This is Chapter 6 of an ongoing series. Other parts of the series are here:
- Greenhouse Gas Theory
- A) Actual Temperature Data; B) Problems with the Surface Temperature Record
- Attribution of Past Warming: A) Arguments for it being Man-Made; B) Natural Attribution
- Climate Models vs. Actual Temperatures (this article)
- Are We Already Seeing Climate Change
- The Lukewarmer Middle Ground
- A Low-Cost Insurance Policy
In some sense, this is perhaps the most important chapter, the climax of all the discussion to this point. It is where we return to climate forecasts and attempt to conclude whether forecasts of catastrophic levels of man-made warming are reasonable. So let's take a step back and see where we are.
Here is the framework we have been working with -- we have walked through in earlier chapters both the "theory" and "observation" sections, ending most recently in chapter 5 with a discussion of how much past warming can be attributed to man.
It is important to remember why we embarked on the observation section. We ended the theory section with a range of future temperature forecasts, from the modest to the catastrophic, based on differing sensitivities of temperature to CO2 which were in turn largely based on varying assumptions about positive feedback effects in the climate.
We concluded at the time that there was not much more we could go with pure theory in differentiating between these forecasts, that we had to consult actual observations to validate or invalidate these forecasts.
We've already done one such analysis when we made two comparisons back in Chapter 4. We showed that temperatures had risen over the last 30 years by only a third to a half the rate projected by James Hanson to Congress...
And that even the IPCC admitted in its last report that temperatures were running below or at best at the very low end of past forecast bands
But in the grand scheme of things, even 30 years is a pretty short time frame to discuss climate changes. Remember that in my own attribution attempt in Chapter 5, I posited an important 66 year decadal cycle, and past temperature reconstructions imply other cycles that are centuries and millennia long.
But there is a way we can seek confirmation of climate forecasts using over 100 years of past temperature data. Let's take our forecast chart we showed above and give ourselves a bit more space on the graph by expanding the timescale:
Here is the key insight that is going to help us evaluate the forecasts: each forecast represents an actual, physical relationship between changes in CO2 concentrations and changes in temperature. If such a relationship is to hold in the future, it also has to be valid in the past. So we can take each of these different forecasts for the relation between temperature and CO2 and run them backwards to pre-industrial times in the 19th century, when atmospheric CO2 concentrations were thought to be around 270 ppm.
The temperature value of each line at 270ppm point represents the amount of warming we should already have seen from man-made CO2
What we see is that most of the mainstream IPCC and alarmist forecasts greatly over-predict past warming. For example, this simple analysis shows that for the IPCC mean forecast to be correct, we should have seen 1.6C of manmade warming over the last century and a half. But we know that we have not seen more than about 0.8C total in warming. Even if all of that is attributed to man (which we showed in the last chapter is unlikely), warming has still been well-short of what this forecast would predict. If we define a range for historic man-made warming from 0.33C (the number I came up with in the last chapter) to 0.8C (basically all of past warming due to man), we get numbers that are consistent with the non-catastrophic, zero-feedback cases
Of course we are leaving out the time dimension -- many of the hypothesized feedbacks take time to operate, so the initial transient response of the world's temperatures is not the same as the longer-term equilibrium response. But transient response likely is at least 2/3 of the full equilibrium value, meaning that my hypothesized value for man-made past warming of 0.33C would still be less than the no feedback case on an equilibrium basis.
It is from this analysis that I first convinced myself that man-made warming was unlikely to be catastrophic.
I want to add two notes here.
First, we mentioned back in the attribution section that some scientists argue that man has caused not all of but more than the total observed historical warming. This chapter's analysis explains why. The fact that climate models tend to overpredict history is not a secret among climate modelers (though it is something they seldom discuss publicly). To justify their high feedback and sensitivity assumptions in their forecasts, they need more warming in the past. One way to do this is to argue that the world would have cooled without man-made CO2, so that man-made CO2 contributed 0.8C of warming in addition to whatever the cooling would have been. It allows attribution of more than 100% of past warming to man.
There are various ways this is attempted, but the most popular centers around man-made sulfate aerosols. These aerosols are byproducts of burning sulfur-heavy fossil fuels, particularly coal, and they tend to have a cooling effect on the atmosphere (this is one reason why, in the 1970's, the consensus climate prediction was that man was causing the world to cool, not warm). Some scientists argue that these aerosols have tended to cool the Earth over the past decades, but as we clean up our fuels their effect will go away and we will get catch-up warming.
There are a couple of problems with this line of thought. The first is that we understand even less about the magnitude of aerosol cooling than we do of CO2 warming. Any value we choose is almost a blind guess (though as we shall see in a moment, this can be a boon to modelers on a mission). The second issue is that these aerosols tend to be very short-lived and local. They don't remain in the atmosphere long enough to thoroughly mix and have a global effect. Given their localization and observed concentrations, it is almost impossible to imagine them having more than a tenth or two effect on world temperatures. And I will add that if we need to take into account cooling from sulfate aerosols, we also need to take into account the warming and ice melting effect of black carbon soot from dirty Asian coal combustion. But we will return to that later in our section on Arctic ice.
My second, related note is that scientists will frequently claim that their computer models models do claim correctly match historic temperatures when run backwards. As a long-time modeler of complex systems, my advice is this: don't believe it until you have inspected the model in detail. At least 9 times out of 10, one will find that this sort of tight fit with history is the result of manual tweaking, usually from the affect of a few "plug" variables.
Here is one example -- there was a study a while back that tried to understand how a number of different climate models could all arrive at very different temperature sensitivities to CO2, but all still claim to model the same history accurately. What was found was that there was a second variable -- past cooling from man-made aerosols, discussed above -- that also varied greatly between models. And it turned out that the value chosen in the models for this second variable was exactly the value necessary to make that model's output match history -- that is why I said that our very lack of knowledge of the actual cooling from such aerosols could be a boon to modelers on a mission. In essence, there is a strong suspicion that this variable's value was not based on any observational evidence, but was simply chosen as a plug figure to make the model match history.
Having gone about as far as we can with the forecasts without diving into a whole new order of detail, let's move on to the final alarmist contention, that man-made CO2 is already changing the climate for the worse. We will discuss this in Chapter 7.