Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 350

factors potentially affecting the circulation are beginning to take place. For example
(just one of many), there has been a decrease since 1950 of cold, dense water from
the Nordic Seas across the Greenland-Scotland ridge. The decrease is of the order
of 20% and if this reduction is not compensated for by increased cold, dense water
from elsewhere then the thermohaline circulation could be weakening (Hansen et al.,
2001). What is known is that the North Atlantic around Iceland is freshening. The
thinning of Arctic ice (itself a result of climate change) could account for part of this
freshening. Sea-level rise has not been as great (considering contributing factors such
as ocean thermal expansion and mountains glacier melt) yet glaciers almost every-
where around the world are in retreat so some of this freshening could come from
North Atlantic continental glacier melt (with the missing sea-level rise perhaps being
accounted for by increased evaporation and precipitation as snow over Antarctica tak-
ing water out of the ocean system). If Arctic melt accounts for much of this freshening
then the problem will be comparatively short-lived as it is likely that the Arctic could
become largely ice-free during the summer in the course of the 21st century. Ice-free
summers mean that the Arctic annual carry-over store of freshwater ice has been
depleted and so major freshening of the North Atlantic from this source would cease.
Meanwhile, in the southern hemisphere the Ross Sea indeed became fresher during
the late 20th century, which itself has thermohaline implications for one of the drivers
of the Broecker conveyor in that hemisphere (Jacobs et al., 2002). Of course, a change
in any single driver of the conveyor has implications for the conveyor as a whole;
hence, remote impacts even in another hemisphere are important. The possibilities
of changes in the Broecker circulation in the North Atlantic are, as mentioned, the
subject of research initiatives in the UK and Norway and these nations have engaged
in joint research programmes, whereas European, North American and Australasian
researchers have undertaken work on the thermohaline circulation in the southern
hemisphere.
6.6.7 Oceanacidity
Human additions of carbon dioxide to the atmosphere affect flows to other carbon
cycle reservoirs and notably flows into the ocean (see section 5.4). This has resulted
in increased ocean acidification, much like that thought to have happened early in
the Eocene. However, there are limits as to how acid the ocean is likely to become;
after all, there have been times both tens and hundreds of millions of years in the past
when atmospheric carbon dioxide was higher than today and calciferous sediments
such as chalk did not dissolve. In part this is because should the ocean (or rather
the ocean/atmosphere interface) become saturated then sea uptake of carbon dioxide
would be reduced. This would mean that the atmospheric increase in carbon dioxide
associated with the burning of a unit of fossil fuel would rise and with it the climatic
impact. Also, acidity would be stabilised by buffering.
With little other than clues from the Eocene thermal maximum and Toarcian
events (see Chapter 3), we do not know exactly what will happen to ocean pH if an
IPCC B-a-U scenario were followed for just three centuries to the year 2300, but a
handful of researchers are beginning to look at the problem. In 2003 Ken Caldeira
and Michael Wickett of the Lawrence Livermore National Laboratory ran the IPCC