The oceans as a carbon sink

Meanwhile, what role are the ocean plants—the phytoplankton—playing in this story? Basically, the answer is "not much''. Plants tend to grow faster with increased C02 levels (Chapter 8) but there is very little room for phytoplankton to benefit from this because their growth is so limited by shortage of nutrients such as nitrogen and phosphorus (and perhaps iron) in the oceans. Furthermore, the plankton cells are already bathed in high concentrations of C02 because the gas is so soluble in sea-water, so a little more does not make so much difference to them. However, the ocean water itself is having a major effect in taking up carbon, independent of whatever the plants are doing. The solubility of C02 in water with bicarbonate present means that

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3,0 Fi I j. Li J. i.l.i.l j J j.Li.1 j .LiLlLi .1 ji 1.1 ,i. 11 J.i J.i. .1. i Jx I.lU.Li. ). i J. I.i.l iJ.jLLtjjJLli.ij Jj.l i.lxl.i Ij-l.i.lj.Li.LiJ.i.ij.L.LiJ.iJj-l.ri 1958 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04

Year

Figure 7.13. The record of atmospheric C02 increase since the 1950s, measured directly at the Mauna Loa Observatory in Hawaii (monthly average C02 concentration).

the oceans have the potential to take up nearly all the CO2 we can throw at them into the alkalinity reservoir. The uptake is occurring most rapidly in particular parts of the oceans where the surface water has cooled as it has drifted up from the tropics, most especially in the North Atlantic. Here, the Gulf Stream water not only cools and takes up CO2 from the atmosphere, but it also sinks down into the deep ocean, effectively trapping the CO2 within it. Overall, CO2 dissolving into ocean waters takes up about a third of the extra carbon that humans add to the atmosphere each year through fuel-burning and deforestation. On the timescale of thousands of years the oceans will eventually take up almost all of it (about seven-eighths of it, to be more exact), but for now they cannot quite keep pace and so CO2 in the atmosphere keeps on increasing.

7.5.2 Seasonal and year-to-year wiggles in CO2 level

As well as affecting the long-term balance of CO2, the world's vegetation also affects the CO2 level in the atmosphere in smaller, more subtle ways that vary with the seasons, and from one year to another. On the seasonal timescale, there is a "wiggle" in the CO2 level (Figure 7.13), with CO2 slightly higher in the winter and early spring than in summer and autumn. In the northern hemisphere, this seasonal variation is greatest in the far north, and it flattens out as one approaches the equator (Figure 7.14). Going south from the equator the seasonal wiggle reappears, but reversed in terms of months of the year and corresponding now to the southern hemisphere summer and winter. It is striking, however, that the seasonal wiggle is much smaller in the southern hemisphere.

What causes the seasonal wiggle in CO2? It is the result of the way that plants change their activities with the seasons. In the spring and summer, plants in the mid and high latitudes are busy photosynthesizing—taking up carbon dioxide into sugars which is stored as starch, or built into the tough cell walls of leaves and wood. Plants in the higher latitudes take a few weeks to unfurl their leaves and start photosynthe-sizing rapidly, so there is a delay during May and June as leaves mature. During this stage, decaying leaf litter on the soil surface pushes out a lot of CO2, so the atmospheric CO2 level in the northern hemisphere reaches a peak in late spring. Then, by late June and July photosynthesis begins in earnest, and so much uptake over such huge areas is enough to produce a noticeable decrease in CO2 level in the atmosphere across the whole hemisphere. When autumn comes, the leaves stop photosynthesizing and are dropped to the ground; trees stand leafless and herbaceous vegetation dies back. As winter cold sets in, the uptake of CO2 from the atmosphere has essentially stopped over vast swathes of the continents. But in mid-latitudes with milder winters where the ground is not totally frozen, leaves now begin to decay on the forest floor, releasing CO2 which joins the steady trickle of CO2 up from the dead organic matter in soils, and the CO2 level goes up a tad.

Why then is there much less of a seasonal wiggle in the southern hemisphere— after all, don't the plants there also experience seasons? This weaker seasonal cycle occurs because there is much less land south of the equator, and much of the land that does occur is arid and sparsely vegetated, so there is less seasonal activity of plants taking up or releasing carbon.

78 80 82 84 86

90 92 94 Year

78 80 82 84 86

90 92 94 Year

Figure 7.14. The seasonal cycle in CO2 concentration varies with latitude. It is strongest in the far north and flattens out towards the equator. It reappears in a much weaker form in the high latitudes of the southern hemisphere. Acronyms/initials refer to the names of the CO2 monitoring stations (e.g., MLO = Mauna Loa Observatory).

Taken from one year to the next, the CO2 level always goes up due to the amount added by humans burning fossil fuels and clearing tropical forests. But, in some years the increase is greater than in others: as much as a two-fold difference. Human activity does not vary enough from one year to the next to explain such variations, so they must have something to do with natural processes. Partly, these year-to-year differences are due to purely physical changes in the temperature and circulation of the oceans. For example, in an El Nino year in the Pacific, upwelling of deep waters off the coast of Peru slows, and less CO2 than usual escapes to the atmosphere. This should tend to make the atmospheric CO2 increase a little less than usual in that year because uptake in other parts of the oceans is no longer partly balanced by this upwelling CO2 source. Despite this predictable influence, something else clearly dominates in an El Nino year because the CO2 level actually goes up by more in such years, on average. Detailed study of the way carbon isotopes in CO2 vary from one year to the next shows that changes in the oceans are not the main cause of year-to-year differences in the rate of increase in C02. Year-to-year differences in C02 are matched by differences in the abundance of the "biological" isotope of C02—carbon-12—that plants are particularly good at taking up. Whenever changes in the photosynthesis or decay of plants (or other organic matter) cause a change in C02 levels, there is a corresponding change in the carbon-12 composition of the atmosphere (see Box 7.1 on carbon isotopes). The fact that inter-annual changes in C02 are paralleled by fairly big changes in carbon-12 suggests that something plants are doing is a large part of the reason C02 varies on this timescale.

Box 7.1 Plants and carbon isotopes

Isotopes are different forms of atoms of the same element, differing in the number of neutron particles in the nucleus of the atom. There are two main stable isotopes of carbon, and one radioactive isotope. By far the most abundant is the lightest form, carbon-12. A small percentage of any sample of carbon consists of the heavier carbon-13. Also, a very small proportion of carbon atoms are the radioactive form carbon-14, which is continuously made at the top of the atmosphere when nitrogen gas is hit by cosmic rays from outer space.

The radioactive carbon-14 form gets into plants when they take up radioactive 14CO2 from the atmosphere. As well as entering all the living plant materials it is passed down the food chain so everything in the ecosystem picks up some of this radioactive carbon. 14C disappears by radioactive decay at a very precise rate, so the level of radioactivity emanating from carbon in a sample of plant or animal material gives an accurate estimate of how old it is. This technique, radiocarbon-dating, is immensely valuable in finding out the age of samples in archeology, and also in dating ecological changes in the past (Chapter 1) from buried fragments of wood, leaves or soil organic matter. However, by about 50,000 years after the material was first fixed by plants taking in CO2 from the atmosphere, all the 14C has gone, so the technique cannot be used back beyond this age.

The two stable carbon isotopes can also reveal a lot about both past and present day ecosystem processes. When plants photosynthesize, the enzyme (called rubisco) in their cells that takes in the CO2 tends to go preferentially for the lighter 12C isotope. So, any living material is slightly enriched in 12C, and depleted in 13C (by about 22 parts in a thousand). This difference carries over into the animals that eat the plants, and into organic matter buried in soils and rocks.

If we study the isotope composition of the CO2 in the earth's atmosphere over the past couple of hundred years, one thing we can tell is that it is getting "lighter"— that is, that more 12C is entering the atmosphere (Figure 7.15). We can tell that this carbon is coming from a source that was once living, because it is a source rich in 12C. This source could be either present-day forest and soils, or fossil carbon like oil and coal. The fact that the 14C content of the atmosphere is also going down rapidly means, even though its rate of production at the top of the atmosphere has stayed constant, it is being diluted by a large portion of very old carbon. This old carbon must be coming from fossil fuels. The combined picture from both the stable and radioactive carbon isotopes tallies with general expectations from observing forest clearance and the rate of use of fossil fuels: that most of the CO2 rise is from fossil fuels with a smaller part from tropical forest clearance.

Carbon isotopes can also reveal broad trends in the global carbon cycle going back many millions of years. In the distant history of the earth, after about 450 million years ago, there was a big decline in the amount of 12C in ocean carbonate minerals. Such minerals reflect the composition of the CO2 "left behind" in the atmosphere after some of it has been extracted by plants. What the decline in 12C

Figure 7.15. "Lightening" of the isotope composition of atmospheric C02 over time, measured from a monitoring station at the South Pole. CSIRO monthly mean flask data—South Pole, Antarctica. Source: CDIAC.

reveals is that a huge proportion of the total carbon in the atmosphere must have been taken out by plants and stored in dead organic carbon reservoirs, probably causing a major decline in atmospheric CO2.

Other changes in the 12 C content of carbonates reveal catastrophic events in earth history. There were several times in the last billion years when the 12C content of carbonates underwent a sudden large increase. What these reveal is that a lot of organic carbon, probably from vegetation, soils and organic-rich marine sediments, had suddenly been released as CO2 into the atmosphere. It seems that for some reason almost all the plants in the world died and decayed, and the ecosystems that depended on them fell apart. These events are sometimes (but not always) associated with "mass extinctions", when a large proportion of the species on earth vanished. A large 12C "blip" corresponds to the end of the Permian period 250 million years ago, when something like 90% of species disappeared. The cause of the end-Permian extinction is unclear (possibly an asteroid impact, possibly a phase of massive volcanic activity). It took hundreds of thousands of years for the functioning of the world's ecosystems to recover, as revealed by the time taken to return to more normal 12C levels.

Other more gentle and subtle changes in the earth's ancient environment are also revealed by 12C changes in buried soils. Plants with different photosynthetic systems, the C3 and C4 systems (see Chapter 8), concentrate the 12C isotope by differing amounts. Because of the peculiar way they fix CO2 in the leaf, C4 plants are not so discriminating about whether they use 12C or 13C. So their carbon is less depleted in 13 C, by only around 8 atoms in a thousand as opposed to 22 atoms in C3 plants. It is easy then to trace whether a soil has had C4 or C3 plants growing in it by measuring the composition of soil carbon derived from the plants that grew there. Around 7 million years ago there was a sudden decrease in 12C in various soils in many parts of the world (Figure 7.16), revealing a widespread shift from C3 plants to C4 plants. It is thought that this switchover reflects a drying of the earth's climate, and possibly also the effects of a decrease in CO2 levels which would also favor C4 plants.

Late Miocene global carbon shift

Late Miocene global carbon shift

• North American horse teeth □ Pakistan mammal teeth -o- Pakistan paleosois Ocean carbonate shift

• North American horse teeth □ Pakistan mammal teeth -o- Pakistan paleosois Ocean carbonate shift

813Cpdb (parts per thousand)

Figure 7.16. A carbon isotope shift around 7 million years ago indicates that C4 plants suddenly became much more common. Source: Cerling et al. (1993).

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