Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 15

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Aquatic Ecosystem Variability and Climate
Change - A Palaeoecological Perspective
Richard W. Battarbee
Introduction
Over the last decade it has become increasingly clear that there is a strong human
contribution to global warming (IPCC 2007). Antarctic ice-core records (e.g. Petit
et al . 1999; EPICA 2004) show that greenhouse gas concentrations are already
higher than at any time in the last 750,000 years, temperatures in the northern
hemisphere are now on average probably higher than the previous 1000 years
(Mann et al . 1998) and climate models can only simulate temperatures accurately
over the last 150 years if greenhouse gases are included as a forcing mechanism
(Stott et al . 2001).
Evidence is also accumulating to suggest that changes in natural systems that
can be unambiguously attributed to rising temperatures are also occurring. In
particular, most mountain glaciers across the world are receding (Oerlemans
2005), there is evidence that the collapse of Antarctic ice shelves in the Antarctic
is unprecedented in the Holocene (Domack et al . 2005) and ecological changes
are taking place in remote Arctic lakes that appear to be outside the range of their
natural variability (Douglas et al . 1994).
The evidence for human impact on the climate system is thought now to be so
compelling that Crutzen has argued that the recent period of earth history dating
from the late 18th century and associated with a significant rise in atmospheric
CO 2 manifested in Antarctic ice cores (Petit et al . 1999) should be given a new
geological name, the Anthropocene (Crutzen & Stoermer 2000). Indeed
Ruddiman (2003) has argued that human activity may have affected atmospheric
greenhouse gas concentrations even earlier as a result of deforestation and land-
cover change associated with early agriculture in the early to middle Holocene,
approximately 5-8000 years ago.