CHELSA is a high resolution (30 arc sec) climate data set for the earth land surface areas currently hosted by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. It includes climatic variables for various time periods.
CHELSA is based on a quasi-mechanistical statistical downscaling of reanalysis and global circulation model output and is freely available  in the download section.

The data is described in this peer reviewed article:

Karger, D.N., Conrad, O., Böhner, J., Kawohl, T., Kreft, H., Soria-Auza, R.W., Zimmermann, N.E., Linder, H.P. & Kessler, M. (2017) Climatologies at high resolution for the earth’s land surface areas. Scientific Data 4, 170122.

A technical specification including the units used can be found here:

CHELSA V1.2: Technical specifications.