## The Azolla Event

My friend Bruce Smith just pointed out something I’d never heard of:

Azolla event, Wikipedia.

As you may recall, the dinosaurs were wiped out by an asteroid about 65 million years ago. Then came the Cenozoic Era: first the Paleocene, then the Eocene, and so on. Back in those days, the Earth was very warm compared to now:

Paleontologists call the peak of high temperatures the “Eocene optimum”. Back then, it was about 12 °C warmer on average. The polar regions were much warmer than today, perhaps as mild as the modern-day Pacific Northwest. In fact, giant turtles and alligators thrived north of the Arctic circle!

(“Optimum?” Yes: as if the arguments over global warming weren’t confusing enough already, paleontologists use the term “optimum” for any peak of high temperatures. I think that’s a bit silly. If you were a turtle north of the Arctic circle, it was indeed jolly optimal. But what matters now is not that certain temperature levels are inherently good or bad, but that the temperature is increasing too fast for life to easily adapt.)

Why did it get colder? This is a fascinating and important puzzle. And here’s one puzzle piece I’d never heard about. I don’t know how widely accepted this story is, but here’s how it goes:

In the early Eocene, the Arctic Ocean was almost entirely surrounded by land:

A surface layer of less salty water formed from inflowing rivers, and around 49 million years ago, vast blooms of freshwater fern Azolla began to grow in the Arctic Ocean. Apparently this stuff grows like crazy. And as bits of it died, it sank to the sea floor. This went on for about 800,000 years, and formed a layer 8 up to meters thick. And some scientists speculate that this process sucked up enough carbon dioxide to significantly chill the planet. Some say CO2 concentrations fell from 3500 ppm in the early Eocene to 650 ppm at around the time of this event!

I don’t understand much about this — I just wanted to mention it. After all, right now people are thinking about fertilizing the ocean to artificially create blooms of phytoplankton that’ll soak up CO2 and fall to the ocean floor. But if you want to read a well-informed blog article on this topic, try:

• Ole Nielsen, The Azolla event (dramatic bloom 49 million years ago).

By the way, there’s a nice graph of carbon dioxide concentrations here… inferred from boron isotope measurements:

• P. N. Pearson and M. R. Palmer, Atmospheric carbon dioxide concentrations over the past 60 million years, Nature 406 (6797): 695–699.

### 40 Responses to The Azolla Event

1. This is stuff denialists love (incl. the word “optimum”). Great you posted the 2nd image, for it gives the first and simplest response to their “reasoning”: It was a different planet back then.

2. Arrow says:

Interesting theory, but I find this sentence objectionable:

“But what matters now is not that certain temperature levels are inherently good or bad, but that the temperature is increasing too fast for life, and human civilization, to easily adapt.”

So far the temperature is certainly NOT “increasing too fast for life, and human civilization, to easily adapt.”

Human civilization never did better, and while some life in general may be in trouble it’s precisely because human civilization is doing so good and not because of temperatures.

If anything what you refer to is just a prediction of some questionable climate models for the future, but as I’m sure you know predictions are very difficult, especially about the future.

• John Baez says:

I have removed “and human civilization” because while I believe some problems have already started for us, they mostly lie in the future.

As for “life”, I will keep that, because I know it’s already having trouble keeping pace with global warming. I’ll present some evidence later, but right now it’s dinner-time!

• “As for “life”, I will keep that, because I know it’s already having trouble keeping pace with global warming.”

Life have all kinds of trouble all the time, but I doubt it has anything to do with the pace of current warming.

About 10,500 years ago, at the end of Younger Dryas average temperatures over Greenland have increased by 15°C in two steps separated by several decades, with magnitudes of 7-8°C and durations less than 10 years each. That’s ten times faster than rate of current change, still, the biosphere has survived somehow.

At the same time sea level went up by dozens of meters (more than 120 meters since 18,000 years ago). It means all the magnificent coral reefs we currently have are younger than ten thousand years; the old reefs have drowned, they had to re-grow over previously dry land. It is practically the same with all our temperate and boreal forests. The area occupied by them right now used to be covered with miles thick ice sheets or subarctic desert.

Life is durable, local ecosystems are not so much.

• Frederik De Roo says:

have increased by 15°C in two steps separated by several decades, with magnitudes of 7-8°C

I looked for a link, but it’s already on the wiki! A paper by Grachev and Severinghaus is linked that claims a $10 \pm 4$ °C change in temperature.

Btw:

the Younger Dryas does not seem to have been a world-wide event (though this has been heavily debated).

• Tim van Beek says:

As an AGW sceptic you could rephrase the sentence and say “modern civilization cannot adapt to climate change as easily as e.g. the hunters and gatherers living during the ice ages could”.

This is a main point of the entertaining book “The Long Summer” of Brian Fagan, which is on our recommended reading page on the Azimuth project.

• John Baez says:

Okay, here’s some evidence that life is already having trouble keeping up with the pace of global warming. Instead of drowning people with evidence, I’d rather focus on one thing at a time — and since this blog entry is about sea life, let’s start there.

Record high ocean temperatures are causing not just the bleaching but now even the death of coral reefs:

• James Cook University Media Office, Worst coral death strikes at SE Asia, ARC Centre of Excellence for Coral Reef Studies, October 19, 2010.

International marine scientists say that a huge coral death which has struck Southeast Asian and Indian Ocean reefs over recent months has highlighted the urgency of controlling global carbon emissions.

Many reefs are dead or dying across the Indian Ocean and into the Coral Triangle following a bleaching event that extends from the Seychelles in the west to Sulawesi and the Philippines in the east and include reefs in Sri Lanka, Burma, Thailand, Malaysia, Singapore, and many sites in western and eastern Indonesia.

“It is certainly the worst coral die-off we have seen since 1998. It may prove to be the worst such event known to science,” says Dr Andrew Baird of the ARC Centre of Excellence for Coral Reef Studies and James Cook Universities. “So far around 80 percent of Acropora colonies and 50 per cent of colonies from other species have died since the outbreak began in May this year.”

This means coral cover in the region could drop from an average of 50% to around 10%, and the spatial scale of the event could mean it will take years to recover, striking at local fishing and regional tourism industries, he says.

NOAA warns: Caribbean’s coral reefs in danger this year, July 23, 2009.

The National Oceanic and Atmospheric Administration released its 2009 outlook for the world’s coral reefs this week, and the results are disturbing.

The temperature patterns and heat stress that scientists are seeing, particularly in the Caribbean, are reminiscent of 2005. That year set records for coral deaths. Across the Caribbean, 25 to 95 percent of the coral colonies were affected. In the U.S. Virgin Islands, nearly 52 percent of the corals died. In Trinidad and Tobago, 73 percent of all Colpophyllia and Diploria brain coral colonies were wiped out.

The damage goes beyond the corals themselves. Reefs provide habitats and ecosystems for tens of thousands of organisms, and they support the fisheries and tourism that some 100 million people worldwide depend on for their livelihoods.

“There’s a lot of similarly between what we’re seeing now and what hindcasts of 2005 showed,” said C. Mark Eakin, coordinator of NOAA’s Coral Reef Watch. “We can’t say whether it’s going to be worse or how they’re going to compare, but we’re looking at the potential for conditions that could lead to coral bleaching.”

In 2005, parts of the Caribbean were hit by a double-whammy: high surface temperatures and a lack of tropic storm activity that could have cooled the water. This year, the global ocean surface temperature reached a record 0.62 degrees Celsius above the 20th century average in July, and the warmer water is accompanied by a storm forecast for the Caribbean that promises little rain or cloud cover.

And then later:

• Stephen Leahy, Record Temperatures Killing Caribbean Corals, 13 September, 2010.

This year many corals are already bleached and dying in the southern Caribbean Sea, especially in the Lesser Antilles, according to Mark Eakin, coordinator of Coral Reef Watch at the U.S. National Oceanographic and Atmospheric Administration (NOAA).

The waters are warmer than they were in 2005 when a severe bleaching occurred across much of the Caribbean. More than 60 percent of corals died in the U.S. Virgin Islands, Eakin told Tierramérica.

[...]

Prior to the 1980s only one large-scale bleaching event had ever been recorded. An increase in water temperature of just one or two degrees Celsius above the average summer peak period is enough for bleaching to begin.

[...]

In Southeast Asia, ocean temperatures were 4.0 degrees above normal in May. Sixty to 80 percent of corals in various regions near Indonesia, Vietnam, Sri Lanka, Thailand, and Malaysia bleached, and some died, according to surveys done by the Wildlife Conservation Society, based in Indonesia.

It is expected that 2010 will be worse than the 1998 bleaching that killed 30 percent of reefs in the Indian and the western and central Pacific Ocean, they reported.

That year, 16 percent of the world’s corals died due to bleaching. Until the last decade or so, overfishing, pollution and coastal development were the biggest killers of coral. Those threats still exist, although there have been attempts to protect corals in marine protected areas (MPAs) and “no-take” fishery reserves.

For more, see Coral reef on the Azimuth Project.

• Arrow says:

Ok, but I don’t see how it supports the notion that “the temperature is increasing too fast for life to easily adapt.”

Sure, some waters may become too hot for corals, but others which were too cold may now become suitable.

If it is indeed the temperature that is the problem (and not for example pollution or some natural disease) then all the corals need to do to adapt is move to cooler waters which is basically the easiest form of adaptation one can think of.

• John Baez says:

It is indeed the change in temperature that is the problem: this has been intensively studied by many researchers, and the precise correlation between events of high sea temperature and massive sudden die-offs of coral reefs should be a clue even to a casual observer.

How long do you think it will take for coral reefs to form in cooler waters, comparable in size to the massive reefs that have died this year? (Perhaps centuries: the Great Barrier Reef is 6-8 thousand years old.) Do you think these cooler “suitable” waters will remain cool long enough for comparable new reefs to grow there? (No: the Earth is warming too fast, and will continue to do so until we drastically reduce carbon emissions.)

Of course another approach is to wait for fossil fuels to run out and let the Earth to reach a new higher-temperature state, say perhaps around 2300 in a business-as-usual-scenario. Any corals still alive at this time will eventually regroup and form new reefs as large as the ones that are dying now. It may take centuries or millennia, but they will eventually bounce back. This has happened before: in fact, during the Permian-Triassic extinction whole orders of coral, the Rugose and Tabulate corals, went extinct. And then, after a mere 10 million years, the Scleractinian corals filled that vacant niche.

If you’re okay with waiting long enough for nature to recover from the blow we’re inflicting on it, well, then everything is fine for you. With a sufficiently long-term, detached attitude one can in fact look calmly upon any event whatsover, and there is no logical way to prove that this attitude is “wrong”. But the idea that corals are keeping up with global warming now — that’s a factual claim, and that, I think, is something we can prove wrong.

3. Tim van Beek says:

Scientists in Columbia have been digging out the remnants of the “first tropical rainforest” that came into being during the Eocene optimum, including the biggest snake that ever lived on Earth, the “titanoboa” (13 meters long, ate crocodiles, did anyone see the movie “Anaconda”?)

Here is a short article in National Geographic: world’s biggest snake.

There is supposed to be an article in the “Proceedings of the National Academy of Sciences” that I did not find.

• Nathan Urban says:

This snake (I think it’s the same snake) has been used to argue that tropical temperatures at that time were hotter than previously thought, because a snake that big needs a hot climate to survive. (See this paper and commentary.) It’s kind of amusing to think of using the size of an animal to estimate past temperatures. People in my old department were making jokes about the “snake paleotemperature proxy” (not a comment on their opinion of the paper). But the paper was subsequently challenged strongly by at least three published comments (here, here, here).

• Tim van Beek says:

Thanks! I wrote my last comment before I saw yours here…

• John Baez says:

Okay, this blog needs a picture of the titanoboa. Here’s an unnamed artist’s conception of it, from the National Geographic article:

13 meters long and over 1000 kilograms! It must have been extremely hot back then.

Click for more, including a video.

• Tim van Beek says:

Click where?

To put the snake into perspective, its size is pretty much that of the anaconda that ate JLo’s friends :-)

• John Baez says:

Now you can click on the cute little snake.

4. Nathan Urban says:

My favorite review of Cenozoic climate and CO2 is Zachos et al. (2001), “Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present”. It doesn’t mention the Azolla event, which I hadn’t heard of. The work on it seems to appear later, around 2006.

My immediate reaction, regardless of the evidence about Azolla productivity, is to be suspicious of the magnitude of the CO2 drawdown mentioned in the blog article (taken from Wikipedia). But that opinion might not be worth much, since I’m not an expert in this area.

My current “go-to” reference for CO2 concentrations around that time period is Royer et al. (2010), Fig. 1. While not definitive, and certainly debatable, it’s a reasonable starting point. It doesn’t show anything like 3500 ppm pCO2 around 49 Ma. But it’s not a very high resolution synthesis record, either. The Pearson and Palmer paper shows a supposed drawdown over a million years or so. Royer’s figure might not even pick up something that “fast”.

I do note that Pearson and Palmer’s pCO2 reconstruction is based on the boron isotope proxy. This proxy has been criticized in Pagani et al. (2005) as giving too-high CO2 estimates, and Royer in the above article simply chooses to exclude all the boron data. I don’t know how controversial this is. I’m sure the boron isotope people contest it, but I have no knowledge of what the “feeling of the paleoproxy community” currently is. There is a comment and response to the Pagani paper.

• Tim van Beek says:

In this week’s edition, the German newspaper der Spiegel published an article about the preceding increase of CO2 in the atmosphere to a level of about 3000ppm within a time span of 10 000 years, resulting in a temperature increase of about 5°C, which supposedly led to an explosion of biodiversity and the creation of the rain forest as we know it today, but of course without any references one could check :-(

At first glance this seems to tell us that an increase in CO2 levels is a “good thing” rather than bad, but the Columbian archaeologist who was interviewed for this article in “der Spiegel” already pointed out that what truly matters is the timescale, with 10 000 years being long enough for a lot of lifeforms to adapt, it would seem. The titanoboa, the biggest snake that ever lived, would need an average temperature of above 30°C, which is a climate that I would not survive (you know, where I live, it rarely gets above 30°C at all).

• Nathan Urban says:

What period of time are they talking about for the fast rise in CO2 and temperature? Is this the Paleocene-Eocene thermal maximum? It sounds later, closer to the early Eocene “optimum” (EECO). Maybe the ETM-2 “Elmo” event? All of those Eocene thermals occur well before the 49 Ma Azolla event, but after the 58-60 Ma snake, so I’m not sure what it could be referring to.

• Tim van Beek says:

I’n not sure, either, but isn’t there a sharp peak of the green curve just right to the label “PETM” in the first graphic of the main post? That could be it :-)

• Nathan Urban says:

I don’t know, maybe they are talking about the PETM (Paleocene-Eocene Thermal Maximum). That’s certainly the most famous hyperthermal.

The graph at the top of John’s post was adapted from the Zachos et al. article I mentioned earlier, by the way.

• John Baez says:

The sharp peak that Tim spotted is the PETM, or Paleocene-Eocene Thermal Maximum, 55.8 million years ago, and lasting for roughly 10-20 thousand years. Maybe that’s what the German newspaper was talking about — when it comes to science, you can never tell.

But as Nathan notes (and you can see from the graph), the PETM was considerably before the Azolla event, which was around 49 million years ago.

Someday I’ll say a bit here about the PETM. It was an amazing thing.

Tim wrote:

… above 30°C, which is a climate that I would not survive (you know, where I live, it rarely gets above 30°C at all).

You’d survive. You’d suffer for a while, but your body would adapt. Here in Singapore I set my air conditioning for 28°C and it feels very cool compared to outside. There are lots of Germans here, and I haven’t seen a single one lying dead on the sidewalk yet.

5. Nathan Urban says:

Poking through the literature, Speelman et al. (2009) looks like it could be an up to date review of the Azolla event.

• John Baez says:

Thanks. One Azimuth Project page coming up: Azolla event. (Wait ten minutes for me to write it.)

6. John F says:

John B,
One of the many reasons that ocean fertilization doesn’t work nearly as well as one might think is that the deep ocean is not anaerobic.
http://en.wikipedia.org/wiki/Oxygen_minimum_zone
If one says typical surface aqueous O2 concentrations are 10 (e.g. mg/L at 1 atm), then deep pelagic concentrations are 3, enough to sustain lanternfish etc. Pelagic benthic sediments are approximately as aerobic/anaerobic as shallow water benthic sediments. Nobody is proposing ocean engineering to make the pelagic zone anaerobic because that is very hard (and scary).

Remember this issue also the problem of trees in forests: they grow for 50 years, then aerobically decompose in 2 years. Those people urging growing many more forests do not want to engineer vast anaerobic swamps. An economic alternative is to harvest, i.e. not just stacking dead plants but using them.

• John Baez says:

John F wrote:

One of the many reasons that ocean fertilization doesn’t work nearly as well as one might think is that the deep ocean is not anaerobic.

Interesting! How much of the problem is due to this? We’ve been building up information about iron fertilization on the Azimuth Project. There seem to be a lot of problems with it, but I haven’t read about aerobic conditions on the sea floor as one of them. If you know any relevant references, I’d love to get ahold of them.

Remember this issue also the problem of trees in forests: they grow for 50 years, then aerobically decompose in 2 years. Those people urging growing many more forests do not want to engineer vast anaerobic swamps. An economic alternative is to harvest, i.e. not just stacking dead plants but using them.

I guess growing new forests draws down carbon, but once a forest is grown, it emits almost as much CO2 as it absorbs… unless large portions are getting sequestered (e.g. in your anaerobic swamps) or getting turned into products that bind carbon for many years!

• Graham says:

John F said “Those people urging growing many more forests do not want to engineer vast anaerobic swamps.”

I don’t know if they are the same or different people, but some do want to (re-)create that kind of carbon sink.

“Peat and Repeat: Can Major Carbon Sinks Be Restored by Rewetting the World’s Drained Bogs?

“Bogs, swamps and mires help keep 500 billion metric tons of carbon out of the atmosphere, so preserving peatlands is emerging as a new priority.”

http://www.scientificamerican.com/article.cfm?id=peat-and-repeat-rewetting-carbon-sinks

• John Baez says:

Great! Maybe we need some other Azimuth Project pages on forms of biocarbon sequestration other than biochar. Getting nature to do a lot of the work could be a good idea, since carbon flows through the biosphere greatly exceed the influx of excess carbon we’re putting into the atmosphere.

• Graham says:

I am writing a page on peat bogs – I live near a lot of them. But the wiki has been down for a while.

• John Baez says:

Andrew Stacey will give me the ability to reboot the server on which the Azimuth Project and nLab wikis reside. This should help.

7. ar18 says:

John wrote:

It was about 12 °C warmer then.

You need to remember that, like today, the vast majority of that 12°C difference was over the North and South Poles.

People not familiar with elementary statistics naively believe that when someone says, “The average temperature of the Earth has increase by 1°C in the last 100 years”, it does not mean that everywhere has increased by 1°C. A 1°C increase would not even be noticed in most geological places today. The variation from day to day and year to year is greater than this.

There were crocodiles and palm trees in Antarctica.

Antarctica also was not located over the South Pole. In fact, if you look at the chart of temperatures in your post, it wasn’t until Antarctica moved in place over the South Pole that the “average global temperatures” started to drop. Again, the vast majority of that temperature drop occurred over Antarctica, while the rest of the world did not change significantly.

The biggest change in “average global temperatures” occurred when South America rotated up into Mexico and cutoff the Atlantic from the Pacific, with a nearly continuous stretch of land from the North Pole down to the South Pole. This resulted in ocean currents from circumscribing the globe as they naturally would, and coincided with the start of the Ice Ages, of which we are still stuck in. All the while this is happening average CO2 levels dropped in response to this, with an average lag between CO2 and temperatures of 800 years. Yes, if you look at the NOAA data for the last 400,000 years (the same data that we see everywhere, including during Al Gore’s speech), CO2 levels lag temperatures by 800 years, proving that CO2 is not the main determinant of temperature. That award belongs to water, especially in the form of negative feedback of clouds. Of the current TOTAL greenhouse effect of 33°C, 3 of those degrees are due to CO2 with the rest determined by H2O.

• Nathan Urban says:

You need to remember that, like today, the vast majority of that 12°C difference was over the North and South Poles.

I am not sure what prompted you to think that John “forgot” about polar amplification.

A 1°C increase would not even be noticed in most geological places today. The variation from day to day and year to year is greater than this.

Here you appear to be repeating your earlier misleading implication that a global mean climate change of a few degrees is of no consequence. The fact that the diurnal and seasonal cycles are larger than this change is a non sequitur. It has nothing to do with the impact of a few degrees warming on shifting ecological climate zones, thermosteric sea level, glacier melting, alterations in atmospheric and ocean circulation and precipitation patterns, changes in the statistics of extreme weather events, etc.

CO2 levels lag temperatures by 800 years, proving that CO2 is not the main determinant of temperature. That award belongs to water, especially in the form of negative feedback of clouds.

This is a collection of potentially misleading statements.

CO2 doesn’t “drive” the glacial-interglacial cycle. The cycles are paced by Milankovitch forcing, and most of the temperature response is from ice albedo changes. However, this does not imply that the contribution of CO2 to temperature is insignificant in the glacial-interglacial cycle. It certainly does not imply that the climate sensitivity to CO2 is less than currently believed. Indeed, a climate insensitive to CO2 fails to explain the climate dynamics of the glacial-interglacial cycle.

You then switch to discussing water vapor, which when juxtaposed with your previous statement, appears to imply that water vapor is the main determinant of temperature changes in the glacial-interglacial cycle. This is not true. It is neither the cause of the cycles nor the largest feedback.

You then correctly note that water vapor has a large greenhouse effect. This produces a large positive feedback to CO2 or other forcings. Again, that does not imply CO2 is unimportant to temperature.

Part of this confusion arises from a failure to distinguish contributions to the base climate from contributions to changes in climate. While much of the baseline greenhouse effect is due to water vapor, this does not mean that most of a given climate change (in the Cenozoic, the Pleistocene glacial-interglacial cycle, or today) is attributable to water vapor.

For example, for a doubling of atmospheric CO2, the combined water vapor+lapse rate feedback roughly doubles the direct greenhouse effect of CO2. It isn’t the vast majority of the warming, as a consideration of the total greenhouse effect may mistakenly imply.

Likewise, you fail to distinguish between forcings and feedbacks. Water vapor doesn’t tend to cause the global temperature to change itself, but it can modify changes due to other causes. In that sense, water vapor isn’t the determinant of temperature changes, which is really what we’ve been talking about.

Oddly, you then try to minimize the importance of water — after claiming it’s the most important determinant of temperature — by switching to the (unsupported by you) possibility of net negative cloud feedbacks. A strongly negative cloud feedback would negate the supposed influence of water on temperature changes, insofar as it would tend to prevent the climate from changing due to any cause (CO2 or not). This is rather difficult to reconcile with the actual magnitude of the climate changes observed in the Earth’s history.

• John Baez says:

ar18 wrote:

People not familiar with elementary statistics naively believe that when someone says, “The average temperature of the Earth has increase by 1°C in the last 100 years”, it does not mean that everywhere has increased by 1°C.

I believe you meant to say:

People not familiar with elementary statistics naively believe that when someone says, “The average temperature of the Earth has increase by 1°C in the last 100 years”, it means that everywhere has increased by 1°C.

Of course, as you point out, it does not mean this. I’ve changed my blog entry to make it clearer that when I say temperatures were 12°C higher in the mid-Eocene, I mean on average. I hadn’t forgotten this, but it’s a good point to emphasize.

Antarctica also was not located over the South Pole.

I did a hasty check, and the Paleomaps website seems to show Antarctica as located over the South Pole in the middle Eocene. Do you have a reference that says otherwise? I’m curious about this.

I’ve deleted this remark of mine:

There were crocodiles and palm trees in Antarctica.

because I can’t find a reference for it. I’ve replaced it with this:

The polar regions were much warmer than today, perhaps as mild as the modern-day Pacific Northwest. In fact, giant turtles and alligators thrived north of the Arctic circle!

Thanks for your help in improving this blog entry! I think Nathan has responded adequately to some of your other points.

• Nathan Urban says:

By the way, the polar amplification issue brings up an interesting puzzle in paleoclimate, which is the meridional or pole-to-equator temperature gradient problem.

If you go back to some greenhouse periods such as the early Paleogene, the poles were much warmer than today, but the tropics weren’t that much warmer. The difference between polar and tropical surface temperatures, or meridional temperature gradient, was smaller back then &mash; maybe a 15 °C difference instead of the modern ~30 °C difference.

The problem is that we are unable to explain the magnitude of this reduced temperature gradient using our climate models, even though those models do predict polar amplification of warming. This implies that we are missing some positive feedback which is active near the poles, or possibly some negative feedback which is active near the tropics. Maybe clouds, or alterations in meridional ocean heat transport.

This is related to hypothesis about a “tropical thermostat”. This is contentious and some have argued that the gradient wasn’t really as small as some people think. One classic paper is Norris et al., one can work forward from there.

Appy Sluijs discussed this problem with respect to the PETM, as has Huber (and a blog entry); also this paper.

I even found a paper discussing its impact on isotope proxies during the Azolla event.

Abbott and Tziperman have discussed polar cloud feedbacks here and here. Kump and Pollard have also discussed cloud feedbacks, more to get the overall magnitude of the climate right than to explain the temperature gradient, IIRC.

This isn’t really my area and so I don’t have a coherent overview or collection of references. These are just papers I’ve happened across.

8. Luís says:

(“Optimum?” Yes: as if the arguments over global warming weren’t confusing enough already, paleontologists use the term “optimum” for any peak of high temperatures. I think that’s a bit silly.

The term “optimum” comes from Climatology and calling it silly is a silliness in itself. Basic knowledge of climate dynamics tells you why.

Regards.

• John Baez says:

Okay, then please give us some basic knowledge! What are you getting at?

9. Phil Henshaw says:

I think there’s almost always a bit of trouble when scientists adopt perfectly good terms from natural language and then think they can just assign abstract meanings pertaining to something else entirely at will, hoping it won’t confuse everyone…

• John Baez says:

Yup. In pure mathematics we do that all the time — but luckily few people pay attention to pure mathematics, so the damage is minimal!

10. Phil Henshaw says:

Well, but… the joke is on us scientists most to the time…

It also means that the math doesn’t refer to the things of the natural world that the terms of natural language refer to, the things of nature our words point to but we can’t explain… I have a current paper showing that the world standard method of measuring the energy use and carbon footprint of businesses is off by nominally 500% because of that, because engineers prefer crisp definitions of things rather than inclusive ones. !-o