Arguments
Does high levels of CO2 in the past contradict the warming effect of CO2?
The skeptic argument...
CO2 was higher in the past
"The killer proof that CO2 does not drive climate is to be found during the Ordovician- Silurian and the Jurassic-Cretaceous periods when CO2 levels were greater than 4000 ppmv (parts per million by volume) and about 2000 ppmv respectively. If the IPCC theory is correct there should have been runaway greenhouse induced global warming during these periods but instead there was glaciation."
(The Lavoisier Group)
What the science says...
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| When CO2 levels were higher in the past, solar levels were also lower. The combined effect of sun and CO2 matches well with climate. | |||||
Over the Earth's history, there are times where atmospheric CO2 is higher than current levels. Intriguingly, the planet experienced widespread regions of glaciation during some of those periods. Does this contradict the warming effect of CO2? No, for one simple reason. CO2 is not the only driver of climate. To understand past climate, we need to include other forcings that drive climate. To do this, one study pieced together 490 proxy records to reconstruct CO2 levels over the last 540 million years (Royer 2006). This period is known as the Phanerozoic eon.
Figure 1: Atmospheric CO2 through the Phanerozoic. Dashed line shows predictions of the GEOCARB carbon cycle model with grey shading representing uncertainty range. Solid line shows smoothed representation of the proxy record (Royer 2006).
Atmospheric CO2 levels have reached spectacular values in the deep past, possibly topping over 5000 ppm in the late Ordovician around 440 million years ago. However, solar activity also falls as you go further back. In the early Phanerozoic, solar output was about 4% less than current levels. The combined net effect from CO2 and solar variations are shown in Figure 2. Periods of geographically widespread ice are indicated by shaded areas.
Figure 2: Combined radiative forcing from CO2 and sun through the Phanerozoic. Values are expressed relative to pre-industrial conditions (CO2 = 280 ppm; solar luminosity = 342 W/m2). The dark shaded bands correspond to periods with strong evidence for geographically widespread ice.
Periods of low CO2 coincide with periods of geographically widespread ice (with one notable exception, discussed below). This leads to the concept of the CO2-ice threshold - the CO2 level required to initiate a glaciation. When the sun is less active, the CO2-ice threshold is much higher. For example, while the CO2-ice threshold for present-day Earth is estimated to be 500 ppm, the equivalent threshold during the Late Ordovician (450 million years ago) is 3000 ppm.
However, until recently, CO2 levels during the late Ordovician were thought to be much greater than 3000 ppm which was problematic as the Earth experienced glacial conditions at this time. The CO2 data covering the late Ordovician is sparse with one data point in the CO2 proxy record close to this period - it has a value of 5600 ppm. Given that solar output was around 4% lower than current levels, CO2 would need to fall to 3000 ppm to permit glacial conditions. Could CO2 levels have fallen this far? Given the low temporal resolution of the CO2 record, the data was not conclusive.
Research examining strontium isotopes in the sediment record shed more light on this question (Young 2009). Rock weathering removes CO2 from the atmosphere. The process also produces a particular isotope of strontium, washed down to the oceans via rivers. The ratio of strontium isotopes in sediment layers can be used to construct a proxy record of continental weathering activity. The strontium record shows that around the middle Ordovician, weatherability increased leading to an increased consumption of CO2. However, this was balanced by increased volcanic outgassing adding CO2 to the atmosphere. Around 446 million years ago, volcanic activity dropped while rock weathering remained high. This caused CO2 levels to fall below 3000 ppm, initiating cooling. It turns out falling CO2 levels was the cause of late Ordovician glaciation.
So we see that comparisons of present day climate to periods 500 million years ago need to take into account that the sun was less active than now. What about times closer to home? The last time CO2 was similar to current levels was around 3 million years ago, during the Pliocene. Back then, CO2 levels remained at around 365 to 410 ppm for thousands of years. Arctic temperatures were 11 to 16°C warmer (Csank 2011). Global temperatures over this period is estimated to be 3 to 4°C warmer than pre-industrial temperatures. Sea levels were around 25 metres higher than current sea level (Dwyer 2008).
If climate scientists were claiming CO2 was the only driver of climate, then high CO2 during glacial periods would be problematic. But any climate scientist will tell you CO2 is not the only driver of climate. Climatologist Dana Royer says it best: "the geologic record contains a treasure trove of 'alternative Earths' that allow scientists to study how the various components of the Earth system respond to a range of climatic forcings." Past periods of higher CO2 do not contradict the notion that CO2 warms global temperatures. On the contrary, they confirm the close coupling between CO2 and climate.
Last updated on 9 July 2010 by John Cook.
I'm wondering a little about why the "skeptic argument" above claims that there was glaciation in the "Jurassic-Cretaceous period". It would indeed be bad for the connection to CO2 if there was widespread glaciation during this period, but I can't find anything about that.
In contrast, we're doubling CO2 on a timescale of a century or so. We're also pumping out CH4, N2O, halocarbons, and other greenhouse gases. Thus, if you look at the actual magnitude of the radiative forcings, over the course of the 21st century the increase in greenhouse gases has a much larger forcing than any changes in TSI, Milankovich, etc.
The general idea is that astronomers think that they have good models for the evolution of stars like the sun, so in particular, they can compute solar output from the age of the sun.
There are a couple of very straight-forward holes in these denialist arguments.
1. Ordovician CO2 over 4000 ppm and glaciation proves CO2 doesn't matter! Nope: Look at the distribution of continental landmasses of the Ordovician (~450 MY). Those "glaciers" were the south-polar ice cap. There wasn't much in the way of land in the northern hemisphere.
2. Warming and cooling is purely cyclical! CO2 variation is natural! Sure, there are natural cycles. But something very important and very obvious changed over the geologic time scales involved that makes such simple comparison irrelevant: Plants. Lots of plants. Gymnosperms (conifers etc) originated in the late Devonian-early Carboniferous (380-300 Mya) and angiosperms (flowering plants) in the Cretaceous (100 Mya). All that carbon in the Carboniferous coalbeds? Dead plants that took CO2 out of the atmosphere. The downward trend apparent in the graph above from the Cretaceous forward? More plants. And now we've turned the downward CO2 trend around despite a world rich in plants... maybe we can hope that a whole new class of plant life comes to our rescue... but that would require evolution and the science is still uncertain on that too.
As a rough calculation, an increase in solar irradiance by 4% over the past 400 million years would yield something like +9 w/m2 forcing. Compare that to the anthropogenic CO2 forcing of something like +1.5 w/m2 ...
"if CO2 were to go over 2000 ppm today most of that ice would (eventually) be gone."
Agreed. And I certainly am not questioning the role of solar irradiance.
But the geological proof that ice once existed at our South Pole -- striated bedrock among other unmistakable features -- would still be there. So any future scientific inquiry -- if there is such an enlightened future -- would say "see, they had 'glaciers' in a time of high CO2!" and conclude that CO2 is unimportant.
"increase in solar irradiance by 4% over the past 400 million years" ... "Compare that to the anthropogenic CO2 forcing" 400MY is time enough for evolutionary changes on the grand scale. Isn't anthropogenic forcing is on a time scale of 100s of years? Not enough time for many organisms to get ready for a warmer environment.
Or, another way of putting it is that a much smaller increase in CO2 today will produce a climate that would have required much higher CO2 to achieve in the Paleozoic.
"a much smaller increase in CO2 today will produce a climate that would have required much higher CO2 to achieve in the Paleozoic."
That's an excellent way of putting it.
The Ordovician's big dropoff in CO2 is usually explained by the massive, continent-wide carbonate banks (Trenton, Knox, Arbuckle, Delaware Basin, etc in the US) deposited in warm, restricted shallow seas.
"These carbonate rocks constitute part of the “Great American Bank” (Ginsburg, 1982) that extended more than 3,000 km (1,864 mi) along nearly the entire length of what was the southern seaboard of the Laurentian continental mass" -- Pennsyvania Geological Survey
The deposition of carbonates (Ca0+CO2->CaCO3, calcite) is linked to climatic change in this paper:
"The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO2. This would have caused an important lowering of the average temperature". We don't see such massive carbonates deposited today.
muoncounter:
Thanks for the link to that Villas et al. 2002 paper. That's really neat. They claim that marine carbonate deposition sequestered a mass of carbon equivalent to 350 times the current quantity of atmospheric CO2! I like their explanation of the mechanisms for both the onset and termination of glaciation.
Those mechanisms are critical to the argument over "high CO2 and glaciation=No". It is certainly clear that widespread carbonate deposition takes up lots of atmospheric CO2, but whether that alone causes an ice age isn't clearly established. It is also clear that the graph of CO2 levels taken from a denialist website, posted above (#6), doesn't take a short-term drop in CO2 due to perfectly valid geological mechanism into account.
I have some difficulty with the mechanisms in the "Mountains that froze the world" article John references at the top of this thread. For one thing, the Appalachians weren't all done in the late Ordovician -- it took another 100 MY or so until the Alleghenian Orogeny was complete. The image below is the mid-Ordovician southern ocean:
-- source
All that light blue is shallow sea -- mostly between 10N and 30S latitude -- perfect environment for carbonate deposition from marine organisms.
For another, the idea that Sr86 in Nevada is runoff from the proto-Appalachians just doesn't seem right -- on the map above, Nevada is on the 'north coast' of Laurentia, while the emerging Appalachians are on the 'south coast'.
Other mechanisms abound in the literature, from a mega-volcano to a gamma-ray burst.
From another key paper on this subject, "the waxing and waning of ice sheets during the Late Ordovician were very sensitive to changes in atmospheric pCO2 and orbital forcing at the obliquity time scale (30–40 k.y.)" I've even seen one author who suggests that the concentration of continental land masses at the south pole would perturb the earth's orbit -- but that's a much longer-time scale event.
Please note that I accidentally italicized the last sentence ("We don't see...") in #10. That was my statement and not part of the referenced article.
Also, re: Please note that I accidentally italicized the last sentence ("We don't see...") in #10. That was my statement and not part of the referenced article.
Yes ... and I solved that by inserting a "/i" tag (in brackets) at the beginning of my comment.
:-)
http://wattsupwiththat.com/2010/08/10/study-climate-460-mya-was-like-today-but-thought-to-have-co2-levels-20-times-as-high/
It refers to a new study in PNAS
http://www.pnas.org/content/early/2010/08/02/1003220107.abstract?sid=08063fb7-c9e9-48d7-a515-b3db8907505c
Hope you can comment on this soon.
Doesn't that make it a little pointless what the CO2 levels were 500 mya?
"When CO2 levels were higher in the past, solar levels were also lower"
Can anyone point to a source for the this? And a nice graph showing solar levels in the past?
faint young sun paradox
Solar models are not complete, our understandings of the inner workings of stars is far from ideal and certainly not complete. There are problems with the SSM (Standard Solar Model) and this may or may not impact our model of the evolution of Stars in general, but especially those with similar properties to our sun. Many papers have been written on this subject in recent years. I would direct anyone interested to this article, Problems for the standard solar model arising from the new solar mixture. by J.A.Guzik 2008
http://articles.adsabs.harvard.edu/full/2008MmSAI..79..481G
Whilst I think it is important and helpful to look at climate data in the past, 400My is taking it to extremes as anything we say about that time is largely guesswork based on assumptions and statistical modelling. Anything more than about 5 million years old, in which we have lots of inter-related indicators of climate in the real world is largely pointless, and I would aim that at both sides of this debate. Wasting time on what may or may not have happened 400My ago is not helpful to anyone IMHO.
I've seen this graphic come up a few times to refute this argument and similar ones.
Here's the original source: http://www.geocraft.com/WVFossils/Carboniferous_climate.html
The page's author, Monte Hieb, is listed at the bottom. Poking around a little more on Google will give you a sense of his paleoclimate qualifications.
Please take some time to get acquainted with SkS. See the newcomers guide, browse the 'Skeptic arguments.' There's a lot to learn; it will take some reading, but if you want to understand what's happening, it's well worthwhile.
As far as the geocraft graph, see prior discussion starting with comment #6 on this thread, in which this graph gets debunked.
Easterbrook GISP2
"If CO2 is indeed the cause of global warming, then global temperatures should mirror the rise in CO2"
No, that would only be true if CO2 were the only thing that affects global temperatures. Nobody would claim that is the case.
"In 1945, CO2 emission began to rise sharply and by 1980 atmospheric CO2. had risen to just under 340 ppm. During this time, however, global temperatures fell about 0.9°F (0.5° C) in the Northern Hemisphere and about 0.4°F (0.2° C) globally."
Sulphate aerosols (which have a cooling effect) also rose in the 1940s, but began to be phased out from the early 70s. Dr Easterbrook is just demonstrating his ignorance of the work that has been done on attribution of climate change in the 20th century. It isn't hard to find, there is a whole chapter on it in the IPCC WG1 scientific basis report. Being skeptical is fine, but you do need to know what it is you are skeptical about.
These two errors ought to be enough to make anyone skeptical of Dr Easterbrooks article, I suspect there are others.
Easterbrook treats a local temperature record as if it were a global temperature record, which is obviously a fallacious method. What is more, he treats the last data point in the ice core record as though it were very recent, whereas it is in fact 1855. Comparison with modern Greenland temperatures show that for most of the ice core record, temperatures have been below modern temperatures (and may have been below for all of it). Further discussion on this point should be taken here where they are already discussed in detail.
here and particularly here.
Of course, dont take a warmist blog word for it. Pull the data, check the references (especially the metadata) and see for yourself.
The drop in TSI of 4% is about 54 W/m^2, three doublings of CO2 would be about 12 W/m^2, so that presumably means that for a global glaciation to happen now, the sun would have to dim by about 42W/m^2 or about 3%. To put that into context, the variation of the 11 year solar cycle is about 2W/m^2 and the difference between glacial and interglacial conditions is apparently about 7W/m^2.
Eccentricity is good for a couple watts as well.
DeltaF = 5.35*ln(C/C0)
which implies that
C = exp(DeltaF/5.35 + log(C0))
so substituting the figures, we get
C = exp(7/5.35 + 5.6384) = 1000ppmv (ish)
That calculation ignores any feedback etc, so if it was within a factor of two of the real answer from a climatologist (who unlike me knows what they are talking about ;o), I would be pleasantly surprised. A value of 500ppmv sounds plausible to me.
The last glacial period of any consequence was the so called KT about 230 mya at the Permian-Triassic boundary. This one coincided with the greatest extinctions in the history of life.
The notable one before that was at the end of the Ordovician about 450 mya. There were extinctions but only a few liverworts and mosses and possibly insects had made it on to land. The really wierd thing is that there are glacial tillites with a carbonate cap in Australia, and if we can believe the apparent polar wander paths Australia was pretty close to the equator then.
There was another glacial period about 650 million years ago in the Proterozoic. Everynoe starts getting really grumpy and calling each other names and the apparent polar wander paths diverge before this.
You can see that glacial periods are rare in earth history, ocurring roughly every 200 my. Between hese periods Gloval Average Temperature and CO2 are thought to be higher than now.
Where dies the 5.35 come from in DeltaF = 5.35*ln(C/C0)?
As for terminology... we are currently in an interglacial (i.e. relatively warm) period of the ongoing ice age (i.e. geological period where large ice caps are present).
What you have probably heard people saying is that raising CO2 to ~500 ppm might prevent the next glacial period entirely.
That is, normally we would expect the current interglacial period to end some time in the next 15,000 years or so and then be followed by a long period of increasing cold which would cause glaciers to spread out from the poles for ~90,000 years and then retreat as the next warming cycle comes around. However, if CO2 were raised to 500 ppm then it would likely take more than 100,000 years to return to pre-industrial levels (barring some new technology to sequester it faster than would happen naturally) and could thus keep the planet warm enough that we skip the next glacial cycle entirely.
That'd actually be a good thing... but given that it is thousands of years away not quite as pressing as dealing with the warming we will see over the next two centuries.
This diagram could not prove anything.
1. It is a moving average - and of how many values, nobody knows.
2. The values themselves used in the Moving Average are also averaged values.
3. It is not even accurately calculated. Does anyone have any vague idea why the temperature saturation in the Jurassic and Cretaceous in the upper version of the Diagram is 22°C and in the lower version is 23° C, and how does this average temperature look like as distinct values.
3. This trend in the end of the diagram (in the last 10 million years, for example) is a masterpiece of misrepresentation:
- What part of this period is with Homo sapiens and what without it (how much is 200 000 of 10 mln)?
- What part of this period is with use of fossil fuels and what without?
This 'prediction' is for another system and for another world (without humans and vehicles, and their fresh ideas of how to control the world).
The guys that put back into the air the carbon (in the form of carbon dioxide) should have any idea of what they are doing and how they will clean up the air and the ocean back in case of 'emergency'.
In the past Nature 'regulated' the concentration of CO2 by extinction of species. Who, how, and when will regulate the CO2 produced by the vehicles, for example and which species will extinct first - humans or their cars.
The dinosaurs 'ruled over' the Earth for 160 mln years by virtually doing nothing 'as regulation'.
We, with our fresh ideas of wasting natural resources, mania to control everything, and dealing with things that we don't fully understand will hardly make a million - seriously.
Care to give a reference to back up that assertion? On a timescale of thousands of years CO2 levels are regulated by ocean-atmosphere transfers, over timescales of tens of thousands of years plus by the chemical weathering thermostat. See e.g. David Archers global carbon cycle primer published by Princeton University Press.
Presumably you are referring to the graph posted in comment #6 and at larger scale in #27?
This graph is a cartoon; it is not from an authoritative source and is not taken very seriously.
I am not specialist in the field, and yet according to Craeme Lloyd, Natural History Museum, London, UK more than 99% of all species ever lived on the Earth are extinct at present ... by one reason or another.
RE: The two versions of the Graph
I cannot dispute that both of the versions are absolute cartoons, but they are presented all over the Internet as 'Evidence No.1' that the CO2 and the global temperature 'are falling'.
That should tell you a lot about the quality of those arguments -- and the folks that present them. I'd say the science of using cartoon graphs in place of real data and observation is the real 'climastrology.'
And that is true, except it says nothing at all about CO2 levels.
This volcanic activity, of course, is what likely led to the high atmospheric CO2 levels in the past but my question is this - volcanoes spew alot more then just greenhouse gases. They will also spew dust and other such particles that would have a cooling effect on the earth.
As such, could that also explain the reason for high CO2 levels during a period of glaciation?
I'm not clear on how much volcanic activity has changed over the past 500 million years but what's really fascinating is you can see in the geologic record almost exactly where the Indian continent started bumping up against the Asian continent to start forming the Himalayas and started a long process where CO2 was pulled out of the atmosphere through rock weathering. And along with that you see the global temperature start a long slow decline from the days where you had crocodiles in the Arctic to modern glacial cycles in the Arctic. All of it a function of the amount of CO2 in the atmosphere.
This all fits well with deep glaciation events (Snowball Earth) where the almost complete ice cover of the planet would prevent any rock weathering and thus cause CO2 to build up to very high levels before raising the temperature enough to melt the ice.
Rob Honeycutt - thanks for the article link!