The Fate of Tropical Rainforests in a Super CO₂-Enriched and Warmer World

The Paleocene-Eocene Thermal Maximum (PETM) of some 56 million years ago, according to Jaramillo et al. (2010), "was one of the most abrupt global warming events of the past 65 million years (Kennett and Stott, 1991; Zachos et al., 2003; Westerhold et al., 2009)." It was driven, as they describe it, by "a massive release of ¹³C-depleted carbon (Pagani et al., 2006; Zeebe et al., 2009)" that led to "an approximate 5°C increase in mean global temperature in about 10,000 to 20,000 years (Zachos et al., 2003)." And during this period of warming, they say it was thought by many that earth's tropical ecosystems "suffered extensively because mean temperatures are surmised to have exceeded the ecosystems' heat tolerance (Huber, 2008)."

But was that really so? Did the ancient warming of the world truly constitute a major problem for the planet's rainforests?

In an attempt to answer this important question, the 29 researchers, hailing from eight different countries, analyzed pollen and spore contents and the stable carbon isotopic composition of organic materials obtained from three tropical terrestrial PETM sites in eastern Colombia and western Venezuela; and this work revealed -- contrary to the prevailing wisdom of the recent past -- that the onset of the PETM was "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." And they determined that this increase in biodiversity "was permanent and not transient."

In discussing their findings, Jaramillo et al. write that "today, most tropical rainforests are found at mean annual temperatures below 27.5°C," and they say that several scientists have argued that "higher temperatures could be deleterious to the health of tropical ecosystems," citing Stoskopf (1981), Bassow et al. (1994), Lewis et al. (2004), Huber (2008, 2009) and Tewksbury et al. (2008) in this regard. In fact, they report that tropical warming during the PETM is actually believed to have produced intolerable conditions for tropical ecosystems, citing the writings of Huber (2008, 2009). Nevertheless, they reiterate that at the sites that they studied, "tropical forests were maintained during the warmth of the PETM (~31° to 34°C)," and they thus conclude that "it is possible that higher Paleocene CO₂ levels (Royer, 2010) contributed to their success."

In regard to this hypothesis, it should be noted that such would indeed appear to be the case, in light of what is now the well-established fact that most plants, including trees, tend to exhibit their greatest photosynthetic rates at ever warmer temperatures as the air's CO₂ content continues to rise (see Interactive Effects of CO₂ and Temperature on Plant Growth in our Topical Archive).

In light of Jaramillo et al.'s impressive findings, therefore, it is becoming ever more clear that greater warmth and atmospheric CO₂ concentrations are not the "twin evils" that the world's climate alarmists typically make them out to be. Quite to the contrary, they are just what the good earth's ecosystems need, in order to make them both more stable and more productive, which characteristics are absolutely essential for sustaining the still-expanding human population of the planet, as well as preserving what yet remains of what we could call wild nature.

Additional References
Bassow, S.L., McConnaughay, K.D. and Bazzaz, F.A. 1994. The Response of temperate tree seedlings grown in elevated CO₂ to extreme temperature events. Ecological Applications 4: 593-603.

Huber, M. 2008. A hotter greenhouse? Science 321: 353-354.

Huber, M. 2009. Snakes tell a torrid tale. Nature 457: 669-670.

Kennett, J.P. and Stott, L.D. 1991. Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature 353: 225-229.

Lewis, S.L., Malhi, Y. and Phillips, O.L. 2004. Fingerprinting the impacts of global change on tropical forests. Philosophical Transactions of the Royal Society B 359: 437-462.

Pagani, M., Caldeira, K, Archer, D. and Zachos, J.C. 2006. An ancient carbon mystery. Science 314: 1556-1557.

Royer, D.L. 2010. Fossil soils constrain ancient climate sensitivity. Proceedings of the National Academy of Sciences, USA 107: 517-518.

Stoskopf, N. 1981. Understanding Crop Production. Prentice-Hall, Upper Saddle River, New Jersey, USA.

Tewksbury, J.J., Huey, R.B. and Deutsch, C.A. 2008. Putting the heat on tropical animals. Science 320: 1296-1297.

Westerhold, T., Rohl, U., McCarren, H.K. and Zachos, J.C. 2009. Latest on the absolute age of the Paleocene-Eocene Thermal Maximum (PETM): New insights from exact stratigraphic position of key ash layers + 19 and - 17. Earth and Planetary Science Letters 287: 412-419.

Zachos, J.C., Wara, M.W., Bohaty, S., Delaney, M.L., Petrizzo, M.R., Brill, A., Bralower, T.J. and Premoli-Silva, I. 2003. A transient rise in tropical sea surface temperature during the Paleocene-Eocene Thermal Maximum. Science 302: 1551-1554.

Zeebe, R.E., Zachos, J.C. and Dickens, G.R. 2009. Carbon dioxide forcing alone insufficient to explain Palaeocene-Eocene Thermal Maximum warming. Nature Geoscience 2: 576-580.