewhat higher level of affluence. Getting back to 350ppm in a reasonable amount of time requires much more aggressive cuts, universally. It would also require that India and China move to a low-carbon economy long before any significant proportion of their population has reached Western levels of affluence.

Personally, I hope Hansen’s most recent testimony is not as prescient as that he gave twenty years ago. If we need to get the planet on a rapid path towards 350ppm, the disjoint between what is physically necessary and what is politically possible is far wider a chasm than has hitherto seemed to be the case.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

13 thoughts on “Hansen on 350ppm”

  1. “We’re actually projecting this year that the North Pole may be free of ice for the first time [in history],” David Barber, of the University of Manitoba, told National Geographic News aboard the C.C.G.S. Amundsen, a Canadian research icebreaker.

    Firsthand observations and satellite images show that the immediate area around the geographic North Pole is now mostly annual, or first-year, ice—thin new ice that forms each year during the winter freeze.

  2. Heat: How to Stop the Planet from Burning

    “Here is a smattering of what he says will be required by 2030:

    * A power grid dominated by renewables and natural gas plants with carbon capture and storage.

    * Dramatically, dramatically tightened building regulations – making most houses either ‘passive’ in their non-use of heating or cooling or capable of producing their heat and power from piped-in hydrogen, possibly supplemented by solar.

    * Most private automobile travel replaced by a buses or non-motorized transport, both within and between cities.

    * An end to cheap air travel: no more low cost flights, with massive total cuts in the number of both short and long-haul flights.”

  3. When the Northwest Passage opened amid the great Arctic melt last summer, many scientists were stunned. James Hansen, our greatest climatologist, was already at work on a paper that would try, for the first time, to assign a real number to global warming, a target that the world could aim at. No more vague plans to reduce carbon dioxide in the atmosphere, or keep it from doubling, or slow the rate of growth—he understood that there was already enough evidence from the planet’s feedback systems, and from the quickly accumulating data about the paleoclimate, to draw a bright line.

    In a PowerPoint presentation he gave at the American Geophysical Union meeting in San Francisco last December, he named a number: 350 parts per million carbon dioxide. That, he said, was the absolute upper bound of anything like safety—above it and the planet would be unraveling. Is unraveling, because we’re already at 385 parts per million. And so it’s a daring number, a politically unwelcome one.

  4. By burning fossil fuels and destroying forests, we are releasing
    greenhouse gases, importantly carbon dioxide (CO2), into
    the atmosphere. These heat-trapping gases are warming the
    planet, setting in motion changes that are taking us outside
    the climate bounds within which civilization developed.

    We cannot afford to let the planet get much hotter. At today’s
    already elevated temperatures, the massive Greenland and
    West Antarctic ice sheets—which together contain enough
    water to raise sea level by 12 meters (39 feet)—are melting
    at accelerating rates. Glaciers around the world are shrinking
    and at risk of disappearing, including those in the mountains
    of Asia whose ice melt feeds the continent’s major rivers
    during the dry season.
    Delaying action will only lead to greater damage. It’s time
    for Plan B.

    The alternative to business as usual, Plan B calls for cutting
    net carbon dioxide emissions 80 percent by 2020. This
    will allow us to prevent the concentration of CO2 in the
    atmosphere, already at 384 parts per million (ppm),
    from exceeding 400 ppm, thus keeping future global
    temperature rise to a minimum.

  5. Although the atmospheres and climates of Venus and Earth differ very greatly today, it is generally believed that the two planets started out in a rather similar state, but subsequently evolved along divergent paths. Venus succumbed early to a “runaway water vapor greenhouse,” in which the increased water vapor content arising from increased temperature reached an end state with much of the ocean evaporated into the atmosphere. Once this happens, it is easy for the water vapor to decompose in the upper atmosphere, whereafter the light hydrogen escapes and oxygen either escapes or reacts with rocks. One hypothesis is that the weak magnetic field at Venus, which otherwise would protect the planet from the solar wind, is one reason for why the oxygen and hydrogen escaped faster into space. Once water is lost, the reaction that turns carbon dioxide into limestone can no longer take place, so CO2 outgassing from volcanoes accumulates in the atmosphere instead of staying bound up in the rocks. The end state of this process is the current atmosphere of Venus, with essentially no water in the atmosphere and essentially the planet’s whole inventory of carbon in the form of atmospheric CO2.

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