Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 179

Chapter 6
Post-glacial climatic change
and variability
6.1 Introduction
An understanding of past climates, climate change, and variability within the
Earth's climate system is essential to help understand current climate variability
and to assist in the prediction of future climate change. Research over the past
50 years or so has demonstrated that changes in climate can be described for
periods up to 420 000 years before present (BP). The very last few thousand
years of record, the period since the end of the last major glaciation, the
Holocene, is described by Petit et al.( 1999 )as''... the longest stable warm
period recorded in Antarctica during the past 420 ky.''
Jones and Mann ( 2004 ) emphasize that the last 12 000 years BP is a short
enough period so that long-term effects on climate, such as changes in the
Earth's orbit and major changes in global ice mass, have no impact. Natural
forces over smaller temporal scales, such as variations in solar radiation output,
dust and gases from irregular volcanic eruptions, ENSO variations (Section 2.8 )
and changes in ocean-atmospheric circulation, become major forcing factors.
After about AD 1850, human influences become much more important, asso-
ciated with excess greenhouse gas emissions, global warming, land use changes,
and cooling from sulfate aerosol pollution.
Thompson et al.( 1993 ) list several reasons why the study of climate during
the Holocene, and especially over the past 1000 years, can provide an important
contribution to understanding of climate variability. There is enough detailed
information available from a range of sources to allow climate reconstructions.
These can include rates of climate change, lags, leads, and annual and seasonal
resolutions. The data show that periods of extremes existed, but also periods
where climate could be considered ''average'' or ''normal.'' Through modeling
and other methods, the causes of Holocene climate change and variability can be
explored, and the details of potential forcing functions can be established.
This chapter describes current knowledge about climate change and variability
during the Holocene. After a short review of the methods used to determine past
climate, the essay by Zielinski focuses on the Northern Hemisphere (NH), where
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