Melting glaciers ... rising sea levels ... inundated coastlines: Those are the nightmare scenarios for global warming. But for now, the investigation into climate change is more like a detective story than a science-fiction tale. Scientists are hunting for clues about the origins and impact of the greenhouse effect — and trying to crack the case before fear takes hold.
There's no disputing that the planet’s climate is changing: Global mean temperatures have risen 1 degree Fahrenheit over the past century. Researchers have declared last year to be the warmest in 120 years. Analyses of ice cores and tree rings have led experts to conclude that the 1990s rank as the warmest decade in 1,000 years. One study even contends, on the basis of readings from a 2.2-mile-deep ice core from Antarctica, that the current climate trend is unprecedented in the past 420,000 years.
Figuring out the cause and effect is trickier, however - and that’s the root of the controversy over climate change.
In the U.S., public opinion is evenly divided over whether anything should be done about global warming, according to an NBC/Wall Street Journal poll: Fifty-one percent of the 500 surveyed in late July said action should be taken, another 43 percent said no action should be taken, and 6 percent were unsure.
But how much do people know about the current bout of global warming? Is it primarily the result of a natural cycle beyond our control, or is it mostly our fault? What’s the potential impact, today and in the longer term? Here’s a progress report on the search for answers:
The cause
The “greenhouse effect” provides the link between human activity and global warming: Scientists know that some gases in the atmosphere - such as carbon dioxide and methane - can retain heat and re-radiate it to Earth’s surface, just as glass bounces heat back within a greenhouse.
Scientific sleuths have compared today’s atmosphere with bubbles of air trapped within layers of ice, building up a “fossil record” of the atmosphere’s composition. Their findings: Greenhouse-gas levels have risen significantly since pre-industrial times. Carbon dioxide, for example, has gone up from about 270 parts per million to more than 360 parts per million.
Other investigators have used carbon-dating techniques and computer simulations to conclude that much of the greenhouse-gas rise is due to human activity, primarily fossil-fuel burning, agricultural practices and deforestation.
This is the basis for making humans the prime suspects behind the rising temperatures. But that doesn’t mean the case has been proven beyond a reasonable doubt. Patterns in solar activity and even the earth’s changing orbit also affect global climate.
Two reports in the Aug. 15 issue of Geophysical Research Letters indicate that solar activity could account for 15 to 50 percent of the temperature rise over the past century or more, based on computer simulations. But both reports also say the solar factor has played much less of a role in recent years - which supports the idea that the human factor is playing much more of a role.
The current computerized models indicate that humans are responsible for two-thirds of the world’s warming trend, says Tom Wigley, senior climate scientist at the National Center for Atmospheric Research.
Simulation efforts
How far can the trend go? To answer that question, scientists develop computer simulations that match past patterns and extend those simulations into the future. The current models come up with a 3- to 5-degree rise in global temperatures over the next century.
Because of the interplay of ocean currents, atmospheric circulation and geography, scientists say a continued warming trend would have a stronger impact on the Northern Hemisphere than on the Southern Hemisphere.
But historically, Mother Nature has found ways to compensate for climate shifts: One study has shown that increased carbon dioxide levels stimulate forest and plant growth, which would convert more CO2 into oxygen. Another study claims that an earlier round of global warming, 8,200 years ago, melted a huge ice dam in North America, releasing a cooling flood.
Wigley says computerized climate simulations take such factors as increased vegetative growth into account. Nevertheless, the complications illustrate that the outlook for future global warming isn’t an open-and-shut case.
Debating the link
“Is Earth’s climate changing? The answer is, of course. It has been and it always will be in the future,” says Florida Tech professor George Maul, who has studied the environmental effects of warmer ocean temperatures. “The real question is whether human influence is causing climate change.”
Maul characterizes himself as a “fence-sitter” on that question, and says better scientific data will have to be developed. Other scientists, however, worry that time is ticking away.
“Even if we stop burning fossil fuel and putting CO2 in the atmosphere, I know from the scientific studies that the lifetime of the CO2 we’re putting in is about a century - so I know we have to live with our mistakes for a long time,” says Ken Denman of Canada’s Institute of Ocean Sciences, who was one of the lead authors for a 1995 international report on climate change.
“In that sense, there’s an urgency.”
The effect
What would a warmer world look like? We’re already living with the effects of warmer global temperatures - and if the greenhouse effect keeps those temperatures on the rise, the changes could be even more dramatic. Other factors complicate the forecast, however:
Weather worries
Summer heat waves and drought ... warmer winters with more precipitation in the upper Midwest and the Northeast. Does that sound like a familiar forecast? Climate scientists say it’s just what you would expect in the United States in a global warming scenario.
The latest computer simulations indicate that the future warming effect in the United States could be even more pronounced than the global average.
Ironically, the models show that sulfur dioxide emissions have a cooling effect on the atmosphere, and as those pollutants are reduced, the warming trend strengthens. Global warming also produces secondary effects in atmospheric circulation patterns, says Drew Shindell of NASA’s Goddard Institute for Space Science.
For example, Shindell’s computer model indicates that warm marine air would blow more rapidly over northern land masses - causing a more pronounced trend of warmer, wetter winters in western North America and Eurasia.
“In 30 years more, we could have not much permafrost left in Alaska,” he says.
On the other hand, some experts contend that global warming could be a good thing for some regions: Stanford scholar Thomas Moore, for example, notes that there would be fewer winter frosts and longer growing seasons in the United States.
In any case, researchers emphasize that cyclical patterns of oceanic and atmospheric circulation can affect weather more dramatically than long-term global warming. El Nino and La Nina patterns are just the most vivid examples of such patterns. Separating out all the influences is devilishly difficult.
“This definitely can give you a foretaste of what it would be like if the climate models are coming true,” says Janine Bloomfield, staff scientist for the Environmental Defense Fund. “But can we say this is definitely global warming? That may be a little premature.”
Water worries
The retreat of glaciers, melting ocean ice and disintegrating ice shelves in the Arctic and Antarctic provide the clearest evidence of extraordinary global warming, Bloomfield says. Some of that melting ice has endured for centuries.
The breakup of Antarctica’s Larsen B Ice Shelf could be just the beginning: Bloomfield points out that some urban areas depend on snowpacks for their water supply. Rapid melting of such snowpacks, plus a greater chance of summer drought, could lead to more frequent water shortages.
Then there’s the threat of higher sea levels, fed by the melting ice. The Environmental Defense Fund has worked out a whole report on how rising sea levels and increased flooding could endanger New York City in 2100.
But Maul says it’s easy to exaggerate the threat. He takes a skeptical view of sea-level projections, pointing out that land masses rise and subside all the time.
Life as we know it
A host of environmental changes have been attributed to warmer temperatures on land and in the ocean.
On the West Coast, the range of some salmon species already has moved northward, Denman says. A NASA study, based on satellite observations, has found that warmer temperatures have pushed the spring growing season a full week earlier from the Great Lakes region to Alaska. The nesting grounds for birds in Britain are slowly moving northward. Coral reefs around the world are showing increased signs of bleaching, an indicator of potentially fatal stress.
In the longer term, warmer oceans could hamper the flow of nutrients upward from the cold, deep ocean - and hurt the marine food chain at its lowest levels. The “comfort zone” for fish, wildlife and crops in the Northern Hemisphere could continue to shift northward, forcing changes in the economic balance of power.
But scientists say their powers of prediction are limited, particularly when it comes to the environmental impacts.
“It’s very difficult to show a relationship between climate per se and human influences, and it becomes much harder when you’re talking about the effects of climate on the biological environment. ... There’s no doubt that things are going to pop up and become painfully obvious to some people somewhere in the not-too-distant future, but pinning down where this is going to happen is somewhat tricky,” Wigley says.
Maul says he believes localized human factors such as urban “heat islands” and water pollution will far outweigh the impact of global climate change in the foreseeable future. Coral bleaching, for example, could be caused by “the critters that we’re pumping into the system, just because we’re flushing toilets,” he said.
Even though he considers himself a fence-sitter, Maul says he still favors strategies to reduce greenhouse-gas emissions and other sources of pollution - “just because it’s good stewardship.”
“This notion of sustainable development is good thinking,” he says. “It’s the right direction to go. But I think the idea that we understand the problem and know how to fix it and take action is more political decision than a scientific decision at this point.”
Alan Boyle is MSNBC’s Science Editor.