UNCERTAIN FUTURE: Because small changes in things like snow cover or greenhouse gas concentrations lead to big climate effects, scientists will never be certain how bad global warming could be.
Image: NASA
The Intergovernmental Panel on Climate Change in its first report in 1990 predicted that temperatures would warm by 0.5 degree Fahrenheit (0.3 degree Celsius) per decade if no efforts were made to restrain greenhouse gas emissions. But the panel of scientists and other experts was wrong: By 2001, the group estimated that average temperatures would increase by 2.7 to 8.1 degrees F (1.5 to 4.5 degrees C) in the 21st century, and they raised the lower end to 3.6 degrees F (2 degrees C) this year in their most recent report. In essence, neither this international team of experts nor any other can say with any certainty just how bad global warming may get.
There is a simple explanation for this, says atmospheric physicist Gerard Roe of the University of Washington (U.W.) in Seattle: Earth's climate is extremely sensitive. In other words, small changes in various physical processes that control climate lead to big results. "If nothing else changed by [warming], a doubling of carbon dioxide would ultimately lead to a temperature change of about 1.2 [degrees] C," [(2.1 degrees F)] Roe says. "In fact, because of internal processes within the climate system, such as changing snow cover, clouds and water vapor in the atmosphere, our best estimate is that the actual warming would be two to four times larger than that."
Some of these feedback processes are poorly understood—like how climate change affects clouds—and many are difficult to model, therefore the climate's propensity to amplify any small change makes predicting how much and how fast the climate will change inherently difficult. "Uncertainty and sensitivity are inextricably linked," Roe says. "Some warming is a virtual certainty, but the amount of that warming is much less certain."
Roe and his U.W. co-author, atmospheric physicist Marcia Baker, argue in Science that, because of this inherent climate effect, certainty is a near impossibility, no matter what kind of improvements are made in understanding physical processes or the timescale of observations.
"Once the world has warmed 4 degrees C [(7.2 degrees F)] conditions will be so different from anything we can observe today (and still more different from the last ice age) that it is inherently hard to say when the warming will stop," physicists Myles Allen and David Frame of the University of Oxford wrote in an editorial accompanying the article. "If the true climate sensitivity really is as high as 5 degrees C [(9 degrees F)], the only way our descendants will find that out is if they stubbornly hold greenhouse gas concentrations constant for centuries at our target stabilization level."
Therefore, waiting for more scientific certainty before acting is a mistake, Roe says. "People are comfortable with the idea that stock markets, housing prices and the weather are uncertain, and they are used to making decisions on that basis," he notes.
But this also means that targets such as stabilizing atmospheric concentrations of CO2 at 450 parts per million (nearly double preindustrial levels) to avoid more than a 3.6 degree F (2 degree C) temperature rise are nearly impossible as well. There is no guarantee that such a target would achieve its stated goal. "Policymakers are always going to be faced with uncertainty and so the only sensible way forward to minimize risk is to adopt an adaptive policy," argues climatologist Gavin Schmidt of the NASA Goddard Institute of Space Studies, "which adjusts emissions targets and incentives based on how well, or badly, things are going."
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4 Comments
Add CommentThis situation reminds me of Micky Mouse in Fantasia, Remember, he could not stop the brooms from moving the water any more then we can understand what we are doing to the earth! All 7 billion of us!
Reply | Report Abuse | Link to thisThis situation (not the article) of when Micky Mouse, in fantasia, tries to stop the brooms from bring in the water, after he had just trained the brooms to bring in the water. What is it we want from the earth?
Reply | Report Abuse | Link to thisThe only thing certain is that the climate will change. The main greenhouse gas is water vapor not CO2. As I have mentioned in the past, an easy way to cool the earth is to start a volcano. I also wan to state that panels voting on climate change have no physical effect on the climate.
Reply | Report Abuse | Link to thisAt first it sounded like Roe was siting chaos theory
Reply | Report Abuse | Link to this"small changes in various physical processes that control climate lead to big results"
Very misleading since weather and climate are based on chaos theory. However, under chaos theory processes are very sensitive at the initial state and can lead to variations over time that are unpredictable. Roe is actually saying that it is predictable since small changes will lead to "big results". Chaos theory shows us that it is unpredictable so that it could be either way.
I would also like to point at an alternative point of view from a previous Scientific American article
http://www.scientificamerican.com/article.cfm?id=climate-heretic&page=3
The uncertainty lies in both the data about past climate and the models that project future climate. Curry asserts that scientists haven’t adequately dealt with the uncertainty in their calculations and don’t even know with precision what’s arguably the most basic number in the field: the climate forcing from CO2—that is, the amount of warming a doubling of CO2 alone would cause without any amplifying or mitigating effects from melting ice, increased water vapor or any of a dozen other factors.
Things get worse, she argues, when you try to add in those feedbacks to project likely temperature increases over the next century, because the feedbacks are rife with uncertainty as well: “There’s a whole host of unknown unknowns that we don’t even know how to quantify but that should be factored into our confidence level.” One example she cites is the “hockey stick” chart showing that current temperatures are the warmest in hundreds of years. If you are going to say that this year or that decade is the hottest, you had better have a good idea of what temperatures have actually been over those hundreds of years—and Curry, along with many skeptics, does not think we have as good a handle on that as the scientific community believes.