In a small college town like Corvallis, Ore., it's not unusual that George Taylor would ride a bike to his job on the Oregon State University campus. He commutes this way for the exercise, he says, but also because it's good for the planet.
Mr. Taylor manages the Oregon Climate Service, and much of his work has to do with global warming. "I'm certainly in favor of doing prudent things to reduce the human impact," he says.
But unlike most climate scientists, he does not believe that anthropogenic (human-caused) greenhouse gases – mainly from coal-fired power plants and motor vehicles spewing carbon dioxide – are the main culprits. In fact, he says, "It's my belief that in the last 100 years or so natural variations have played a bigger role."
Among the forces of nature he cites are changes in solar radiation, "very significant influences" of the tropical Pacific (El Niño and La Niña events in decades-long cycles), as well as changes in Earth's tilt and orbit over cycles lasting thousands of years.
Above all, says Mr. Taylor, who is past president of the American Association of State Climatologists, "The climate system is very, very complex, and the more we learn, the more we see that we really don't understand it."
Taylor may be in the minority among climate experts, but he is not alone.
Other planets in our solar system have expanding and contracting ice caps, too, other skeptics point out, and those worlds have no people as far as we know – certainly no gas-guzzling muscle cars and trucks. Antarctica and Greenland at times have been warm and green before humankind invented machines, indicating to these skeptics that this is just a natural cycle.
In Phoenix, where it's been very hot indeed this summer, Warren Meyer has written "A Skeptical Layman's Guide to Anthropogenic Global Warming." He is not a professionally trained climate scientist, but he studied physics and engineering at Princeton University, then earned an MBA at Harvard University before entering the business world.
Like Taylor, Mr. Meyer cites other possible factors – ocean oscillations and currents, sunspot cycles, and recovery from the "Little Ice Age" (which ran for roughly three to four centuries, up to the mid-19th century) – to argue that "we are a long way from attributing all or much of current warming to man-made carbon dioxide."
He says he's carefully studied the official reports and assertions about global warming and come to the conclusion that "it's a funny sort of anthropomorphic hubris to say that we know what 'normal' is or even know what the cycles are.
"Look, there's a lot going on here that we've observed for a very short time," Mr. Meyer says. "We have all these complicated cycles happening, and many of them last for thousands or millions of years. And we've observed them carefully for – what? – 30 years?"
Is climate 'feedback' positive or not?
Meyer's engineering background is in feedback and control theory, so he especially takes issue with the belief among many climate scientists (as well as activists such as former Vice President Al Gore) that what had been a long-term, stable climate system is now dominated by "positive feedback ." Positive feedback means that as temperatures rise in extraordinary fashion there will be a tendency for global warming to speed up. One example is when light-colored sea ice melts to reveal darker ocean water, which in turn absorbs more heat, which melts more ice.
Meyer contends that in physics (and in nature) the tendency is just the opposite: a "negative feedback" will occur as CO2 levels rise – in other words, cooling mechanisms will set in. In the case of carbon dioxide and global temperature, "future CO2 has less impact on temperature than past CO2," he says.
One bit of recent research may give some weight to Meyer's argument.
Researchers at the University of Alabama's Earth System Science Center in Huntsville studied heat-trapping tropical clouds thought to result from global warming. They found an apparent decrease in such clouds as the atmosphere warms, allowing more infrared heat to escape from the atmosphere. The cloud decrease appears to be "negative feedback," meaning that as warming continues it sets off another process that counters its effects.
Neither Taylor nor Meyer (nor most other climate change skeptics, some of whom call themselves "global-warming optimists") deny that modern human development in the form of additional greenhouse gases has played a role in warming the planet.
But most of them agree with Meyer when he says, "To this day, there's still no empirical proof of how much warming is coming from CO2. There's a lot going on, and it's almost impossible to pick effects out."
Warming oceans tell a different tale
That flies in the face of what most scientists say. "For me, the most compelling single data set that undermines that suggestion is the increase in the heat content of the oceans," says Daniel Lashof, science director for the climate center at the Natural Resources Defense Council in Washington, an environmental lobbying group.
Another environmental group, the Union of Concerned Scientists, notes that higher temperatures have been found 1,500 feet deep in the ocean. It calls ocean warming the No. 1 "human fingerprint … well outside the bounds of natural climate variation."
"There's just no argument about it," Dr. Lashof says. Heat "is going down hundreds of feet … an accumulation of heat that there's just no other explanation for other than that the earth has been driven out of energy balance with the sun by this accumulation of heat-trapping gases in the atmosphere."
James Wang, a scientist in the climate and air program at Environmental Defense in New York, points out the close correlation between computer model predictions of how much heat has been trapped by greenhouse gases with measurements of how much heat has been stored in the global oceans over the past few decades.
"The earth has gotten warmer over the past 30 years, and a lot of that additional heat has gone into the oceans," Dr. Wang says. "We find that none of the natural factors, such as sunlight and volcanoes, can explain the warming that has occurred over the past 30 years or so."
Electricity, cars, and cement's effect
In a recent report, Wang and colleague Bill Chameides, chief scientist at Environmental Defense, write:
"Independent measurements demonstrate that the increased CO2 in the atmosphere comes from burning fossil fuels and forests. The isotopic composition of carbon from these sources contains a unique 'fingerprint.' The only quantitative and internally consistent explanation for the recent global warming includes the intensified greenhouse effect caused by the increase in CO2 and other greenhouse gases."
This is essentially what the United Nations Intergovernmental Panel on Climate Change (IPCC) concluded in its most recent report issued earlier this year. The report notes that concentrations of CO2 and of methane (another major greenhouse gas) today are "both very likely much higher than anytime in at least 650,000 years."
Burning fossil fuels to heat and light homes and offices, power vehicles, and even to manufacture cement (which releases high amounts of CO2) accounts for more than 75 percent of the increase in CO2 concentration during the industrial age, according to the IPCC, with the rest attributable to such land use changes as deforestation and biomass burning.
The type of CO2 in the atmosphere today, "in particular the ratio of its heavy to light carbon atoms," identifies it as having come from the burning of fossil fuels, the report says.
Climate change skeptics aren't convinced. Neither Taylor nor Meyer deny that ocean temperatures have been rising. But they say the picture is more complicated and inconclusive than Drs. Wang and Chameides do.
Looking at the data on temperature predictions from the IPCC, Meyer of the "Skeptical Layman's Guide" concludes that "if anything, the United Nations is overstating it. Even taking the worst it could be, it's not going to be that bad," he says, referring to what he sees as alarmist forecasts by Mr. Gore and others. "Therefore under no circumstances should we be doing things [to combat climate change] that are extraordinarily intrusive."
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