Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 18

also entirely possible that human activity has modified or is modifying the climate
system itself, even to an extent that a return to glacial conditions, as considered
inevitable only a decade ago, may be prevented by the current and projected rise
in greenhouse gas concentrations (Crucifix 2008).
Although the Holocene is a warm period, on average 7°C-8°C higher than the
mean for the last glacial period (cf. Lowe & Walker 1997), there have been
significant changes in climate within it, driven by a range of different natural
forcing mechanisms. These have operated on different timescales and have
affected, and will continue to affect, both temperature and precipitation patterns
across the world. The principal external forcings are (i) orbital change, mainly
related to changes in precessional changes of the earth's axis as it orbits the sun;
(ii) solar variability, related to cyclical variations in solar activity through time;
and (iii) volcanic activity, related to the scattering and absorption of incoming
radiation caused by changes in volcanic dust concentrations in the stratosphere.
Climate also varies in a quasi-cyclical way on a range of timescales purely in
response to the internal dynamics of the climate system itself. The best known
and important of these modes of internal variability is the Southern Ocean
Oscillation, ENSO, although in higher latitudes of the northern hemisphere, the
Arctic Oscillation (AO) and the associated North Atlantic Oscillation (NAO) can
be equally or more important. Changes in these modes can also occur and can be
influenced by changes in external forcing (e.g. Shindell et al . 2004). In addition
to these natural forcings, there is now very good evidence (cf. IPCC 2007) that
anthropogenically produced greenhouse gases are playing an important role in
altering the climate system and are beginning to warm the planet, potentially to
higher temperatures than have hitherto been caused by natural factors. However,
such is the complexity and variability of the climate system that attributing
changes in natural ecosystems at the present time solely to a rise in greenhouse
gas concentrations is not easy (cf. Battarbee & Binney 2008).
Multi-millennial scale change
Temperate latitudes
Palaeoclimate reconstructions of sea-surface temperatures provide evidence of
warmer conditions than at present prevailing in the northern hemisphere in the
early Holocene (e.g. Jansen et al . 2008). The early warming followed by a
progressive cooling to the present day agrees closely with the expected decrease
in northern hemisphere insolation based on known precessional changes in the
orbit of the earth around the sun. Perihelion (when the earth's orbit takes it
closest to the sun) now occurs in January, whereas it occurred in July 11,000
years ago. Although proxy records vary, the cooling experienced in medium to
high latitudes suggests a decrease in mean annual temperature over the Holocene
of approximately 2°C (e.g. Seppä et al . 2005) sufficient to cause major southwards
shift in the northern limit of plants and animals, including a depression of the
northern timber line (Birks & Birks 2003). However, demonstrating the response
of aquatic organisms to this long-term cooling is not easy, partly because human