EKF400 version 2 is a monthly resolved paleo-reanalysis covering the period 1603 to 2003. Early instrumental temperature, surface pressure and precipitation (new in version 2) observations, temperature indices derived from historical documents and temperature and moisture sensitive tree-ring measurements (largely increased number in version 2) were assimilated into an atmospheric general circulation model ensemble using a Kalman filtering technique. This data set combines the advantage of traditional reconstruction methods of being as close as possible to observations with the advantage of climate models of being physically consistent and having 3-dimensional information about the global state of the atmosphere at 2x2 degree resolution for various variables and at all points in time.

While version 1 was using time-dependent covariances from the 30-member ensemble, version 2 uses a blended covariance matrix. It consists of 50% time-dependent and 50% climatological covariances. This leads to a better covariance estimation, not only between various locations in space but in between various variables, too.