WHAT WAS THE EARTH'S CLIMATE LIKE WHEN THE GLOBAL MEAN TEMPERATURE WAS LAST AT 22° CELSIUS?

15 July 2017

I've been doing a little bit of research, based on the observation that humans are ice age creatures, even though ice ages have made up only 5% or so of our current era (roughly the past half billion years or so). So today's topic is, "What was the earth like during the much warmer climatic periods during which humans and our precursors hadn't yet evolved?" An implication of this discussion is that, due to human-initiated massive carbon release, we might be headed back to such conditions sooner rather than later (that is, in a few hundred years, vs. several million years).

The underlying question we're asking is, "Did the earth have to cool before humans could emerge?" Our working hypothesis is that humans are specialists in handling ecological and climatic diversity, and that the "hot" earth that is more typical of the last several hundred million years lacked the diversity that may have been needed for humans and our precursors to evolve. It is notable that even our evolutionary forebears don't show up in the fossil record until the earth transitioned into its most recent ice age (we're technically still in it) about 6 million years ago. Homo Sapiens has about a 200,000 year history, and our genus (homo) has been around for only about 2-1/2 million years.

Well, let's take as an example the late Cretaceous period, roughly 65 to 100 million years ago, and just preceding the extinction of the dinosaurs: In general, the climate of the Cretaceous Period was much warmer than at present, perhaps the warmest on a worldwide basis than at any other time during the past 542 million years (the Phanerozoic Eon). No ice existed at the poles. The oceans were stagnant and similar to hot springs in temperature. Dinosaurs migrated between the Warm/Hot Temperate and Cooler (extreme north and south) Temperate Zones as the seasons changed. High temperature conditions were almost constant until the end of the period. The warming may have been due to intense volcanic activity which produced large quantities of carbon dioxide.

Floral evidence suggests that tropical to subtropical conditions existed as far as 45° N, and temperate conditions extended to the poles.

Large magma deposits were sufficient to raise sea levels to extremely high elevations, creating vast, shallow seas across the continents. The Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole.

An equable temperature gradient from the equator to the poles (one-half that of the present) meant much less climatic variability than today, and weaker global winds, which drive the ocean currents, resulted in less upwelling and more stagnant oceans than today. This is evidenced by widespread black shale deposition and frequent anoxic events. Sediment cores show that tropical sea surface temperatures may have briefly been as warm as 42° C (108° F), 17° C (31° F) warmer than at present, and that they averaged around 37° C (99° F). Meanwhile, deep ocean temperatures were as much as 15 to 20° C (27 to 36° F) warmer than today's.

As to geography, the continents had differentiated from Pangaea, but were bunched together more closely than today. A vast watery channel divided North America north to south, with only the Rocky Mountains above the sea in the west. Despite sea levels more than 200 feet higher than today, Antarctica and Australia were still one continent. India was an island located east of Madagascar. There was much more sea surface, and much less land surface.

So, there are two questions to wrap up: (1) Is there any particular reason that our human precursors waited until the climate described above had cooled by about 10° C before showing up? (2) Are humans and other species going to adapt well to a planet that is 7-8° C warmer than today?

The scientist I have so far identified who seems most interested in this question is Dr. Rick Potts at the Smithsonian Institution. The following is an abstract for one of his journal articles.

THE RICK POTTS HYPOTHESIS

Variability selection (abbreviated as VS) is a process considered to link adaptive change to large degrees of environment variability. Its application to hominid evolution is based, in part, on the pronounced rise in environmental remodeling that took place over the past several million years. The VS hypothesis differs from prior views of hominid evolution, which stress the consistent selective effects associated with specific habitats or directional trends (e.g., woodland, savanna expansion, cooling). According to the VS hypothesis, wide fluctuations over time created a growing disparity in adaptive conditions. Inconsistency in selection eventually caused habitat-specific adaptations to be replaced by structures and behaviors responsive to complex environmental change. Key hominid adaptations, in fact, emerged during times of heightened variability. Early bipedality, encephalized brains, and complex human sociality appear to signify a sequence of VS adaptations—i.e., a ratcheting up of versatility and responsiveness to novel environments experienced over the past 6 million years. The adaptive results of VS cannot be extrapolated from selection within a single environmental shift or relatively stable habitat. If some complex traits indeed require disparities in adaptive setting (and relative fitness) in order to evolve, the VS idea counters the prevailing view that adaptive change necessitates long-term, directional consistency in selection. © 1998 Wiley-Liss, Inc.
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