because it got rid of the Medieval Warm Period and to a lesser degree the Little Ice Age. why is that so important? science couldnt explain why the MWP happened using known natural variables. if they didnt have to admit they didnt understand the reasons for that warm period then they could turn the null hypothesis on its head in the 20th century and say that known factors cant explain the present age warming so it must be CO2! it was clear that the first and second IPCC reports understood that there were large gaps in understanding the mechanisms that control climate. since the 2001 report and Mann's graph they have not been forthcoming about their ignorance. we know that Mann's Hockey Stick was pseudoscience and an affront to the scientific method but the damage has been done. it created a paradigm of using CO2 as the scapegoat and climate models as data. I have no idea how we can rehabilitate the reputation of climate science but having a real investigation into Climategate would be a good start.