While Tans et al. (1990) concluded that the terrestrial biosphere
is a much bigger sink for atmospheric CO
than the ocean,
Keeling et al. (1989) drew a somewhat different conclusion. In
their 1989 paper and in earlier work (Keeling and Heimann, 1986)
Keeling and co-workers argued that anthropogenic CO
is being
added to a system in which the atmosphere naturally transports
CO
from the southern hemisphere to the north. The release of
fossil fuel CO
in the northern hemisphere currently
overwhelms the natural interhemispheric cycle and reverses the
atmospheric gradient.
Broecker and Peng (1992) suggested that the atmosphere's pre-
industrial south-to-north transport could be a straightforward
consequence of the ocean's thermohaline circulation. They
calculate that the ocean naturally takes up about 0.6 GtC/yr of
atmospheric CO
during the formation of North Atlantic Deep
Water. This CO
is transported across the equator at depth
and is released again to the atmosphere in the southern hemisphere.
In pre-industrial times a net north to south transport in the ocean
would be balanced by a south to north CO
transport in the
atmosphere. If Broecker and Peng's conjecture is correct, the flux
of CO
into the southern hemisphere ocean predicted by ocean
models may be consistent with atmospheric constraints after all:
the flux of anthropogenic CO
into the southern hemisphere
ocean is simply opposed by the outgassing of CO
as part of a
natural cycle.
One of the most convincing pieces of evidence for this possibility
is the fact that the interhemispheric CO
gradient has been
growing with time. During the early 1960s, the earliest time at
which the interhemispheric CO
gradient was measured, the
atmospheric CO
gradient was only 1 ppm. By 1980, the
gradient had grown to some 3 ppm (Keeling and Heimann, 1986). This
suggests that sometime shortly before 1960 a south-to-north
transport driven by ocean circulation may have exactly balanced the
north-to-south transport driven by anthropogenic releases.