The difference is that we generally know the reasons behind earlier climate changes. The medieval warming period, for example, was due to increased solar radiat

The difference is that we generally know the reasons behind earlier climate changes. The medieval warming period, for example, was due to increased solar radiation [1]. It's not that there have not been lots of major climate change before; it's that this time it's caused by humans. [1] http://www.skepticalscience.com/medieval-warm-period.htm http://www.skepticalscience.com/medieval-warm-period.htm