BERKELEY -- One in a new generation ofcomputer climate models that include the effects of Earth's carboncycle indicates there are limits to the planet's ability to absorbincreased emissions of carbon dioxide.
If current production ofcarbon from fossil fuels continues unabated, by the end of the centurythe land and oceans will be less able to take up carbon than they aretoday, the model indicates.
"If we maintain our current course of fossil fuel emissions oraccelerate our emissions, the land and oceans will not be able to slowthe rise of carbon dioxide in the atmosphere the way they're doingnow," said Inez Y. Fung at the University of California, Berkeley, whois director of the Berkeley Atmospheric Sciences Center, co-director ofthe new Berkeley Institute of the Environment, and professor of earthand planetary science and of environmental science, policy andmanagement. "It's all about rates. If the rate of fossil fuel emissionsis too high, the carbon storage capacity of the land and oceansdecreases and climate warming accelerates."
Fung is lead author of a paper describing the climate model results that appears this week in the Early Online Edition of the Proceedings of the National Academy of Sciences(PNAS). Fung was a member of the National Academy of Sciences panel onglobal climate change that issued a major report for President Bush in2001 claiming, for the first time, that global warming exists and thathumans are contributing to it.
Currently, the land and oceansabsorb about half of the carbon dioxide produced by human activity,most of it resulting from the burning of fossil fuels, Fung said. Somescientists have suggested that the land and oceans will continue toabsorb more and more CO2 as fossil fuel emissions increase, makingplants flourish and the oceans bloom.
Fung's computer model,however, indicates that the "breathing biosphere" can absorb carbononly so fast. Beyond a certain point, the planet will not be able tokeep up with carbon dioxide emissions.
"The reason is verysimple," Fung said. "Plants are happy growing at a certain rate, andthough they can accelerate to a certain extent with more CO2, the rateis limited by metabolic reactions in the plant, by water and nutrientavailability, et cetera."
In addition, increasing temperatures and drought frequencies lowerplant uptake of CO2 as plants breathe in less to conserve water. Asecond study she and colleagues published last week in PNAS reportevidence for this temperature and drought effect. Since 1982, agreening of the Northern Hemisphere has occurred each spring and summer(except for 1992 and 1993, after Mt. Pinatubo erupted) as the climatehas steadily warmed. As a result, there is a small but steady declinein atmospheric CO2 each growing season due to increasing photosynthesisat temperate latitudes in the northern hemisphere. When Fung and a teamof her former and current post-doctoral fellows took a detailed look atthis phenomenon, however, they discovered that since 1994, enhanceduptake of CO2 as photosynthesis revved up in the warm wet springs wasoffset by decreasing CO2 uptake during summers, which becameincreasingly hot and dry - an unsuspected browning in the past 10years.
"This negative effect of hot, dry summers completelywiped out the benefits of warm, wet springs," Fung said. "A warmingclimate does not necessarily lead to higher CO2 growing-season uptake,even in temperate areas such as North America."
In the climatemodeling study published this week in PNAS, she and colleagues foundthat similar water stress could slow the uptake of CO2 by terrestrialvegetation, and at some point, the rate of fossil fuel CO2 emissionswill outstrip the ability of the vegetation to keep up, leading to arise in atmospheric CO2, increased greenhouse temperatures andincreased frequency of droughts. An amplifying loop leads to everhigher temperatures, more droughts and higher CO2 levels.
Theoceans exhibit a similar trend, Fung said, though less pronounced.There, mixing by turbulence in the ocean is essential for moving CO2down into the deep ocean, away from the top 100 meters of the ocean,where carbon absorption from the atmosphere takes place. With increasedtemperatures, the ocean stratifies more, mixing becomes harder, and CO2accumulates in the surface ocean instead of in the deep ocean. Thisaccumulation creates a back pressure, lowering CO2 absorption.
Inall, business as usual would lead to a 1.4 degree Celsius, or 2.5degrees Fahrenheit, rise in global temperatures by the year 2050. Thisestimate is at the low range of projected increases for the 21stcentury, Fung said, though overall, the model is in line with otherspredicting large ecosystem changes, especially in the tropics.
Withvoluntary controls that flatten fossil fuel CO2 emission rates by theend of the century, the land and oceans could keep up with CO2 levelsand continue to absorb at their current rate, the model indicates.
"Thisis not a prediction, but a guideline or indication of what couldhappen," Fung said. "Climate prediction is a work in progress, but thismodel tells us that, given the increases in greenhouse gases, the Earthwill warm up; and given warming, hot places are likely to be drier, andthe land and oceans are going to take in carbon at a slower rate; andtherefore, we will see an amplification or acceleration of globalwarming."
"The Earth is entering a climate space we've never seenbefore, so we can't predict exactly what will happen," she added. "Wedon't know where the threshold is. A two degree increase in globaltemperatures may not sound like much, but if we're on the threshold, itcould make a big difference."
Fung and colleagues have worked forseveral decades to produce a model of the Earth's carbon cycle thatincludes not only details of how vegetation takes up and releasescarbon, but also details of decomposition by microbes in the soil, thecarbon chemistry of oceans and lakes, the influence of rain and clouds,and many other sources and sinks for carbon. The model takes intoaccount thousands of details, ranging from carbon uptake by leaves,stems and roots to the different ways that forest litter decomposes,day-night shifts in plant respiration, the salinity of oceans and seas,and effects of temperature, rainfall, cloud cover and wind speed on allthese interactions.
"This is a very rough schematic of the life cycle of the ecosystem," she said.
Fiveyears ago, she set out with colleagues Scott C. Doney of Woods HoleOceanographic Institution in Massachusetts, Keith Lindsay of theNational Center for Atmospheric Research (NCAR) in Boulder, Colo., andJasmin John of UC Berkeley to integrate the carbon cycle model into oneof the standard climate models in use today - NCAR's Community ClimateSystem Model (CCSM). All of today's climate models are able toincorporate the climate effects of carbon dioxide in the atmosphere,but only with concentrations of CO2 specified by the modelers. Fung'smodel does not specify atmospheric CO2 levels, but rather predicts thelevels, given fossil fuel emissions. The researchers used observationsof the past two centuries to make sure that their model is"reasonable," and then used the model to project what will happen inthe next 100 years, with the help of supercomputers at NCAR and theNational Energy Research Scientific Computer Center at LawrenceBerkeley National Laboratory (LBNL).
The climate model coupledwith the carbon cycle has been her goal for decades, as she tried toconvince climate modelers that "whether plants are happy or not happyhas an influence on climate projections. To include interactivebiogeochemistry in climate models, which up to now embrace primarilyphysics and dynamics, is new."
She admits, however, that muchwork remains to be done to improve modeling. Methane and sulfate cyclesmust be included, plus effects like changes in plant distribution withrising temperatures, the possible increase in fires, disease or insectpests, and even the effects of dust in the oceans.
"We havecreated a blueprint, in terms of a climate modeling framework, thatwill allow us to go beyond the physical climate models to moresophisticated models," she said. "Then, hopefully, we can understandwhat is going on now and what could happen. This understanding couldguide our choices for the future."
The studies were supported bythe National Science Foundation, the National Aeronautics and SpaceAdministration, LBNL and the Ocean and Climate Change Institute of theWoods Hole Oceanographic Institution.
Her colleagues on the paperlooking at spring and summer CO2 uptake in northern climes were A.Angert, S. Biraud, C. Bonfils, C. C. Henning and W. Buermann of theBerkeley Atmospheric Sciences Center; and J. Pinzon and C. J. Tucker ofNASA Goddard Space Flight Center in Greenbelt, Md.
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