Geoscience Reference
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decadal timescales. Although knowledge is still imperfect, climate scientists are
beginning to establish how the timing and amplitude of climate change have
varied in the past on these different timescales and in response to these different
forcing mechanisms (cf. IPCC 2007).
A central objective now is to understand how such climate change has influenced
natural ecosystems in the past and the extent to which such an understanding
might provide insights into how climate change might influence freshwater
ecosystems in the future. Palaeoecology offers a potentially powerful methodology
to address this issue, although a degree of caution is needed, not only because of
the fragmentary nature of the fossil record but also because changes in aquatic
sediments deemed to provide evidence of the response of ecosystems to climate
change are often themselves used as a proxy source of information for reconstructing
past climate change. Palaeoecological interpretations and arguments can easily be
open to the risk of circular reasoning. Despite these important caveats, a number
of principal conclusions can be made.
First, the palaeo-evidence suggests that small changes in climate forcing, either
in moisture or in temperature, can cause major changes in ecosystem response.
The mid-Holocene aridification in low latitudes occurred in response to a
relatively small change in insolation, but for some regions, it caused widespread
desiccation and salinization, and for lakes close to the threshold between positive
and negative moisture balance, the palaeo-evidence shows that only very small
shifts in hydrology are needed to switch lakes between fresh and saline states.
Likewise, for temperature, the large swings in the organic content of lake
sediments in mid and high latitudes indicate how small temperature shifts may be
magnified by in-lake feedbacks between nutrients, oxygen concentration and
organic matter preservation.
Secondly, mid- and high-latitude northern hemisphere freshwaters experienced
July temperatures 7-8000 years ago approximately 2°C higher than today. The
cooling since then caused a range contraction in the northern limit of thermophilous
taxa, but the rate of change was probably sufficiently slow for organisms and
ecosystems to adapt without causing extinctions. In contrast, the future
temperature increase expected from global warming may reach 2°C over the
1960-90 baseline within two decades, and the warming projected to the end of
the century will be unprecedented both in rate and in magnitude. The probability
that this will cause local and regional extinctions and major readjustments in the
functioning of freshwater ecosystems is very high, and the extent to which these
climate projections and associated ecosystem responses lie well beyond the range
of natural variability is evident from the palaeo-record.
And thirdly, despite the probability of such unprecedented change in the
future, it is still difficult to attribute observed changes in freshwater ecosystems
over recent decades unambiguously to greenhouse-gas-forced warming. This is
due to problems in separating evidence for greenhouse-gas forcing from forcing
caused by natural variability and the difficulty in identifying the influence
of climate on freshwater ecosystems that are already modified by pollution and
other kinds of human activity. The palaeoecological record shows that cyclical
changes in lake behaviour are continually taking place on decadal to centennial
timescales, possibly related to variability in solar irradiance or other modes of
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