Previous warm periods don’t mean we’re not responsible for this one
For those interested in temperature reconstructions of past climates, in particular the kerfuffle over the hockey stick, I recently found a pretty good website. It contains a load of useful information, some of which I did not know. For example, consider this famous plot from the IPCC’s First Assessment Report:
Skeptics have used this plot to argue that today’s warmth cannot be caused by humans because it was warmer one thousand years ago. The website does a good job of laying out the history behind the plot. For example, I learned that:
The graph was created by climatologist Hubert Lamb to show the temperature of Central England over the last 1,100 years. For these purposes, Central England is the triangular area enclosed by Bristol (west), London (east), and the county of Lancashire (north). Everyone agrees that there was a MWP in England and other areas near or in the North Atlantic.
The fact that this is not a global plot greatly undercuts the skeptics’ argument. The website also does a good job discussing the rest of the hockey stick controversy.
For those not immersed in the minutia of climate science, the most recent thinking on temperature reconstructions is that it is likely that the last few decades are the warmest of the last thousand years. “Likely” here means about a two out of three chance that this statement is correct, which makes this a highly uncertain statement.
However, it is important to remember that it is well established that the Earth has been much warmer in the past than it is today. For example, 55 million years ago, in the Eocene, the Earth was so warm that it was completely ice free, with crocodiles and ferns and other frost intolerant species living in the Arctic. However, none of that means that the warming we are experiencing today is not caused by humans.
To understand why, imagine that a skeptic’s car does not start one day. He or she takes it into the garage and the mechanic informs the skeptic that the problem is a faulty fuel pump. The skeptic objects, however. The last time the car did not start, he or she tells the mechanic, it was the battery. Therefore, it cannot be the fuel pump now. It must be the battery.
That’s essentially the argument that they make when they say, “because the Earth was warmer in the past, today’s warming cannot be caused by humans.” Do yourself a favor and ignore that argument.