A fair wind for raiding
Equally, if you dig down into a peat bog, you will find that there are horizontal layers, caused by variations in climate - in dry summers the surface of the bog will dry out and compact, whereas high rainfall will encourage the peat to grow. Plants can also provide evidence. They have different needs in terms of temperature and moisture, and the presence or absence of the pollen of certain plants on an archaeological site can indicate what the climate was like when people were living there. A cool and wet climate, for instance, will encourage the growth of ferns.
So we know that the summers were getting warmer at the time of the Viking raids, but that was not the whole story. For Norwegians intent on reaching Scotland and the honeyed seaway to Ireland, the wind was on their side too. The prevailing winds helpfully blew westwards into their sails in springtime, to take them to the Scottish islands, and eastwards in the autumn to bring them home.