(CNN) -- Is global warming really a threat?
Absolutely, respond most scientists, but they have only recently been able to approach a basic agreement about our changing climate.
First, the Earth has gotten warmer. Since 1850, average global temperatures have risen about .6 degrees Celsius, the United Nations says. Greenhouse gases such as carbon dioxide released by humans burning fossil fuels and clearing land are the likely culprits. Sea levels have also risen about 4 to 8 inches during the past century, according to the U.S. Environmental Protection Agency.
Second, the concentration of greenhouse gases (or GHG) in the atmosphere is near its highest point in recorded history. Since the Industrial Revolution, concentrations of carbon dioxide, a potent greenhouse gas, have risen 30 percent.
Based on studies of air bubbles trapped in ancient ice, today's levels are higher than any time in at least 420,000 years, said David King, chief science adviser for the British government. If GHG concentrations rise, as expected, concentrations could cross what some consider a "dangerous" threshold, although that designation is contentious.
Finally, almost every scientist agrees upon one thing: the future is highly uncertain. While most scientists support projections by the United Nation's Intergovernmental Panel on Climate Change that temperatures will rise 1.4 degrees Celsius (2.5 degrees Fahrenheit) to 5.8 degrees Celsius (10.4 degrees Fahrenheit) by 2100, the scientific consensus shows cracks beyond this point.
John Christy, director of Earth System Science Center and critic of severe warming predictions, says forecasting the future "gets messy quickly."
"The Earth system has more unknowns that we are generally willing to acknowledge," he told CNN via e-mail. "It is very difficult for [scientists] to say, 'I don't have a clue.'...Our pronouncements often express more confidence than is warranted given the level of ignorance in which we presently operate."
Climate models inherit this uncertainty. The first crude models -- spinning aluminum dishpans in the 1950s -- have evolved into some of the world's most sophisticated computer simulations replicating the interaction between the atmosphere, oceans and continents. Yet the system's complexity -- a mathematical swamp of biological cycles, ocean circulation, geologic emissions and even solar activity -- injects guesswork into the science.
Despite the uncertainties, says Drew Shindell, a NASA climate modeler at the Goddard Institute for Space Studies in New York, climate models, at least in the short term, are good and "getting better."
"The results become steadily less reliable as you go out in time," Shindell said. "We can do a pretty good job 25 to 30 years out; we have a rough idea 50 years out. By the time you get into 100 years, there's a lot of things we can't really say."
The biggest questions in the climate equation are water vapor and airborne particles called aerosols. Water vapor acts like a huge sponge soaking up energy absorbed by greenhouse gases such as carbon dioxide. It is a wild card for climate modelers because water vapor may also cool the planet by increasing cloud cover. Aerosols -- which can also cool or warm the planet -- may have other unknown effects.
But Shindell says dozens of climate models run by scientists around the world convincingly describe a world sweating under the influence of greenhouse gases which trap the sun's energy.
"I think we can say very clearly, with the same amount of energy going in and less going out, [the Earth] has to warm up," he said. "That's elementary physics."
This growing confidence is the result of progress being made with climate models and deciphering cryptic clues about ancient climate in tree rings, lake sediments and ice cores. Paleo-climate measurements, once unattainable, now offer a record of global temperatures stretching back 750,000 years.
"There is no doubt that humans are warming the planet. That's very clear now," says Jeffrey Severinghaus, a geoscience researcher at the Scripps Institute of Oceanography. "The data is beautiful. It's very strong. Humans are changing the climate, and we're expected to change it a lot more in the future."
Global warming 'alarmists'?
Scientists were not always so convinced.
As early as 1979, the U.S. National Academy of Sciences reported with "high confidence" that a 1.5 to 4.5 degree Celsius temperature increase was likely if carbon dioxide levels doubled. It was greeted by a chorus of skepticism.
However, the past two decades have also seen the retreat of once noisy critics. BP, a major energy company, says it is now taking "precautionary action" against climate change by cutting greenhouse emissions and investing in mitigation of greenhouse gases.
Nonetheless, a minority of scientists reject what they call "alarmist" global warming on scientific grounds. They raise three major objections, which most researchers agree remain troublesome.
Natural climate variability is not well understood and may be greater than once thought.
Computer models are oversimplifications that cannot simulate the complexities of the real climate.
Temperature extrapolations of the past are not precise enough to make dire conclusions about "normal" warming.
Richard Lindzen, a respected meteorologist from the Massachusetts Institute of Technology, says in light of these uncertainties, pronouncements about climate change are both self-serving and unscientific.
"Scientists make meaningless or ambiguous statements. Advocates and media translate statements into alarmist declarations. Politicians respond to alarm by feeding scientists more money," said Lindzen at a scientific conference this January. He added that the accepted evidence is "entirely consistent with there being virtually no problem at all."
This sentiment is in the extreme minority of the scientific community, said Richard Sommerville, meteorologist at the Scripps Institution of Oceanography, who drew a parallel with proposing that HIV does not cause AIDS.
"[Lindzen] is taken seriously because he's capable of excellent science," Sommerville said. "[But] most of the scientific community thinks he's mistaken... People are given a fair hearing and then we move on."
New research
Plenty of questionable scientific claims muddy the discussion on climate change. Extreme weather events such as last year's hurricane season in the Atlantic are not conclusively linked to global warming, say scientists at NASA's Goddard Space Flight Center. It is exceedingly difficult to establishing a causal link between global warming and these events.
Even melting glaciers, such as the rapidly receding ice cap on Africa's Mount Kilimanjaro or the collapse of an Antarctic ice shelf, while consistent with climate change, cannot be decisively linked to the phenomenon.
But major studies released this year appear to buttress the belief the Earth is undergoing significant warming and will continue to do so.
Tim Barnett, a researcher with the Scripps Institute of Oceanography, told the American Association for the Advancement of Science in February that the world's oceans are heating up from the top down. Barnett found the world's six ocean basins show a .5 degree Celsius increase since the 1940s in a pattern that could only be explained by human-induced warming.
An Oxford University report, published in the journal Nature, used computers networked over the Internet to conduct one of the most powerful computer climate simulations ever attempted. It appeared to confirm that predictions of warming of at least 2 degrees Celsius -- and perhaps as high as 11 degrees Celsius -- were possible.
Ultimately, scientists who believe global warming is underway say uncertainties do not undermine the significance of the research.
"When you go to your doctor, and she says you're due for a heart attack, you don't turn around and say medicine is imperfect even if she can't predict the date of your heart attack," Sommerville said. "You take it seriously. I think climate science is in that position now."
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(j/k, after all that's half the fun)
