Monday, November 15, 2010

No Tropical Drought in the Paleocene-Eocene Thermal Maximum

There is a paper that just came out (subscription required) in the current issue of Science that is encouraging with respect to the possibility of widespread drought under global warming.  The paper concerns the Paleocene-Eocene Thermal Maximum.  This is an episode about 55 million years ago when, for reasons that aren't altogether clear, there was a large release of carbon into the atmosphere.  The leading theory is that it might have been caused by volcanic intrusion into carbon rich sediments in the North Atlantic.  At any rate, over the course of about 20,000 years, global temperatures rose about 6oC (11oF) from a base already warmer than today.  There was a global extinction event, with large amounts of ocean flora disappearing.  There was also a large number of new species created, including many new types of mammals.

The episode is of obvious interest as a prototype for what is presently happening with human-caused CO2 emissions, though it clearly isn't a perfect analogy.  It happened a long time ago in a world that was quite different in important respects, and the rate of emissions was significantly slower than modern anthropogenic emissions.

In any case, the new paper concerns what happened in the South American tropical forests during the PETM, and comes from analyzing pollen from sediments at three sites in Columbia and Venezuela.  The good news is twofold:

1) There was no large scale die-off of tropical vegetation.  It had earlier been speculated that the PETM might have been too hot for the survival of tropical plants, but at least in Columbia/Venezuela, this doesn't appear to have been the case. The tropical forest persisted, and in fact became more diverse:
We assessed changes in floral composition by using a detrended correspondence analysis (DCA) and agglomerative clustering (9). The DCA shows a gradual change in flora during the PETM in the three studied sites (Figs. 2B and 3B) that is confirmed by the clustering analysis (9) (fig. S13), indicating that there was a significant change in the overall plant assemblage across the PETM. Even though we still do not know the affinities of ~85% of the flora, we were able to compare the remaining 15% (9). Diversity remained unchanged from the Paleocene into the Early Eocene for many families, including Polypodiaceae (ferns), Podocarpaceae (gymnosperms), Onagraceae, Ctenolophonaceae, Annonaceae, Moraceae, Rhizophoraceae, and Ulmaceae. However, diversity decreased in Proteaceae and increased in Arecaceae (palms), Bombacoideae, Fabaceae, Araceae, Poaceae, and Convolvulaceae. New families also appeared (i) during the uppermost Paleocene, including Myrtaceae, Sapotaceae, and Passifloraceae; (ii) within the PETM, including Sterculioideae, Euphorbiaceae, and Pellicieraceae; and (iii) during the Early Eocene, including Olacaceae and Ericaceae. Most of these originations, such as Sapotaceae, Passifloraceae, Ericaceae, Sterculioideae, Euphorbiaceae, and Pellicieraceae, represent the oldest pollen records for each family in the neotropics (12). Relative abundances show a similar pattern (Fig. 2).

The rate of extinction increased slightly during the PETM at a rate comparable to that during intervals within the Early Eocene (Fig. 4), with the gradual extinction of a small proportion (~5%) of Paleocene flora. However, extinction does not appear different from the background extinction rates of the entire sequence (Fig. 4). In contrast, per capita rate of origination increased significantly within the PETM interval (Fig. 4). Although the rates of origination continued to be high into the Early Eocene (10), the increase in rate began within the PETM.

Overall diversity and composition analysis suggest that the onset of the PETM is concomitant with an increase in diversity produced by the addition of many taxa (with some representing new families) to the stock of preexisting Paleocene taxa. This change in diversity was permanent and not transient, as documented for temperate North America (6). Interestingly, phylogenetic molecular studies of extant ephypitic ferns (which are mostly restricted to tropical rainforest canopies) and orchids indicate a major radiation at the onset of the Eocene (13, 14).
2) There is no evidence of increased drought stress from the composition of plants seen in the sediments analyzed:
Temperature and precipitation are important factors affecting plant communities. Estimates of PETM mean annual temperature (MAT) from tropical latitudes are scarce. By using both published literature and TEX86 values from a nearby marine core [P2 core (Fig. 1 and fig. S14) (9)], we estimate that tropical temperatures during the PETM increased by ~3°C in the northern Neotropics and that mean temperatures were between 31° and 34°C (±2°C) during the peak of global warmth [see (9) for a detailed explanation]. To reconstruct regional hydrology, we classified plant families according to the rainfall preferences using Gentry’s neotropical plant data set (15) into dry- versus wet-preferred habitats (9) (table S9). Both Paleocene and Eocene are dominated by families indicating wet habitats, with no significant difference across the Paleocene-Eocene (Paleocene = 64%, Eocene = 61%, P < 0.49, df = 32.5), and low abundance of dry elements (e.g., Poaceae) that represents <2% of the assemblage (Paleocene = 0.7%, Eocene = 2%), all suggesting that no increase in aridity occurred across the PETM. This conclusion is further supported by stable carbon and hydrogen isotope (D/H) compositions of higher-plant–derived n-alkanes at site Mar 2X.
Overall, the paper concludes:
Today, most tropical rainforests are found at MAT below 27.5°C. Many have argued that tropical communities live near their climatic optimum (19) and that higher temperatures could be deleterious to the health of tropical ecosystems (8, 19–23). Indeed, tropical warming during the PETM is surmised to have produced intolerable conditions for tropical ecosystems (8, 21), although 31° to 34°C is still within the maximum tolerance of leaf temperature of some tropical plants (24). We recognize that further studies toward the center of the South American continent need to be performed in order to understand the effects of warming in more continental tropical settings. However, at our sites in northern South America, tropical forests were maintained during the warmth of the PETM (~31° to 34°C). Greenhouse experiments have shown that high levels of CO2 together with high levels of soil moisture improve the performance of plants under high temperatures (25), and it is possible that higher Paleogene CO2 levels (26) contributed to their success. Higher precipitation amounts could have been as important as high CO2. Precipitation reconstruction from a nearby Late Paleocene site, Cerrejon, indicate high precipitation regimes: about 3.2 m of rain per year (11). Our data, including a –35% shift in leaf-wax D values, no increase in plant abundance of dry indicators (e.g., Poaceae), absence of a large plant extinction, high plant diversity, and high abundance of families typical of wet tropical rainforests (such as Annonaceae, Passifloraceae, Sapotaceae, Araceae, and Arecaceae), suggest that precipitation in the northern Neotropics during the PETM was either similar to Paleocene levels (3.2 m/year) or higher. Indeed, it is possible that rainforest families in general, which have been present in the Neotropics since the Paleocene (11), have the genetic variability to cope with high temperatures, CO2, and rainfall (25).
This paper is quite significant in the context of the papers on drought in IPCC climate models that I've been discussing on this blog. The areas covered by this paper are projected to get much more drought prone in the 21st century, so the fact that that didn't occur in the PETM is certainly interesting and important.  Of course, it's only one small area of the globe, so that limits how general a conclusion can be drawn.  Still, I'm encouraged.

Note: This post is part of the Future of Drought Series on Early Warning.


Greg said...

Off topic slightly Stuart, but you got a mention on Climateprogress today:

(blockquote) PAUL DONOHUE says:
November 15, 2010 at 1:09 pm

What about the work of Staniford that questions the paper by Zhao and Running as not being statistically valid? One of my students brought this to may attention and I have to answer him Wednesday.

[JR: Haven't seen it. Was it published? That's what the scientific literature is for. You should email the authors.

And this is why I posted so many studies here. A couple may not pan out.]


(blockquote)rustneversleeps says:
November 15, 2010 at 1:32 pm

Thanks for the summary.

With respect to the Saniford commentary, it is here. As far as I know, he did not follow up with Science formally, but he makes a pretty good case. He also details his correspondence with Running and Zhao. So, on this one, I think you should be cautious about the conclusions.

[JR: Thanks for this. Three points. First, this commentary doesn't actually disprove the results of the original study. Second, it does look to me like the authors should have put in more of a disclaimer about statistical uncertainty. Third, I viewed the original results as credible because they were consistent with the findings of the Global Carbon Project -- see slide 26 here, which is based on this 2009 Nature Geoscience article. I probably should have put that in the original post.]

Stuart Staniford said...

Yeah, thanks Greg - I saw the effect in my blog stats. Joe Romm added an update in the body of his post also. I'll have to take a look at the Nature Geoscience paper he mentions.

Greg said...

And now a more on-topic comment.

Yes, this is encouraging for the long run, but I don't think it tells us anything useful about the immediate future, when the earth-system is far out of equilibrium.

The risks are all about rate of change, and we seem to be on course for a 6 degree Celsius change over 200 years, rather than over 20 000 years. That's more than significantly different, to me; it's like comparing a stroll with young kids to driving down the back straight in a Formula 1 race.

Oh, and the PETM rain-forest didn't have bulldozers and chainsaws eating it away while it adapted to the temperature changes. (I recall reading an assertion that clearing the South Amazon for farms could trigger desertification even in the absence of further climate change. I don't know whether that was ever corroborated or rejected.) We're poking the angry beast with more than one stick.

So yes, it's reassuring that the North Amazon climate ends up similar or wetter. But what path does it take between now and then? Geology can't help us; 200 years of events isn't going to show up in the record from 55 million years ago. We really need better climate models.

Stuart Staniford said...


Yes, I haven't (yet) been able to get straight how much of the drought effect in models might be due to out-of-equilibrium effects, and how much is equilibrium. Note that the first paper in this vein: Wang, Agricultural drought in a future climate: results from 15 global climate models participating in the IPCC 4th assessment was comparing 20th century with post CO2 increase climate stabilization, so presumably should not have been subject to the out-of-equilibrium effects, but still claimed a lot of drought (albeit with a lot of uncertainty):

Since soil moisture indicates water availability for agriculture and the increase of soil moisture in northern middle to high latitudes occurs during the non-growing season, the seasonal pattern of predicted soil moisture changes suggests that there will be a worldwide agri- cultural drought. The results therefore have significant implications for evaluating the impact of climate change on water resources management and planning, agricul- ture, economy development, and terrestrial ecosystem. However, at this stage, quantitative evaluation of soil moisture changes therefore evaluation of climate change impact is difficult due to the lack of model consistency in many regions in predicting the direction of hydrological changes and due to the wide range of magnitudes of soil moisture changes. Significant effort on inter-comparison of land surface parameterizations (e.g., Chen et al. 1997) is needed to fully understand the differences in soil moisture changes predicted by different models. At a minimum, it should be treated as a high priority for IPCC’s next assessment to minimize obvious differences in land model structures (e.g., number of soil layers, depth of rooting zone, etc.) to reduce the model dependence related to land surface schemes.

OTOH, it does seem qualitatively reasonable that a situation in which the ocean lags the land significantly would lead to drier land. So you'd expect the out-of-equilibrium effects to tend in the direction of dryness. But I haven't done anything to quantify that at this point (or found any published discussion of the issue).

solon said...


I've enjoyed you blog for about a year now, have a physical sciences background academically, research oil/gas for a living, and think about many of the same issues you discuss regularly. One area where I'm still completely in the dark is climate change. Much of what I read seems more emotional/religious in nature than scientific, but when I see someone like you (who requires rigor) with a strong view point on the topic, I feel the need to press on with my own learning. What were the papers/books that most convinced you man-made climate change is a real issue? And that really helped you understand the science. Thanks, I bet you've already posted this somewhere in the past, so apologies for any redundancy.