Geology Reference
In-Depth Information
membrane. You've been cycled out of the atmosphere and back again in a molecule of
carbon dioxide within a single year.
In summer, photosynthesis takes out much more carbon dioxide than is released from
the soil by microbes breaking down dead leaves and other organic material, producing
the troughs in the Mauna Loa graph. In autumn and winter, photosynthesis shuts down
in the high latitudes as plants go into a deep dormancy. But the soil microbes continue
their work of breaking down organic carbon despite the low temperatures, and so carbon
dioxide wafts out of the soil and into the air, showing up as the peaks in the graph. This
regular, yearly breathing is more marked in the northern latitudes because there is more
land mass on which the vegetation can grow—in the southern hemisphere there is far
less land and so the breathing is weaker. The upward trend in carbon dioxide concentra-
tion is primarily due to our emissions of this gas.
Although the soil is a great reservoir of carbon fixed by photosynthesis, it seems that
what happens in the soil could, paradoxically, help to warm Gaia if we continue to emit
carbon into the atmosphere at very high rates. Recent models from the Hadley Centre
in the UK show that the activity of the soil microbes increases dramatically when atmo-
spheric carbon dioxide is at about 550 parts per million—a situation we will most likely
reach within a few decades if emissions continue unabated. Higher levels of carbon di-
oxide in the air increase temperatures in the soil, which in turn stimulates the growth
and activity of soil microbes. This leads to a massive breakdown of soil organic carbon,
and hence to an outflow of carbon to the atmosphere that outstrips the inflow of carbon
into the green world from photosynthesis. Here we see an important feature of non-lin-
ear systems such as our planet—what was once a negative feedback can easily become
a dangerous positive feedback if the system is forced beyond a critical tipping point.
So far, we've looked at how carbon moves in and out of soils and land plants, but car-
bon's journeys through the oceans are also hugely important. The ocean absorbs around
30% of the carbon dioxide currently emitted into the atmosphere by humans. Scient-
ists talk about three major 'pumps' by means of which the oceans remove carbon diox-
ide from the air—the solubility pump, the biological pump and the physical pump. The
solubility pump doesn't directly involve life—it works simply because carbon dioxide
dissolves in sea water. Waves break on the ocean surface, and as they ripple and swirl,
carbon dioxide from the atmosphere is folded into the ocean much as a baker kneading
bread wraps air from the kitchen into her creation. Because the gas molecules have less
energy available for leaping back into the air if the water is cold, more carbon dioxide
enters the ocean through this route in the high latitudes. In fact, carbon dioxide has a
great talent for dissolving itself in water—it is twice as soluble as oxygen at 20
0
C—a
property that makes a huge difference to Gaia's temperature. The ocean surface is like a
membrane across which carbon moves between air and sea. If carbon dioxide is added
