Geoscience Reference
In-Depth Information
3.10 Uncertainties
Many uncertainties remain regarding ocean changes. Limitations in sampling, especially in
the Southern Hemisphere, mean that decadal variations can only be evaluated with moder-
ate confidence. Observed changes in the thermohaline circulation of the North Atlantic are
inconclusive due to large decadal variations. Climate models suggest that the climate system
responds to changes in that circulation and also that it might gradually decrease during this
century due to warming and freshening of the North Atlantic surface layer. However, there
is low confidence in the trends of thermohaline circulation and the global ocean freshwa-
ter budget. Global average sea-level rise for the past 50 years is likely to be larger than ex-
plained by warming and ice melting, since it is not possible over this period to satisfactorily
quantify the known processes causing sea-level rise. The longer term perspective of consid-
erable sea-level rise and associated unstable coastal conditions can however not be doubted
if the current situation continues.
Future changes in ocean circulation and stratification are highly uncertain. The overall
reaction of marine biological carbon cycling to a warm and high CO
2
world is not well un-
derstood. Several small feedbacks could add up to one large feedback. The response of mar-
ine biota to ocean acidification is not clear, either with regard to individual physiology or
ecosystem functioning. Potential impacts are serious, particularly for those species depend-
ing upon calcium carbonate, some of which may even become extinct in the current cen-
tury. Finally, palaeontology, using different techniques, has elucidated many previous cli-
mate changes. However, although we can identify many positive feedbacks, we still do not
know what first triggered the changes. Could they be related to varying heights of the con-
tinents, allowing more or less coverage of snow and ice? Great uncertainties and a lack of
understanding of critical processes also lead to very grave concern about large geo-engin-
eering projects under consideration to address the climate change problem.
There is an increasing need to maintain high productivity, biodiversity, and vulnerable
habitats, particularly in coastal areas often subject to large land reclamation projects, in view
