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Changes have always been happening sure -- but how FAST were the changes in the past? Going up or down a degree C in 100,000 years is easy, but having that change in ~100 years is another thing altogether.
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Not so. Google "8.2k event". During the Holocene Climatic Optimum (the warm period mentioned in my post above), there was a climatic, uh, bump in the road. As ice sheets retreated, a large North American fresh water lake suddenly drained into the North Atlantic, generating a large scale shift in ocean currents. The fallout was this: in a five year time span, the earth cooled by an astounding
6ºC and remained this cold for about 60 years. Also, to counter jamesqf's belief that a nomadic culture is happier, this event seems to coincide with the collapse of the first human settlements (archeological sites in Iraq, Israel, and Jordan) and their remission into hunter-gatherer culture. When the climate did recover from this cold snap, it then warmed up 6ºC in just 100 years. By comparison, some of the
worst-case climate models show us warming by 5ºC in the next 100 years (as a note, these models are already showing divergence from actual conditions, ie, we're not as warm as we should be). The return to warm conditions coincides with the rise of the ancient cultures of the fertile crescent we know so much about. And again, global temperatures were
higher than the present day.
I won't go so far as to say "arbitrary" - but I don't believe that
350ppm is scientifically justified. Why? Simply this: Water vapor and clouds are the dominant players in the greenhouse game. They account for 66-85% of the greenhouse effect. CO2 contributes about 9-25%, with methane and other GHGs filling out the rest. (FYI - The variability in the percentage is largely dependent upon relative humidity and cloud cover.) The CO2 reinforcement of the positive feedback loop of the water vapor cycle is mitigated by two forces: 1) dust generation in dry conditions leads to cloud formation and 2) the power of evaporative cooling. On the first, clouds reflect solar radiation even while they absorb re-radiated heat from earth. On the second, yes, higher atmospheric temperatures mean a larger capacity for water vapor, but getting more vapor in the atmosphere takes tremendous amounts of solar energy, which puts negative feedback into the loop.
Please do not misunderstand or misrepresent my opinions here. I do believe that we must work on being better stewards of the Earth and our resources. I do believe that we should develop cleaner and more efficient energy sources. I also believe that cheap, clean and efficient energy sources has the potential to raise billions of people out of poverty - meaning that developing this clean energy is imperative from a humanitarian perspective. While I do believe in anthropogenic climate changing forces, I do not believe that anthropogenic warming is, as of yet, the dominant force in our global climate. I believe this to be the case because of the past.
Climate changes in the past have been sudden and dramatic. Did humans, 130,000 years ago, cause the temperature peak shown in this graph? Unless our great(5x 10^3)grand-daddy Ugg the Clumsy accidently started the largest forest fire ever, obviously not. No, other forces, Milankovitch cycles for example, are a bigger player than we could ever pretend to be.