8. Ocean and response time (Climate change)
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This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
8. Ocean and response time (Climate change)
It is not known whether it will take decades or centuries until
equilibrium is approached for a given enhanced level of greenhouse
gases. Much of this uncertainty stems from poorly known behavior of
the ocean. The ocean covers about 70 % of the globe, it transports
large amounts of heat, and it is the major source of atmospheric
water vapor. The atmosphere and land are affected by variations of
the ocean surface only, which in turn depend on the coupling between
the ocean surface and the deeper ocean. With its huge heat capacity,
the ocean slows down climate change. On the other hand, due to the
deep ocean's slow response, temperature may continue to rise for
centuries after stabilization of greenhouse gas levels.
The topmost so-called `mixed layer', being warmer and less dense than
the deeper layers, tends to stay on top. Cool, particularly salty
(thus dense) surface water sinks and deep water forms in the northern
North Atlantic and near Antarctica. Subsurface water wells up near
eastern margins of oceans. For other regions of the ocean, the extent
to which surface and deeper waters are exchanged is less clear.
The replacement time for the deep ocean is many centuries. In heat
capacity, a water column of about 2.5 m depth matches the atmosphere
lying above it. Less than 2 m of water match an average land surface.
[Hartmann, p 84-5, chapter 7] [IPCC 95, p 210-4, 290].
How much heat will the ocean's deeper layers store before things get
really going ? Already in a "stable" climate, ocean circulation is
likely to vary a good deal. A changing climate may entail major
changes in ocean currents. For instance, North Atlantic deep water
formation may decline or become more variable, which, in this region,
may inhibit warming or even produce cooling. Unfortunately, not even
the ocean's present state is fully known. This should improve over
the next decade, but tracking down natural variations lasting decades
to centuries may be not so easy. Exchange processes between surface
and deeper layers of the ocean are among the ocean models' weaknesses.
Improving the models is difficult, as the dearth of observational data
hinders judging whether a given model behavior is reasonable [IPCC 95,
p 166-7, 210-4, 266-7, 302-4, 317, 346, 526, 530]. At this point,
the above question is unanswerable.
For illustration, imagine a CO2 rise to 560 ppmv (twice the preindus-
trial level) until about 2050, with CO2 remaining constant thereafter.
Assume that other greenhouse gases and human-made aerosols remain at
their 1990 levels. For this scenario, 15 out of 16 leading US climate
scientists offered a best guess of between 2 and 4 o C surface warming
by the year 2300, with widely varying time responses. The sixteenth
expert estimated 0.3 o C and didn't provide a time response. By 2050,
9 of the 15 respondents expected roughly 50 to 70 % of the eventual
warming, in line with recent estimates from climate models [IPCC 95,
p 297-300]. The remaining 6 divided equally between swifter and
slower warming. By 2100 most participants expected 80 % or more of
the eventual warming, two suspected a sluggish response of below 25 %
[Morgan, p 469A, 472A, figure 5].
These numbers shouldn't be taken too seriously, yet they highlight
the pickle. By the way, all 16 researchers estimated some chance,
between 8 and 40 %, that uncertainty about climate sensitivity could
grow by a quarter or more after a 15-year research program [Morgan,
table 1].
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