Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 326

raising atmospheric carbon dioxide levels from around 400 ppm (soon to be reached
early in the 21st century) to 700-1000 ppm during the early 22nd century.
There is therefore little doubt that biome shifts and ecosystem change of the sort
described so far will continue to be seen both while atmospheric carbon dioxide
continues to increase and also after carbon dioxide levels have stopped increasing
and while ecosystem communities stabilise, adapting to the new conditions. However,
there will be other changes. As noted in Chapter 2 with, for example, leaf shape and
climate, species themselves will change. Some of these transformations will manifest
themselves genetically and others will be physiological, with regards to the way carbon
is metabolised (see Chapter 1).
A recent instance of genetic change under circumstances analogous to 22nd-century
carbon dioxide conditions (namely, carbon dioxide at 1050 ppm) was found in 2004
when Canadian biologists Sinead Collins and Graham Bell grew a protoctist species
of Chlorophyta (green algae) called Chlamydomonas reinhardtii over about 1000
generations. C. reinhardtii was used because it has been studied extensively to elu-
cidate photosynthetic mechanisms. Like most other eukaryotic algae, it is known to
have a carbon-concentrating mechanism (CCM) that increases the concentration of
carbon dioxide near the enzyme Rubisco (see Chapter 1). After 1000 generations
at elevated carbon dioxide levels there was less need for a CCM. So the CCMs in
Chlamydomonas became less efficient and its cells became smaller but with higher
rates of both photosynthesis and respiration. The researchers tentatively concluded
that as carbon dioxide levels increase so many Chlorophyta will have less-effective
CCMs and changes in photosynthetic and respiratory rates. This, they note, would
affect global processes by changing the rate of carbon turnover in aquatic and perhaps
terrestrial ecosystems. It needs to be remembered that very roughly a quarter of the
Earth's photosynthetic primary production is undertaken by algae, so even if other
photosynthetic species do not see such a radical response to elevated carbon dioxide
(although some change is likely), there will be a major change in carbon transfer-
ences around sources, sinks and reservoirs and not just changes part-mediated by
alterations in physiology. These unknowns further confound those developing com-
puter models that forecast climate change from the effects of greenhouse gases. They
will, though, be important should we enter a warming period analogous to the Initial
Eocene Thermal Maximum (IETM).
6.6 Possiblesurpriseresponsestogreenhousetrendsinthe
21stcenturyandbeyond
The IPCC recommendations that provided the scientific consensus on climate change
were summarised in the last chapter (see section 5.3). Despite the broad picture (green-
house gases are increasing, the Earth is warming up and sea levels are rising) the IPCC
acknowledge that there are uncertainties and that in addition to heeding their main
conclusions we also need to be aware of surprises. As we have seen from the above
three case studies, at the present time the global IPCC broad picture at best translates
to very basic scenarios, as do other perspectives that are able to draw on additional