Core Evidence That Humans Affect Climate Change
Ice drilled in Antarctica offers the fullest record of glacial cycles and greenhouse gas levels.
By Usha Lee McFarling
Times Staff Writer
November 25, 2005
AN ICE CORE ABOUT two miles long — the oldest frozen sample ever drilled from the underbelly of Antarctica — shows that at no time in the last 650,000 years have levels of the greenhouse gases carbon dioxide and methane been as high as they are today.
The research, published in today's issue of the journal Science, describes the content of the greenhouse gases within the core and shows that carbon dioxide levels today are 27% higher than they have been in the last 650,000 years and levels of methane, an even more powerful greenhouse gas, are 130% higher, said Thomas Stocker, a climate researcher at the University of Bern and senior member of the European team that wrote two papers based on the core.
The work provides more evidence that human activity since the Industrial Revolution has significantly altered the planet's climate system, scientists said. "This is saying, 'Yeah, we had it right.' We can pound on the table harder and say, 'This is real,' " said Richard Alley, a Penn State University geophysicist and expert on ice cores who was not involved with the analysis.
Previous records, from an ice core drilled at the Russian Antarctic station Vostok, extended back 440,000 years. Extracting and analyzing that core was a major achievement, but the core stopped short of a time period scientists are anxious to study because it was like today's.
Climate scientists called the analysis of the older records spectacular because they were so clear and said they would become "canonical" additions to the climate record. "It's really important," Ed Brook, an ice core expert at Oregon State University said of the new research. "Those 200,000 years were a lot harder to get than the previous 400,000 — and those were hard enough."
Ice cores are plugs drilled from glaciers and ice sheets. They are composed of tens of thousands of layers of fallen snow and air bubbles compressed over time. Ice cores are among the most powerful tools available to climate scientists. The chemistry of the ice reveals temperatures from the distant past, while bubbles within the ice are minuscule time capsules that capture air and greenhouse gases as they existed hundreds of thousands of years ago.
The ice core was drilled by the European Project for Ice Coring in Antarctica from a high plateau in East Antarctica called Dome C that rises about two miles above sea level. It is one of the driest, coldest parts of the continent, where summer temperatures can fall to 50 degrees below zero. Temperature records from the core were published in 2004, and scientists have been waiting for an analysis of the core's gases.
The last time carbon dioxide levels were as high or higher than today was probably tens of millions of years ago, Alley said. Over millions of years, carbon dioxide levels shift because of slow geological processes, such as weathering of rocks, swallowing of crust into subduction zones and the release of gases from volcanoes. But these processes are much slower and more gradual than the current rapid increase of carbon dioxide into the atmosphere from the burning of fossil fuels, Alley said.
Scientists are enthusiastic about the ice core because it includes about eight full glacial cycles. The Vostok sample had four. Glacial cycles occur roughly every 100,000 years and include long periods of cold, when ice ages occur, and brief, warm interglacial periods, such as the current one. The cycles are controlled by shakes, wobbles and tilts in the Earth's orbit around the sun that determine the amount of sunlight falling on and warming the planet.
The Vostok core showed that warm interglacial periods lasted about 10,000 years. Because the current temperate interglacial period has lasted about 12,000 years, many scientists had speculated that the planet was overdue for an ice age.
But the new core shows that the interglacial period of 440,000 years ago, when the Earth's position relative to the sun was similar to what it is today, lasted nearly 30,000 years and was not ended by natural decreases in carbon dioxide, Stocker said. The work suggests that the next ice age is about 15,000 years away.
"Anyone counting on an ice age to head off global warming, or hoping to justify human greenhouse-gas emissions as a useful attempt to head off the next ice age, will find no comfort in the ice-core record," Alley said.
The latest findings also run counter to a theory presented two years ago by William Ruddiman, a professor emeritus of environmental sciences at the University of Virginia, that humans who lived five thousand or more years ago are responsible for delaying the next ice age because their activities — forest clearing and rice growing — started to raise greenhouse gas levels when they should have been naturally declining.
"This claim can no longer be upheld," said Stocker, because the ice core shows greenhouse gases do not naturally decline after 10,000 years in the longer interglacial periods like today's.
Scientists are eager to look further back into earth's climatic past. About a million years ago, the earth shifted from ice age cycles that were 40,000 years long into cycles that were 100,000 years long. This shift from a "40K world to a 100K world" is a major mystery, said Oregon State's Brook, and will require a core that reaches deeper into the ice and much further back in time.
Brook is co-chairman of a joint European and American group that hopes to start drilling in coming years a core that could produce ice and bubbles that are 1.2 million to 1.5 million years old.