Although the anthropogenic perturbation seems small compared to the natural cycling of carbon between ocean,
atmosphere and terrestrial systems, models and observations suggest that the increasing CO2
levels in the atmosphere are causing an increase in global temperature (Fig. 2).
While the evidence is rapidly growing for a causal relationship, it has not been unambiguously established yet.
The perturbation is also showing effects on terrestrial and oceanic ecosystems.
Based on carbon and carbon isotopic records in ice cores and tree rings we know that the atmospheric CO2 levels remained very constant at 280 ± 5 parts per million (ppm) for the millennium prior to the industrial revolution. The remarkably constancy of atmospheric CO2 despite large exchanges between the major reservoirs suggests a well-balanced global carbon cycle prior to the industrial revolution. However, we also know from the paleo-records that atmospheric CO2 levels varied in proportion to global temperatures between ice ages and warmer periods. Therefore it seems quite possible that the current dramatic atmospheric CO2 level rise will have a significant effect on climate and ecosystems. Moreover, although we have good paleo-records of climate in low CO2 environments we have very limited information of how the earth responds to the current unprecedented high CO2 levels and anticipated increases in the next century.
Our current knowledge of the fate of the anthropogenic CO2 released to the atmosphere is based on models; atmospheric observations of CO2 , carbon isotopes and small decreases in oxygen levels; terrestrial measurement of biomass inventories and primary productivity; and oceanic measurements of CO2 inventories and fluxes between air and ocean. Less than a decade ago there were significant discrepancies between estimates leading to the popular notion of the "missing carbon sink" there now is a broad agreement that the "missing sink" is uptake by the terrestrial ecosystems based on disparate methods as summarized in Table 1. As the table indicates, our level of confidence in different observations ranges from a general good knowledge of the annual changes in some reservoirs, to highly uncertain estimates in others. Annual releases due to fossil fuel burning and cement production, and annual atmospheric CO2 increases are the most constrained. Decadal changes in the ocean carbon inventory have recently been established with reasonable confidence. Changes in the terrestrial biosphere have been more difficult to pinpoint. From a variety of observations we now have a reasonable estimate of the partitioning of the fossil fuel carbon between reservoirs over the last two centuries with roughly 50 % ending up in the ocean. The terrestrial systems released CO2 over this same period. Over the last two decades, however, the terrestrial systems appear to have taken up CO2 but the magnitude, cause, and particularly the longevity of this sink remains in great doubt. Significant efforts, such as those proposed in the North American Carbon Plan (NACP), are underway to directly determine CO2 sources and sinks in the terrestrial system. However, in the foreseeable future the best approach for constraining the net terrestrial flux will be from the difference between atmospheric and oceanic observations and model calculations.
Table 1. Global inventory of anthropogenic CO2 for the past 200 and 20 years | ||
CO2 Sources | 1800-1994 [Pg C] |
1980-1999 [Pg C] |
Constrained sources and sinks |
(1) Emissions from fossil fuel and cement production | 244 ± 20 | 117 ± 5 |
(2) Storage in the atmosphere | 165 ± 4 | -65 ± 1 |
(3) Uptake and storage in the ocean | -118 ± 19 | -37 ± 8 |
Inferred net terrestrial balance | ||
(4) Net terrestrial balance = [-(1)-(2)-(3)] | 39 ± 28 | -15 ± 9 |
Terrestrial balance | ||
(5) Emissions from land use change | 100 to 180 | 24 ± 12 |
(6) Terrestrial biosphere sink = [-(1)-(2)-(3)]-(5) | -61 to -141 | -39 ± 18 | From Sabine et al., 2004 |
The need for an integrated investigation of the carbon cycle has been well articulated in the US Carbon Cycle Science Plan (Sarmiento and Wofsy, 1999). Through efforts of the Interagency Carbon Working Group and the Scientific Advisory Committee, science and implementation plans have been developed for subcomponents of the program including the NACP Science Plan, the NACP Implementation Strategy, the Ocean Carbon and Climate Change Implementation Strategy, and the Large Scale Carbon Observing Plan (LSCOP)(Bender et al. 2001). The LSCOP plan in particular focuses on the implementation and justification for sustained ocean observations. All of the plans address the central tenets of the Carbon Cycle Science Plan which focuses on the "excess carbon", that is the carbon produced by fossil fuel burning and other activities of mankind releasing CO2 such as land use change:
Table 2: Summary of estimated global CO2 fluxes using different gas transfer velocities but the same ΔpCO2 climatology | |
Parameterization | Uptake (Pg C/yr) |
Wanninkhof, 1992 | -1.6 |
Wanninkhof&McGillis, 1999 | -1.9 |
Nightingale, 2000 | -1.2 |
Liss and Merlivat, 1983 | -1.0 |
All these values were obtained using the ΔpCO2 climatology of Takahashi et al. 2002 and
41-year climatological 6-hour winds from the NCAR/NCEP reanalysis project. The divergence of values illustrates that
besides determining seasonal ΔpCO2 fields the gas transfer velocity needs to be better constrained.
References for the relationships: Liss, P.S., and L. Merlivat, Air-sea gas exchange rates: Introduction and synthesis, in The Role of Air-Sea Exchange in Geochemical Cycling, edited by P. Buat-Menard, pp. 113-129, Reidel, Boston, 1986. Nightingale, P.D., G. Malin, C.S. Law, A.J. Watson, P.S. Liss, M.I. Liddicoat, J. Boutin, and R.C. Upstill-Goddard, In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers, Global Biogeochem. Cycles, 14, 373-387, 2000. Wanninkhof, R., Relationship between gas exchange and wind speed over the ocean., J. Geophys. Res., 97, 7373-7381, 1992. Wanninkhof, R., and W.M. McGillis, A cubic relationship between gas transfer and wind speed, Geophys. Res. Let., 26, 1889-1893, 1999.
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The first three cruises of the repeat hydrography program were completed in 2003 focusing on the North Atlantic to provide a
constraint for the NACP program. The initial highlights are that the ventilation pattern/circulation in the North Atlantic thermocline
has changed based on a significant change in oxygen content (Fig. 4). Also, we have been able to unambiguously determine an
increase in total carbon content in the upper ocean over 6 to 10 years suggesting that uptake of anthropogenic CO2
continues unabated and that we can detect anthropogenic carbon increase in the ocean on decadal timescales. (Fig. 5)
Background
To assess changes in exchanges between reservoirs on sub-decadal timescale we have to determine the fluxes. The fluxes can be determined from measuring the partial pressure differences of CO2 between surface ocean and lower atmosphere, Δ pCO2 , and a quantity referred to as the gas transfer velocity that is related to physical forcing and often parameterized with wind speed. Thus, if ΔpCO2 fields can be determined and used in combination with wind fields, regional fluxes can be obtained.
Creation of flux maps This approach has been applied successfully using a global climatology of Δ pCO2 painstakingly developed based on 40-years of ΔpCO2 data from many investigators (Takahashi et al., 2002). Uptakes based on this climatology range from 1 to 1.9 Pg C yr-1 depending on the relationship between gas exchange velocity and wind speed (Table 2). This approach will be used to quantify regional fluxes on seasonal timescale. The implementation will require a significant increase in ΔpCO2 observations, development of methods to interpolate ΔpCO2 in time and space, and improvement of algorithms to quantify the gas transfer from wind or other relevant parameters, such as surface roughness, that can be directly observed from remote sensing.
Following a recommendation in the LSCOP plan a surface ocean flux observing system is being put in place with autonomous
instrumentation on volunteer observing ships VOS, research ships, and buoys. The LSCOP plan lays out an observing strategy
based on scaling analysis that involves sampling of the ocean roughly at 10 degree spacing and monthly intervals. By coordinating
efforts with international and national partners this goal will be attainable in the next decade for the North Atlantic, North Pacific and
Equatorial Pacific, particularly if we develop methods to increase time and space scales of observation through use of remotely sensed
observations. The scheme of implementing such a system utilizing in situ and remotely sensed data is outlined in Figure 6.
Determining and attributing changes in ΔpCO2
The approach of utilizing remote sensing, algorithms of ΔpCO2 and gas exchange
with remotely sensed products has been utilized in test beds in the Equatorial Pacific and Caribbean Sea. Flux map products
for these regions are shown in figures 7 and 8. For the Equatorial Pacific work the algorithms are used in a retrospective fashion to
determine the large variations in air-sea flux due to the ENSO Cycle.
Limited time series records of surface water pCO2 levels have shown that for much of the ocean the surface water pCO2 rises roughly at the same rate as the atmospheric increase implying that the global air-sea flux remains the same. However, changes in the rate of increase are a sensitive indicator of changes in the uptake of the ocean and perturbations in the biogeochemical cycles. Using a historical database of ΔpCO2 for the Equatorial Pacific Takahashi et al. (2003) determined significantly slower increases in the 80-ties than in the 90-ties that were attributed to a climatic re-organization in the North and Equatorial Pacific referred to as the Pacific Decadal Oscillation (PDO).
Table 4. Operational milestones of the CO2 - CLIVAR Repeat Hydrography Program | |
Summer 2003 | Organize and complete the A16N cruise in the North Atlantic and provide leadership (chief scientist), CTD, oxygen, nutrient, total carbon and pCO2 analysis. |
Winter 2003/2004 | Provide final CO2, oxygen, CTD data to the repeat hydrography data center at Scripps. |
Summer 2004 | Analyze total inorganic carbon on the P2 cruise |
Winter 2004/2005 | Provide final total CO2 data to the repeat hydrography data center at Scripps |
Winter 2004/2005 | Organize and complete the A16N cruise in the North Atlantic and provide leadership (chief scientist), CTD, Oxygen, nutrient, total carbon and pCO2 analysis. |
Winter 2004/2005 | Analyze total inorganic carbon on the P16S cruise |
Spring 2006 | Organize and complete the P16N cruise in the Pacific and provide leadership (chief scientist), CTD, Oxygen, nutrient, total carbon and pCO2 analysis. |
The COSP CO2 flux map effort focuses on the ΔpCO2
observations needed to create the seasonal maps. The initial lines in the North Atlantic are shown on figure 9.
The implementation schedule is presented in Table 5 with the italicized text that will be proposed in FY-05. The
effort is starting to incorporate time series on moorings that are critical to determine the higher frequency (< 1-month)
temporal variability. Particularly in the coastal oceans and Equatorial Pacific large changes can occur on weekly timescales.
The exact balance and number of fixed pCO2 observing sites vs. ship based
(moving) observing platform has not been firmly established. Analysis of the results of the initial surface
pCO2 observing system will be used to optimize spacing, frequency, and mix of
observing methods. Optimizing the observing system requires inclusion of measurements of biogeochemical and
physical parameters that influence pCO2 as well in order to investigate extrapolation
routines. The added benefit will be that these parameters yield mechanistic information that can be used in prognostic
models and interpolation schemes utilizing satellite data.. An to end-to end iterative effort starting from observations to
interpretation and analysis feeding into improved observing system design and assessing the state of the ocean carbon
cycle is critical at this point and attainable within national and international frameworks.
At a national level the CO2 COSP effort is part of the US Carbon Cycle Science Plan. Its critical role in the overall US ocean science effort is outlined in the multi-agency implementation plan, the Ocean Carbon and Climate Change plan (Doney, 2004). Information about the programs linked to, or which are a part of COSP-CO2 can be found in Table 6.
Table 6: Web sites of the CO2/COSP program and program partners: Links open new browser window | |
Data sites for pCO2 data from ships: | |
AOML | http://www.aoml.noaa.gov/ocd/gcc |
PMEL | http://www.pmel.noaa.gov/co2/uwpco2/ |
LDEO | http://www.ldeo.columbia.edu/res/pi/CO2/ | Program sites |
CLIVAR: | Climate Variability and Predictability: http://www.clivar.org |
SOLAS: | Surface-Ocean Lower Atmosphere Study: http://www.uea.ac.uk/env/solas/ |
IOCCP: | International Ocean Carbon Coordination Project http://ioc.unesco.org/ioccp |
IGBP: | International Geosphere-Biosphere Program: http://www.igbp.kva.se/ |
IMBER: | Integrated Marine Biogeochemistry and Ecosystem Research http://www.igbp.kva.se/obe/recentupdates.html |
WCRP: | World climate research Program: http://www.wmo.ch/web/wcrp/wcrp-home.html |
GCP: | Global Carbon Project: http://www.globalcarbonproject.org/ |