At the Kyoto climate conference in December 1997, policymakers, climate experts, and industrial leaders came together to seriously consider future global climate. Their goal: to negotiate limits on the future release into the atmosphere of carbon dioxide (CO₂), the most plentiful greenhouse gas, in order to prevent further humandriven climate change. The conferees took to heart the latest scientific findings on global climate processes that have resulted, in large part, from climate models.

Climate models are a major tool that scientists use to understand the complex web of climate mechanisms. However, important uncertainties remain. Scientists at Lawrence Livermore are actively seeking answers to the questions of how much climate is changing and how much CO₂ is stored naturally in the atmosphere and oceans, ultimately helping to resolve the larger uncertainty: what effect human activities have on our environment.

Climate modeling began in the 1960s, when the National Oceanic and Atmospheric Administration initiated development of general circulation modelscomplex mathematical descriptions of the physics and dynamics of the atmosphere and oceans. Since then, climate scientists here and elsewhere have been refining these models and formulating an entire hierarchy of other models to elucidate global climate. Their ultimate goal is a virtual global climate system, a model that will allow them to confidently predict future global and regional climate.

Development of such a model is far from complete. But with global climate change at the forefront of public attention and policymaking agendas, climate scientists must provide input to policymakers based on today's imperfect models while pressing forward to improve them. Indeed, they are working under some urgency, for the world now appears ready to take action against global warming.

One important question that models of the carbon cycle answer is what the future atmospheric concentrations of CO₂ may be. Carbon dioxide, a major greenhouse gas, is important to Earth's energy balance and climate. Because CO₂ concentrations have been increasing rapidly, climatologists have pinpointed CO₂ as a major cause of the increase in global temperatures observed since 1860, when complete temperature records began. It is no wonder, then, that the global carbon cycle has become a vigorous area of study for the Climate System Modeling Group in Lawrence Livermore's Earth and Environmental Sciences Directorate. (See S&TR, October 1996, for an overview of climate work in the Atmospheric Sciences Division, much of which stems from Livermore's multidisciplinary capabilities in scientific computation, nuclear testing, and atmospheric science.) The group also collaborates with Department of Energy sponsors and universities.

Determining Carbon's Fate

In nature, carbon is plentiful and dynamic. Its natural cycle is thought to have been in a relatively steady state for thousands of years until the industrial revolution. Then, humans perturbed the carbon cycle by clearing forests and burning fossil fuels (coal, oil, and natural gas), which increased the CO₂ content of the atmosphere. How has this anthropogenic change disrupted Earth's climate? To find out, scientists are obtaining a better understanding of how the climate system functions and tracking CO₂ released into the environment by humans.

Philip B. Duffy, group leader of the Climate System Modeling Group, says that their work includes modeling how a portion of anthropogenic CO₂ remains in the atmosphere and how the rest of it is taken up by the oceans and terrestrial ecosystems. Tracking CO₂ is neither simple nor easy: measurements of CO₂ in the ocean and terrestrial biosphere are scarce; CO₂ released by humans is indistinguishable from naturally occurring CO₂; and the natural fluxes of carbon between ocean, atmosphere, and terrestrial biosphere are poorly understood.

Lawrence Livermore modelers have been formulating and testing a suite of models to reproduce carbon absorption, transport, and storage processes. The models cannot possibly incorporate all climate factors everywhereeven Livermore's advanced supercomputers cannot provide that resolutionso modelers must select the most important climatic factors and influences and represent them as well as possible.

The goal of the carbon cycle models is to predict the future behavior of carbon in the climate system. The models are tested by "hindcasting" the behavior of the carbon cycle from the historical past to the present. If model simulations match past records and observations, then model calculations predicting the future will merit confidence.

The Ocean Carbon Cycle

The Climate System Modeling Group's work involves both terrestrial and ocean carbon cycles, with current emphasis on the ocean. The ocean is an important regulator of climate as well as a major absorber of excess CO₂. Livermore modelers, having access to powerful, nextgeneration computers from the Department of Energy's Accelerated Strategic Computing Initiative, are able to formulate models that represent ocean currents, eddies, and interactive processes with more fidelity than previously possible. Their most recent ocean models reflect the advance of climate simulation techniques and skills. The projects described here demonstrate the approaches that modelers use to test hypotheses about carbon cycle dynamics, understand process interactions, and refine representations of the processes to derive accurate and useful models.

Improving Convection Models
The ocean is a large "sink" for anthropogenic CO₂. A model that can accurately simulate how the ocean absorbs anthropogenic CO₂ is prerequisite to predicting future atmospheric CO₂ concentrations and rates of global warming. But according to climate modelers Duffy and Ken Caldeira, current ocean models may overpredict the amounts of CO₂ that oceans absorb. One reason is that models do not accurately represent convection, the rapid vertical mixing of water that occurs when dense water overlies less dense water. They saw the effects of that inaccuracy in models involving seaice formation, which is a major cause of ocean convection.

When sea ice is forming, it expels salt into surrounding surface water.
The saltier, denser water triggers convection, sinking the surface water and CO₂ it contains into the depths of the ocean. Surface ocean concentrations of CO₂ are thus reduced, and more CO₂ transfers from the air to the sea.

In the real ocean, convection occurs in horizontal regions that are meters or hundreds of meters across. Because ocean models cannot accurately represent such small features, some significant inaccuracies can arise in their simulations. Duffy and Caldeira thought that the modeled transfer of CO₂ from air to sea was probably too great because the models simulated excessive convection. If the instabilities resulting from expelled salt were treated more carefully, model results might better represent reality.

The two modelers tested their hypothesis by performing a pair of simulations that compared the standard treatment of convection to a "test" simulation in which the model's convection mechanism was partially suppressed. In the standard "control" simulation, salt released during seaice formation is placed in the model's top layer (which is 25 meters thick), and the model's convection mechanism mixes it into the rest of the ocean. Because the convection mechanism is clumsy, excessive convection occurs. This in turn causes excessive ocean absorption of carbon.
In their test simulation, Duffy and Caldeira dispersed expelled salt uniformly over a broader areafrom the surface down to a depth of 160 meters and suppressed the model's convection mechanism (Figure 1). This simulation produced much more realistic results for simulated convection, salinity, circulation, and absorption of chlorofluorocarbons (CFCs).

Marine biological processes play a large role in ocean carbon cycle dynamics, but they have been incorporated only recently into climate models. The inherent difficulties of mathematically describing their spatial and temporal effects have been a challenge. As a result of inadequate modeling of these processes, climate scientists have not been able to study the influences of marine biology and feedback responses to them. They have not known how the processes would affect the ocean carbon cycle.
Caldeira has a project under way to investigate some of these processes and clarify some of the uncertainties surrounding ocean and marinebiology interactions. He is using recently collected remote sensing and satellite data to model the interplay between sunlight, plankton (barely moving plant and animal aquatic organisms), and the ocean's absorption of CO₂.

Earlier models have established connections among solar radiation, plankton, and ocean circulation dynamics. With newly available data, Caldeira is studying these intertwinings further, considering feedback interactions in the process. For example, when sunlight penetrates the ocean layers and heats the deeper waters, convective mixing results and causes two other effects. First, CO₂ in the surface waters downwells and must be replaced by CO₂ transferred from the air to the sea; second, nutrients from the depths well up to the surface. Additional nutrients increase plankton growth. In time, the increased volume of plankton blocks solar penetration, so the sunlight heats only the surface waters. This inhibits CO₂ downwelling, nutrient transport to the surface, and plankton growth. Does the ocean then return to its steady state, or does the cycle continue through other ecosystem dynamics? And what is the cumulative effect on the ocean's absorption of CO₂?
Caldeira's study will systematically simulate the interactions among solar radiation, plankton, and ocean dynamics; the feedback resulting from those interactions; and the impacts of the feedback on the predicted response of the ocean carbon cycle to climate change. The simulations will replicate the time frame from the preindustrial ocean carbon cycle (before 1765) to the present. The simulations will also calculate future atmospheric CO₂ content based on several emission scenarios that may result when CO₂ stabilization policies are implemented.

CO2 by Radiocarbon Proxy
Radiocarbon, or ¹⁴C, has provided much of our knowledge about the rates of carbon exchange from the atmosphere to the oceans and land, and it yields important information about the ocean circulation. The radioactive halflife (5,730 years) of ¹⁴C is comparable to the time span taken by surface ocean water to circulate to the ocean bottom and back. This means that the spatial distribution of ¹⁴C in the water yields significant knowledge about ocean circulationwater that has not been near the surface recently has significantly lower ¹⁴C concentrations because of radioactive decay.
For climate modelers, data on ¹⁴C distributions took on additional usefulness as a result of the atmospheric nuclear tests conducted mostly in the period from 1954 through 1963. The concentration of ¹⁴C in atmospheric CO₂, which had remained fairly constant for the previous thousand years, was doubled by those nuclear tests in less than 10 years. Since the end of the tests, ¹⁴C concentrations have been decreasing as atmospheric CO₂ moves into other carbon reservoirs. This infiltration of nucleartesting ¹⁴C into carbon reservoirs can provide valuable information on carbon exchange. In particular, if the rate of nucleartesting ¹⁴C transfer from the atmosphere to the oceans and land could be accurately predicted, climate scientists would be able to predict the rate at which the oceans and land absorb humaninduced CO₂ because essentially the same physical rules govern the transfer processes. Thus, understanding ¹⁴C transfer is another route to determining rates of future greenhouse warming.
Models to simulate this transfer process make use of the estimated amounts of ¹⁴C created by nuclear tests; they also make use of nucleartesting ¹⁴C data that were actually measured in the troposphere and stratosphere. If the models are accurate, the total amount of nucleartesting ¹⁴C transferred to the oceans, land, troposphere, and stratosphere should equal the estimated inventory of nucleartesting ¹⁴C, and the amounts transferred to the troposphere and stratosphere should approximate the measured data.

Early models simulating this transfer matched observed data only in part. In 1995, Duffy, five collaborators from Lawrence Livermore, and one from the University of Illinois tackled the problem of transfer. They used different and relatively more sophisticated multidimensional models of the ocean, stratosphere, and land, which they ran using observed data on tropospheric concentrations of ¹⁴C as the model boundary conditions. Their simulations, for the period from 1955 through 1990 (Figure 3) are well within previously recognized uncertainties. This exercise proved that contemporary models can accurately account for all the bombproduced ¹⁴C in the climate system.

Tom Brown of Livermore's Center for Accelerator Mass Spectrometry, working with the Livermore ocean modelers and ocean and fishery scientists from the University of Washington, came up with a way to provide the "prebomb" ¹⁴C data. In 1997, they measured the ¹⁴C content of archived salmon scales using Livermore's accelerator mass spectrometry capability (see S&TR, November 1997), which can measure minute isotopic quantities with high precision. The salmonscale measurements are proxy indicators for the ¹⁴C levels, and even the ¹³Cto¹²C ratios, of the surface waters of the oceans during the time period the salmon dwelled in the waters.

Fishery scientists have been collecting and archiving salmon scales for nearly a hundred years for various research purposes. The scales are particularly suitable for developing timehistory data of ocean waters because the salmon's seasonal migration patterns are known. And because salmon live and feed on plankton and small fishes in the uppermost surface waters of the ocean, the scales can be equilibrated with the ¹⁴C content of the surface waters. Furthermore, the sections of the scales that grow while salmon live in the open ocean are identifiablethey appear as bands of different thickness roughly corresponding to seasons (Figure 4).

Burning fossil fuel may appear to have no effect on atmospheric radiocarbon content because fossil fuel contains no ¹⁴C. However, global processes are rarely that simple or linear. Caldeira and Duffy collaborated with Greg Rau from the University of California at Santa Cruz to formulate a model to quantify radiocarbon fluxes. Their model predicted significant, though indirect, effects of fossilfuel burning on global distributions of ¹⁴C. More important, if its prediction of increased ¹⁴C levels by 1998 is correct, this model will soon become a test for global carboncycle models.
The model's calculations of changes induced by land clearing, fossilfuel burning, and atmospheric nuclear tests show that, in the very near future, ¹⁴C in the atmosphere will begin to increase. Even though ¹⁴C, which increased between 1954 and 1963, has been steadily declining, that decline will be reversed if humans do not change their habits in burning fossil fuel.
Caldeira, Duffy, and Rau used a simplified model of the atmosphere, land, and ocean. The modeldriven by carbon fluxes from land clearing, fossilfuel burning, and atmospheric nuclear testswas used to simulate changes that would occur in the face of isotope decay, continuing CO₂ emissions, radiocarbon exchanges into the oceans and atmosphere, and estimates of future biomass. Nuclear test radiocarbon inventory in 1975, as well as data on both natural and bomb radiocarbon collected through GEOSECS (Geochemical Ocean Sections Study, an ocean data collection program), were used as reference points for adjusting model parameters.
Simulations were performed to separately assess the changes caused
by (1) land clearing and fossilfuel burning, (2) land clearing only, (3) fossilfuel burning only, and (4) both plus observed atmospheric data until 1990, which includes nucleartesting ¹⁴C levels. The last simulation indicates the model's ability to portray past trends and predict future ¹⁴C fluxes.
The simulations indicate what percentages of increases in atmospheric CO₂ can be attributed to deforestation and fossilfuel burning. The data indicate that, prior to about 1910, most of the carbon entering the ocean resulted from deforestation; since that date, the carbon flux has been dominated by the effects of fossilfuel burning.
The modelers' interpretation of the trends led them to the unexpected prediction that ¹⁴C levels in the atmosphere will begin to increase as a result of fossilfuel burning. The modelers explain it this way: When atmospheric CO₂ content increases, the ocean's absorption of it increases. In the case of fossilfuelcaused increases, because fossil fuel contains no radiocarbon, the ocean is absorbing CO₂ that consists primarily of ¹²C, a weak acid. A more acidic ocean tends to reject carbon in all its isotopic forms. So the ¹⁴C component of ocean CO₂ is rejected along with the other carbon isotopes, adding to the atmospheric ¹⁴C content and reversing the decline that began in the 1960s after the end of atmospheric testing. The model indicates that ¹⁴C levels will begin increasing as early as 1998 and, by 2015, the fossilfuelinduced radiocarbon flux out of the ocean will exceed the nuclearexplosion radiocarbon flux into the ocean, so the ocean's ¹⁴C mass will then begin to diminish.
The Caldeira, Duffy, and Rau model is noteworthy because the prediction that the ¹⁴C flux into the ocean will be reversed early in the next century indicates that human impacts on the global carbon cycle are significant on geologic, not just human, time scales.

Closing In on Global Climate
The growing consensus that fossilfuel use is causing climate change and the recent effort to formulate international treaties to limit greenhouse gas emissions lend urgency to understanding how carbon moves within the climate system. If indeed humans have been responsible for changing the climate, climate models must accurately and conclusively portray this cause and effect. Then we will have the understanding needed to begin mitigating the effects and assuring a better future for the environment.
Gloria Wilt