Science Briefs

Can Ocean Carbon Uptake Keep Pace with Industrial Emissions?

The oceans play a central role in the global carbon cycle, absorbing more than a quarter of the carbon dioxide (CO2) that humans put into the air ("anthropogenic carbon"), primarily by fossil fuel burning. Now, the first observationally based estimate of the history of anthropogenic carbon uptake by the ocean suggests that the oceans are struggling to keep up with rising emissions, a finding with potentially wide implications for future climate. The study was performed by researchers at Columbia University, the University of California-Irvine, and NASA's Goddard Institute for Space Studies and was recently published in the journal Nature.

The human perturbation to ocean carbon is notoriously difficult to measure, despite the ocean's large role in buffering the build-up of atmospheric CO2. The difficulty arises from the inhomogeneity of ocean carbon and from the fact that anthropogenic carbon has increased ocean carbon by only 1-2%, even while it is has increased atmospheric carbon by about 38%. The only global observational estimate previously made of anthropogenic carbon in the ocean was a snapshot in time for 1994 made by Sabine et al. (2002). In the new study, we used novel indirect techniques to tease out the signal over the entire industrial era. We used observations of inert and radioactive chemical constituents ("tracers") to estimate the rates at which the ocean transports material from the surface to the interior. The analysis was formulated to allow for the effects of both turbulent mixing and large-scale currents, with the relative importance of these transport mechanisms constrained solely by the tracers. We then applied these rates to the well-known industrial-era history of atmospheric anthropogenic carbon to determine the evolution of the total amount and distribution of oceanic anthropogenic carbon.

Figure 1 shows our estimate of the ocean uptake history. Driven by the exponential increase in the atmosphere the oceans have taken up an ever-increasing amount of anthropogenic carbon, with a particularly rapid increase after 1950. A record amount of 2.3 billion metric tons of carbon was absorbed in 2008. Today, the oceans hold about 150 billion tons of industrial carbon (150 Pg C) — a third more than in the mid-1990s.

Line plot of ocean uptake of anthropogenic carbon, 1775-2000

Figure 1: The black line represents the annual rate of anthropogenic carbon uptake by the ocean (left axis), with shaded area indicating the error envelope. The dashed red line is the atmospheric history of anthropogenic CO2. Also shown with blue circles and vertical error bars are the decadal average uptake rates adopted by the IPCC Fourth Assessment Report. (View larger image.)

Importantly, however, the rate of increase of the ocean's uptake appears to slow after 1980, and even more so after 2000. In other words, the fraction of anthropogenic CO2 emissions entering the ocean appears to be slowing, even while the absolute tonnage increases. Some climate models have predicted such a slowdown, but this is the first time a slowdown has been inferred from observations. Some global climate models attribute the change to global warming-induced shifts in ocean circulation. However, our study precludes this mechanism, because it assumes steady circulation. Instead, the study suggests the slowdown is due to natural chemical limits on the oceans' ability to absorb carbon.

We have found evidence that a reduction in the ocean's capacity to absorb anthropogenic CO2 is ominous; a larger fraction of anthropogenic emissions will remain in the atmosphere, exacerbating the global warming due to industrial activity. The nature of carbon chemistry in seawater is such that as the ocean's carbon concentration in solution increases the seawater becomes more acidic and consequently less able to absorb additional CO2. This chemical effect has long been known, and increases in ocean acidity have been well publicized recently. We found evidence that the increased acidity is reducing the oceans capacity to absorb new CO2.

We have deduced several other features of the ocean's role in the perturbed carbon cycle. About 40 percent of the carbon enter the oceans through the waters of the Southern Ocean, around Antarctica, because CO2 dissolves more readily in cold seawater than in warmer waters. Although the Southern Ocean was suspected to play a large role in carbon absorption, this is the first quantitative estimate based on observations.

Line plot of anthropogenic carbon sources and sinks, 1775-2000 (View larger image.)

Figure 2: History of the accumulated source and sinks for anthropogenic carbon. The accumulated fossil fuel source (including the small contribution from cement production) is shown as positive value (blue). The accumulated anthropogenic carbon in the atmosphere (green) and ocean (red) are negative. The remainder is the accumulated atmosphere-land exchange (black). Uncertainties are shown as dashed lines.

We have also estimated carbon uptake on land by taking the known amount of fossil-fuel emissions and subtracting the oceans' uptake and the carbon accumulated in the atmosphere. Figure 2 shows the accumulated anthropogenic carbon source and the accumulated exchanges among the atmosphere, ocean, and land. Prior to 1940 (give or take two decades), the global land surface was a source of anthropogenic carbon to the atmosphere. Afterwards, the land absorbed anthropogenic carbon from the atmosphere, as shown by the turnover in the land curve of Fig. 2. The net result over the industrial era is that the land surface has been neutral or a net weak source to the atmosphere. (The land curve is positive in 2008, but the uncertainty range includes zero.)

Our analysis cannot discriminate among land-carbon mechanisms. The early source was likely due to logging and slash-and-burn agriculture. Deforestation continues today, but it may be that after 1940 atmospheric CO2 levels reached a level that stimulation of new plant growth overtook forest burning. Carbon is a raw material for photosynthesis, and new carbon may accelerate plant growth, resulting in additional uptake.

Features of the ocean-carbon cycle that we did not account for may also be changing and could influence ocean carbon uptake in the future. Changes in ocean circulation could either enhance or inhibit ocean uptake. (Some model studies suggest a reduction, as noted above.) Changes in upper-ocean biological productivity could also enhance or inhibit ocean uptake. (A warmer upper ocean is a more vertically stable ocean, possibly reducing the upwelling of nutrients to surface-water plankton, and reducing their photosynthetic uptake of carbon.) These topics are intensely active areas of research. Our new study, however, is so far the most comprehensive observationally based estimate of the ocean's past and present role in the perturbed carbon cycle.

Reference

Khatiwala S., F. Primeau, and T. Hall, 2009: Reconstruction of the history of anthropogenic CO2 concentrations in the ocean, Nature, 462, 346-349, doi:10.1038/nature08526.

Sabine, C.L., R.A. Feely, N. Gruber, R.M. Key, K. Lee, J.L. Bullister, R. Wanninkof, C.S. Wong, D.W.R. Wallace, D. Tilbrook, F.J. Millero, T.-H. Peng, A. Kozyr, T. Ono, and A.F. Rios, 2002: The ocean sink for anthropogenic CO2, Science, 305, 367-371. doi:10.1126/science.1097403.

Related Link

NASA "What On Earth?" blog: Are the Oceans Really Stuffed to the Gills with Carbon Dioxide?

Contact

Please address all inquiries about this research to Dr. Timothy Hall.