Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 269

(with low vertical mixing) are in the warmer low latitudes, where the average sea
surface temperature is greater than 15 ◦ C. Upward vertical mixing brings nutrients
from the abyssal depths to the surface, and cooler waters facilitate gaseous exchange.
Phytoplankton concentrations vary by a factor of around 100 in different parts of
the Earth's ocean surface. In the stratified, low-latitude, areas of the ocean Behrenfeld
et al. found that there was an inverse relationship between sea surface temperature and
primary productivity. It is in the high latitudes where net primary productivity seems
to best correlate with sea surface temperature and it is this that (from the limited time
window this unique study shows) seems to drives the overall loose proportionality of
global temperature with ocean net primary productivity. (See also a summary review
with some discussion of the Behrenfeld et al. paper provided by Scott Doney [2006]
in the same issue of Nature .)
The prognosis for the future in a warming world, should these trends continue
(and remember that it is early days for this type of global perspective), is that marine
biological productivity will decline in the tropics and mid-latitudes, and productivity
at high latitudes will increase.
Primary productivity is an ecological bottom line, but the physiology (health-
related aspects) of primary producers is also of interest. There have been some
climate change physiological studies but no global, let alone global-time-series, ana-
lysis. However, one study of freshwater plankton that illustrates an avenue of likely
considerable importance was its response to a simulated 4 ◦ C warmer world. This
saw reduced mean and maximum size of phytoplankton by approximately one order
of magnitude. The observed shifts in phytoplankton size structure were reflected in
changes in phytoplankton community composition, although zooplankton species
composition was unaffected by warming. Furthermore, warming reduced the total
community biomass and total phytoplankton biomass, although zooplankton biomass
was unaffected. This resulted in an increase in the zooplankton to phytoplankton
biomass ratio in the warmed simulations, which could be explained by faster phyto-
plankton turnover. Overall, warming shifted the distribution of phytoplankton size
towards smaller individuals with rapid turnover and low standing biomass, resulting
in a re-organization of the biomass structure of the food webs. These results suggest
that future warming may have profound effects on the structure and functioning of
aquatic communities and ecosystems (Yvon-Durocher et al., 2010).
We have seen in this section that the fingerprint of global climate change can
be ecologically discerned. The fingerprint can be discerned in many different ways.
The effects of climate change are many. They are both biotic (for example, species'
ranges shift) and abiotic (glaciers retreat and so forth). Such phenomena have been
studied many times and also (less often) over long time periods. The global climate
has also been monitored instrumentally, with greater accuracy in recent years. So
how many of these long-term studies of biotic and abiotic systems show shifts in
the direction we would expect with recent anthropogenic climate change? Step up
Cynthia Rosenzweig, David Karoly, Marta Vicarelli and colleagues, who in 2008
attempted to answer just this question.
Rosenzweig et al. first needed to see what climate change is taking place and so
divided the world into North America, Europe, South America, Africa, Asia and
Australasia. They then looked at the climate change taking place between 1970 and