Heated dispute; Scientists grapple with conflicting measurements but agree that Earth's temperature is on the rise

By Eric Niiler
Copyright 1998 San Diego Union-Tribune
September 30, 1998

Taking the Earth's vital signs would seem to be an easy task.

Backyard weather-watchers check their thermometers daily, while oceangoing drifters, weather balloons and orbiting satellites record the planet's pulse far from human hands.

Every day, these readings are plugged into mathematical models that simulate the Earth's complex climate machine. Yet despite the latest technology and fanciest computer models, scientists still don't understand basic questions about how the Earth's climate works and, more specifically, the global warming trend that they believe is under way.

Is this warming -- about 1 degree Fahrenheit since the late 19th century -- out of the ordinary? If so, is it caused by the waste gases of global economic activity? And why don't temperature reports from satellites, weather ballons and ground stations always add up?

At the moment, this past August ranks as the warmest month on record, followed by July 1998. The warmest years since the late 1800s are 1997, 1990 and 1995. Indirect measurements indicate that, overall, this century is the hottest in 600 years.

A bit of warming might seem benign, but the implications of global climate change are staggering. Scientists already are predicting rising sea levels, coastal flooding, droughts, heat waves, shifting agricultural growing zones and more powerful tropical storms that would make this year's El Nino- generated rains look like a spring shower.

These events may be in store if the Earth's average temperature rises 2 to 6 degrees in the next century, as predicted by some researchers. The rise is linked to a doubling of so-called greenhouse gases -- carbon dioxide and other byproducts of fossil fuel burning -- in the lower atmosphere.

But researchers say gaps in both the measurement and understanding of today's climate -- as well as the climate of the past -- continue to plague them.

For example, most of the land-based temperature records come only from meteorological stations in the Northern Hemisphere. Records go back only a few decades or years in South America, Asia and Africa.

Ice cores that give researchers important clues about the temperature in Greenland and Antarctica thousands of years ago say nothing about the tropics, a region where much of the global warming turmoil may take place and where the Earth's climate engine gathers heat.

And just getting accurate, up-to-date satellite temperature records for the past two decades has turned into a scientific brouhaha slugged out in the pages of academic journals.

"None of the tools we have are great," said Ellen Mosley-Thompson, a climatologist at Ohio State University who is studying the past climate through ice-core measurements.

Despite the drawbacks, Thompson believes that by adding up the pieces, a clear picture of human-induced climate change is emerging.

"We know the weaknesses in our data and we present the caveats," Mosley- Thompson said. "What the public walks away with is that scientists don't know what they're doing, but science is a constantly evolving process. The longer our records get, the closer we will get to answering questions to how unique the 20th and 21st century (climate) is. Our records are so short right now."

Ice, coral and ring tales

To better understand what's happening today, scientists are trying to reconstruct the climate record of the past. Consistent temperature records based on thermometer readings go back only about 100 years.

To learn more of the distant past, investigators must rely on indirect measures: ice cores that trap ancient atmosphere, tree rings that indicate warm, wet years vs. cooler, drier ones and coral reefs that capture a temperature signal of the sea surface centuries ago. Other researchers, meanwhile, dig up fossilized pollen captured in lake sediments or ancient marine fossils for meteorological clues.

One of the best records of the paleo-climate, or past climate, are ice cores. Tiny air bubbles trapped in the vast ice fields of Greenland and Antarctica contain traces of nitrogen, carbon dioxide and methane from as far back as 400,000 years. These core bubbles preserve the past temperature, like a cellar preserving last winter's chill.

To determine historical temperatures, scientists drill several miles into an ice sheet, extracting a long, thin cylinder of ice that is sliced into sections and shipped back to labs in airborne coolers.

In a basement laboratory at the Scripps Institution of Oceanography, Jeff Severinghaus holds up a 4-inch-long, 20,000-year-old hunk of ice. It looks like frost-covered crud chipped out of an old icebox.

At $20 million per core drilling, this is "pretty precious ice," said Severinghaus, a young climatologist who recently came to Scripps from the University of Rhode Island.

Severinghaus and his colleagues melt the ice in a vacuum chamber to release the trapped air. This air is piped through a device called a mass spectrometer to determine its molecular makeup.

The ratio of argon and nitrogen isotopes in the trapped bubbles gives a proxy record of how cold it was when the ice formed. Severinghaus recently discovered that Greenland warmed up 18 degrees in as little as three years at the end of the last ice age, some 11,600 years ago.

Previously, scientists had believed it took thousands of years for Greenland and the rest of the Northern Hemisphere to thaw out. The quick warming was probably accompanied by violent, unstable weather.

Such quick reheating, Severinghaus said, "would be much more destructive to society than warming that took 50 to 100 years."

Severinghaus' efforts to map out Greenland's prehistoric climate have implications today. Computer models predicting future global warming are run using data from the past. Unfortunately, the models don't always agree with the ice-core record. They underestimate the strength of past climate changes and may underestimate the size of future climate changes.

"Most of the models fail," he said. "It's a powerful indictment of their reliability. But that said, the models are getting better."

Upstairs from Severinghaus' lab, Franco Biondi slices up slabs of dead trees. Under his microscope, he looks for a pattern of tree rings getting bigger during warm or wet years and smaller when there is less moisture or cooler.

Biondi has been studying trees from Idaho, California, Arizona, Baja and mainland Mexico. Each kind of tree has a different record. He matches recent meteorological records to recent tree rings to see if they match. If so, he can then go back and use older rings to measure older climate.

The few remaining Torrey pines just north of La Jolla give a record stretching back 300 years. Douglas firs in Idaho go back 900 years, and some Bristlecone pine in the Sierras date back 4,700 years.

Biondi said tree ring studies are best for determining patterns in the climate rather than actual temperature or rainfall numbers.

Another drawback is that most tropical trees don't produce rings, like pine trees. And while some equatorial zones have mountainous regions with trees producing annual rings, the gaps in the climate record at the tropics continue to confound researchers.

Of all the climate scientists, Scripps geophysicist Chris Charles may have one of the more the enjoyable jobs. He and his colleagues travel around the Pacific, plunge into tropical waters with mask and fins, and sample coral reefs with a drill. After taking a sample, they plug the hole with cement to prevent parasites from destroying the living coral.

Upon returning to Scripps, they grind up small pieces of the coral and zap them in a mass spectrometer. By comparing ratios of two oxygen isotopes, they get a precise correlation of the changes in sea surface temperature over the years. Some corals live 300 to 400 years, long before mariners began taking ocean temperatures on sea voyages.

"Coral is just sitting there bathed in sea water," Charles said. "So it really is a beautiful archive we're waiting to tap."

Past and present

These indirect measurements -- ice, tree rings and corals -- are filling in gaps in the the Earth's climate history. In the technologically advanced 20th century, you would think that scientists would have nailed down the temperature record. But it has yet to happen.

Merchant and Navy ships began taking consistent readings at the beginning of the century, but they used several, sometimes conflicting, methods. British meteorologists began launching weather balloons to measure the atmosphere in 1948, but their record-keeping methods weren't consistent, and the readings haven't been reliable as a whole.

Ocean drifters and satellites began scanning the oceans and atmosphere in the 1960s, but only recently have they been able to cover the entire globe.

"Every data set has its problems," said Rob Quayle, chief of the global climate lab, at the National Climactic Data Center in Asheville, N.C. "You need multiple observing systems, and if as a group they are telling about the same thing, that gives you added confidence."

Satellites hold out promise as a high-tech way to measure the temperature of the atmosphere or oceans. They hover above the Earth and can take a wide- angle snapshot of various environmental indicators.

Since 1979, satellites operated by the National Oceanic and Atmospheric Administration (NOAA) have been sampling the troposphere, a layer of the atmosphere that extends from the surface to six miles above it. The orbiting satellites beam microwaves through this layer, measuring tiny vibrations of oxygen molecules given off when exposed to the sun's rays.

Land, ocean and deep-ground measurements have risen in the past century, but this satellite record has bucked that trend. Skeptics have used this contradiction to bolster their argument that global warming is fictional, or so insignificant as to not worry about.

Last month, two California scientists looked at the satellite record and found that the satellites' orbits had decayed by more than mile as they have circled the globe, throwing off their ability to measure temperatures near the Earth's surface.

After correcting for the bias, the scientists said the satellite record showed a slight warming of about .13 degrees per decade. That's compared to the .09 degree per decade cooling reported earlier.

Kevin Trenberth, a climatologist at the National Center for Atmospheric Research in Boulder, Colo., noted that individual satellites only operate three or four years, making direct comparisons over a long time period difficult. Satellites also drift, wobble, their instruments go bad (one device was pointed at outer space instead of Earth) and they are only replaced when they actually fail.

"Satellites are great for giving global perspective, they are absolutely useless in giving long term trends," Trenberth said.

Trenberth also had harsh words for James Christy, a former graduate student of his who put together the "cooling" satellite record along with Roy Spencer of NASA's Marshall Space Flight Center.

"Somebody besides Spencer and Christy should be doing it," Trenberth said. "They can't admit they are wrong. The answer they are giving doesn't make sense with surface measurements and water vapor measurements."

Christy defends his work. He said he has corrected the correction by the California scientists and that his new information shows a slight cooling in the troposphere.

Trenberth "is not a satellite meteorologist," Christy said. "He wouldn't understand. The proof of pudding is independent comparisons."

Christy has resubmitted his work to several research groups for peer review and publication in a journal, the way that scientists validate each other's work.

Many scientists say the lesson of the satellite dispute is that better instruments are needed to measure climate change. Satellites have broad coverage, but their accuracy is in question and clouds or dust can interfere with their sensors.

Drifting buoys take on-site readings of ocean temperature, they are being used to verify satellite readings. There are now more than 600 such bobbing buoys -- resembling 30-foot-long socks full of holes -- drifting throughout the world's oceans.

Scientists recognize that gaps in their knowledge about the climate machine are being used by critics to stall global warming treaties, and they are working hard to eliminate them.

Ralph Kahn, a climate scientist at the NASA/Caltech Jet Propulsion Laboratory in Pasadena, has built a new satellite device that will measure the Earth's climate when it is launched next year.

"We aren't measuring temperature accurately enough and in enough places to know what's going on," he said. "But it's getting better and we keep diminishing the uncertainties."

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