Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 267

Oscillation index, humidity, temperature and snowfall interact to affect the hardness
of snow. This affects the lemming population which in turn impacts on predators such
as stoat ( Mustela erminea ) and weasel ( Mustela nivalis ), which affects other prey such
as birds (Kausrud et al., 2008; and see also the review by Coulson and Malo, 2008).
Finally, in a warmer world with elevated levels of carbon dioxide we would expect
increased levels of photosynthesis. Although regionally there may be differences, with
some areas seeing decreases in photosynthesis (for example, due to water restriction),
and regionally there would be differences in fixing and retaining carbon (both from
plants and soils), in purely photosynthetic terms the reasonable expectation would
be to see an increase in global primary production (see the definition of production
in the Glossary, Appendix 1). Satellite observation of global vegetation between
1982 and 1999 suggests that globally terrestrial (excluding oceans and freshwater
systems) net primary production has increased by 6% (3.4 GtC) over 18 years.
The largest increase was in tropical ecosystems and the study suggested that the
Amazon rainforest accounted for 42% of the global increase in net primary production
(Nemani et al., 2003). This ties in with the evidence for the increase in global
primary production noted in section 5.3.1. Here we should remember that whereas
global climate-induced changes in primary production will not meaningfully affect
atmospheric concentrations of carbon dioxide as far as policy-makers (who seek
large changes over a short time) are concerned, changes in primary production are
a regionally important mechanism for carbon transportation, via the atmosphere,
between ecosystems and, of course, primary production is of ecological interest as
well as of relevance to biosphere carbon cycling which, as already noted, is increasing
in our warming world.
In accountancy terms, gross primary productivity might be considered as analogous
to the all-important, proverbial bottom line: what was the net productivity of the
ecosystem (how much living matter grew in the year minus that which died)? Yet for
ecologists (and indeed entrepreneurs, to continue with the accountancy analogy) of
equal interest is how we arrive at that bottom line: what types of primary producer
species will thrive as the world warms? You will recall from section 3.3.11 that plants
use a number of photosynthetic pathways and the two most common categories
are C 3 and C 4 , and that the C 4 pathway includes a carbon dioxide-concentrating
mechanism so that the plant can photosynthesise at lower atmospheric concentrations
of the gas. Indeed, as discussed, atmospheric carbon dioxide levels were falling in
the Miocene and this gave C 4 plants an increased advantage over C 3 plants. Now,
it might reasonably be thought that leaf photosynthesis and water-use efficiency in
C 4 grasses benefit less from higher carbon dioxide in a greenhouse world than in C 3
grasses, but is this true in practice? In the wild, where C 3 and C 4 plants grow side
by side, there will also be competition for water, especially in a warmer world with
higher evaporation from land. In 2011 a team of researchers based in the USA plus
one based in Australia, and led by Jack Morgan, Daniel LeCain and Elise Pendall,
examined just this problem in what was called the Prairie Heating and Carbon Dioxide
Enrichment (PHACE) experiment. The PHACE experiment evaluated the responses
of native mixed-grass prairie to 1 year of carbon dioxide enrichment (from the then
present ambient 385 parts per million by volume [ppmv]) to elevated levels (600
ppmv), as well as C 3 and C 4 plants by themselves. This was followed by another