Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 351

1992a scenario (IS92a) through Livermore's ocean general-circulation model. This
assumed that fossil fuel consumption (and greenhouse gas emissions) would continue
to grow much as they have from 1950 to date through to the year 2100 and then begin
to stabilise logistically to 2300 as the economically recoverable reserves were used up.
The IS92a scenario is, in terms of the IPCC Special Report on Emission Scenarios
(SRESs) used for their 2001 assessment (and with respect to their 21st-century
forecasts), a middle-of-the-road B-a-U scenario.
Caldeira and Wickett's results suggest that the atmospheric carbon dioxide level
would exceed 1900 ppm by 2300. At first this might seem acceptable; after all,
some 500 mya the biosphere had atmospheric concentrations up to some 7500 ppm
(see Figure 3.1) and calcareous marine species survived that. However, the current
anthropogenic carbon dioxide pulse is different from that early Palaeozoic rise. That
peak was reached after many millions of years of carbon dioxide build-up; conversely,
the current human B-a-U rise would be literally millions of times faster. When an
atmospheric carbon dioxide change of such magnitude takes place in less than 1000
years then geological weathering and the resulting buffering cannot take place. For
greater buffering it is thought that such a fossil-fuelled carbon dioxide increase would
need to take place over tens of thousands of years or longer. Consequently, Caldeira
and Wickett's simulation predicts that the pH reduction would be 0.7. Currently there
is no evidence that ocean pH has been more than 0.6 pH units lower than today. Their
conclusion is that unabated carbon dioxide emissions over the coming centuries may
produce changes in ocean pH that are greater than any experienced since 300 mya,
with the possible exception of those resulting from some mass-extinction events. This
suggests that a marine mass-extinction event, certainly of calcareous forams, would
be a possibility.
Then in 2005 an international team led by James Orr used 13 ocean carbon cycle
models to assess ocean chemistry, especially with respect to calcium carbonate and
its associated ions. As with the Caldeira and Wickett team, the Orr team were again
relating ocean chemistry to the IPCC's IS92a scenario. Using pteropods, a group of
species of gastropod mollusc that have calcareous shells, in tanks of water matching
the chemistry of the IS92a scenario they attempted to see whether there were any
adverse effects. They found that water simulating the IS92a scenario up to the year
2100 did affect species. This was particularly a problem at high latitudes when the
sea water becomes undersaturated with aragonite (a metastable form of calcium
carbonate). According to the models, aragonite undersaturation is likely to take place
around the middle of the century at some high latitudes and by 2100 throughout the
entire Southern ocean and in the sub-Arctic Pacific. The researchers exposed a small
population of a live pteropod ( Clio pyramidata ) to water simulating IS92a-forecast,
high-latitude ocean waters. They observed 'notable dissolution' of their calcareous
shells (see section 5.4).
The ecological consequences of ocean acidification are likely to be significant. The
researchers suggested that 'conditions detrimental to high latitude ecosystems could
develop within decades [of 2005], not centuries as [previous work] suggested' (Orr
et al., 2005).
Pteropods, especially at high latitudes, form integral components of food webs.
Species high in the food web also rely on pteropods and include North Pacific