Huber on "A Hotter Greenhouse?"

Half of Earth's surface area is in the tropics, so changes and uncertainties in tropical temperatures dominate any climate sensitivity estimate. If SSTs were truly ~35°C at times in Tanzania (19°S) or New Jersey (~30°N), some tropical regions must have been much hotter. This has thought-provoking implications for paleoclimate, vegetation, and carbon cycle evolution.

First, tropical temperatures above 31°C offer no evidence for a climate thermostat, that is, a strict mechanism that maintains tropical SSTs in the modern range; climate dynamicists trying for decades to explain thermostats may have been chasing a chimera. Second, climate models might be able to reproduce warm poles and warm extratropical continental winters, given that these new tropical SSTs imply closer to modern temperature gradients (5).

Third, during the warmest parts of the past 65 million years--that is, the Paleocene-Eocene Thermal Maximum (PETM) and subsequent brief, sudden "hyperthermal" phases of the Early Eocene Climate Optimum (17)--tropical vegetation may have been above the upper limits of its thermal tolerance (18). Most plants, especially the C3 plants that comprised Eocene floras, have physiological mechanisms that break down in the 35° to 40°C range (18, 19); in particular, they can die because photorespiration dominates over photosynthesis (18-20). Annual mean temperatures greater than 35°C can be plausibly reconstructed to have been widespread equatorward of 35° latitude (8, 9, 21, 22), so floras may have been thermally stressed, and perhaps undergoing water stress in the warmest intervals.There is some evidence of tropical floral extinctions during the warmest periods (23, 24), while forests thrived at higher latitudes.

This scenario may be a missing link in the hypothesis (25) that carbon cycle and climate changes during the PETM were caused by oxidation of the terrestrial biosphere. It is well established that a major tropical vegetation die-off in a global warming world has profound temperature, precipitation, and carbon feedbacks (20). Carbon cycle modeling (26) suggests that the terrestrial carbon pool could have been much larger than modern, ~6000 gigatons of carbon. The gradual warming preceding the PETM may have loaded a terrestrial carbon storage gun, and crossing the 35°C threshold may have triggered it. Tropical die-back after an initial warming (22) might have added thousands of gigatons of carbon into the atmosphere and further increased temperatures by radically reducing evapotranspirative fluxes that normally cool tropical landmasses. Tropical heat death helps resolve two mysteries: the magnitude of the carbon and climate excursion at the PETM, and the fact that these abrupt warmings occur during broader intervals of extreme warmth, rather than in cold intervals as expected from methane degassing (27).

The recent results suggest that, rather than being a stable cradle for tropical life, the tropics may have been a crucible; during warming, many taxa may have been forced to flee poleward, innovate, or face extinction (28). These far-ranging implications are a lot to place on the narrow shoulders of the few published proxy records, but they highlight the importance of the next challenge: collecting more tropical multiproxy records and establishing the accuracy of existing ones.

Dr Matt Huber of Purdue is someone I have spoken to on occasion about paleoclimate matters. We started to talk about doing a collaboration, but it fell apart when we were both not able to sync up. Alas. That said, he was looking into, a couple years ago, doing some simulations on the Pliocene. The world at that point had a climate of 2.5 C warmer than now. IDK if Matt ever got around to doing the simulations or not, but it seems someone else now has an interest. Unfortunately, Scotese lacks a Pliocene climate map. Otherwise I'd point you there. Well, the Miocene may not be that far off.

That said, it appears that the tropics are a highly unstable region ecologically. During the Pleistocene, it was shattered into small forests all over the place. During the Eocene, if Matt's hypothesis is right, the tropics became too hot for C3 plants to survive. This idea that the tropics have been radically unstable over time is interesting because it means that it would explain why the tropics always have the highest extinction rates: their environment rather than being the most stable is the most volatile. And due to the fact that most tropical critters are not large and the fossil record biases towards the largest critters anyways, this may explain why there are some questions about the which area has the highest origination rates.

That said, if the tropics of tomorrow are still wet and the temperatures that much higher, could we see the first great tropical forests of grass and succulents? That'd be interesting!

Hey, didn't the succulents originate in the tropics? I seem to recall as a child hearing that and it being something of a surprise. Perhaps this is why. The succulents present there now are survivors from a time when the tropics were more arid.