Climate Science Flurry: Cooling vs. Warming et al. Hot Items

by Bill Chameides | May 5th, 2009
posted by Erica Rowell (Editor)

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Despite some cool years over the past century, the overall temperature trend is still on an upward climb. So the question remains, how do we deal with this. A flurry of new science papers offer up some suggestions.

This past week has been chock-full of new stuff on climate change. Let’s take a gander at a few.

Is It Warming or Cooling?

Over the past 10 years the increase in global temperatures has slowed (and maybe even stopped), but the overall warming trend is still very much alive and kicking. I’ve written a number of posts on this subject: Did Climate Scientists Flip Flop?, 2008 Does Not a Climate Trend Make, Global Warming and Predictions of an Impending Ice Age: Global Warming Since 1998.

A new paper in Geophysical Research Letters confirms what I’ve been arguing: A 10-year period is far too short a time to infer a climate trend. The paper’s authors, David Easterling of the National Climatic Data Center and Michael Wehner of the Lawrence Berkeley National Laboratory, point out that between 1975 and today there have been two other periods with essentially no temperature trend (1977–1985 and 1981–1989) even though over the entire period there was an unmistakable warming.

The same reasoning that leads to conclusions that global warming has now stopped would have produced similar conclusions that the warming had halted in 1985 and 1989. As the longer temperature record has shown, those conclusions would have been wrong.

Interestingly, Easterling and Wehner suggest, on the basis of climate model simulations, that the coming century will have more decades-long periods with no warming or even some cooling even as the overall trend will be warming. As if getting the world to agree on a climate change policy wasn’t difficult enough…

How Much CO2 Is Too Much CO2?

The world community has agreed that when it comes to global warming the objective is to avoid “dangerous anthropogenic interference” with climate. And, on the basis of our best guesses about how the globe will respond to warming temperatures, it is generally agreed that avoiding dangerous climate change requires not allowing global temperatures to rise more than two degrees Celsius (or 3.5 degrees Fahrenheit) above pre-industrial temperatures. (Currently, global temperatures have risen by about 0.8 degrees Celsius.)

The trick from a policy standpoint has been translating that two-degree ceiling into a pathway for greenhouse gas emissions that ratchets them down by large enough amounts and rapidly enough to avoid such warming. Thus far, policy discussions have focused on a target for carbon dioxide (CO2) concentrations (actually equivalent CO2 concentrations) of 450 parts per million of CO2.

The problem with the 450 ppm CO2 target is that there’s a vast number of emission pathways that can take us from the present to some future world with CO2 concentration of 450 ppm or less. Because there is no a priori reason for choosing one path over another, choosing emissions targets such as an 80 percent reduction in emissions by, say, 2050 becomes arbitrary and difficult to justify.

A New Way to Look at the CO2 Target: Total Instead of Annual Emissions

In separate papers published in Nature, Myles Allen of the University of Oxford and colleagues and Malte Meinshausen of the Potsdam Institute for Climate Impact Research and colleagues propose a different, more direct approach: placing limits on the total cumulative emissions rather than the emissions in any given year. In other words rather than focusing on emission rates and CO2 concentrations, focus on the total amount of CO2 emitted into the atmosphere over the years.

The advantages are two-fold:

  1. The total cumulative emissions, the researchers show, robustly predict the maximum global temperature increase, thus avoiding the potentially obfuscating debate about specific emission targets.
  2. A limit on total cumulative emissions provides a clear picture of the trade-offs and risks of delaying emission cuts. The longer a country (or the world) delays cutting emissions, the less of the total cumulative emissions will be left in the out-years and thus the larger the required emissions cuts will be then. If you believe that new technologies will make those cuts easier, delaying might seem like a good bet. But wait too long and achieving the required large cuts may prove to be economically disastrous. (For an illustration, see this earlier post from

Thus far we have emitted about 1.8 trillion tons of CO2 into the atmosphere. Allen and colleagues estimate that to avoid a two-degree Celsius increase in global temperatures, we can emit no more than another 1.8 trillion tons over the next century or two. Meinshausen and colleagues take that one step further, and estimate that to keep total anthropogenic emissions below 1.8 trillion tons and have a 50 percent chance of avoiding that two-degree Celsius increase in global temperatures, we must limit total emissions between now and 2050 to about 1.1 trillion tons.

To put those numbers into context, between 2000 and 2006 cumulative global emissions were about 0.23 trillion tons; at that rate we would use up the 1.1 trillion tons by 2043 and our total allotment of 1.8 trillion by 2055. Coming in under those caps doesn’t look impossible but will definitely require bringing emissions down. The sooner we get started the easier it’s going to be later — you know, pay me now or pay me later.

Jim Hansen Takes an Axe to Cap and Trade

Jim Hansen of the NASA Goddard Institute for Space Studies, arguably the most well-known climate scientist, has been in the news recently. Hansen, who sounded the global warming alarm in 1988, well before most other climate scientists (including me) were ready to do so, is now arguing that a CO2 concentration of 450 ppm is too high to avoid dangerous climate change. Instead, he argues that we must bring CO2 concentrations down from their current level of 385 ppm to 350 ppm. Doing so would require not only a rapid decrease in CO2 emissions to zero, but also implementing land management practices and technologies to scrub CO2 out of the atmosphere.

Jim is also outspoken on the policy side, fervently against new coal-fired power plants and strongly in favor of carbon taxes over cap and trade. (In a talk last month at the University Corporation for Atmospheric Research, he explained his fears: that cap and trade could be “loaded with escape valves” and inadvertently “end up making Wall Street millionaires at public expense.”) Last weekend, on May 2, speaking at Columbia University’s 350 Climate Conference, he took his cap-and-trade opposition up a notch, stating that he hoped the cap-and-trade bill being considered in Congress will “fail.”

Such a position requires an awful lot of faith that a carbon tax bill will rapidly rise from the ashes of a failed cap-and-trade bill. Given the need for the United States to get a comprehensive climate policy in place I am not nearly so sanguine. It seems unlikely to me that any climate legislation will see the light of day while the environmental community favoring such legislation cannot reach consensus.

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1 Comment

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  1. Dan
    May 5, 2009

    The complexities of the current situation (that’s why I opt for using the term ‘global climate change’): Cheers!

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