However, whilst the variation over large time-scales is extreme (deviating from the long-term average by as much as ten degrees Celsius at a time), the changes are usually very gradual and take place over many millennia. | However, whilst the variation over large time-scales is extreme (deviating from the long-term average by as much as ten degrees Celsius at a time), the changes are usually very gradual and take place over many millennia. |