Geoscience Reference
In-Depth Information
29. Deep ocean circulation changes depending on freshwater inputs
NADW = North Atlantic Deep Water; and AABW = Antarctic Bottom Water.
A counter scenario is that, if the Antarctic ice sheet starts to melt significantly before the
Greenland and Arctic ice, things could be very different. If enough melt-water goes into
the Southern Ocean, then AABW will be severely curtailed. Since the deep-water system
is a balancing act between NADW and AABW, if AABW is reduced then the NADW will
increase and expand (
Figure 29c
)
. The problem is that NADW is warmer than AABW,
and because if you heat up a liquid it expands, the NADW will take up more space. So
any increase in NADW could mean an increase in sea level. Computer models by Dan
Seidov (now at the National Oceanic and Atmospheric Administration) and myself have
suggested that a melt-water event in the Southern Ocean could cause a reduction in the
AABW and the expansion of the NADW, and would result in an average sea-level in-
crease of over 1 m. It has been over 20 years since the possibility of a catastrophic shut
down of the deep ocean circulation was suggested and there has been a huge amount of
work on it. The consensus in the very latest IPCC science report is that this collapse is
highly unlikely in the 21st century. The evidence is from detailed monitoring of the
oceans and climate models, and a greater understanding of the past changes in ocean cir-
culation. Put simply, there will not be enough freshwater entering either the Nordic Seas
or the Southern Ocean in the next 100 years or so to switch off the circulation. However,
if emissions keep rising as they have over the last 10 or 20 years then by the next century
when both Greenland and the Western Antarctica are committed to full melt then we could
see huge amounts of freshwater entering the oceans and deep-ocean circulation will be
disrupted.
