The aim of the NERC funded Euroclim500 project (Causes of change in European mean and extreme climate over the past 500 years) was to understand the causes of variability and change in mean and extreme temperature, and changes in mean precipitation over the last 500 years, largely focusing on Europe. Spatial patterns of climate change and changes in extremes are important since climate affects society and the natural world on regional scales. By understanding why temperature and temperature extremes as well as precipitation have varied in the past, the project sought to improve our ability to predict how it might vary in the future. The project aimed to achive this by having improved our knowledge of what mechanisms were important in changes in the past, and tested the ability of climate models to reproduce what has happened in the past.

Researchers have reconstructed European temperature variability over the last 500 years, and long homogenized instrumental records are now available that go back into the 18th century, and in some cases have daily resolution. Also, some reconstructions of precipitation and circulation are becoming available that go back reliably about 250 years, and with more limitations, 500 years. Other investigators have reconstructed changes in solar activity, the timing and magnitude of major volcanic eruptions and the composition of the atmosphere using data recorded in the ice sheets. The most important natural 'forcing' of climate has been thought to be changes in radiation from the Sun. This is reconstructed from measures of solar activity such as sunspots. However, another possibly more important climate forcing is the effect of large explosive volcanic eruptions. These inject sulphate aerosol into the upper atmosphere, which reflect more sunlight, causing cooling. Since the 18th and 19th centuries human forcings have become important. Human emissions of carbon dioxide and other greenhouse gases are well mixed, so cause roughly equal warming everywhere. From the 18th century on substantial deforestation took place. The removal of forest makes the land reflect more sunlight back to space, causing a cooling. The main fossil fuel burnt in the 19th century was coal, whose burning causes sulphur to be emitted to the atmosphere. The sulphur forms a short-lived aerosol which acts to cool regions close to where it is emitted. Thus, human forcings would be expected to have a rather complex regional affect with, in the Northern Hemisphere, warming from greenhouse gases being, partly to fully, offset by cooling due to deforestation and aerosols.

To find causes of the observed patterns of climate change over Europe and the Northern hemisphere the Euroclim500 project used climate models that can simulate what would have happened due to each individual 'forcing' described above. Then using mathematical techniques the participants compared these 'fingerprints' with reconstructions of past climate change and climate variability and determine which changes have been caused by external influences, and which are just a representation of a naturally varying climate. The main focus of the work was on patterns of temperature and precipitation change over Europe since 1500, but also explored the causes of changes over the entire millennium.

The results of the comparison were then to informed us of the relative contribution of each forcing to past climate change, and how much change is left unexplained and may have occurred spontaneously due to chaotic variability in the climate. Since climate models, like the real world, have chaotic variability, several simulations were used to isolate the predictable component. Using models and reconstructions, the participants also sought to explore the mechanisms responsible for the historical changes.