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Amplification of Cretaceous Warmth by Biological Cloud Feedbacks

By Joe Romm

"Amplification of Cretaceous Warmth by Biological Cloud Feedbacks"


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That’s the title of an article in Science today (available here with subs.) I’m giving a talk this morning so don’t have a lot of time to comment on it, but here’s the abstract:

The extreme warmth of particular intervals of geologic history cannot be simulated with climate models, which are constrained by the geologic proxy record to relatively modest increases in atmospheric carbon dioxide levels. Recent recognition that biological productivity controls the abundance of cloud condensation nuclei (CCN) in the unpolluted atmosphere provides a solution to this problem. Our climate simulations show that reduced biological productivity (low CCN abundance) provides a substantial amplification of CO2-induced warming by reducing cloud lifetimes and reflectivity. If the stress of elevated temperatures did indeed suppress marine and terrestrial ecosystems during these times, this long-standing climate enigma may be solved.

Wow. If climate warming “did indeed suppress marine and terrestrial ecosystems,” which leads to “reduced biological productivity” then we get fewer clouds and more absorption of the sun’s heat. Ocean acidification and widespread drought, wildfires, and pests, anyone?

Oh, what the heck, here’s the rest of this very upbeat article — it’s short and worth a read:

During supergreenhouse intervals of the geologic past, both tropical and polar temperatures were considerably warmer than today, and the gradient between the two was reduced. To even approach these equable climate states with climate models, atmospheric CO2 levels must be specified that significantly exceed most proxy estimates for the Cretaceous and the Eocene (1). Thus, climate modelers have invoked viable but hard-to-evaluate hypotheses of elevated atmospheric methane levels, greater poleward oceanic heat transport, and enhanced polar stratospheric clouds (2).

An unexplored alternative involves planetary albedo, the fraction of the incoming solar radiation that is reflected to space, which is largely dependent on cloud cover and cloud albedo. A major determinant of cloud properties is the abundance of cloud condensation nuclei (CCN). When CCN are abundant, many small cloud droplets form, creating optically dense, high-albedo clouds; when abundance is low, fewer and larger droplets form, creating optically thinner, lower-albedo, and, importantly, shorter-lived clouds (3). Today, pollution dominates continental CCN, producing abundances of thousands per cm3. In remote oceanic regions, biological release of dimethylsulfide is the major pathway for CCN production. Andreae (4) concludes that biological productivity determined the CCN concentrations over prehuman unpolluted land and sea, ranging from a few tens per cm3 in low-productivity regions to a few hundred per cm3 in high-productivity regions, supporting the notion of a prominent role for the biota in climate regulation on the prehuman Earth (5). If CO2-induced warming during supergreenhouse intervals reduced global primary productivity by temperature stress and enhanced vertical stratification of the ocean, causing a reduction in CCN concentration, would lower cloud amounts and albedo have caused further warming?

To explore this hypothesis, we used a global climate model (GENESIS version 3.0) (GCM) to simulate middle Cretaceous [~100 million years ago (Ma)] climate with various atmospheric CO2 amounts. This GCM has a slab mixed-layer ocean and prognostic cloud water amounts, and version 3 uses the National Center for Atmospheric Research (NCAR) Community Climate Model 3 (CCM3) radiation code with prescribed cloud droplet radii re (3). Cloud droplet radii mainly affect cloud optical depth, infrared emissivity, and precipitation efficiency, Pe, the rate at which cloud water is converted to precipitation. Modern large-scale observations and theory suggest that for ~10- to 100-fold global reductions in past aerosol and CCN amounts, ~30% (over ocean) to ~100% (over land) increases in liquid droplet radii are plausible (3). We simulate the Cretaceous climate with these increases in re and with Pe increased for liquid clouds by a factor of 2.2, reflecting the maximum likely effect of extreme global warmth on marine and terrestrial biological productivity and CCN production rate.

Our Cretaceous model results are shown in Fig. 1. [JR: Reprinted at bottom] In common with previous GCM studies, increasing CO2 from 1x to 4x preindustrial atmospheric level (PAL) (Fig. 1, A and B) fails to produce the extreme high-latitude warmth implied by temperature proxy data (Fig. 1D). We then performed another 4x PAL simulation with the increases in re and Pe described above (Fig. 1C). Global cloud cover is reduced from 64 to 55%, and the less extensive and optically thinner clouds reduce planetary albedo from 0.30 to 0.24. The ensuing warming is dramatic, both in the tropics and in high latitudes, where it is augmented by surface albedo feedback of almost vanishing snow and sea-ice cover. (Other feedbacks due to changes in cloud types and levels are minor.) High-latitude continental temperatures remain above or very close to freezing year round, in better accord with proxy evidence.

Our results support the hypothesis that widespread increases in re can explain the drastic warming and equable high latitudes during supergreenhouse intervals of the Cretaceous and early Cenozoic. The increases in re could plausibly have been caused by an order of magnitude decrease in CCN concentrations, which we suggest was caused in turn by declines in biological productivity triggered by the climatic consequences of high CO2 levels of ~4x PAL.



  • 1. K. L. Bice et al., Paleoceanography 21, PA2002 10.1029/2005PA001203 (2006). [CrossRef]
  • 2. L. C. Sloan, D. Pollard, Geophys. Res. Lett. 25, 3517 (1998). [CrossRef] [ISI]
  • 3. Materials and methods are available on Science Online.
  • 4. M. O. Andreae, Science 315, 50 (2007).[Abstract/Free Full Text]
  • 5. R. J. Charlson, J. E. Lovelock, M. O. Andreae, S. G. Warren, Nature 326, 655 (1987). [CrossRef]
  • 6. J. E. Francis, I. Poole, Palaeogeogr. Palaeoclim. Palaeoecol. 182, 47 (2002). [CrossRef]
  • 7. R. A. Spicer, R. M. Corfield, Geol. Mag. 129, 169 (1992).[Abstract]
  • 8. This work was supported in part by grants from NSF’s Carbon and Water in the Earth System (to L.R.K.) and Paleoclimate History (to D.P.) programs.

Supporting Online Material








Fig. 1. Annual mean surface-air temperatures (°C) in GCM simulations of Middle Cretaceous (~100 Ma, low sealevel stand) and zonal averages (A) with CO2 concentration 1x PAL (280 parts per million by volume), (B) with 4x PAL CO2, and (C) with 4x PALCO2 and increased liquid-cloud re and Pe. (D) Zonal average temperatures for land and ocean, land only, and ocean only, with ocean (1) and terrestrial (6, 7) proxy temperature data for the Middle Cretaceous shown as solid rectangles. Dotted line indicates data from simulation with 1x PAL CO2; dashed, with 4x PAL CO2; and solid, with 4x PAL CO2 and increased liquid-cloud re and Pe.

JR: I hope that’s all clear to you!


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24 Responses to Amplification of Cretaceous Warmth by Biological Cloud Feedbacks

  1. Nylo says:

    Unfortunately, the most glorious life-full times in Earth history (from when we get nowadays’ petrol) happened to be quite hotter than now, which hardly supports the idea that an increase of CO2 may have a possitive feedback on temperatures because of the death of animal and plant life.

    It can ba admitted that the loss of ecosystems means less clouds, but it has to be proved that the increase of CO2 or temperatures will lead to that loss of ecosystems creating the positive feedback. All the past data leads to the opposite conclussions.

  2. Dano says:

    All the past data leads to the opposite conclussions.

    Joe’s and Nylo’s implied points – opposite of one another – need to be clarified.

    65 MYA the planet was very different. Can we make assumptions based on events then? Yes. Can we make policy? I’m not sure.

    However, it is abundantly clear that humans are stressing ecosystems – there is no doubt of this – and additional stressors could lead not to a loss of ecosystems, but a loss in their regimes whereby they flip to new states. States with which we are not familiar, making planning and even day-to-day life much more difficult.

    Now, should we base policy on events from more than 65 MYA with this context?



  3. Paul K says:

    Isn’t the real question do we base policy on the 1.5 – 4.5C IPCC projection or the 6 – 9C warming Joe and Dr Hansen are promoting? Also, much of the stress on ecosystems comes from land use and non CO2 pollutants. I think too little attention is paid to land use both as a cause and a solution to rising CO2 concentrations.

  4. exusian says:

    Nylo Said: “Unfortunately, the most glorious life-full times in Earth history (from when we get nowadays’ petrol) happened to be quite hotter than now, which hardly supports the idea that an increase of CO2 may have a possitive feedback on temperatures because of the death of animal and plant life.”

    What matters is not increasing temperatures, but the rate of temperature increase. Ultimately higher temperatures may very well lead to increased density of life, but many current species will simply not be able to adapt fast enough to rising temperatures and northwards shifting ecosystems and climate zones, so the species that make up that increased density will undoubtedly be different.

  5. David B. Benson says:

    exusian wrote “What matters is not increasing temperatures, but the rate of temperature increase.” Both matter. Looking at the temperatures of the Cenazoic, 65 mya to now:


    we see that the globe is now quite cold and has been throughout the evolution of genus Homo and even earlier man-like species, say the last 6–5 million years. So Homo spaiens sapiens (modern humans) is a specians evolved for a cold climate.

    Even more telling, our agriculture is only adapted to the remarkably stable climate of the Holocene. Moving out of that risks all agriculture.

    Also, higher temperatures may very well lead to more desert area. This does appear to be happening. So I will doubt that hotter is better; indeed, the is good reason to believe that slightly colder is better.

  6. exusian says:

    Ah, but David, I did not exclude Homo spaiens sapiens from those species that might not be around in some future warmer world. In fact my point was that although life will go on, it most likely will not be life as we know it.

  7. David B. Benson says:

    exusian wrote “… although life will go on, it most likely will not be life as we know it.” Yes. I prefer life as we know it.

  8. David B. Benson says:

    Spelling correction: Cenozoic.

  9. Harold Pierce Jr says:

    The rate of rotation of the earth decreases about 0.2 milliseconds/100 yrs or 2 sec /1 million yrs, due to friction with the oceans. 100 million years ago the rate rotation of the earth for one day would be about 20.7 hrs. This is probably the real reason for the warmer climate during the Middle Cretaceous. At the start of the Cretaceus, the lenght of the day would have been about 19.5 hrs.

  10. Tom says:

    How do you get more heat from less sun exposure?

  11. David B. Benson says:

    Harold Pierce Jr — How long ago was ‘snowball earth’? What was the rate of rotation then?

  12. Harold Pierce Jr says:

    If the earth’s rotation is faster, there is less time for cooling. Due to thermal inertia of the oceans and some types of land, these would accumulate energy and heat up. The tropics stay warm because the sun is overhead, the day and night are about equal in lenght and this light-dark cycle is pretty constant.

    Imagine the earth as spicy meatball on a skewer, and you want to cook it uniformly. How do you do this? You put it in a rotissorie with a radiant heater and adjust the rate so that it is heats up nice and slow and is continouly bathed in its own juices. If the skewer didn’t rotate, one side would heat up and burn, and the other side would stay cool.

    Have you guys ever roasted hotdogs and marshmallows on a stick over a campfire? Or are you 100% downtown city slickers?

    Snowball earth is likely a consequence of Milankovitch cycles.

    Go to wikipedia and look up “rotation of earth”. I was quite suprised to learn about the numerous factors that effect the speed of the earth’s rotation such as the tides and the varing positions of the planets in the solar system.

  13. David B. Benson says:

    Harold Pierce Jr wrote “Snowball earth is likely a consequence of Milankovitch cycles.” Nope, landmass distribution.

    My point was it was a long time ago and so the rotation was even faster. But more to the point is the simplest (zero-dimensional) model of the earth’s average temperature:


    which shows thaat the insolution (on the left) is balanced by the emission (of IR) of the right. Won’t heat up by rotating faster.

  14. Tom says:

    If the earth’s rotation is faster, there is also less time for heating.

  15. Harold Pierce Jr says:

    ATTN: David!

    The snowball earth is just a hypothesis, and there is little verifiable evidence that it ever occured.

    ATTN: Tom!

    Why is the earth any different than a meatball on a skewer? Answer: None!

    Suppose the earth rotated ten times faster than it does now? What would the effect? We would be a carbonized meatball because the earth would never have any time to cool off.

    Ever watch a scientific glassblower work molten glass? I have, lots of times. To heat a work piece such as globe, he rotates it manually very fast in the flame of the torch so that it uniformily heated. If the piece is a lathe, he will rotate it very quickly to get it to working temperature, and then slow that rate of rotation to perform a particular operation such as attaching another piece of glass e.g , a piece of tubing or ground-glass joint.

    After performing a certain operation, he will then heat the piece in a large bushy flame to uniformily heat it to relieve any stress with cooler portions of the piece. He will then put it an annealing oven where it is slowly heated to a specific temperature at which all stess and strain will be relieved. After a certain time this temperaure, the automatic contols slowly cool the oven down to room temperature. This process is usually carried out overnight. He will then examine the piece thru a stain viewer to unsure that it was properly annealed.

    BBQ season is coming. Why don’t you guys do some experiments to explore the “Rotisserie Effect? What’s nice about these exp’s is that you get eat the test material if the exp is sucessful. Otherwise, you can feed the failures to the dog!

  16. exusian says:

    “The snowball earth is just a hypothesis, and there is little verifiable evidence that it ever occured.”

    So which is it, Herald, likely a consequence of Milankovic cycles, or just some hypothesis that is not worth accounting for in your hypothesis of Earth as rotisserie?

    “Why is the earth any different than a meatball on a skewer?”

    Ummm, because it isn’t a meatball on a skewer in a rotisserie? It also doesn’t rotate 10 time faster than it does. Nor one tenth as fast. Nor has it ever stood still. It’s also not a piece of lab glassware exposed to the direct flame of a torch.

    I think you’ve been watching too many ads for cooking gadgets on late night tv.

  17. Tom says:

    Why don’t you distance your meatball in the same ratio from your heat source as the earth is from the sun…

  18. David B. Benson says:

    Harold Pierce Jr — Actually, the evidence for snowball earth is about as good as it gets in paleogeology. The only remaining point of debate is whether or not the ice sheet totally convered the earth, or only most of it. The evidence appears to point to the later.

  19. exusian says:

    Evidence? Herald don’t need to see no stinking evidence!

  20. Harold Pierce Jr says:

    The article in Wikipedia says there is no conclusive evidence for the Snowball Earth.

    ATTN: Euxsian!

    Call up the Geology Dept at a nearby college or univ, and ask a geologist to comment about my claim that the earth probably rotated faster than it does now about 100 milion years ago. Also some geologist believe that the axis of rotation of earth was tipped much greater from the vertical perpendicular to the plane of its orbit.

    ATTN: Tom!

    You buy the meatballs and the radiant heater, and I’ll do the experiments.

    I’ll bring a couple of cases of Candian beer and ale and many bottles of wine from the Okanagan!

  21. exusian says:

    I already know Earth’s rotation was was faster 100 million years ago and that it is very slowly slowing down, Herald. I also know that the sun was not as bright 100 million years ago and that it is very slowly getting brighter as well.

    What I called you on was your hyperbole. It was you who asked us to suppose the earth rotated ten times faster than it does now, after all, which is a bit much even for your imagination.

    As Tom pointed out, when the earth’s rotation was faster, the amount of time any given point on the the surface was exposed to the sun, and thus to direct warming, per rotation was also less. The flaw in your logic is that the surface is ALWAYS cooling off, by both radiating infrared and by convection, not just at night, even in the day time.

    Back to the drawing board, Herald.

  22. Tom says:

    Of course you’re doing said experiments in a vacuum?

    BTW I drink Rickards Red…

  23. David B. Benson says:

    Harold Pierce Jr wrote “The article in Wikipedia says there is no conclusive evidence for the Snowball Earth.” Goes to show that Wikipedia is not to be trusted. There was a very good article about it in ‘Scientific American’ a few years ago. Subsequently there is evidence strongly suggesting that not the entire globle was covered with ice.

  24. John Bailo says:

    At first this sounds like more evidence for Svensmark’s theory of Cosmoclimatogy. They do however turn a lot of cartwheels to involve “biological” agents (reminds me of the use of the homunculus in early brain studies).

    Svensmark’s work still contains the simplest…and best…answer to warming.