Note: See the update at the end of the post for a test involving 1000 (rather than 100) Monte Carlo simulations.
If a temperature event like we witnessed in the last century — a warming of around 0.9 deg.C in about 100 years — had happened at some other time in the last 11,300 years, would it have left some trace in the recent paleoclimate reconstruction of Marcott et al.?
Some believe that it wouldn’t because their estimate is based on an average of 1000 “perturbed” results. The perturbations include “smearing” the age estimates (introducing random changes to see how that affects the result), simply because the ages are, after all, uncertain. For each proxy, each age was offset by a random amount based on its estimated uncertainty. Then these perturbed ages were used to compute past temperature, forming a single “realization” of the perturbation process. A thousand perturbations were then averaged to create the final estimate by Marcott et al.
The belief of many is that this process of “smearing” ages would so smooth out any spike which may have occurred in the past, that it wouldn’t show in the Marcott reconstruction. This means, so they say, that warmings like we saw in the 20th century could have happened multiple times in the past, and the Marcott work doesn’t provide any evidence against that.
Let’s find out, shall we?
I created an artificial temperature signal consisting of a temperature spike like that of the 20th century, followed by a return to “normal.” The spike is a rise of 0.9 deg.C over a span of 100 years, followed by a return to zero over the next 100 years. I put in not just one, but three spikes, since the age uncertainties are different at different times so I wanted to know how spikes might be smoothed out at different times in the past. The spikes were centered at 7000 BC, 3000 BC, and 1000 AD.
I then took the proxy data sets used in Marcott et al. and added to them this artificial temperature signal. That enabled me to compute a temperature reconstruction (using the “difference method”) based on the Marcott proxies plus the artificial signal. Here it is:
Note that the spikes in the reconstruction are smaller than they are in real life, even though the ages have not been perturbed. That’s because some of the proxies are totally unaffected by the artificial signal, because they don’t include any observation times which occur during any of the spikes.
Nonetheless the spikes are abundantly clear. But what if we perturb their ages so different proxies record them at different times? For each proxy, I perturbed the ages with Gaussian random noise which had the same standard deviation as the “age model uncertainty” given in the Marcott data. Then I computed the reconstruction using the perturbed ages. One such realization looks like this:
We can compare it to the reconstruction using the unperturbed ages:
Clearly the spikes are reduced in size by the smearing of age estimates. But they’re still there. All three of them.
OK — but Marcott et al. didn’t just create a single perturbed record. They made a thousand, and averaged their final results. I didn’t do a thousand because that would have taken a lot of computer time, but I did do a hundred and averaged them. This is the result:
The spikes are still there. Plain as day. All three of ’em. We can compare this to the reconstruction using unperturbed ages:
The spikes are a lot smaller than with no age perturbations, which themselves are smaller than the physical signal. But they’re still there. Plain as day. All three of ’em.
My opinion: the Marcott et al. reconstruction is powerful evidence that the warming we’ve witnessed in the last 100 years is unlike anything that happened in the previous 11,300 years.
The idea so terrifies those in denial of global warming, that they have undertaken a concerted effort to “smear” this research. That’s because it clearly implies that modern global warming is unprecedented, and shines a light on the folly of throwing a monkey wrench into the climate machine. And that means we ought to change our ways, which just happen to involve some of the biggest money-making ventures in the history of humankind.
The idea also terrifies me. For a different reason.
I went ahead and repeated the experiment using 1000 (rather than 100) perturbed records. It doesn’t change the conclusion: