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ORIGINAL: HamOnWholeWheat
Your argument seems to be “Warming in the dark ages was almost certainly nonanthropogenic, therefore the current warming is also almost certainly not.” Do you honestly think that’s a logical conclusion?
No. Again, I point out to you the work of Mann et al. It has largely been his work used by proponents of AGW to declare the issue settled.
Let me try again. It has been hypothesized that large amounts of CO2 emission over the past 150 years has led to unprecedented increases in the mean global temperature.
To prove this hypothesis, one must show that global climate over the past 150 years has significantly deviated from more remote climactic behavior.
If the data show no significant change in climate over extended periods of time, then one cannot conclude that elevated CO2 has caused recently observed upward trends in global temperatures (this does not imply that CO2 has no effect whatsoever on mean temperatures or climate, it just means it is not the primary causative agent of recently observed changes).
To use your example of chest pain, consider unstable angina versus longstanding stable angina. The hypothesis of AGW poses a similar question, namely does recently observed behavior of the global climate system truly represent significant change from past behavior.
If somebody presented with chest pain which had been going on for years and had not materially changed in frequency or severity, you would conclude that the latest episode of chest pain did not relate to a significant change in the status of their coronaries. But if they clearly have unstable angina then you are dealing with a different situation. It is imperative therefore to determine if the current chest pain significantly differs from prior episodes of chest pain.
Likewise, if the variation of mean global temperature has not significantly changed over the past several millenia (i.e. we include the MWP in the Mann hockey stick), then we cannot conclude recent increased atmospheric CO2 is major cause of recent climate alterations.
Simply put, we are asking “is the climate truly changing”. This is the crux of the debate.
Unfortunately it is very difficult to answer these questions because the data are so poor. Only recently have accurate data become available. Everything else is limited, particularly data obtained before the mid 19th century. For those more remote time periods you must use proxy data (i.e. tree rings and ice cores).
Again, you cannot simply assume the climate is changing in some unusual, unprecedented fashion. That is not logically consistent. You must first prove the climate is unequivocally diverging from prior behavior. This is what Mann’s hockey stick was purported to prove. It is only after you have proven this to be true that you can then move to the next question of “what is causing this change?”.