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 6^oC (11^oF) 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.