Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 347

worded conclusion they said that, assuming the rates of fresh water observed between
1970 and 1995 continued, it would take a century for enough fresh water (9000 km 3 )
to accumulate to affect the ocean that is exchanged across the Greenland-Scotland
ridge and two centuries to stop it. Consequently disruption of the ocean thermo-
haline circulation did not appear imminent. However, they noted that this assumption
would not hold if enhanced high-latitude northern hemisphere precipitation and/or
glacial melt increased. That these are predicted in many climate-model forecasts for
the 21st century means that disruption of the thermohaline circulation cannot be
ruled out.
If the currently prevailing view is that the North Atlantic driver of the conveyor
(Atlantic meridional overturning circulation) will only modestly weaken - a view
of the 2007 IPCC assessment - what is the evidence for this and for the notion of
perhaps an even greater conveyor shutdown with further warming? As stated above,
it is known that in the 20th, but also early in the 21st, century key parts of the North
Atlantic have been freshening. Part of this is thought to be due to Greenland ice-cap
melt. However in 2005 research, funded by the UK Department for Environment, Food
and Rural Affairs (DEFRA) and carried out by the Met Office's Hadley Centre for
Climate Prediction and Research in Exeter by Peli Wu and colleagues, showed that the
Arctic Ocean is freshening due to an increasing hydrological cycle (Wu et al., 2005).
Remember, in a warmer planet there is more ocean evaporation and hence rainfall.
What Wu and colleagues did was to look at data from the six largest Eurasian rivers
flowing into the Arctic Ocean since the mid-1930s, namely the Yenisey, Lena, Ob,
Pechora, Kolyma and Severnaya Dvina. This showed that river flows had increased
by about 2 10 3 km 3 ) over a 140-year historic

average of 2.3 mBe year − 1 . This was a very good agreement with the predictions
from runs of the HadCM3 climate model of 1.8

0.7 mBe year − 1 ± (1 Be or Bering =

0.6 mBe year − 1 . With this qualified
validation of the model, they then ran the model to forecast the future to the end of the
21st century assuming standard IPCC predictions. It showed that the discharge from
Eurasian rivers may increase by between 20 and 70% during the course of this century.
This is quite a range and so the modelling still needs to be improved. Nonetheless,
such results have implications for the Broecker circulation and this line of research is
among that being considered by compilers of the fifth IPCC assessment due in 2013.
However, it is likely that we will still need to accrue at least a decade or so of data
before we gain a clear view based on field evidence rather than a perspective derived
from theoretical computer models.
The UK's Natural Environment Research Council (NERC) Rapid Climate Change
programme used a North Atlantic array of sensors to measure the circulation in the
field. It found that between March 2004 and September 2006 the Atlantic meridional
overturning circulation went through an annual cycle varying in strength from winter
weaks of less than 10 Sv to summer strengths of more than 30 Sv. The success
of the NERC pilot scheme (2004-8) ensured the future of the RAPID-Watch array
until 2014 (Natural Environment Research Council, 2008). So the question is, will
increased peri-Arctic melt run-off alter this, and if so by how much?
Changes in atmospheric circulation are also possible and again these would mainly
have regional consequences and be less likely to have as pronounced an effect on
the Earth's overall temperature. Nonetheless, regional consequences could be severe,

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