Earth's uneasy breathing measured on Niwot Ridge

Gases collected at 11,500 feet reveal nature's responses to warming world

Jim Erickson, Rocky Mountain News

Published December 13, 2005 at midnight

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NIWOT RIDGE — Searching for signs of global climate change in nature goes beyond studies of receding mountain glaciers, thinning Arctic sea ice, shifting tree lines or quirky animal behavior.

Invisible gases trapped in glass flasks also have a story to tell.

Samples of thin mountain air collected on this windswept crest west of Boulder, and gases monitored continuously with tower-mounted sensors in the forest 1,500 feet below, are revealing nature's responses to a warming world, climate scientists say.

Some of the trends Colorado scientists have spotted could be bad news for the future health of the state's mountain forests, they say.

Analyzing the chemical fingerprint of forest and mountaintop gases is like holding a giant stethoscope to the planet and hearing its every heartbeat and breath. And some of the planet's vital signs are wavering.

"The breathing of the biosphere is what you're seeing there at Niwot Ridge," said James Randerson, an associate professor of earth system science at the University of California at Irvine.

"In the northern part of the Northern Hemisphere, there's a strong signal of a lengthening of the growing season," Randerson said. "It's a clear signal that ecosystems are responding to changes in climate, and you're tapping into that at Niwot."

The Niwot Ridge flasks have been collected just below the summit, at 11,500 feet, since 1968. That data archive constitutes the world's third- longest record of atmospheric carbon dioxide, one of the heat-trapping "greenhouse" gases blamed for global warming.

And in the conifer forest below the Niwot summit, at 10,000 feet, sensors mounted on a 100-foot tower track with precision the carbon dioxide pulled in by the trees. In the presence of sunlight, trees and other plants combine carbon dioxide and water to make sugars and other carbohydrates in a process called photosynthesis.

To biologists, spring begins when plants emerge from winter dormancy and fire up the photosynthesis factory, sucking carbon dioxide from the air and launching the annual growth cycle.

At the so-called Ameriflux tower in the forest below Niwot Ridge, that transition to spring has been arriving earlier and earlier.

The findings suggest that between 1980 and 2002, the biological start of spring advanced by about 10.5 days, according to David Schimel, an ecologist with the National Center for Atmospheric Research in Boulder.

At a weather station several hundred yards from the tower, average spring temperatures rose 5.2 degrees Fahrenheit between 1968 and 2003, according to records maintained by the University of Colorado's Mountain Research Station.

So the trees seem to be responding to earlier, warmer springs.

Similar trends have been reported at widely scattered monitoring stations across the Northern Hemisphere. The observations, taken together, suggest that anthropogenic, or human-caused, climate change is partly responsible, Schimel said.

"I believe that the signals we're seeing in CO₂(carbon dioxide) are a signature of anthropogenic global climate change," Schimel said. "I think they're fully consistent with that hypothesis."

Early springs are bad news

But other researchers aren't convinced that the forest-growth trends can be linked to tailpipes, power- plant smokestacks and other sources of heat-trapping emissions produced by the burning of fossil fuels.

CU ecologist Russell Monson works with Schimel at the Ameriflux tower and has pored over the very same data.

"Things are warming up earlier," he said. "Is it due to anthropogenic emissions of carbon compounds and changes in the atmosphere because of that, or is this due to some natural climatic cycle?

"That's where you begin to cross from objective analysis into more subjective interpretation on some of this stuff."

Whatever is causing the shift to earlier springs, the trend is bad news for Colorado forests, Monson and Schimel agree.

Earlier springs are usually associated with hotter, drier summers, Schimel said. That leads to weakened, water-stressed trees, which are more susceptible to diseases, insect predation and wildfires.

In addition, water-stressed trees can't photosynthesize at full throttle, so they're not pulling as much carbon dioxide from the air as they normally would. In the coming decades, if carbon dioxide levels continue to mount as predicted, water-stressed forests will be less able to remove unwanted gases from the air.

"Earlier springs are not good for this forest," Monson said. "Earlier springs actually cause our forest to accumulate less carbon dioxide during the entire year."

The trends seen at the Ameriflux tower prevail over much of the Northern Hemisphere, Schimel said. And the samples from the top of Niwot Ridge provide a glimpse at what's happening in forests hundreds — even thousands — of miles away.

The telltale Niwot flasks

Every Tuesday, rain or snow or shine, the Mountain Research Station's Mark Losleben and his colleagues travel four miles to the top of Niwot Ridge, along the Continental Divide northwest of Nederland, to fill flasks and gather weather data.

The soft hum of a solar-powered electric pump breaks the mountaintop stillness as Losleben flushes fresh air through a 2.6-quart flask, then stoppers it and pumps in air from the 11,500-foot monitoring site.

The same process is completed at more than 60 sites around the globe. All the flasks are sent to the U.S. government's Earth Systems Research Laboratory in Boulder, where levels of carbon dioxide and other climatically important atmospheric gases are painstakingly measured.

The Boulder lab's records chronicle the steady global rise of atmospheric carbon dioxide since chemist Charles David Keeling's first flask measurements atop Hawaii's Mauna Loa volcano in 1958. Most of the increase is attributed to the burning of fossil fuels like coal and oil.

Remarkably, the carbon dioxide records also show that plants in the Northern Hemisphere seem to be responding to warming temperatures by reawakening earlier each spring and remaining active later into the fall. In research papers published in the late 1990s, Keeling and his colleagues found that the overall growing season had lengthened about 12 days in recent decades.

The change is apparent in carbon dioxide records from far-off sites like Mauna Loa, Alaska's Point Barrow and Midway Island in the mid-Pacific, said the University of California's Randerson.

But the growing-season shift also can be seen in data collected in Denver's backyard at Niwot Ridge, said Pieter Tans, head of the carbon cycle and greenhouse gas group at the Boulder lab, which is part of the National Oceanic and Atmospheric Administration.

At the request of the Rocky Mountain News, Tans analyzed the Niwot Ridge carbon dioxide record to see if it showed significant changes in the length of the growing season.

He found that spring now arrives much earlier than it did in 1968, fall lingers significantly later, and the overall growing season has lengthened by about a month.

"It's shocking. It's a full month. It was more than I expected," Tans said of his preliminary and still unpublished findings.

But the air that passes over Niwot Ridge, where prevailing winds are from the west, holds a mix of carbon dioxide signals from both local and far-off sources.

So it's telling scientists what's happening in the Colorado Rockies and the West, as well as places as far away as Siberia, Randerson said.

Niwot Ridge sits at about 40 degrees north latitude. Randerson analyzed the carbon dioxide signal from Niwot Ridge and determined that two-thirds of it originates in regions between 40 and 60 degrees north latitude. The 60-degree latitude band passes near Anchorage, Alaska; Leningrad, Russia; and Oslo, Norway.

Evidence for global climate change has been strongest in the Arctic and the high latitudes of the Northern Hemisphere.

"The ecosystems are responding to the increases in temperature that have been measured across the northern part of the Northern Hemisphere," Randerson said.

"And Niwot Ridge, because it's so high up, is actually getting contributions from that whole northern part of the Northern Hemisphere," he said. "So it's a unique record. . . . It's a compelling record of change."

Seasonal cycles

How can atmospheric carbon dioxide levels reveal changes in plant behavior?

Remember how photosynthesis works.

Each spring, when leafy plants bud and conifers emerge from winter dormancy, they suck up carbon dioxide, so atmospheric levels plunge. When photosynthesis fizzles in the fall, atmospheric carbon dioxide levels soar.

Global carbon dioxide levels are steadily rising and have jumped from 278 parts per million in pre-industrial times to about 380 ppm today. Air bubbles trapped in Antarctic ice cores — and the nation's largest repository of those cores is at the Federal Center in Lakewood — show that current carbon dioxide levels are higher than they have been in more than 450,000 years.

But when the recent CO₂ surge is plotted on a graph, the relentless rise forms a jagged, sawtooth pattern, not a smooth diagonal line.

The serrations reflect the seasonal rise and fall in carbon dioxide each year. The jagged pattern has long been known to scientists as the Keeling Curve, after the climate science pioneer who died in June.

In a paper published in 1996, Keeling first noted that the springtime carbon dioxide drop was occurring earlier than it used to in the Northern Hemisphere.

Those early results were viewed as "very speculative" by other climate scientists, Schimel said. But the findings have been corroborated by more recent studies, he said.

As carbon dioxide levels continue to rise, Schimel, Monson and other researchers will monitor the forest's response at the Ameriflux tower. And on the ridge 1,500 feet above, Losleben and his colleagues will fill glass flasks every Tuesday at a site called the T-Van.

Losleben heads the Mountain Research Station's climate program, which has its roots in a network of backcountry weather stations CU biologist John Marr began establishing in 1951.

A few years later, Marr commandeered several surplus Army vans left over from the Korean War. The vans were driven up Niwot Ridge and parked at various elevations. Their wheels were removed and they became shelters used by mountain researchers.

One of those vans, the T-Van, is parked at 11,500 feet above sea level. T-Van may be short for tree-line van — no one's sure about the origin of the name, Losleben said. A solar-powered pump, its battery and other flask-sampling paraphernalia are stored in a plywood shack next to the van.

"All of the flasks from around the world are sent to Boulder," Losleben said, gazing down at the sprawling urbanized flatlands to the east. "It's just that our mailing costs are a lot less."

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