The next set of evidence frequently cited in support of CAGW is that some aspect of weather, climate, or whatever is unprecedented. Essentially, the argument is that if something significant happens with the weather which has not happened before, then that thing is likely to be a result of mankind’s activities.
However, on closer inspection, these “unprecedented” claims usually turn out to be either (1) false; (2) unsupported by the evidence; or (3) not based on a sufficiently long time period to mean much, if anything.
For example, it is often said that temperatures in the 1990s were the highest since instrumental temperature monitoring began in the 19th century. Assuming this is true, the time period in question does not seem long enough to mean much, since we have been coming out of the Little Ice Age.
Similarly, it has been claimed that the 1990s were the warmest decade in 1000 years. However, the evidentiary support for this is rather weak. Since temperatures were not measured with thermometers 1000 years ago, you have to guesstimate temperatures using “proxies,” like tree rings. How do we know that we are interpreting these proxies correctly? Or even that they can be used to measure past temperatures at all?
The obvious way to check these proxy methods is to use the same methods to estimate recent temperatures, and then to compare the results to intstrumental records. As far as I know, proxies have not done so well when checked this way. This is the so-called “divergence problem.” So any claim that current temperatures are unprecedentedly warm is at best a guess.
At the same time, there does exist evidence of warm temperatures in the past. The classic example of this is the VIking colonization of Greenland during the Medieval Warm Period. Of course, one can debate the significance of this and similar evidence. But the bottom line is that there is not a good case at this point that recent temperatures are unprecedentedly warm.
Objections / FAQ
5.1 Even if one proxy study is flawed, we can overcome this problem by averaging the results of many studies.
We cannot for the same reason as that set forth in 4.4.
5.2 Ok, so maybe recent temperatures are not unprecedentedly warm — what matters is the rate of change. Recent temperatures are rising at an unprecedentedly high rate.
Show me proof. And don’t compare proxy measures with instrumental measures. That’s not an apples-to-apples comparison. If proxy measures are understating the magnitude of recent temperature increases, it’s likely that they are understating the magnitude of past increases. Indeed, this would make sense as proxy measures – such as tree rings – are based on biological systems, subject to homeostasis.
5.3 Well, but what about X, which has never happened before?
By all means, show me proof. I would be fascinated to look at it.