Demystifying the Cretaceous hothouse
Fifty-five million years ago, the Earth was ice-less. Winters were balmy. Palm trees flourished all the way to the poles. As evidenced by fossils, crocodiles and broad-leaved, water-loving plants existed north of the Arctic Circle. This world had warmer oceans, featuring deep ocean temperatures 12 degrees Celsius higher than now. For any climate scientist who enjoys stretching the limits of current theory by imagining ancient worlds, the ever-warm polar regions of the mid-Cretaceous have long presented a paradox.
Unlike the present-day climate, only a small difference (gradient) in temperature existed between the equator and the poles of the ice-less Earth. Theoretically, that weak temperature gradient results from the increasing efficiency of the atmosphere at transporting heat from the equator up to the poles in a warmer climate. However — and here's where climate scientists get annoyed — the atmospheric turbulence required for that increased heat transport itself demands a high, not low, temperature gradient. "That's the confusing part. To explain the weak gradient you need a strong gradient," says David Ferreira, research scientist in the Department of Earth, Atmospheric, and Planetary Sciences at MIT, whose potential solution to this paradox appears this week in Journal of Climate.
Ferreira and colleague Brian Rose PhD '10, a visiting scholar in the Department of Atmospheric Sciences at the University of Washington, decided to find their way out of the paradox. "One way around it," Ferreira says, "could be if the ocean warms the poles efficiently without transporting heat directly to them." So, they created a model ice-free aquaplanet to look for a mechanism by which the ocean would transport heat from the tropics into the poles indirectly.
They found that the mechanism involves water vapor. In their model ocean, the ocean transports heat poleward, but not all the way. Instead, the ocean releases the heat into the atmosphere at mid-latitudes. There, the warming sea surface leads to more evaporation and thus more water vapor.
As Rose and Ferreira increased the amount of water vapor in the lower atmosphere, they observed increased injection of warm moist air into the upper troposphere. The water vapor, which is a potent greenhouse gas, caused an enhanced greenhouse effect of warming that extended all the way to the poles. Essentially, the water vapor acted like an atmospheric bridge, picking up the process of heat transport where the ocean's job ends.
The word to describe this research is "creative," according to Dorian Abbot, assistant professor of geophysical sciences at the University of Chicago, who also models exotic climates: "This paper is an excellent example of the harvest one can reap by approaching climate research from a ‘scientia gratia scientae’ perspective, allowing oneself to play with abstractions and follow them where they may lead rather than being tied too closely to explaining the details of specific phenomena."
As Rose and Ferreira's work explored a warm ocean on a planet comfortably hosting palm trees at high latitudes, their model can't offer insight into the present-day or future ocean. But, as Abbot implies, this work isn't only about explaining the causes of real-world phenomena. It's aimed at deepening a general understanding of fundamental ocean-atmospheric mechanisms that will be used over and over again in many areas of future academic study. Certainly, few climate scientists will look at past warm climates with the same frustration again.