Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 16

Yet, despite the strength of the evidence for human-induced change, climate-
change sceptics still remain, arguing that the role of natural variability is being
underestimated. It can indeed be maintained that changes in aquatic ecosystems
described in this topic, as shown by long-term records (e.g. ice-cover loss on
lakes over the last two centuries (Magnuson et al . 2000) or the observed increase
in river and lake temperatures over recent decades (Hari et al . 2006) ), are still
within the long-term natural range of the climate system, if viewed on centennial
timescales. Despite their quality, even these multi-decadal long-term data sets
cover too short a time span to differentiate a recent global warming component
from changes that might be caused by natural variability.
In this chapter, I examine evidence for changes in aquatic ecosystems from the
palaeoecological record in an attempt to define the natural variability of aquatic
systems as a baseline against which the impact of recent and projected future
climate changes can be assessed. I also examine the impact of past warmer periods
on surface waters as a guide to what might happen in future, although once
global mean temperatures exceed +2°C, the past is unlikely to hold useful
analogues as temperatures greater than about +2°C have not occurred previously
for at least 100,000 years.
I describe briefly how climate has changed over the Holocene and then
summarize palaeoecological evidence for the response of freshwater eco-
systems, principally lakes, to climate change over different timescales, from
multi-millennial to seasonal. Two principal effects of climate change are described:
those that are driven by changes in temperature and those that are driven by
changes in effective moisture (precipitation minus evaporation). These are
considered in both high- and low-latitude settings. The chapter ends by setting
evidence for greenhouse-gas-forced climate change against other causes of
ecological change, specifically those associated with human activity. Note,
however, that the evidence presented here for the effects of past climate change
on freshwater ecosystems is entirely inferential, as inevitably is the case for all
palaeoecological interpretations. Moreover, in some cases, it is difficult to avoid
problems of circular reasoning as changes in the biological history of lakes
revealed from sediment records are often used to reconstruct past climate changes
rather than the response of lake ecosystems to climate change.
Climate over the Holocene
How far back should we go in considering past climate change and its impacts?
Although most lakes in high latitudes are relatively young, formed after the
recession of the last ice sheets approximately 15,000-10,000 years ago, lakes
outside the glacial margin, mainly but not exclusively at lower latitudes, have
survived repeated switches from glacial to interglacial conditions and contain
sediment records spanning many hundreds of thousand years. A few (e.g. Lake
Baikal) span the entire Pleistocene period and are of considerable interest amongst
palaeoclimatologists as well as palaeoecologists as rare archives of environmental
change, comparable to those found in deep oceans, and as in situ centres of
evolution. Here I restrict the time span under consideration to the Holocene