Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 263

such tropical trees may be slowed by the tendency of canopy and emergent trees to
produce wood of lower density as their size and growth rate increases. Further, such
species shade the more densely wooded sub-canopy species that were seen to have
lower growth and were declining. Indeed, a sufficiently warmer climate may turn
a tropical forest into a carbon source as opposed to a sink. Drought and extended
dry seasons with climate change would serve to release carbon, whereas increased
carbon dioxide concentrations would increase tropical forests as a carbon sink: the
balance between these two factors is unknown. Currently there is a research priority to
ascertain whether such carbon changes are taking place in the tropics and to confirm
their cause as well as to try to forecast the effects of warmer climates on existing
tropical forests.
6.1.3 Somebiologicaldimensionsoftheclimaticchangeingerprint
The above dendrochronological responses to current climate change (sections 6.1.1
and 6.1.2) are but one aspect of the complexities of biological response to climate
change. We have previously noted others, including with regard to climate-driven
species migration (both horizontal and vertical), extinctions and evolution. We have
also seen (Chapter 2) how individual species can serve as indicators of past climate.
So the question arises as to whether there is any general effect of current warming
across natural systems. In other words, is it possible to discern a global warming
fingerprint?
In 2003 biologist Camille Parmesan (who had contributed to the Intergovernmental
Panel on Climate Change [IPCC] 2001 Working Group II report) and economist
Gary Yohe published the results of their attempt to discern such a fingerprint. They
noted that the discussion surrounding the 2001 IPCC Working Group II's analysis
(IPCC, 2001a) of climate impacts and vulnerability had featured a 'divergence of
opinion' from its contributors with disparate academic views from science to the
social sciences. This was in no small part because of the differences in the way
economists and biologists work. Economists tend to look at aggregate data over a
long time. Biologists, conversely, recognise that biological change is multi-factorial
and that much is to do with non-climatic human intervention (such as land-use change)
and so they tend to examine unperturbed systems. The economists view this as data
selection, whereas biologists consider it a quality-control filter. However, Parmesan
and Yohe combined biological and economist approaches, adopting IPCC criteria,
to look at natural (as opposed to human-managed) systems. They looked at data
sets as well as individual cases found in the scientific literature. They then assessed
these using three variables: the proportion of observations matching climate change
predictions, the numbers of competing explanations for these observations and the
confidence of relating each observation to climate change. Finally, to help overcome
any literature bias (scientists might prefer to write up only positive correlations), they
used only multi-species studies that reported neutral and negative climate correlations
as well as positive ones. The brunt of their focus was on phenological effects: those
of season changes on the lives of plants and animals.
They found that the range boundaries of 99 species of northern hemisphere tem-
perate bird, butterfly and alpine herb, at the end of the 20th century, had moved on