The Climate of the Last Millennium

We are living in unusual times. Twentieth century climate was dominated by near universal warming with almost all parts of the globe experiencing temperatures at the end of the century that were significantly higher than when it began (Figure 6.1) (Parker et al. 1994, Jones et al. 1999). However the instrumental data provide only a limited temporal perspective on present climate. How unusual was the last century when placed in the longer-term context of climate in the centuries and millennia leading up to the 20th century? Such a perspective encompasses the period before large-scale contamination of the global atmosphere by human activities and global-scale changes in land-surface conditions. By studying the records of climate variability and forcing mechanisms in the recent past, it is possible to establish how the climate system varied under “natural” conditions, before anthropogenic forcing became significant. Natural forcing mechanisms will continue to operate in the 21st century, and will play a role in future climate variations, so regardless of how anthropogenic effects develop it is essential to understand the underlying background record of forcing and climate system response.