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Geographic 25 Oct 07
Global Warming's Long-Term Effect Uncertain, Study Says
Mason Inman for National Geographic News
How much Earth's climate will change due to global warming is inherently unpredictable, a new study argues.
The claim has big implications for the way decisions will be made that affect the climate over the coming century, some researchers say.
Scientists have been trying to pin down how much the world will warm up over the coming centuries if people continue emitting carbon dioxide (CO2) and other greenhouse gases into the atmosphere.
Experts often make their estimates by using computer simulations to gauge how average temperatures would eventually change if CO2 emissions were to double their preindustrial levels.
Various models agree that this doubling would likely raise Earth's temperature by 5 Fahrenheit (3 Celsius) over the coming centuries.
But there's still a fair chance the increase could be much more than this.
For the last three decades, climate researchers have estimated that there's a one-in-three chance that the warming from a doubling in CO2 would be more than 8 Fahrenheit (4.5 Celsius).
There's also a 5 percent chance of much more drastic warming, around 12 Fahrenheit (7 Celsius), the models suggest.
"You'd hope that if you throw more computers and people at the problem, your confidence would increase," said Gerard Roe of the University of Washington, who led the new study.
"[But ] this range in uncertainty hadn't been going down over the last 30 years."
Roe's team's research, published today in the journal Science, provides a simple explanation as to why.
The Feedback Effect
The new study, co-authored by Marcia Baker of the University of Washington, argues that feedback processes that are fundamental to our climate make it hard to predict how the climate will behave in the long run.
There are many ways in which a little bit of warming causes feedback, so small changes can become amplified.
When CO2 emissions rise, this greenhouse gas traps more heat from the sun within Earth's atmosphere, for example.
But warmer air is also able to hold more water vapor, which is also a greenhouse gas, so this feedback enhances the effects of the CO2 alone.
And when snow and sea ice melt, they expose land or water that soaks up solar heat, instead of reflecting it like ice does.
Researchers can't measure all these feedback processes perfectly, so there is always some uncertainty about how much of an effect they have.
"If we have any error, this will also be amplified," Roe said.
The new study showed, using simple mathematical models, that the uncertainty of these long-range predictions will remain about the same as it is now.
"[Roe's researchers] have provided a simple and beautiful explanation" to an issue that has vexed climate scientists, said Reto Knutti of the Swiss Federal Institute of Technology in Zurich.
"[The new study] does not undermine our understanding of climate change," Knutti said.
"[But it shows] it is hard to exclude the possibility of large [amounts of] warming."
If we can't be sure of how temperatures will rise, how can we manage CO2 emissions to avoid the worst outcomes?
The solution is to be flexible, argue Myles Allen and David Frame of the University of Oxford in the United Kingdom.
In a commentary in Science, they say that we should work to reduce emissions now and meanwhile keep an eye on the warming.
If the world is warming more or less than we expected, then we can adjust our CO2 emissions.
The authors call on policymakers to "resist the temptation to fix a [CO2] concentration target early on. Once fixed, it may be politically impossible to reduce it."
Mark Cane, a climate scientist at Columbia University in New York City, said, "A lot of the rhetoric about climate change has said we shouldn't do anything [about it] because it's uncertain."
But with this new study, he said, "we now know that this uncertainty will not go away."
Experts agree that they can still improve shorter-term predictions of climate change for the next several decades and better forecast how particular regions will fare.
So whether we could improve the predictions of what will happen over the next few centuries is less important when it comes to framing policy, argued Tim Palmer of the European Centre for Medium-Range Weather Forecasts.
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