The good old times when carbon dioxide and methane levels as well as temperatures were low

The previous post was about “the most popular contrarian argument” according to skepticalscience (“climate changes before, so current climate change is natural”) and what they seem to consider a live example of such a claim. I then proposed in that post that it actually was not a good example of what they want to prove.

What I didn’t discussed yet was how skepticalscience “debunked” this most popular contrarian argument. They did this in the “Climate’s changed before” myth page that was apparently based on this example.

They “debunked” this “myth” by stating that the climate is indeed always changing, but the difference is that it is changing much faster now than in the past because of our increasing emissions. This is how it starts:

Greenhouse gasses – mainly CO2, but also methane – were involved in most of the climate changes in Earth’s past. When they were reduced, the global climate became colder. When they were increased, the global climate became warmer. When CO2 levels jumped rapidly, the global warming that resulted was highly disruptive and sometimes caused mass extinctions. Humans today are emitting prodigious quantities of CO2, at a rate faster than even the most destructive climate changes in earth’s past.

Basically, climate changed naturally before, but now we are changing it because of our rapid emissions.

It was not really clear in the basic rebuttal what they based themselves upon to prove this claim. However, in the intermediate version of the rebuttal it was illustrated with this graph:

It visualizes the temperatures related to the CO2 and CH4 (methane) levels in the atmosphere over the last 800,000 years, showing those values are in lockstep with each other. When CO2 and CH4 go up, temperature also goes up. When CO2 and CH4 go down, temperature also goes down. Just as was explained in the basic version of the rebuttal.

The most important thing happened at the right side of the graph. The current levels of CO2 and CH4 are shown there and at first glance it seems pretty clear that the CO2 level never went above 300 ppm during those 800,000 years, while reaching 386 ppm a couple years ago. The same with the CH4 level that never went above 800 ppb and reached 1,790 ppb a couple years ago.

Somehow suggesting that the current levels are unprecedented over the last 800,000 years and that, because these greenhouse gases moved in lockstep with temperature in the past, temperature will follow.

However, the big question is whether it really could be reliably concluded from this data?

It would be convincing if it was an apples-to-apples comparison. Which it is clearly not. We are reliably measuring temperature and CO2/CH4 levels in the atmosphere since half a century to a century by now and I am pretty sure that early humans had other worries than to measure these levels in prehistoric times. So the graph consists of two parts: proxy data combined with current measurements of CO2, CH4 and temperature.

The problem I have with this graph is that there is no distinction between both parts, the proxy data as well as the current measurements have the exact same color. The graph lines flow fluently from proxy data into the current measured values. Giving the impression that the current levels are unprecedented for the past 800,000 years.

Could that be true? I see two issues with this.

Firstly, proxy data and the currently measured data are two different things. Are these good proxies for CO2, CH4 and temperature in the last 800,000 years? In other words, could the signal of those parameters be filtered out from the prevailing environmental conditions (and possible other issues)?

Secondly, even assuming for the sake of the argument that these are good proxies for the three parameters, there is still another important issue: the difference in resolution. Proxy data is not sampled for a specific year, but many years are pooled. The sampling rate of proxy data is in the range of hundreds of years. Such low resolution has the effect of dampening the extremes, so it is misleading to suggest that current levels are unprecedented by comparing them with a dataset that has the extremes removed due to the sampling method.

CO2 in our atmosphere started to rise significantly since the 1950s. That is about 60 years. There is no way that we could ever find a similar 60 years increase in a dataset that has a sampling rate of at least double the period under investigation. I am also not going to speculate whether similar increases happened in the past or not, a dataset with such a low resolution will not be able to prove that either way.


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