Holocene Climate Change

Concern about current and future climate change has increased interest in understanding past climate changes, and particularly periods when temperatures are considered to have been warmer than the present. One of the most recent and widely recognized of these was during the early-mid Holocene (~9000-5000 years ago), when widespread warming is thought to have occurred, mainly over high latitude areas of the Northern Hemisphere. Our work is involved in reconstructing the climate of the Northern Hemisphere extra-tropics throughout the Holocene in order to better understand the nature and causes of this warming, as well as other climate changes that occurred during this period. This work is primarily based on newly developed pollen databases, which allow us to reconstruct Holocene climate in unprecedented detail across the major Northern Hemisphere continents based on thousands of sites. Most previous work has been concentrated in high latitudes using summer or annual temperature proxies, but an important aspect of our work is to understand temperature changes at both high and low latitudes, as well as those that occurred in winter as well as summer. A direct application of this work is to evaluate the ability of climate models to simulate Holocene climate change (see image), providing an important test of model reliability that is distant enough in the past to be well outside of modern experience, but close enough that the forcings are well known and sufficient data is available that we can reconstruct the climate in considerable detail.

The summer temperature of Europe 6000 years ago (expressed as an anomaly compared to the present). Left: reconstructed from pollen data, Right: simulated by the latest CMIP5 climate models (14 model ensemble mean).

Climate models show a uniform summer warming across all of Europe, but the data shows a very different pattern, with warming confined to Northern Europe and cooling over much of Southern Europe. Models indicate that warming was a simple direct response to the increase in summer insolation at this time, whilst the data suggests a more complex response that can be related to changes in atmospheric circulation, which brought warmer air north and cooler air south.