Climate Variability in Europe and Africa: a PAGES-PEP III Time Stream I Synthesis

In each of the six major study regions constituting the PEP III Europe-Africa transect, the principal trend of Holocene climate change reflects the global climate forcing that is exerted by variation in the seasonal distribution of solar insolation received at the Earth surface, due to precession of the Earth’s orbit around the Sun. In the Northern Hemisphere, summer insolation was above-average between about 15 and 5 kyr (kyr here denotes 103 calendar years ago), peaked at 10 kyr, and is currently near its minimum; in the Southern Hemisphere, summer insolation has been above average for the past 5 kyr, and is currently near its peak. However, the regional expression of this forcing is modulated by a multitude of amplification, damping, and feedback processes involving all four components of the climate system: atmosphere, ocean, continents, and cryosphere. Consequently, regional histories of Holocene temperature and rainfall change often deviate significantly from the perfect hemispheric anti-phasing which orbital forcing would predict. In addition, superimposed on the long-term climate trends attributed to orbital forcing are various modes of Holocene climate variability operating at inter-annual to millennial time scales. Some of these can be linked to other external forcing mechanisms, such as volcanic eruptions and variations in the radiation output of the Sun, but others appear to result from poorly understood periodicity in the internal dynamics of the climate system. These processes have generated tremendous complexity in Holocene climate history at both regional and continental scales, so that the challenge to document this complexity in enough detail to elucidate the exact mechanisms involved constantly strains the possibilities of available climate-reconstruction methods. This Time Stream 1 synthesis chapter serves two purposes. First, a summary of the main patterns of Holocene climate change across the PEP III transect aims to draw attention to the (real or apparent) synchrony and time-lags between distinct climatic anomalies in the different regions, in the hope that it may help generate the sequence of speculation, modelling, and more detailed reconstruction efforts which typically yield exciting new insights. Second, it aims to draw attention to some of the more persistent problems in highresolution climate reconstruction, in the hope that efforts to address these problems face-on may lead to an improved methodology of climate reconstruction which, when applied to both new and existing data, will advance our understanding of exactly how the world’s climate system operates.