enough fossil fuels to seriously influence the climate over the next 1000

Where is the carbon going

Figure 31.3 is a gross simplification. For example, humans are causing ad-
ditional flows not shown on this diagram: the burning of peat and forests
in Borneo in 1997 alone released about 0.7 Gt C. Accidentally-started fires
in coal seams release about 0.25 Gt C per year.

Nevertheless, this cartoon helps us understand roughly what will hap-
pen in the short term and the medium term under various policies. First, if
carbon pollution follows a “business as usual” trajectory, burning another
500 Gt of carbon over the next 50 years, we can expect the carbon to con-
tinue to trickle gradually into the surface waters of the ocean at a rate of
2 Gt C per year. By 2055, at least 100 Gt of the 500 would have gone into
the surface waters, and CO2 concentrations in the atmosphere would be
roughly double their pre-industrial levels.

If fossil-fuel burning were reduced to zero in the 2050s, the 2 Gt flow
from atmosphere to ocean would also reduce significantly. (I used to imag-
ine that this flow into the ocean would persist for decades, but that would
be true only if the surface waters were out of equilibrium with the atmo-
sphere; but, as I mentioned earlier, the surface waters and the atmosphere
reach equilibrium within just a few years.) Much of the 500 Gt we put into
the atmosphere would only gradually drift into the oceans over the next
few thousand years, as the surface waters roll down and are replaced by
new water from the deep.

Thus our perturbation of the carbon concentration might eventually be
righted, but only after thousands of years. And that’s assuming that this
large perturbation of the atmosphere doesn’t drastically alter the ecosystem.
It’s conceivable, for example, that the acidification of the surface
waters of the ocean might cause a sufficient extinction of ocean plant-life
that a new vicious cycle kicks in: acidification means extinguished plant-life,
means plant-life absorbs less CO2 from the ocean, means oceans become
even more acidic. Such vicious cycles (which scientists call “positive
feedbacks” or “runaway feedbacks”) have happened on earth before: it’s
believed, for example, that ice ages ended relatively rapidly because of
positive feedback cycles in which rising temperatures caused surface snow
and ice to melt, which reduced the ground’s reflection of sunlight, which
meant the ground absorbed more heat, which led to increased temperatures.
(Melted snow – water – is much darker than frozen snow.) Another
positive feedback possibility to worry about involves methane hydrates,
which are frozen in gigaton quantities in places like Arctic Siberia, and
in 100-gigaton quantities on continental shelves. Global warming greater
than 1 °C would possibly melt methane hydrates, which release methane
into the atmosphere, and methane increases global warming more strongly

Figure 31.4. Decay of a small pulse of CO2 added to today’s atmosphere, according to the Bern model of the carbon cycle. Source: Hansen et al. (2007).