If current production of carbon from fossil fuels continues unabated, by the end of the century the land and oceans will be less able to take up carbon than they are today, the model indicates.
"If we maintain our current course of fossil fuel emissions or accelerate our emissions, the land and oceans will not be able to slow the rise of carbon dioxide in the atmosphere the way they're doing now," said Inez Y. Fung at the University of California, Berkeley, who is director of the Berkeley Atmospheric Sciences Center, co-director of the new Berkeley Institute of the Environment, and professor of earth and planetary science and of environmental science, policy and management. "It's all about rates. If the rate of fossil fuel emissions is too high, the carbon storage capacity of the land and oceans decreases 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 on global climate change that issued a major report for President Bush in 2001 claiming, for the first time, that global warming exists and that humans are contributing to it.
Currently, the land and oceans absorb about half of the carbon dioxide produced by human activity, most of it resulting from the burning of fossil fuels, Fung said. Some scientists have suggested that the land and oceans will continue to absorb more and more CO2 as fossil fuel emissions increase, making plants flourish and the oceans bloom.
Fung's computer model, however, indicates that the "breathing biosphere" can absorb carbon only so fast. Beyond a certain point, the planet will not be able to keep up with carbon dioxide emissions.
"The reason is very simple," Fung said. "Plants are happy growing at a certain rate, and though they can accelerate to a certain extent with more CO2, the rate is limited by metabolic reactions in the plant, by water and nutrient availability, et cetera."
In addition, increasing temperatures and drought frequencies lower plant uptake of CO2 as plants breathe in less to conserve water. A second study she and colleagues published last week in PNAS report evidence for this temperature and drought effect. Since 1982, a greening of the Northern Hemisphere has occurred each spring and summer (except for 1992 and 1993, after Mt. Pinatubo erupted) as the climate has steadily warmed. As a result, there is a small but steady decline in atmospheric CO2 each growing season due to increasing photosynthesis at temperate latitudes in the northern hemisphere. When Fung and a team of her former and current post-doctoral fellows took a detailed look at this phenomenon, however, they discovered that since 1994, enhanced uptake of CO2 as photosynthesis revved up in the warm wet springs was offset by decreasing CO2 uptake during summers, which became increasingly hot and dry - an unsuspected browning in the past 10 years.
"This negative effect of hot, dry summers completely wiped out the benefits of warm, wet springs," Fung said. "A warming climate does not necessarily lead to higher CO2 growing-season uptake, even in temperate areas such as North America."
In the climate modeling study published this week in PNAS, she and colleagues found that similar water stress could slow the uptake of CO2 by terrestrial vegetation, and at some point, the rate of fossil fuel CO2 emissions will outstrip the ability of the vegetation to keep up, leading to a rise in atmospheric CO2, increased greenhouse temperatures and increased frequency of droughts. An amplifying loop leads to ever higher temperatures, more droughts and higher CO2 levels.
The oceans exhibit a similar trend, Fung said, though less pronounced. There, mixing by turbulence in the ocean is essential for moving CO2 down into the deep ocean, away from the top 100 meters of the ocean, where carbon absorption from the atmosphere takes place. With increased temperatures, the ocean stratifies more, mixing becomes harder, and CO2 accumulates in the surface ocean instead of in the deep ocean. This accumulation creates a back pressure, lowering CO2 absorption.
In all, business as usual would lead to a 1.4 degree Celsius, or 2.5 degrees Fahrenheit, rise in global temperatures by the year 2050. This estimate is at the low range of projected increases for the 21st century, Fung said, though overall, the model is in line with others predicting large ecosystem changes, especially in the tropics.
With voluntary controls that flatten fossil fuel CO2 emission rates by the end of the century, the land and oceans could keep up with CO2 levels and continue to absorb at their current rate, the model indicates.
"This is not a prediction, but a guideline or indication of what could happen," Fung said. "Climate prediction is a work in progress, but this model tells us that, given the increases in greenhouse gases, the Earth will warm up; and given warming, hot places are likely to be drier, and the land and oceans are going to take in carbon at a slower rate; and therefore, we will see an amplification or acceleration of global warming."
"The Earth is entering a climate space we've never seen before, so we can't predict exactly what will happen," she added. "We don't know where the threshold is. A two degree increase in global temperatures may not sound like much, but if we're on the threshold, it could make a big difference."
Fung and colleagues have worked for several decades to produce a model of the Earth's carbon cycle that includes not only details of how vegetation takes up and releases carbon, but also details of decomposition by microbes in the soil, the carbon chemistry of oceans and lakes, the influence of rain and clouds, and many other sources and sinks for carbon. The model takes into account 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 all these interactions.
"This is a very rough schematic of the life cycle of the ecosystem," she said.
Five years ago, she set out with colleagues Scott C. Doney of Woods Hole Oceanographic Institution in Massachusetts, Keith Lindsay of the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and Jasmin John of UC Berkeley to integrate the carbon cycle model into one of the standard climate models in use today - NCAR's Community Climate System Model (CCSM). All of today's climate models are able to incorporate the climate effects of carbon dioxide in the atmosphere, but only with concentrations of CO2 specified by the modelers. Fung's model does not specify atmospheric CO2 levels, but rather predicts the levels, given fossil fuel emissions. The researchers used observations of the past two centuries to make sure that their model is "reasonable," and then used the model to project what will happen in the next 100 years, with the help of supercomputers at NCAR and the National Energy Research Scientific Computer Center at Lawrence Berkeley National Laboratory (LBNL).
The climate model coupled with the carbon cycle has been her goal for decades, as she tried to convince climate modelers that "whether plants are happy or not happy has an influence on climate projections. To include interactive biogeochemistry in climate models, which up to now embrace primarily physics and dynamics, is new."
She admits, however, that much work remains to be done to improve modeling. Methane and sulfate cycles must be included, plus effects like changes in plant distribution with rising temperatures, the possible increase in fires, disease or insect pests, and even the effects of dust in the oceans.
"We have created a blueprint, in terms of a climate modeling framework, that will allow us to go beyond the physical climate models to more sophisticated models," she said. "Then, hopefully, we can understand what is going on now and what could happen. This understanding could guide our choices for the future."
The studies were supported by the National Science Foundation, the National Aeronautics and Space Administration, LBNL and the Ocean and Climate Change Institute of the Woods Hole Oceanographic Institution.
Her colleagues on the paper looking at spring and summer CO2 uptake in northern climes were A. Angert, S. Biraud, C. Bonfils, C. C. Henning and W. Buermann of the Berkeley Atmospheric Sciences Center; and J. Pinzon and C. J. Tucker of NASA Goddard Space Flight Center in Greenbelt, Md.