The Global Carbon Cycle
USGCRP Fiscal Year 2003 Accomplishments

USGCRP
Program Elements

Atmospheric Composition

Ecosystems

Global Carbon Cycle

Decision-Support Resources Development and Related Research on Human Contributions and Responses

Climate Variability and Change

The Global
Water Cycle

Observing and Monitoring the Climate System

Communications

International Research and Cooperation

Recent trends in greenhouse gases quantified:

Based on monitoring since the late 1970s, data from more than 50 global sites of the Global Cooperative Air Sampling Network show continuing increases in the atmospheric concentrations of CO₂, methane, nitrous oxide (N₂O), and other greenhouse gases. Percentage increases of CO₂ and N₂O are similar. The methane increase has slowed considerably, and the most recent measurements indicate it has leveled off. CO₂ accounts for more than 60 percent of the calculated direct radiative forcing for these gases, and methane is now less than 20 percent. The time-averaged atmospheric CO₂ concentration increase has been approximately 1.5 ppm per year over the past several decades. While there is large year-to-year variation, and a very small annual rate of increase, 1.5 ppm per year would be most appropriate for use as the CO₂ forcing in current climate modeling applications. (See Figure 18)

AmeriFlux measures terrestrial carbon sinks:

Measurements of the net exchange of CO₂ between terrestrial ecosystems and the atmosphere — referred to as net ecosystem exchange (NEE) — confirm that most terrestrial ecosystems in the United States are assimilating CO₂ and are important sinks for atmospheric CO₂. While NEE measures vary according to properties of ecosystems and their environments, over a 3-10 year period of observation annual net carbon uptake ranged from 2 to 4 tons per hectare for forests, and about 1 ton or less per hectare for agriculture and grassland. In addition to the unique NEE results, AmeriFlux sites are producing systematic biological and micro-meteorological data for understanding both terrestrial carbon cycling processes and the biophysical controls on them.

Climate-driven increases in terrestrial productivity:

New analyses of 18 years of climatic data and satellite observations of vegetation indicate that changes in climate have eased several critical climatic constraints to plant growth around the world. Global terrestrial net primary productivity has increased 6% (3.4 petagrams of carbon over 18 years), with 25% of the global land area showing significant increases and 7% showing significant decreases. Ecosystems in all tropical regions and in the high latitudes of the Northern Hemisphere accounted for 80% of the increase in productivity. Tropical increases were attributed to decreased cloud cover and the resulting increase in solar radiation. Increases in other regions were due to the combined effects of increasing temperature, changes in rainfall, and changes in solar radiation. (See Figure 19)

Conservation Reserve Program (CRP) lands are removing greenhouse gases from the atmosphere:

Results across a 13-state region of the United States show that CRP lands sequester about 910 kg of carbon per hectare in the top 20 cm of soil each year. This translates to 5.1 million metric tons of carbon removed from the atmosphere and sequestered into the soil each year in the 5.6 million hectares (about 13.8 million acres) of CRP land. The research demonstrates a clear role for farmers and ranchers in carbon sequestration and possibly climate change mitigation – in addition to the conventional CRP benefits of improving soil, water, and wildlife resource conservation.

Ocean Inventory of Anthropogenic Carbon:

Estimates of the current oceanic anthropogenic CO₂ inventories and transports have been greatly improved using data from the global surveys of the World Ocean Circulation Experiment (WOCE), the Joint Global Ocean Flux Study (JGOFS), and the Ocean Atmosphere Carbon Exchange Study (OACES). Between 1991 and 1998 these programs produced a large number of high-quality measurements of important tracers for anthropogenic carbon, including nearly 100,000 dissolved inorganic carbon (DIC) samples as well as a large number of other high-quality measurements of important anthropogenic carbon tracers such as chlorofluorocarbons (CFCs), 13C and 14C of DIC, and other chemical species important in the study of biogeochemical cycling.

Analyses of these data indicate a total uptake of approximately 117 ± 19 Pg C from anthropogenic sources and large regional differences in its horizontal and vertical distribution in the world’s oceans. The reconstructed distribution of anthropogenic CO₂ in the oceans shows large differences between the North Atlantic, where anthropogenic CO₂ can be traced down to the bottom, and the tropical Pacific, where no anthropogenic CO₂ can be detected below 600 m. Despite the predominantly Northern Hemisphere source of fossil fuel CO₂, approximately 60% of the anthropogenic CO₂ is located in the Southern Hemisphere associated with the subtropical convergence zones. This distribution is consistent with that expected based on current knowledge of large-scale ocean circulation. (See Figure 20)

Effect of climate variability on air-sea exchange of CO₂:

Measurements from studies in the Equatorial Pacific Ocean show a large shift in the surface water partial pressure of CO₂ (pCO₂) levels and CO₂ fluxes to the atmosphere from the 1980s to the 1990s. The surface water pCO₂ levels increased much more slowly in the 1980s than in the 1990s, with the change in trend occurring around 1990. This timing corresponds with a change in the Pacific Decadal Variability and is consistent with the hypothesis that natural climate variations have a major effect on air-sea CO₂ fluxes. This is the first documentation of an effect on the ocean carbon system by a timescale oscillation with a periodicity longer than that of the El Niño-Southern Oscillation (ENSO).

Ocean color calibration refinement improves carbon estimates:

Observed ocean color variability, as measured by satellites, was modified based upon on new radiometric characterizations of the in situ calibration sensors on the Marine Optical Buoy (MOBY), which is used for on-orbit calibration of satellite sensors such as SeaWiFS and MODIS. A new, portable, tunable-laser system was used to improve characterization of the MOBY sensors, and a new algorithm was developed to correct for stray light effects. These adjustments have improved the SeaWiFS calibration, resulting in reductions in derived global mean chlorophyll concentrations of about 6 percent, which, in turn, reduced global ocean biomass and primary productivity estimates, yielding a more accurate understanding of the oceans' role in Earth's carbon budget.

Carbon On-Line Estimation (COLE):

A new computer tool has been developed to provide forest carbon estimates for user-defined areas of the conterminous United States. The project is an effort to make Forest Inventory Analysis data readily accessible. COLE should allow the user to harness these data and use them in a number of complex queries involving estimates of sequestered carbon. COLE gives the user an interactive, online, database query capability, and has been implemented with a database for the eastern United States. (See Figure 21)

Disturbance and seasonal dryness reverse seasonality of carbon exchange in moist tropical forests:

Recent results from eddy covariance flux and biometric measurements in two old-growth Amazonian forests indicate that the seasonality of carbon exchange is exactly the opposite of what conventional knowledge and models predict. Carbon was lost during the 7-month wet season and gained during the 5-month dry season. The short dry season strongly limits respiration due to desiccation of surface detrital materials, but only weakly affects photosynthesis because there is adequate moisture at depth. Decomposition of the large amounts of coarse woody debris in these forests, present due to past disturbances, predominates after the rains resume. These are also the first eddy covariance measurements that document a net carbon loss to the atmosphere from old-growth forests in the Amazon.