Many Scientists Say Temperature Drop From Recent Record Highs Is a Blip, While a Few See a Trend; Inexact Climate Models
Two years ago, a United Nations scientific panel won the Nobel Peace Prize after concluding that global warming is "unequivocal" and is "very likely" caused by man.
Then came a development unforeseen by the U.N.'s Intergovernmental Panel on Climate Change, or IPCC: Data suggested that Earth's temperature was beginning to drop.
That has reignited debate over what has become scientific consensus: that climate change is due not to nature, but to humans burning fossil fuels. Scientists who don't believe in man-made global warming cite the cooling as evidence for their case. Those who do believe in man-made warming dismiss the cooling as a blip triggered by fleeting changes in ocean currents; they predict greenhouse gases will produce rising temperatures again soon.
The reality is more complex. A few years of cooling doesn't mean that people aren't heating up the planet over the long term. But the cooling wasn't predicted by all the computer models that underlie climate science. That has led to one point of agreement: The models are imperfect.
"There is a lot of room for improvement" in the models, says Mojib Latif, a climate scientist in Germany and co-author of a paper predicting the planet will cool for perhaps a decade before starting to warm again -- a long-term trend he attributes to greenhouse-gas emissions. "You need to know what you can believe and can't believe from the models."
The renewed discussion of inherent shortcomings in climate models comes on the cusp of potentially big financial commitments. In five weeks, diplomats from around the world will meet in Copenhagen to try to hash out a new agreement to curb global greenhouse-gas emissions. The science continues to evolve.
The goal of climate models is to project how rising greenhouse-gas emissions will interact with natural forces to affect the global temperature. The models are technological marvels. Using supercomputers, they divide the world into grids of roughly 4,000 cubic miles apiece. The grids are stacked, one on top of the other, up through the atmosphere.
It is complicated stuff. The models consist of dozens of equations written to reflect how liquids and gases move about the planet. Just as a symphony's sound is affected by the crash of symbols or the pluck of a violin string, the planet's future temperature is influenced by powerful ocean currents and tiny specks of sea salt. In between are other players, such as sunlight, clouds and rain.
Added to the equations are such measurements as past temperatures, barometric pressure and sea salinity. Calculations about the influence of sunlight are entered. Then various projections of greenhouse-gas emissions are factored in. The computers run the equations and generate projections of global temperatures.
The models are only as good as the information they are fed. One big uncertainty is ocean temperature. Oceans trap huge amounts of heat, and they process by which they release it over time affects the temperature of the planet. But there isn't a lot of actual data, because the vastness of the oceans makes gathering temperature data costly and arduous.
The success of the models also depends on the soundness of their assumptions. The effects of clouds, for example, are unclear. Depending on their shape and altitude, clouds can either trap heat, warming the earth, or reflect it, cooling the planet. The way that greenhouse gases affect cloud formation -- and how clouds in turn affect temperature -- remains a subject of debate. Different models treat these factors differently.
On a graph, the models' temperature projections ultimately point upward, signifying warming. But along the way, each line has dips -- temporary periods of cooling. The timing and depth of the drops differ from model to model.
Most climate scientists have regarded these zigs and zags as noise. Their models are designed to project how greenhouse gases will affect the global thermostat over a century, not what temperatures will be in any year or even in any decade.
"We care about the climate in the 2080s. We don't care about the climate on Aug. 15, 2084," says Andrew Dessler, a climate scientist at Texas A&M University.
The models' focus on century-long trends is in part a function of limited data. Predicting short-term temperatures requires more measurements than projecting long-term trends. But such data have been lacking. "These long-term climate projections are a much easier problem than these shorter-term climate projections," says Mr. Dessler. "It's sort of counterintuitive."
Though often overlooked in the debate about man-made warming, natural factors have contributed to record high temperatures. The year 1998, for example, was widely noted as the hottest year on record, intensifying concerns about global warming and people's role in it. But one reason that 1998 set a record is that a strong shift in ocean temperature known as El Niño occurred that year. "1998 was a very hot year because it was an El Niño year," says Mr. Dessler.
The 2007 U.N. report included in its widely read summary a chart of projected temperatures that lacked visible periods of cooling. That is because it was an average of the lines from many different climate models. As averages do, it looked smooth. And it pointed up, indicating rising temperatures.
Yet as the report was released, the global average temperature was below what it had been in 2005, which along with 1998 was one of the two hottest years on record. Even so, the average temperature in 2006 and 2007 remained among the 10 highest ever recorded.
About a year of the U.N. report's release, researchers in Germany published a paper in the journal Nature that attributed the cooling to the enigmatic ocean currents.
The paper was based on a model that used new ocean-temperature measurements. It concluded that a shift in ocean currents was counteracting the warming from greenhouse gases. And that is causing the planet, on balance, to cool.
The paper argues that intermittent cooling from natural factors such as ocean currents will prove less significant in the long term than continued warming from greenhouse-gas emissions. But climate scientists acknowledge that those natural variabilities aren't fully understood. "This is pioneering work," says Mr. Latif, one of the authors of the authors of the German paper. "I won't say our forecast will be correct."
A separate study by researchers in the U.K., published in 2007 in the journal Science, also says the cooling will soon be outweighed by warming from greenhouse gases.
Unsurprisingly, the research hasn't settled the debate. Scientists who have long questioned man-made global warming cite the temperature drop that began in 2006 as more evidence the models are wrong. "They were predicting warming," says Richard Lindzen, a climate scientist at the Massachusetts Institute of Technology.
Mr. Lindzen's work, regarded as leading the research challenging man-made warming, suggests that natural factors such as clouds generally inhibit, rather than intensify, greenhouse-gas warming. He wrote in a recent article that the study from the U.K. admits that the kind of climate model cited in the U.N.'s IPCC report "did not appropriately deal with natural internal variability, thus demolishing the basis for the IPCC's iconic attribution" linking greenhouse-gas emissions to climate change. He added that "even when all models agree, they can all be wrong."
The researchers behind those studies strenuously reject that description. But they disagree among themselves on how long the cooling will last. The British paper says warming will resume as early as this year. The German paper says warming won't resume for perhaps a decade.
Such disagreements aren't unusual in a nascent science. "I don't think anybody is surprised that we're going to get one model that suggests it's going to cool and another that suggests it's going to warm," says Vicky Pope, a scientist at the Hadley Center, the U.K. institute where the research for the British paper was done. "That's consistent with where we are with the science."
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