The academic blogger who goes by the slightly pretentious name of … andthentheresPhysics is a dogged character who never concedes any point. When you think you have him pinned down, he wriggles out and insists on having the last word. I can’t understand how he has the time, both on his blog and on twitter simultaneously! It is a mystery. Anyway here is an example on his recent blog post on ‘Lukewarism’, which starts as follows.
I’ve noticed a slight resurgence in those claiming to be lukewarmers and trying to argue that this is some kind of reasonable middle ground. …. they take something that is possible (low climate sensitivity) and argue that it is almost certain
Clive,
It is a mistake to set goals based on temperature rise rather than on CO2 levels. Instead of the Paris agreement limiting temperature rise to 2C, it should have limited CO2 levels to say 500ppm.
I don’t think there is really much of a difference. A temperature target and a CO2 level target will both be associated with carbon budgets (i.e., how much more we can emit).
@ATTP There is a difference because 2C may be reached at 500ppm or only at 1000ppm. We just don’t know. All we can say is that it is most likely to be reached 100 years after we stabilise CO2 levels at ~500ppm
Clive,
CO2 levels were probably higher because the earth was warmer and not the other way round.
Huh? Without CO2 in the atmosphere, the Earth would probably be a snowball. It’s probably correct that the drawdown of CO2 has lead to a long-term cooling, but that does not mean that you can’t use past CO2 levels and global temperatures to say something about climate sensitivity today. Also, the Pliocene was only a few million years ago, so I don’t think the arrangement of the continents was all that different to today.
@ATTP
Huh? Without CO2 in the atmosphere, the Earth would probably be a snowball. It’s probably correct that the drawdown of CO2 has lead to a long-term cooling, but that does not mean that you can’t use past CO2 levels and global temperatures to say something about climate sensitivity today. Also, the Pliocene was only a few million years ago, so I don’t think the arrangement of the continents was all that different to today.
What came first the egg or the hen?
Yes CO2 is a greenhouse gas but we know that during Ice Ages levels falls below 200ppm. CO2 does not cause ice ages, which are forced by Orbital changes. However, CO2 may accentuate them. Likewise CO2 did not cause the cooling of the Earth over the last 5 million years, but its decrease may have accentuated it.
The Geological weathering thermostat seems to set long term CO2 levels for any epoch. What we don’t understand is exactly what sets the thermostat temperature. The position of the continents and ocean circulation must be a strong effect. The closing of the Panama Isthmus cut off the Atlantic from the Pacific about 3 million years ago and this is a possible cause.
What is occurring today is completely different. Humans are artificially increasing CO2. This has only happened once before at PETM 56 Million years ago when the world was much warmer and natural CO2 levels were near 1000ppm. The pulse of CO2 then was more than if we were to burn all known fossil fuel reserves. DT was ~5C. This is the only example where we could try to measure ‘anthropogenic’ climate sensitivity.
Clive,
Yes CO2 is a greenhouse gas but we know that during Ice Ages levels falls below 200ppm. CO2 does not cause ice ages, which are forced by Orbital changes. However, CO2 may accentuate them. Likewise CO2 did not cause the cooling of the Earth over the last 5 million years, but its decrease may have accentuated it.
Again, huh? If we know there was a period in the past when the continents were in a similar configuration to today and the atmospheric CO2 was ~ 400 ppm, then we can use information from that period to infer what might happen now if atmospheric CO2 concentrations remain at ~ 400ppm. It doesn’t really matter what caused atmospheric CO2 to be ~ 400ppm.
What is occurring today is completely different.
The cause of the change in atmospheric CO2 might be different, but the effect is not likely to be very different.
No @ATTP you are making a false conclusion which goes to the heart of the problem.
You are assuming that there is a direct causal one way relationship between CO2 and temperature. Something like if CO2 = 400ppm then T = 290C
You reject that 3 million years ago the natural value of T was instead 290C so then CO2=400ppm. However this must have been the case since unless you can find natural source for increasing CO2, the geological thermostat would have pumped out any excess CO2 within a thousand years.
It is obvious that the GAIA (geology + life) set temperature of earth varies with time.
Life itself starts to falter if CO2 levels fall significantly below 200ppm at which point the deep ocean sink starts to fail, restoring temperatures conducive to life.
Clive,
Can I maybe ask that you refrain from telling me what I assume and reject? If I want people to know what I assume and reject, then I’ll tell them.
All I’m suggesting is that all else being equal, a 400ppm world in the past should have a similar equilibrium climate state to a 400ppm world today. I don’t think this is all that controverisial a point. Of course, if the solar forcing is different, or the arrangement of the continents is different, then the states may differ. If, however, the solar forcing is similar, and the distribution of the continents is similar, then we can use a 400ppm world in the past, to infer what such a world would be like today.
@ATTP @BBD @Guthrie
CO2 is not the only show in town.
The Northern Hemisphere is more problematic. From sediment cores and other data, we know that until about 5 million years ago, North and South America were not connected. A huge gap—the Central American Seaway—allowed tropical water to flow between the Atlantic and Pacific Oceans.
A growing body of evidence suggests that the formation of the Isthmus of Panama partitioned the Atlantic and Pacific Oceans and fundamentally changed global ocean circulation. The closing of the Central American Seaway initially may have warmed Earth’s climate, but then set the stage for glaciation in the Northern Hemisphere at 2.7 million years ago.
Read more here:
http://www.whoi.edu/oceanus/feature/how-the-isthmus-of-panama-put-ice-in-the-arctic
Clive,
CO2 is not the only show in town.
I didn’t say it was the only show in town. Strawman much?
I have a funny feeling that Clive is arguing that geological processes can act to draw down (or release) CO2 so as to cool, or warm, the climate. Well, yes, but this tends to take a long time (thousands of years) and doesn’t change that, all else being equal, a 400ppm world in the past can be used to inform what we might expect for a 400ppm world today.
@David Bensen
Thanks for the explanation. If I understand you correctly you are implying that CO2 is the only determinate of global temperatures. Past warmer climates were due to more vulcanism increasing the natural CO2 levels. So the climate has entered a critical stage in the Pliocene whereby Milankovitch cycles alone were able to trigger glaciations, whereas before then higher CO2 levels avoided this.
Thanks also for identifying a true climate alarmist – Mark Lynas :
A three-degree rise would spell the collapse of the Amazon rainforest, disappearance of Greenland’s ice sheet, and the creation of deserts across the Midwestern United States and southern Africa. A six-degree increase would eliminate most life on Earth, including much of humanity.
Don’t forget that most of North America and Northern Europe were under 3km of Ice just 20,000 years ago. Somehow humans and other life managed to adapt and survive. Quite possibly we have inadvertently delayed the next glacial cycle because obliquity is reducing following the Holocene optimum.
Clive,
Don’t forget that most of North America and Northern Europe were under 3km of Ice just 20,000 years ago. Somehow humans and other life managed to adapt and survive.
So, 20000 years ago, the ~7 billion people on the planet, a significant fraction of whom had moved into cities and urban regions, and who relied on a complex infrastructure that provided food, shelter, entertainment, etc, managed to adapt easily to the arrival of 3km ice sheets across most of North America and Northern Europe……oh, hold on??????
So, 20000 years ago, the ~7 billion people on the planet, a significant fraction of whom had moved into cities and urban regions, and who relied on a complex infrastructure that provided food, shelter, entertainment, etc, managed to adapt easily to the arrival of 3km ice sheets across most of North America and Northern Europe……oh, hold on??????
No but that is exactly what 9 billion people will have to do when another ice age begins.
Clive,
No but that is exactly what 9 billion people will have to do when another ice age begins.
According to this paper, we’ve already delayed the next ice age by about 50000 years. That would appear to be a good deal longer than your “Don’t worry. Even at 3mm/year it will take 8000 years to reach 25m ! So that would be in 300 generations time.”
In 5000 years time people may even be grateful we kept CO2 levels at around 400ppm !
Well, an issue is that if we continue as we are, it’s not going to be at around ~400ppm, it will be considerably higher. Also, we might benefit from not using up all the available fossil fuels now, just in case there is some need to maintain CO2 levels at 350 – 400ppm.
@ATTP,
I more or less agree this time.
However surely it is a good thing if we can delay the next ice age altogether?
Of course I meant Pleistocene not Pliocene !
?
Clive,
However surely it is a good thing if we can delay the next ice age altogether?
I doubt this is actually possible; in the sense that there will almost certainly be another ice age at some point in the future. I find it slightly odd that you seem comfortable being rather alarmed about an ice age that we’ve almost certainly delayed by thousands of years, while completely blase about possibly changing the climate by a magnitude that is higher, and a rate that is faster, than at any time in human history.
at which point I more or less gave up as it is like playing ping pong.
