Science & the Public: Study recalibrates trees' carbon uptake

Finding, based on using heavy oxygen as a photosynthetic yardstick, could alter climate projections.Web edition : Wednesday, October 5th, 2011 If trees inhale faster . . . Apparent earlier underestimate of how quickly trees and other plants take in carbon for photosynthesis could have climate implications.iStockphoto

Photosynthesis appears to be somewhat speedier than conventional wisdom had suggested, a new study finds.  If true, this could mean that computer projections are at risk of overestimating the potential for forests to sop up carbon dioxide, a major greenhouse gas.

The new study did not measure photosynthesis directly. It instead deduced photosynthetic rates from subtle variations in the molecular weight of carbon dioxide molecules in air. Samples had been collected from across the globe during a 30 year period.

Some CO2 molecules tip the scales more than usual because one of their oxygen atoms has a molecular weight of 18, not 16. (The heavies pack an extra pair of neutrons.) The likelihood that an oxygen atom in CO2 will be an O18 will depend on the proportion of heavy oxygen in a region’s water, explains biogeochemist Lisa Welp of the Scripps Institution of Oceanography in La Jolla, Calif. That ratio can vary by soil moisture and weather conditions, she explains.

As plants breathe CO2 into their leaves, that CO2 will exchange its oxygen atoms with those in water, Welp notes. A substantial amount of that CO2 will eventually be released back into the air, now bearing an O18-to-O16 ratio reflective of the plant’s water.

In the September 29 Nature, Welp — and colleagues on three continents — report finding a subtle change in the proportion of CO2 molecules hosting heavy oxygen. This anomaly appeared to start in the tropics and then quickly spread across the planet. The pattern then repeats, almost in waves.

Each wave lasted about 18 months. And the start of a new wave every four years or so generally coincided with the emergence of a prolonged spell of unusually warm ocean temperatures in the Equatorial Pacific — a climatic event known as an El Niño. Explains Welp, “We find that water’s oxygen isotopes get heavier during El Niños in the tropics.”

Using the average ratio of heavy-to-normal oxygen in CO2 (linked to an El Nino event that could be timed), Welp’s group now had a means to evaluate how long it takes plants to transfer an anomalous oxygen signature into atmospheric CO2 — and then wash it away again once an El Niño ended. With this information, the researchers attempted to validate a fairly well accepted estimate of the global rate of carbon taken up by photosynthesis in plants each year —120 petagrams (peta being 1015).

Their assessment found the 120-petagram figure looked short — by about 25 to 45 percent. As to how such a revision in the rate of carbon cycling through plants might alter estimates of future long-term carbon sequestration by forests, Welp emphasizes: "We don't know yet. It's way too early to tell."

But Matthias Cuntz, a biogeochemist with the UFZ-Helmholtz Centre for Environmental Research in Leipzig, Germany, argues that in fact, carbon-storage implications of the new numbers are not that hard to fathom.

The global value for carbon sequestered long term in plant tissue each year is fairly well established at about 1.6 billion tons, he says. That’s almost 2 percent of the 120 petagram estimate. So if the carbon throughput is revised upward by 25 to 45 percent, then the amount of carbon being sequestered long term must be substantially less than 2 percent, Cuntz says – perhaps “only about 1 percent.”

In a commentary accompanying the new Nature paper, Cuntz likens the old 120-petagram figure to a “gold standard” for the annual rate at which land plants take in carbon for photosynthesis. If Welp’s team is right, he now argues, it has just put “a dent” in that gold standard.

The new estimates in the Nature paper do, however, rest on a lot of assumptions – albeit smart ones, Cuntz explains in his commentary. So the rather unexpected result that Welp and her colleagues report will certainly need confirmation.

But if the new numbers hold up, he told me, it could mean that current computer climate models rely on overly optimistic estimates about how efficiently trees can sequester carbon. “If you change photosynthesis a little, like this,” he says, “it could lead to huge differences [in projections of how climate might change in the future].”

Indeed, it would argue that reining in global warming will prove much harder than biologists had led us to expect.

Found in: Chemistry, Climate Change, Earth Science, Environment, Molecules and Science & Society

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Science & the Public: Sarah’s tale of Arctic warming

Observations from one remote village as its climate changesWeb edition : Thursday, October 20th, 2011 Recalling cooler timesSarah James brings stories of change affecting her Caribou People in Alaska.J. Raloff

MIAMI Sarah James remembers playing outdoors as a child even in the dead of winter. It often hit -70 degrees Fahrenheit, although not for prolonged periods. But she and the other kids, tired of being house-bound, just bundled up in snow suits and boots made from caribou skins. Even such frigid temps never put life — or play — on hold, recalls this 67-year-old resident of Arctic Village, Alaska.

Today, winter cold snaps seldom dip below -40 degrees, she says. And snows, that used to begin falling in August or September, now typically arrive in late October or November.

Over a half-century or so, her town of some 150 Athabascan Indians has watched as the formerly extreme but fairly predictable climate in this amazingly remote region of inland Alaska has become warmer and more erratic. Overall, that’s definitely not been a change for the better, James says.

She ventured to South Florida this week — and the Society of Environmental Journalists’ annual meeting — to describe what it’s like to weather life on the frontlines of climate change.

James grew up on a flat tundra range above the Arctic Circle. In her youth, the local vegetation was several inches to a few feet tall. And just inches down, the soil was permanently frozen. No more. Cottonwoods and willows have migrated in, sending up trunks 10 feet high or more. Understory shrubs followed. With the taller, woodier vegetation came an influx of bears and beavers.

That at least has brought some benefits, James claims. Like more meat. In her community with no running water, no agriculture to speak of and no roads, people still largely live off of the land. That means eating caribou, moose, ducks, fish and small mammals. Including beaver.

The villagers catch game whenever it’s available, then smoke it or freeze it for meals throughout the rest of the year. Arctic Village built a huge solar-powered municipal walk-in freezer during the 1970s, about 24 feet on a side. It circulated a fluid that was chilled as it ran through the frozen soil and then released that cold to keep the stored meats rock solid. But after two decades this cold locker had to be abandoned. The melting permafrost wasn’t keeping the chiller chilly enough. Today, James says, people use diesel-powered electricity to run home freezers. And it’s expensive, since that diesel fuel all arrives by plane.

Another byproduct of the melting permafrost: erosion of shorelines along lakes and creeks.

Always fairly drought-ridden, inland Alaska increasingly has also been plagued by fires — especially now that the invasion of trees had added extra tinder. Smoke associated with the fires can shut down air traffic through the area. And that can delay deliveries of fresh produce and fuel — or medical evacuations of the sick or injured. Arctic Village is one of the most remote spots in North America, James says. And the closest hospital, she notes, is in Fairbanks, a two-hour flight away.

For decades, scientists have been explaining that any global warming will be exaggerated in the polar regions, especially the Arctic. But reading about that in dry research papers and reports by the Intergovernmental Panel on Climate Change are quite different from listening to a woman describe how the norms of climate in her part of the world have been turned topsy turvy. “Sometimes in rains in January,” James says, “or snows in July. We just don’t know what to expect from one day to another,” she told me across the breakfast table this morning.

“We’re people of the caribou,” the quiet-voiced woman explains. “We believe our people were put on this Earth to protect our caribou.” And to date, she maintains, her Gwich’in tribe of some 8,000 individuals  —  spread across 15 villages in northeastern Alaska and northwestern Canada — think they’ve been managing pretty well.

In 1988, “our tribal elders came together to protect the coastal plain of the Arctic National Wildlife Refuge, the birthing ground of our caribou,” she says. Even back then, she says, “they were concerned about the many changes that were happening. They were talking about climate change and warming — and that’s one reason they organized against oil and gas development.”

Her community is still arguing against it. It’s also one themes James plans to talk about during an October 21 session, here, at the SEJ meeting about climate impacts on indigenous communities.

Found in: Climate Change, Environment and Science & Society

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Science & the Public: Arctic ozone: ‘Hole’ or just not whole?

Some scientists argue the far North’s ozone merely thinned.Web edition : Tuesday, October 4th, 2011 This past spring, the Arctic stratosphere’s ozone layer suffered unprecedented depletion. But whether the record loss constituted a “hole” depends on which experts you consult.

In a Nature paper published online earlier this week, Gloria Manney of NASA’s Jet Propulsion Laboratory in Pasadena and more than two dozen coauthors describe the 2011 loss as “an Arctic ozone hole.” Other renowned scientists have been weighing in — and some argue that as dramatic as this year's thinning was, a hole it wasn't.

Reports of a putative hole in the far North’s ozone are far from new. A quarter century ago to this day, Science News ran a story noting that “while everyone’s attention has been riveted on the atmosphere above Antarctica, a NASA researcher has discovered what he believes is another ozone cavity that forms each [winter] on the other side of the world. . . . This Arctic ozone hole is ‘not as large in magnitude, but it’s unquestionably there.'"

Since then, descriptions of the recurrent depletion of Arctic ozone have been scaled back to more of just a demonstrable thinning. There's been little question that its triggers, however, are identical to those that seasonably eat away huge portions of Antarctica’s stratospheric ozone.

What made 2011 different — and a watershed — argues Michelle Santee (a JPL colleague of Manney's and coauthor on the new paper), is that at long last, “the magnitude of the [Arctic] loss is comparable to that in the early Antarctic ozone holes in the mid 1980s.”

Santee observes that “the actual definition of an ozone hole has never been codified, even for the Antarctic.” Over the past several decades, however, a de facto rule of thumb has developed. It's measured in terms of the total ozone throughout a column of the atmosphere spanning from Earth’s surface up to satellite height. Such composite measurements are logged in Dobson units, and 220 is about the minimum needed to constitute a hole.

Ordinarily, Santee says, 450 Dobson units “is sort of the canonical value for the Northern polar region’s ozone — what would be your sort of basal level.” This year, she notes, total-column Arctic ozone values “were below 250 Dobson units for nearly a month — and reached 220 to 230 for about a week.”

Geir Braathen, senior scientific officer with the World Meteorological Organization in Geneva, concurs that “scientists have not agreed on any threshold ozone loss, like 250 or 260 Dobson units [for a hole].” Still, this atmospheric chemist cautions, “I would be careful about calling the Arctic depletion an ozone hole” because it might lead people to think it's comparable to what emerges in the Antarctic. And it isn’t.

Antarctica's hole recurs annually, whereas mega-thinning in Arctic ozone is novel. Antarctica’s ozone also thins at some point to zero in a band many kilometers high. At no altitude has Arctic ozone ever fallen to zero — even in 2011. Finally, Braathen points out, the aerial expanse and depth of the Antarctic hole greatly dwarfs the Arctic region that experienced substantial thinning earlier this year.

“Going into this Arctic spring, many of us — myself included — really thought this might be the year that we would see a real Arctic ozone hole,” observed Susan Solomon, of the University of Colorado, Boulder, at the recent American Chemical Society meeting in Denver. "But in the end," she says, "I think it’s fair to say that we didn’t.”

It may be a matter of semantics, she concedes, but there was a rapid resupply of ozone from outside the Arctic vortex (that swirling wall of winds in the stratosphere that largely corrals a patch of atmosphere, rendering it vulnerable to ozone-destroying chemical reactions). Such a resupply does not occur in the Antarctic vortex, she notes; and that's what permits its stratospheric ozone concentrations to plummet to zero over a several-kilometer height.

So, although the new paper clearly demonstrates that at some altitudes Arctic ozone was efficiently destroyed, Solomon says, “I wouldn’t call this an ozone hole.” 

Whatever you call 2011's Arctic ozone depletion, “I consider this to be a ‘big deal,’” says Ross Salawitch of the University of Maryland in College Park. Moreover, there’s no reason to suspect that in some future years, the losses won't be substantially worse, he says. 

Warming surface temperatures can cool the stratosphere, promoting conditions that accelerate ozone depletion, he notes. It takes prolonged cold temperatures in the winter and spring stratosphere to maximize ozone losses. Although such conditions have not been recurring annually in the Arctic, they have returned at three- to four-year intervals. And each new cold snap has been more extreme than the last, Salawitch points out.

Reinforcing concerns about future ozone depletion, Braathen says, is recognition that for many years to come there will be more than enough chlorine- and bromine-based pollutants in the stratosphere to allow for the possibility of “complete destruction of ozone — even in the Arctic — if it’s cold enough.”

Found in: Earth Science, Environment and Science & Society

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Science & the Public: HIPPO reveals climate surprises

Swooping pole-to-pole plane flights uncover unexpected trends in pollutant releases and spreadWeb edition : Thursday, September 8th, 2011 Pollutant snifferThis aircraft sampled air at different altitudes along paths that ran from pole to pole looking for climate-altering pollutants.©UCAR/Carlye Calvin

A major pollution-mapping program that ends September 9 has turned up startling trends in climate-warming gases and soot. The data it collected over the past five years from a National Science Foundation aircraft show the tropics periodically belch huge plumes of nitrous oxide — a potent greenhouse gas — into the upper atmosphere. Arctic measurements show that the recent record summer retreats of ice cover have allowed seas there to exhale unexpected amounts of methane, another potent greenhouse gas.

Then there’s soot. Parts of the supposedly pristine Arctic skies host dense clouds of these black carbon particles. During some flights, “We were immersed in essentially clouds of black carbon that were dense enough that you could barely see the ground,” recalls Stephen Wofsy of Harvard University, a principal investigator in the program. “It was like landing in Los Angeles — except that you were 8 kilometers above the surface of the Arctic Ocean.”

Until a few years ago, scientists interested in mapping global emissions of climate-altering pollutants had to rely on Earth-based sensors or satellites’ eyes on the skies. Neither could identify at what altitude the pollutants tended to congregate. They also missed many highly localized or seasonal plumes of natural pollutants.

That all changed when a federal-university research partnership got access to NSF’s research plane: HIAPER (for High Performance Instrumented Airborne Platform for Environmental Research). Throughout a number of periodic runs, this aircraft repeatedly swooped up and down — from 150 meters above Earth’s surface to heights sometimes exceeding 13.7 kilometers (45,000 feet). All along the way, its instruments measured more than 50 greenhouse gases and black carbon.

The unparalleled altitude- and latitude- specific data collected as part of this program — named HIPPO (for HIAPER Pole-to-Pole Observations) — will soon be made available to researchers generally, notes Wofsy. He expects scientists will mine its data for many years, looking for additional climate trends.

Sky-truthing carbon dioxide levels
A primary goal of HIPPO was to investigate how well airborne pollutant concentrations match what computer models had predicted should exist. In some cases, as for soot, HIPPO data pointed to serious problems — oversimplifications — in those models. In other instances, such as for oxygen movement in and out of oceans, the new data generally validated computer predictions.

Currently, land plants and the oceans absorb roughly half of all carbon dioxide emitted, notes Britton Stephens, a scientist with the National Center on Atmospheric Research in Boulder, Colo. But details on which parts of which ecosystems do it, under what circumstances and how efficiently remains somewhat of an open book. Simply put: “We don’t understand their behavior at the current time well enough to predict their behavior into the future,” he says.

So airborne observations have been repeatedly compared to what computer models predict. And one example of where the models need fine tuning involves carbon dioxide, HIPPO indicates.

Little ice, lots of methaneHIPPO ties unexpected release of methane to loss of summer Arctic sea-ice cover (and 2011 may be record loss, Wofsy says).iStockPhoto

It revealed “large plumes of carbon dioxide over the Arctic,” Stephens reported Sept. 7 at a news briefing. These plumes didn’t come from the Arctic, he says, but bled into Arctic skies from industrial centers throughout the Northern Hemisphere.

“This was a bit of a surprise,” he says, because models had suggested that much of the carbon dioxide should have been sucked up by plants and seas close to where the gas was being emitted.

Another instance of where the models appear to fall short is on how well the mixing of near-surface parcels of air homogenize carbon dioxide concentrations.

Stephens pointed to data collected earlier this week by the HIAPER aircraft during a run from Kona, Hawaii, to Anchorage, Alaska. “This is the time of year when we see peak uptake [of the gas]” as a result of photosynthesis in land plants, he explains. And HIPPO indeed observed a large depletion of carbon dioxide near the surface, he notes — except “we were measuring it over the middle of the Pacific Ocean.”

Computer analyses had predicted a greater degree of mixing of clean and polluted air parcels, he says. Instead, there were sharp gradients in the gas among closely sampled regions.

Elsewhere, HIPPO offered welcome confirmation of a different model prediction: large plumes of oxygen coming out of the southern oceans during the austral summer. Stephens attributes these massive releases to the uptake of carbon dioxide by photosynthetic bacteria in the warming seas. Currently, it’s winter in the South. And HIPPO has just measured the opposite trend, Stephens says: a seasonal absorption of oxygen by oceans there.

Similar trends for these gases have been observed before throughout the Northern Hemisphere. But HIPPO shows that this normal pattern of winter absorption of oxygen and summer absorption of carbon dioxide “is somewhat decoupled” in the southern oceans. Indeed, Stephens concludes, at times “it was almost more significant that we measured an anti-correlation between oxygen and carbon dioxide than the actual numbers [of how much of either was present].”

Other revelations
More unexpected, Wofsy says, was the March 2010 finding of “a significant excess over the tropics of greenhouse gases — especially nitrous oxide — very high up in the atmosphere. That hadn’t been predicted by any models.”

So radical were the data that his team rushed them into print. Those data show a “bulge” in nitrous oxide emissions between the equator and 20° North latitude.

“It is clear that the enhanced nitrous oxide seen at altitude is a product of tropical emissions lofted to the middle and upper troposphere by convection,” the authors conclude in the Aug. 6, 2011, Geophysical Research Letters.  HIPPO data alone cannot confirm whether the release of this gas represents a “winking on and off” of emissions on time scales of days to weeks, the researchers said, or whether the releases occur more chronically but only occasionally shoot up to altitudes of between 2 kilometers and 14 kilometers.

The best explanation for these data, Wofsy and his coauthors write, is that rainfall or regional flooding spurs production of the gas (probably by soil microbes) — and when this coincides with sharp atmospheric updrafts, the pollutant is propelled high into the skies.

Something too new to fully understand (although a report on it is being prepared for publication), Wofsy says, is a finding of notable concentrations of methane in the Arctic’s atmosphere that trace back to the sea.

“Oceanographers have known for some time that there is production of methane in surface waters of the Arctic,” he says, but “it’s never been observed in the atmosphere.” Those oceanographic data, he says, suggest a source for this methane other than sediments or the melting of icy gas hydrates.

The phenomenon also appears very widespread. “We observed that the ocean surface releases methane to the atmosphere all over the whole of the Arctic Ocean,” Wofsy says.

Climate scientists have been concerned about whether the Arctic Ocean's loss of summer ice cover might lead, through some feedback mechanisms, to boosting the release of methane. Concludes Wofsy: Thanks to HIPPO, “This hypothesized feedback has been observed for the first time.” And there are hints, he adds, that methane’s source may be something other than melting of gas hydrates.

One notable take-home message from HIPPO: Climate-altering pollution from the Northern Hemisphere — home to 95 percent of humanity — has been migrating everywhere, even into southern skies, says James Elkins of the National Oceanic and Atmospheric Administration, in Boulder. The data are “just very persuasive.”

Indeed, Wofsy adds, after reviewing HIPPO data, you’re left with an impression that pollution associated with human activities has exerted “an overwhelming influence” on Earth’s atmosphere. And that’s not, he adds, reassuring.

Found in: Climate Change, Earth, Environment and Molecules

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Science & the Public: Microbes may sky jump to new hosts

Terrestrial microorganisms may have adapted to use clouds as a way station.

Scientists have been probing the role of microbes in cloud formation and precipitation, something discussed in a May 24 session at the American Society for Microbiology meeting, in New Orleans. But this bioprecipitation may not be an accident of chemistry so much as an evolutionary adaptation by certain bacteria and other nonsentient beings, argues Brent Christner of Louisiana State University in Baton Rouge.

He’s referring to the ability of certain one-celled organisms to foster the nucleation of ice crystals.

Even an airborne dust mote can serve as the surface on which water vapor condenses and freezes. But he notes that the most efficient ice nucleators — micro-particles that can catalyze freezing at the highest temperatures — are living organisms. Some bacteria, like Pseudomonas syringae, for instance, can serve as nuclei for ice formation at temperatures as warm as -2 degrees Celsius, more than 10 degrees warmer than the ice-forming limit for dust motes.

Christner now thinks it’s probably no accident that “the most active ice nucleators are biological.” As part of a survival strategy, he contends, many microbes may have evolved “to essentially piggyback on the hydrological cycle.”

The plant pathogen P. syringae “can probably be found on any plant in your back yard,” Christner says. But if winds fling this germ high enough into the air, it can be entrained by currents for up to a week or more. During that time, the fragile microbe faces a risk of deadly desiccation or irradiation by damaging solar ultraviolet rays. Safety, from its perspective, is a moist leaf back on the ground.

This bacterium can return to Earth, he says, by fostering the nucleation of moisture that will eventually rain out as liquid or frozen precipitation. (And don't worry about their getting cold along the way. Christner and other biologists have isolated live germs from precipitation — including the heart of a fallen hailstone). 

The LSU scientist's postulation of bioprecipitation as a survival tactic is certainly tantilizing. I can even picture the graphic novel story line he might offer students: wind-kidnapped microbes that turn on the synthesis of ice-nucleating proteins in hopes of skydiving back home.

Found in: Agriculture, Chemistry, Earth Science, Ecology, Environment and Life

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