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500 million years debate

Page history last edited by PBworks 15 years, 6 months ago

9 Nov 2006

Broadly Misleading

Filed under: Climate Science Paleoclimate Climate modelling Reporting on climate— raypierre @ 12:57 pm

Just when we were beginning to think the media had finally learned to tell a hawk from a handsaw when covering global warming (at least when the wind blows southerly), along comes this article 'In Ancient Fossils, Seeds of a New Debate on Warming' by the New York Times' William Broad. This article is far from the standard of excellence in reporting we have come to expect from the Times. We sincerely hope it's an aberration, and not indicative of the best Mr. Broad has to offer.

 

Broad's article deals with the implications of research on climate change over the broad sweep of the Phanerozoic -- the past half billion years of Earth history during which fossil animals and plants are found. The past two million years (the Pleistocene and Holocene) are a subdivision of the Phanerozoic, but the focus of the article is on the earlier part of the era. Evidently, what prompts this article is the amount of attention being given to paleoclimate data in the forthcoming AR4 report of the IPCC. The article manages to give the impression that the implications of deep-time paleoclimate haven't previously been taken into account in thinking about the mechanisms of climate change, whereas in fact this has been a central preoccupation of the field for decades. It's not even true that this is the first time the IPCC report has made use of paleoclimate data; references to past climates can be found many places in the Third Assessment Report. What is new is that paleoclimate finally gets a chapter of its own (but one that, understandably, concentrates more on the well-documented Pleistocene than on deep time). The worst fault of the article, though, is that it leaves the reader with the impression that there is something in the deep time Phanerozoic climate record that fundamentally challenges the physics linking planetary temperature to CO2. This is utterly false, and deeply misleading. The Phanerozoic does pose puzzles, and there's something going on there we plainly don't understand. However, the shortcomings of understanding are not of a nature as to seriously challenge the CO2.-climate connection as it plays out at present and in the next few centuries.

 

Let's take a closer look at the question of CO2 variations over deep time. In contrast to the situation for the late Pleistocene, there is no one method for reconstructing CO2 at earlier times which is fully satisfactory. Methods range from looking at carbon isotopes in microfossils to looking at the density of pores on fossil leaves, with many other exotic geochemical tracers (e.g Boron) coming in in recent times. There is also some data for the very early Earth associated with the CO2 conditions under which certain exotic minerals (uraninites and siderites) form. None of the methods is unambiguous, and none provide information about other greenhouse gases that might be playing a role (though there may be some hope to do something abou methane). As an example of the difficulty faced by the field, take a look at the compilation of various estimates of CO2 since the Permian presented in the following figure (From Donnadieu et al, G3, in press; the red squares come from an attempted geochemical model fit to the data).

 

 

By the time one gets back to the Permian, the error bars are huge. At earlier times, the estimates are even more problematic. Broad's article does make reference to a very interesting paper by MIT's Dan Rothman, writing in PNAS. This paper attempts a peek at the CO2 over the past 500 million years, using a clever and novel reconstruction technique. It is innovative, but far from the last word on the subject. Broad inappropriately cherry-picks Rothman's statement that there appears to be no clear connection between warm climates and CO2 (except in "recent" times, about which more anon). However, Broad's article neglects all the caveats in the paper, which clearly point to the real problem being that the reconstructions of CO2 and climate over such time scales are so uncertain that it's not clear that the data is up to the task of teasing out ssuch a connection.

 

Even in Rothman's reconstruction, during the past 50 milllion years -- when the data is best and continents are most like the present -- the long term cooling trend leading into the Pleistocene is clearly associated with a long term CO2 decline. This is not our main reason to infer that increasing CO2 will warm the climate in the future, but insofar as the data supports CO2 decline as a main culprit in the long slide from the Cretaceous hothouse climates of 60 million years ago to the cold Pleistocene climate, it also lends weight to the notion that as industrial activity busily restores CO2 to levels approaching those of the Cretaceous, climate is likely to turn the climate clock back 60 million years as well. From Broad's flip dismissal of the CO2-climate connection in the "recent" part of the record, the reader would never guess at the length and particular significance of this period.

 

And then, too, the tired old beast of Galactic Cosmic Rays (GCR) raises its hoary head in Broad's article. The GCR issue has been extensively discussed elsewhere on RealClimate (e.g. here and here) On one level the GCR idea is another instance of the problem that Phanerozoic climate variations may have had many causes, giving rise to a false appearance of decorrelation between climate and CO2. Whatever role GCR may have played in deep time climate, the climate of the past century and its attribution to CO2 is a wholly different kettle of fish, since in modern times we have direct observations of GCR and they are not doing anything of a sort that would cause the observed warming -- to say nothing of the fact that one would still have to argue away the basic radiative physics which makes CO2 affect the planet's radiation budget. We repeat: There has been no recent trend in cosmic rays that could conceivably account for the recent warming, even if the GCR proponents were right about the physical mechanism underpinning their theory. This is made abundantly clear in this recently published article. Further, whatever was going on in the past, the present observations do not support the supposed cloud-GCR connection that is supposed to mediate the climate effect. That's not the end of the story, for there are also severe methodological difficulties in the way the GCR proponents have attributed Phanerozoic change to GCR rather than CO2, and also severe conceptual difficulties in the supposed physical link between clouds and GCR.. Some of these difficulties may ultimately be resolved and allow a more fair test of the possibility that GCR influences played some role in the past. Surely, the play given to Veizer and Shaviv in the context of Broad's article is an instance of false balance of the worst sort. The possibility that the GCR theory may play some role in deep-time Phanerozoic climate is eminently worthy of further consideration, but the way its major proponents have used the theory in attempts to undermine forecasts of near-term warming is unjustified.

 

Besides the broad-brush errors discussed above, Mr. Broad commits a number of lesser climatological faux pas, in areas where he really ought to know better. He refers to CO2 as "blocking sunlight" (whereas it's actually thermal infrared which CO2 affects). He says that CO2 traps heat "in theory." This is a lot like saying that a bowling ball dropped from an airplane will fall to the ground "in theory." There is indeed a theory involved in both cases, but the use of the phrase gives a completely wrong picture of the certainty of the phenomenon. There is no more doubt about the heat-trapping effect of CO2 than there is about the physics that causes a bowling ball to fall. Broad also says that the greenhouse effect of CO2 "plateaus" at high levels. This is a botched attempt to describe the well-known logarithmic radiative forcing of CO2, incorporated in every climate model since the time of Arrhenius. There is no "plateau" where CO2 stops being important. Every time you double CO2, you get another 4 Watts per square meter of radiative forcing, so that the anticipated climate change between present CO2 and doubled CO2 is comparable to that between doubled CO2 and quadrupled CO2. In fact, as one goes to very high CO2 levels (comparable to the Early Earth), the radiative forcing starts to become more, rather than less, sensitive to each further doubling (something that can be inferred from the radiative forcing fits in Caldeira and Kasting's 1992 paper in Nature).

 

Let's not lose sight, however, of the essential conundrum posed by Phanerozoic climate, particularly by the warm climates of the Cretaceous and Eocene. Current climate models do not reproduce the weak pole to equator gradients believed to characterize these climates, and have trouble warming up the polar climates enough to melt ice and eliminate continental winter without frying the tropics more than data seems to permit. Maybe there's something wrong with the data, or maybe there are currently unknown amplification mechanisms that make the switch from a moderate Holocene type climate to a hothouse more catastrophically sensitive to CO2. This truly must give us pause as we contemplate the experiment of doubling CO2 in the next century. It's certainly an experiment that would help to resolve some of the mysteries of Phanerozoic climate, but we'd on the whole prefer to see the mysteries resolved by improved studies of past climate instead.

 

 

 

 

 

18 Comments »

Surely it isn't true that "Every time you double CO2, you get another 4 Watts per square meter of radiative forcing," Eventually, the CO2 IR bands are full and adding more CO2 has no effect. The question is, when is "eventually"?

 

[Response: The CO2 bands may fill up at low levels, but the trick is that there's always a part of the atmosphere tenuous enough that the CO2 bands are unsaturated, and the radiation comes from up there (where it's cold). A complication in thinking about the effect of CO2 on outgoing infrared is that adding CO2 actually tends to make the stratosphere colder, which gives you an additional reduction in OLR (for fixed surface temperature). Long before you get to the point where the stratosphere is optically thick, though, you enter the regime where the weak CO2 bands start to become important, and the radiative forcing per doubling increases -- that's at around 20% CO2 in the atmosphere, which is relevant to the Early Earth, but not to the near (or even far) future. By the way, it takes a whole darn lot of CO2 to saturate all the CO2 bands. Even Venus isn't really saturated. On Earth and other wet planets, the thing that saturates the absorption is the water vapor, which starts to be completely opaque for infrared for saturation vapor pressures corresponding to temperatures around 300K. --raypierre]

 

Comment by John Gribbin — 9 Nov 2006 @ 2:07 pm

 

Nice post. I too had noticed that he falsely states that as CO2 levels rise, the climate effect reaches a "plateau." I also noticed that the headline and first several paragraphs give the impression that new results will overturn modern climate theory, but the latter part of the article is much more moderate.

 

You guys at RC have been busy lately! Thank goodness, because the contrarian forces have been busy too.

 

Comment by Grant — 9 Nov 2006 @ 2:09 pm

 

I wonder just how much the emergence of the first terrestrial rain forests in the Late Devonian and Carboniferous had a bearing on the glaciations that occured around that time, and that set in with earnest in the late Mississippian (Carboniferous). I am also tempted to think that the switching from primary aragonite and primary calcite marine cements during the Carboniferous may also have a relationship to ocean pH and therefore CO2.

 

Anyone have any views?

 

[Response:An excellent point about the rain forests. The albedo feedback and evapotranspiration could be playing a role, but my feeling is that the biggest effect of vegetation in warm rainy areas would be via silicate weathering. Standard wisdom has it that vascular land plants greatly increase silicate weathering, which would draw down CO2, all other things being equal. The effect of varying vegetation on weathering is not yet in the model in our G-cubed paper. Yannick Donnadieu and I are busily trying to learn enough about vegetation modelling that we can put this effect in the geochemical model. --raypierre]

 

Comment by Nick Riley — 9 Nov 2006 @ 2:10 pm

 

I just had a chance to check last month's Scientific American out of the library, found a pretty mind-boggling article on the mechanism that now seems likely to have caused most of the big mass extinctions of the past. It starts with unusual volcanic activity raising the atmospheric CO2 level to about 1000 ppm... leading to a sudden change in ocean chemistry, vast anaerobic bacteria blooms that pour H2S into the air, catastrophic ozone depletion (and it's a miracle anything survives.)

 

I'm wondering if the total amount of carbon available on the earth's surface, in forms that could rapidly get into the atmosphere, might have formerly been less (?) Less carbon tied up in methane clathrates in precarious locations, maybe? Then concentrations of CO2 could have remained high for long periods, making for more uniform temperatures, without (often) passing that 1000 ppm overload mark. (?)

 

And of course I also wonder how this affects recent discussions here of "how much climate change can we afford?" Temperature increases are one constraint, eventual sea level rises another--but how much CO2 ends up in the ocean might put the problem in a whole different worm-can.

 

Comment by Forrest Curo — 9 Nov 2006 @ 2:15 pm

 

Raypierre

Hmmm?.

 

The relationship between weathering and vegetation is a tricky one. It is well documented in modern day Sumatra.

 

In terms of sediment flux- it's actually semi-arid climates that supply the large sediment fluxes. This is because dense rain forest vegetation binds sediment/regolith and protects it from erosion. Peat is particularly difficult to erode. Also many of the rainforests in the Carboniferous were ombrogenous bogs, with the trees rooted into the underlying peat- not bed rock or sediment.

 

What is clear is that those Carboniferous rain forests rooted in sandy soils- end up leaching the soil almost to pure qaurtz- giving rise to silcrete palaeosols- known here in Britain as ganisters.

 

What do you think about the primary aragonite/calcite marine cement flips?.

 

Comment by Nick Riley — 9 Nov 2006 @ 2:51 pm

 

Very interesting, Ray. Have you gotten in touch with Broad? He is a very professional journalist (a co-recipient of the Pulitzer) and I'm sure would be grateful for constructive criticism. I always hope that when I err in something I write -- as we all do -- someone would work with me to improve.

George

 

[Response: Good point. I like to think that journalists covering climate change are routinely reading RealClimate already, but with time pressure and deadlines and all, that's not a completely safe assumption. I've sent Broad an email pointing out our commentary, in case he hadn't noticed. --raypierre]

 

Comment by George Musser — 9 Nov 2006 @ 4:19 pm

 

"The worst fault of the article, though, is that it leaves the reader with the impression that there is something in the deep time Phanerozoic climate record that fundamentally challenges the physics linking planetary temperature to CO2. This is utterly false, and deeply misleading."

 

What would also be misleading would be giving policy makers and the public an impression that we now understand all of the complex, non-linear interactions between all the forcings and feedbacks significantly well to forecast future climate with skill across multi-decadal time. Just because Stefan-Boltzmann and Wein are beyond dispute, the best geological evidence available (most of the last 25% of earths history) tells us that CO2 is not the only game in town. It is the very tricky interaction of CO2 with a number of the other forcings that is really the question. To give one example, the role of aerosols and their associated feedbacks is still incompletely understood, but we know to some extent there is a cooling effect. There might be high aerosols and high CO2 (both can be natural and or man-made), and maybe a cooler than expected climate. What about the feedbacks associated with both? How do they all interact?

 

[Response: You're not getting the point. All the uncertainties you point are real - but ask yourself whether they can possibly be resolved using data from the early Phanerozoic with (at best) million year sampling? Obviously not - there are many, many more uncertainties in using deep time records than there are in using the relatively well sampled glacial period, or the present. Thus it will always be the case that while deep time may provide tests for our understanding (the PETM, Eocene, snowball earth etc.), it will rarely (if ever) be able to reduce uncertaintites in present climate - and certainly not for changes at the decadal or century scale level. Others might be able to add to this, but I can only think of one thing that deep time has told us that is of directly relevance to reducing uncertainties today, and that is the 100,000 year timescale for the removal of the excess carbon at the PETM. All the other information is more of a 'wow, that's an interesting thing to have happened' kind. Anyone got anything else? - gavin]

 

[Response: I myself wouldn't go quite so far as Gavin. I'd agree with him that deep time isn't likely to be useful for direct estimates of climate sensitivity in the sense that one teases such estimates out of the Pleistocene. However, the Eocene is the closest thing we have to an example of what a warm world with near-current continental configurations and (probably) high CO2 is like, so the quest to understand it is likely to tell us something about the risks going into a high CO2 world in the future. Its role is to shake loose ideas. For example, if it turns out that polar stratosphericc clouds or enhanced hurricanes are the key to the warm Eocene poles, that gives some additional confidence to our projections about how such things might behave in the future. The way I would put it is that it is unreasonable to demand that we solve all of the grand challenges of climate science (faint young sun, origin of oxygen, snowball Earth, Cretaceous warmth, PETM, warm wet early Mars ...) before the models are deemed an adequate basis for justifying action on CO2 abatement. That would take centuries to sort out, and given the lifetime of CO2 in the atmosphere and the rate of growth of emissions, we don't have the luxury of waiting that long. The examples we have -- the current climate, the past century, the Holocene, the Pleistocene -- provide more than enough checks and confidence in the prediction that the future warming is real and substantial. --raypierre]

 

Comment by Bryan Sralla — 9 Nov 2006 @ 5:16 pm

 

Comment #2 above states

"I also noticed that the headline and first several paragraphs give the impression that new results will overturn modern climate theory, but the latter part of the article is much more moderate."

 

This raises one of the biggest challenges for writers of all types, but especially science journalists: how to hook the reader in at the beginning, even if your story's only about an incremental achievement....isn't it a good thing that climate change gets so much space in the NYT in the first place?! Compare it to the spin on climate in WSJ editorials...

 

Comment by LucysGranddaughter — 9 Nov 2006 @ 5:16 pm

 

Re Forrest Curo (4)

 

I'm wondering if the total amount of carbon available on the earth's surface, in forms that could rapidly get into the atmosphere, might have formerly been less (?)

 

Less than what?

 

From what I know, deserts and glaciated areas were much smaller in the Paleocene than today, subtropical forests extended as far north as Greenland, and the polar regions were cool and temperate.

To me this implies significant;y *more* carbon in the biosphere.

 

However, other than tectonic activity, there was nothing bringing the carbon trapped in fossil fuels up to the surface. As to methane clathrates - I guess their modern distribution is not well-characterized, and less is known about their distribution in the Paleocene.

 

Comment by llewelly — 9 Nov 2006 @ 6:43 pm

 

Great post! When I read that NY Times piece the other day, I also felt like the connection to modern issues was being oversold. It brings me back to a couple of years ago when I was hanging out on a message board on the Michael Crichton website and a geologist there was harping on this idea that we don't understand the climates of hundreds of millions of years ago. I am glad to see that the basic arguments I came up with at the times are the ones you give here (data is not very good, lots of other confounding factors like different continent locations, ...).

 

Another thing I also liked to point out, which you have mentioned but could probably use re-emphasis, is the vast difference in timescales. We are trying to make predictions on timescales on the order of 10^2 years. Do we really need to understand things on timescales of 10^8 or 10^9 years in order to do this? Sure, it would be nice to understand everything. But, it seems sort of ridiculous to say that we can't make predictions on a timescale of 10^2 years without being able to understand and explain what goes on at timescales 6 orders of magnitude longer than this. It seems impressive enough to me that we now have quite good data and some reasonable understanding (albeit with some gaps) of what goes on at timescales up to 10^6 years.

 

[Response:Your point about timescales is right on. Yes.--eric]

 

[Response: The greatest overselling of the link between modern climate change and some vague (and statistically not significant) correlation on the 100-million-year time scale must be this infamous press release by cosmic ray proponent Nir Shaviv. -stefan]

 

Comment by Joel Shore — 9 Nov 2006 @ 7:00 pm

 

Re 5:

If vegetation reduces sediment flux, then why does the Fly river, which drains mostly PNG rainforest, have such a huge sediment flux? Isn't total rainfall and terrain relief the dominant factor there (and in other high sediment drainages like the Amazon, Ganges, etc.)?

 

As for carboniferous forest type, isn't that a factor of preservation? Obviously subsiding bogs with sedimentary accumulation will preserve their forest much better than eroding hillsides.

 

[Response: In these discussions of vegetation and sediment flux, please keep in mind that I was talking about chemical weathering (silicate to carbonate, primarily), not physical weathering. Vascular land plants affect chemical weathering by pumping CO2 into the soil, and also by changing the acidity of the soil through humic acids. They probably do a lot of other things to the chemical and microbial environment as well. All of this is different from the sort of thing that determines how much sediment is washed into a river. --raypierre]

 

Comment by C. W. Magee — 9 Nov 2006 @ 7:33 pm

 

Re #11:

 

The Amazon and Ganges have their headwaters in the Andes and Himalayas respectively. Erosion is very rapid in both of those mountain ranges.

 

Comment by yartrebo — 9 Nov 2006 @ 8:45 pm

 

> I can only think of one thing that deep time has told us that is of directly relevance to reducing uncertainties today, and that is the 100,000 year timescale for the removal of the excess carbon at the PETM. ...Anyone got anything else? - gavin

 

It seems to show that complex life can survive several thousand ppm CO2. That should counter claims that 600ppm is likely to make humans extinct.

 

[Response: I was thinking of uncertainties a little more at the forefront of serious research.... - gavin]

 

[What the record may show is that life can evolve to adapt to pretty large changes, given enough time (hundreds of thousands of years or more). No serious person is predicting "extinction of humans". The question is not whether humanity will "survive", but what we will go through to survive. --eric]

 

Comment by Steve Reynolds — 9 Nov 2006 @ 8:52 pm

 

Re: #13

 

It seems to show that complex life can survive several thousand ppm CO2. That should counter claims that 600ppm is likely to make humans extinct.

 

It also shows that climate catastrophe can drive a large fraction of complex life forms to extinction.

 

Comment by Grant — 9 Nov 2006 @ 9:09 pm

 

>Response: I was thinking of uncertainties a little more at the forefront of serious research.... - gavin

 

RealClimate does a good job of presenting clear scientific arguments of why GW is real, but I think countering some of the hysteria of exagerated claims of doom is also important. Judging by some of the comments at RC, it is clearly needed.

 

[Response: We do spend a fair amount of time countering exaggerated claims of doom. Many of us have tried to set the record staight on the THC/Ice-age catastrophe idea, and I myself have spend much time pointing out why the Earth is not going to succumb to a Venus type runaway greenhouse. Some of you, however, seem to think that any prediction of severe impact is so unpleasant (or has such unpleasant policy implications) that it must, de facto, be an "exaggerated claim of doom." There's not much constructive I can do with that. Very few, if any, scientists are predicting the extinction of human life at 6oo ppm, though once you start messing so much with the whole biosphere, you do have to leave the door open for unanticipated consequences that can be far worse than we think. The predictions of severe impoverishment of the ecosystem and big impacts on the world socioeconomic structure are very legitimate, and in fact are quite legitimate claims of doom. A certain amount of hysteria is justified, indeed encouraged. --raypierre]

 

Comment by Steve Reynolds — 9 Nov 2006 @ 9:58 pm

 

I read the article when it came out in the NYT. I was wondering what was big deal. I was going to write a comment on RC and ask about it. You must have read my mind!

 

From what I learned about climate science and I know about the press, I guessed that it was a minor issue in the scientific community, but the article made it seem like a big drama to get the readers attention. RealClimate confirmed what I thought.

 

Grant (#2) is right, you guys at RC have been busy lately and its good to see.

 

[Response: Thanks. It's nice to be appreciated. I've been a bit quiet recently on RC because there's been a huge upsurge of public interest in climate change, and I've been using more of my time on live appearances. --raypierre]

 

Comment by Joseph O\’Sullivan — 9 Nov 2006 @ 10:42 pm

 

Re-greetings Raypierre, may this be a bad interpretation: CO2 being well mixed throughout the atmosphere can't have all its molecules saturated at the same time.?? Another question; if there are more molecules of CO2 in the air would this mean more or less OLR in the long run? I am thinking clouds which trap radiation, and tying it with CO2, the problem with clouds is there should be less OLR above them, but with more CO2 the atmosphere should be warmer... Glad I can ask these questions..

 

[Response: I'm sorry, but I am having trouble understanding your question. Note that when we speak of saturation, we are referring to saturation of the ability to absorb infrared at some wavelength -- once you absorb everything, adding more CO2 doesn't cause any more absorption, since you can't absorb more than everything. Because of a thing called Kirchoff's Law, in this circumstance adding more CO2 also doesn't change emission from the layer, unless you change the temerature. Also, note that when we say CO2 is well mixed, we don't mean that the number density of CO2 is independent of height, but that the mixing ratio (ratio of CO2 molecules to the rest of the air molecules) is constant. For fixed mixing ratio, there are fewer CO2 molecules per unit volume in the tenuous stratosphere than near the surface. To get an idea of how all this interacts with the temperature profile, take a look at Chapter 3 of my Climate Book. --raypierre]

 

Comment by wayne davidson — 9 Nov 2006 @ 11:41 pm

 

Boy... when I read that article, I immediately wondered what RC would be saying about it.

 

So... do the people that know what they are talking about now get in touch with this NYT author & let him know?

 

I have a friend who works for the NYT, in their Technology section... If you need an "in" to voice your dissatisfaction with the article, perhaps I could get the message there. Or maybe just writing the editor would work?

 

Comment by Matt — 9 Nov 2006 @ 11:50 pm

 

 

 

posted to ClimateConcern Group

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