From the Seasonal Attribution Project Website:

What is the climateprediction.net Seasonal Attribution Project?

The climateprediction.net Seasonal Attribution Project uses computing time donated by the general public to run state-of-the-art high-resolution model simulations of the world's climate. These simulations are used to determine the extent to which the risk of occurrence of extreme weather events is attributable to human-induced climate change.

We focus on extreme weather events that occur on a seasonal timescale, and in our current project we focus specifically on the United Kingdom floods of Autumn 2000 which occurred during the wettest autumn ever recorded, causing widespread damage and an estimated insured loss of £ 1.3 billion.

• Half of the climate model simulations we run are of the Autumn 2000 period, specifically including within them the effects of human-induced climate change caused by the emission of greenhouse gases. We call these the "Industrial Autumn 2000" simulations.
  
• The other half will simulate a representation of the the Autumn 2000 climate had there not been any human-induced emissions of greenhouse gases over the last century. We call these the "Non-Industrial Autumn 2000" simulations.
  
• By then comparing the results of these Industrial and Non-industrial simulated climates, and recording the occurrence of floods like that of Autumn 2000 in each of them, we can determine how the frequency of occurrence (or 'risk') of such a flood has changed, and therefore how much risk is attributable to human-induced emissions of greenhouse gases over the last century.

There is also an article with more details about the science behind this project.

We are also collaborating with other research groups who are interested in using our simulations to perform similar attribution studies, for snowmelt in western North America, and heatwaves in South Africa and India.