A Wetter World During The Eocene
It is well known that water, H2O, is the single most important greenhouse gas. But water also plays a central role in determining the delicate balance of energy and mass that regulates the temperature of Earth. A wide range of predictions have been made regarding water in a warming climate, ranging from catastrophic droughts to increased monsoon rains and tropical storms. Conventional wisdom states that a warmer world is a wetter world. In a newly published paper in the journal Science, two researchers examine the Eocene (∼56 to 34 million years ago), looking for clues to the tropical climate–water relationship. Annual global temperatures during the Early Eocene Climatic Optimum (EECO) around 50 million years ago were as much as 12°C higher than modern values. The new results provide compelling evidence that the tropical engine of the water cycle was more active than predicted by current climate models.
Climate science is one of the disciplines that does not have the ability to test its grand theories with large experiments—no test Earths exist to play around with. Instead, scientists look to ages past for clues about how climate functions under different conditions. Given the numerous and often contradictory predictions regarding the impact of global warming, it is prudent to examine the last period of warm climatic conditions, the period called the Eocene Epoch. The Eocene was Earth's most recent global greenhouse interval, a long stable period that predates the global cooling which led up to the Pleistocene Ice Age and our much cooler modern world.
The global physical geography of the Eocene was fairly similar to current day Earth, but the climate was significantly warmer. Polar regions lacked major ice sheets and were home to cold-intolerant plants and animals, while tropical oceans experienced temperatures approaching 40°C. These differences were reflected by atmospheric CO2 concentrations around five times preindustrial levels. Though we know quite a bit about conditions during the Eocene, details regarding the hydrologic cycle in the tropics have been lacking. In “Latitudinal Gradients in Greenhouse Seawater δ18O: Evidence from Eocene Sirenian Tooth Enamel,” Mark T. Clementz and jacob O. Sewall provide an innovative approach to fill this gap by examining atmospheric water cycling on the oceans. As with most such research, it involves the use of isotopes as proxy measurements.
Using a water isotope proxy to reveal greenhouse climate conditions millions of years ago is complicated by the fact that the isotopic composition of ancient surface waters can only be measured indirectly. The most common way to do this is to measure the oxygen isotope composition of fossil shells produced by marine microorganisms. These are widely available and reflect the isotopic composition of the water in which they formed. Unfortunately, their isotopic composition is strongly influenced by the water temperature when they were formed. Separating temperature effects from other influences is difficult at best.
Instead of the traditional method, Clementz and Sewall measured oxygen isotopes found in the carbonate of tooth enamel from fossil sirenians: sea cows, dugongs, and manatees. These mammals presumably maintained their body temperatures at constant values reducing uncertainty caused by fluctuation in ambient temperature.
Study sample locations.
As can be seen from the map above, samples were collected from widely separated geographic locations. Fossil locality information can be downloaded from the Paleobiology Database (www.pbdb.org). The researchers generated coarse records of meridional gradients in surface water oxygen isotope ratios (δ18Osw) for the Paleogene and Neogene by analyzing fossil enamel δ18O values for a group of marine mammals recovered from part of the Atlantic Ocean and former Tethys Sea. Their findings were as follows:
Enamel δ18O values for sirenians from multiple localities were strongly correlated with paleolatitude and did not cluster according to region; this further corroborates our assertion that these data represent broad hemispheric trends in Atlantic/Tethyan surface-water composition and not regional riverine input. Within a given time interval, fossil sirenians sampled from low (<20°N) and mid- (~40° to 50°N) latitudes had significantly lower enamel δ18O values than those for sirenians sampled from localities falling in-between these extremes (20° to 40°N). Enamel δ18O values for Eocene-aged sirenians were also significantly lower (by at least 1.0‰) than were enamel δ18O values for younger sirenians (Oligocene and Neogene) sampled from the same latitude.
From these results, the authors were able to conclude that values from early to middle Eocene sirenian tooth enamel indicate an identifiable latitudinal gradient in δ18Osw. Moreover, this gradient is associated with an enhanced tropical hydrologic cycle that is persistent throughout the Eocene Epoch. “These results support not only the long-held belief that the greenhouse climate of the early Paleogene was characterized by an enhanced, but balanced, subtropical hydrologic cycle and wetter mid-high latitudes than are seen under modern conditions,” the researchers write, “but also suggest that the early Paleogene tropics had substantially decreased evaporation and increased precipitation that both contributed to much lower δ18Osw values than those that exist today.”
The authors further suggest that the enhanced hydrologic cycle persisted in the face of dropping temperatures and CO2 levels and that the threshold value of such conditions is ~1.0% lower than expected in the tropics. The leads to a warning that traditional foraminifera tests systematically overestimate Eocene tropical sea-surface temperatures by up to 4°C. Since such readings are used to help set climate model sensitivity to CO2 this implies that the models need to be changed—again.
The tropics are the driver of the global water cycle. Water cycles through the tropical atmosphere about twice as fast as it does in the mid and high latitudes, largely due to high precipitation and evaporation rates. This takes place within the circulation pattern known as a Hadley cell. Convection within the tropical rising branch of the Hadley cells plays an important role in controlling the atmospheric energy balance by delivering water vapor to the upper troposphere, where it contributes disproportionately to the greenhouse effect. The differences between an icehouse climate and a hothouse configuration is illustrated below.
Predicted changes from a cooler icehouse world include: enhanced vertical and poleward vapor transport in the Hadley cells; poleward expansion of the subtropical arid zones associated with the subsiding branch of the Hadley cells; and increases in tropical precipitation and subtropical evaporation rates. Outside the tropics things are warmer and wetter but now, according to Clementz and Sewall, “the Eocene tropics were not only wetter but may have been cooler than foraminiferal δ18O data have previously indicated.” So things in the tropics may not get quite so toasty.
In an accompanying perspective article, Gabriel J. Bowen of Purdue University states, “The tropics are also a dominant source of moisture to the higher latitudes, and although the strength of this tropical moisture pump is sensitive to a number of competing influences, most models suggest that more water vapor will be exported from the tropics in a warmer world.” Bowen summarizes the state of understanding in light of the new work by Clementz and Sewall:
The new results offer compelling evidence that the tropical engine of the water cycle revved faster during past greenhouses, but the implications for understanding past and future climate states still hinge on a number of unknowns. Both the natural range of sirenians and the availability of fossil collections limit the distribution of samples in this study to coastal regions. As a result, the isotopic data do not directly represent the massive gyres and regions of the intertropical convergence zone where the most intense precipitation-evaporation imbalances occur. Records directly representing these regions would strengthen the case for globally significant changes in tropical water cycling in the Eocene. In addition, the surface-water proxy approach applied here does not clarify the dynamic role of enhanced water cycling in the Eocene climate, and it does not account for important factors such as the relationship between faster cycling and cloudiness, water transport to the extratropics, and precipitation intensity.
Bowen concludes, “these are challenging problems that both the modern and paleoclimate communities will continue to struggle with.” Part of the problem with drawing lessons from the past is that present day Earth is not the same planet. There have been subtle changes to the configuration of continents and oceans that altered global circulation patterns since the Eocene. This makes extrapolating paleoclimate data to fit modern day regions effectively impossible. On top of that, think how tenuous our current understanding of Eocene climate must be when it can be substantially rewritten by a few scattered samples of tooth enamel.
What lessons can be drawn from this refined examination of the warmer Earth of ancient times? First, it is hard to figure out past climatic conditions. Second, proxy data are notoriously unreliable indicators. Third, predicting the future from what we think we know of the past is a very risky business. When it comes to predicting climate, science may have as much success divining heavenly portents or reading tea leaves.
Finally, even though the Eocene was undeniably hotter than today's climate, the tropics were probably not as warm as previously thought. That, plus a stable but enhanced hydrologic cycle, made Earth a more congenial planet to live on.
In fact, about 55 million years ago a brief but intense episode of global warming at the Paleocene/Eocene (P/E) boundary caused a radical reshuffling of Earth's biota. That ancient burst of rapid global warming was fleeting, but its biological effects were permanent. A wave of anatomically modern mammals appeared during the Eocene, replacing archaic animals that became extinct. Without global warming, we would not be here. Tell me again why global warming is so bad?
Be safe, enjoy the interglacial and stay skeptical.