Past, Present, and Future Climate Change: Can Palaeoecology Help Manage a Warming World? - Biodiversity Conservation and Environmental Change - page 91

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Figure 5.4 The rate of change in the combined radiative forcing by CO 2 , CH 4 , and nitrous oxide (N 2 O) for
the last 20 kyr reconstructed from Antarctic and Greenland ice and firn data and direct atmospheric
measurements. The arrow shows the peak in the rate of change in radiative forcing after the anthropo-
genic signals of CO 2 , CH 4 and N 2 O have been smoothed. Rate of change above the arrow can be attrib-
uted to forcing by anthropogenic greenhouse gases (Jansen et al. 2007 Climate Change 2007: The
Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change, Figure 6.4(d), Cambridge University Press).

climate models suggest that while most pre-1850 climate change, including the MWP, can be
explained by changes in sunspot activity and volcanism, only 25% of twentieth century warm-
ing can be explained by these two factors (Figure 5.4). Twentieth century warming can only
be simulated with the addition of anthropogenic forcing by greenhouse gases. There is there-
fore convincing evidence that the causes of current warming trends are largely driven by
human activities (Crowley 2000, Jansen et al. 2007).

Past interglacials

The current interglacial period, the Holocene, began about 11,000 years ago, but at least 20
glacial-interglacial cycles have taken place in the last 2.5 million years (the Quaternary
Period). Vital information about the effects of warming climates can be gained by studying
past interglacial periods. Glacial-interglacial cycles, driven by changes in the Earth's orbit
around the Sun, are apparent in the palaeoenvironmental records from ice-cores, marine
cores, and lake sedimentary sequences. Orbital forcing is one of the few climatic drivers that
are understood with high scientific certainty, and glacial periods can be successfully 'hind-
cast' or 'retrodicted' by modelling orbital effects. As a result, we know that the present-
day glacial retreat cannot be explained by orbital factors, and that the twentieth century
change in temperature is inexplicable unless the effects of anthropogenic greenhouse gas