Introduction - Climate Change Impacts on Freshwater Ecosystems - page 4

societies will react as the first of the changes are experienced. There is a problem,
however, in these predictions. They all hold to the former model of living systems
responding to imposed conditions. They are models of simple physicochemical
control. They do not allow for the likelihood of positive ecological feedbacks.
Temperature influences many biological processes, but not in a linear way. More
usual is some sort of exponential relationship in which the process accelerates or
decelerates to a point of death as temperature changes linearly. A key process in
regulating the carbon dioxide content of the atmosphere is the storage of carbon
as organic matter in soils and peat deposits or as calcite in the ocean sediments,
derived from the scales of planktonic coccolithophorids or the matrices of corals
(Lovelock 1988). If the temperature change induces more carbon dioxide or
methane release, through increases of respiration using organic matter stored in
soils and sediments, for example, or through inhibition of calcite formation in
the walls of marine organisms, a positive feedback on further temperature increase
may be induced and the greenhouse effect may be reinforced. Temperature
changes predicted for the future may thus have been underestimated, and climate
modellers are now attempting to rectify this.
The system that maintains the non-equilibrium, equable state of the planet is the
biosphere. The biosphere has, for convenience, been divided up into atmosphere,
hydrosphere and lithosphere: air, ocean and land. And the lithosphere is thought of
in terms of biomes: tundra, coniferous forest, deciduous forest, tropical forest, scrub
savannah, grassland and desert. In turn, these may be divided into constituent
ecosystems, which Arthur Tansley (1935) defined as more or less self-contained
systems of living organisms, and their biologically produced debris, in their
physicochemical setting. In truth, this idea was an artefact of working in the greatly
subdivided landscape of the British Isles, where several thousand years of human
activity have entirely compartmented the landscape. Our upland shepherd, with his
walls, in a sense influences our ecological as well as climatic thinking. For convenience
we nonetheless talk of woodland, heath, saltmarsh, river and lake ecosystems. But the
pristine biosphere was ultimately a continuum that adjusted mutually, gradually and
in many dimensions to changing climatic and geological conditions, and in considering
freshwaters in particular, the greatest understanding comes from seeing them as
intimately linked with the land and atmosphere. It is sometimes convenient, however,
for the process of accounting for change to see the parts rather than the whole.
A report as authoritative as that of the IPCC, the Millennium Ecosystem
Assessment , appeared in 2005. It received much less publicity, for though weather
is immediately noticeable to people everywhere, the fate of distant oceans,
tundras and savannahs is not, unless you are a deep sea mariner, Inuit hunter or
Masai herder. But major changes (Fig. 1.3) have happened to most natural
ecosystems, and are continuing to happen to most of them, as a result of climate
change and also because of many other, independent drivers that depend on the
workings of global economics and the needs of a rising population. It is expected
that we will have lost over half of the world's land ecosystems to agriculture or
development by 2050. The urbanites may not be noticing this but the consequences
will nonetheless be huge, for it is these natural ecosystems that regulate the
nature of the biosphere. We have absolutely no idea how much of them can be
damaged without serious consequences for human survival. All we know is