So I need to get this off my chest. As I explained previously, there’s a disagreement among climatologists, who are roughly divided into two camps. The “climate moderates”, who think climate sensitivity is around 3 degree and the “climate radicals”, who think it is higher, with Hansen arriving at 4.5 or 4.8, depending on the method.
The question that’s on my mind is which of these camps is closer to the truth. When humans double CO2 concentrations in the atmosphere, do we face ~3 degree of global warming, or ~4.8 degree? And why can’t the scientists arrive at agreement on which of these numbers is correct?
Well, to start, let’s just ask ourselves what criticism the “climate moderates” have on the models that suggest a high number. Here is Zeke Hausfather:
In her video, Dr. Hossenfelder suggests that the “hot models” include better physics of cloud behavior that can in turn result in more accurate short-term weather forecasts, citing a paper by Williams et al 2020 assessing HadGEM3 model (ECS 5.5C). However, other models that include updated cloud physics have lower climate sensitivity, and this one test does not obviate the other challenges facing high sensitivity models – their poor agreement with historical temperatures, their inability to reproduce the last glacial maximum, and their inconsistency with other lines of evidence constraining climate sensitivity.
Here you have a graph of the estimates from the different models, from the most recent generation:
We found that effective climate sensitivity (#ECS), when averaged across all #CMIP6 models* (orange), was larger than in #CMIP5 (blue) by about 0.6 deg C (about 1 deg F).
*that were available by the end of 2019 pic.twitter.com/Eva8rT5jq1
— Mark Zelinka (@mzelinka) January 7, 2020
Left you have CMIP5, right you have CMIP6, the newest generation of climate models. You see that the estimate of warming moves from 3 to 4 degrees. So, how did the IPCC authors respond to this? Well, they have decided that the high estimates from these models are just unlikely and so when calculating what the actual climate sensitivity is, they are just being weighed less.
That way, you can keep the comfortable number of 3, rather than ending up having to say that it’s higher than you thought. Incidentally, this also means you can keep telling the policymakers that “there’s still time to keep warming below 1.5 degree, but you have to hurry”, rather than having to tell them: “Hey it’s too late by now.”
It’s kind of like you’re taking 20% of the population and making their vote count for only half in an election. This is why you’re getting people like Sabine Hossenfelder, scientists with experience in different scientific fields, who are looking at this stuff and basically saying: “Hey um, you seem to be engaging in funny business here.”
In simple English, the mainstream climatologists who dominate the IPCC process don’t really want to seriously consider the possibility that climate change is actually going to be much more severe than they told the policymakers. Whenever they see numbers that are far higher than they’re used to, they say “well that can’t be right because X Y and Z, so let’s not take those numbers too seriously”.
And I know what you’re thinking now: “Well, why aren’t those arguments against the high sensitivity estimates valid?” Well, that’s what I have been wondering. Why can’t the two camps agree? Why don’t the arguments of either side convince the other side?
And I think I found a part of the puzzle, that helps me understand why they arrive at different numbers.
Remember, climate sensitivity is how much the Earth responds in temperature, to a doubling of CO2. We have different ways to arrive at that number, one important way is by looking at how the Earth responded in the past to a doubling.
But there’s the thing: Depending on the period you look at, a doubling of CO2 will have been from a different baseline. There’s no inherent reason why going from 180 parts per million to 360 needs to result in the same increase in temperature as going from 400 to 800 parts per million.
If you assume that during the glacial stages, climate sensitivity is lower than during interglacial stages, you end up addressing the argument of climate moderates like Zeke Hausfather.
I did not invent this idea myself. No, instead I found a study that suggests this is the missing answer to the disagreement. I quote:
In research published in 2016, Friedrich et al. show that climate models may be underestimating
climate sensitivity because it is not uniform across different circumstances, but in fact higher in
warmer, interglacial periods (such as the present) and lower in colder, glacial periods.69 Based on a
study of glacial cycles and temperatures over the last 800,000 years, the authors conclude that in
warmer periods climate sensitivity averages around 4.88°C. The higher figure would mean warming
for 450 parts per million (ppm) of atmospheric CO2 (a figure on current trends we will reach
within 25 years) would be around 3°C, rather than the 2°C bandied around in policy making circles.
Professor Michael Mann, of Penn State University, says the paper appears “sound and the conclusions
quite defensible”.70
You can find the paper here. They write:
For warm climates, the value (Swarm) is more than two times larger, attaining 1.32 K W−1 m2 or 4.88 K per CO2 doubling. The average of S over the entire 784-ka range can be calculated from a linear regression of the SAT/radiative forcing data set. It amounts to 3.22 K per CO2 doubling. Comparing the mean of S to Swarm, it becomes apparent that this long-term mean value substantially underestimates Swarm and thus should not be used to assess future anthropogenic warming.
If you accept what it says, then you would have to conclude that the difference in climate sensitivity argued by different camps of climatologists are largely just due to the fact that they’re emphasizing different periods in geological history. But for our current situation, the correct number would be the higher range, around the 4.8 degree Celsius per doubling that Hansen estimates.
But if you assume this estimate to be correct that we will be hitting 3 degrees of warming in response to 450 parts per million of CO2, that’s a big problem, as we could hit 450 ppm as early as 2035. The temperature would not rise to 3 degree overnight though probably, that would probably take a few more years. 3 degree by 2050 unfortunately looks very plausible.
By the way, the important thing to remember about climate sensitivity is that it holds CO2 stable. In other words, it excludes carbon cycle feedbacks. In other words, if we hit 450 parts per million by 2035 and have zero human emissions afterwards, you still need to wait and see how much warming happens in response to further carbon emissions from the Amazon, the permafrost, the boreal forests etc.
But anyway, to me the mystery is solved now. I expect global warming will continue to unfold much faster than the “climate moderates” anticipate. Compare for example, the year 2023 to what the moderates projected:
The year 2023 shattered consensus estimates, but for the “radicals”, the temperatures we saw make perfect sense.
So based on what I’ve read, I get the impression the “radicals” with their high estimate of ~4.8 per doubling of CO2 are the ones who get it right for our current climatic conditions. When you take their argument, that climate sensitivity was simply lower during cold periods than during the current warm period, you address the argument of moderates like Zeke Hausfather.
That’s not great news of course, it means we don’t have a future. But for me, I finally understand why the two camps disagree with each other: They’re trying to figure out the same number, while looking at different periods.