Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures

155

148

Fig. 1. The phylogenetic distribution of ABO phenotypes and genotypes. Shown is a phylogenetic tree of primate species, with a summary of phenotypic/ genotypic information given in the first column, and the genetic basis for the A versus B phenotype provided in the second column (functionally important codons at positions 266 and 268 are in uppercase letters). See Dataset S1 for the source of information about phenotypes/genotypes. Only species with available divergence times are represented here (34 of 40). The phylogenetic tree is drawn to scale, with divergence times (on the x axis) in millions of years taken from ref. 29. OWM, Old World monkeys; NWM, New World monkeys. Under a model of convergent evolution, these data suggest that A is the ancestral allele, and a turnover (e.g., a neutral substitution) occurred on the branch leading to Old World monkeys. If instead, B were ancestral, all Old World monkeys would have had to serendipitously converge from ATG to TTG to encode a leucine, whereas all New World monkeys and hominoids would have had to converge to the CTG codon.

Section: Recovery and Radiationmentioning

confidence: 99%

Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary

Longrich

Bhullar

Gauthier

2012

155

148

“…The last time temperatures approached values considered here is the Paleogene, when global-mean temperature was perhaps 10°C (26) and tropical temperature perhaps 5-6°C warmer than modern (27,28), implying T W of up to 36°C with a most-common T W ðMaxÞ of 32-33°C. This would still leave room for the survival of mammals in most locations, especially if their core body temperatures were near the high end of those of today's mammals (near 39°C).…”

Section: Discussionmentioning

confidence: 99%

An adaptability limit to climate change due to heat stress

Sherwood

Huber

2010

843

647

Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wetbulb temperature T W , is surprisingly similar across diverse climates today. T W never exceeds 31°C. Any exceedence of 35°C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7°C, calling the habitability of some regions into question. With 11-12°C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12°C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.climate impacts | global warming | mammalian physiology | paleoclimate

“…Tropical sea surface temperatures (SSTs), previously thought to be similar to or even cooler than modern SSTs, were 3-10°C warmer than today (8)(9)(10)(11)(12), although uncertainty on these values remains large (13). Extratropical surface temperatures were 10-40°C warmer than modern extratropical surface temperatures in continental interiors (14)(15)(16)(17)(18), along polar margins (19,20), and over the oceans (21).…”

mentioning

confidence: 99%

State-dependent climate sensitivity in past warm climates and its implications for future climate projections

Caballero

Huber

2013

199

195

Projections of future climate depend critically on refined estimates of climate sensitivity. Recent progress in temperature proxies dramatically increases the magnitude of warming reconstructed from early Paleogene greenhouse climates and demands a close examination of the forcing and feedback mechanisms that maintained this warmth and the broad dynamic range that these paleoclimate records attest to. Here, we show that several complementary resolutions to these questions are possible in the context of model simulations using modern and early Paleogene configurations. We find that (i) changes in boundary conditions representative of slow "Earth system" feedbacks play an important role in maintaining elevated early Paleogene temperatures, (ii) radiative forcing by carbon dioxide deviates significantly from pure logarithmic behavior at concentrations relevant for simulation of the early Paleogene, and (iii) fast or "Charney" climate sensitivity in this model increases sharply as the climate warms. Thus, increased forcing and increased slow and fast sensitivity can all play a substantial role in maintaining early Paleogene warmth. This poses an equifinality problem: The same climate can be maintained by a different mix of these ingredients; however, at present, the mix cannot be constrained directly from climate proxy data. The implications of strongly state-dependent fast sensitivity reach far beyond the early Paleogene. The study of past warm climates may not narrow uncertainty in future climate projections in coming centuries because fast climate sensitivity may itself be state-dependent, but proxies and models are both consistent with significant increases in fast sensitivity with increasing temperature.superrotation | hyperthermal T he early Paleogene (∼65-35 Mya) was the most recent time when the atmospheric carbon dioxide (CO 2 ) concentration was in the ≥1,000-ppm range that may reoccur over the next several centuries (1). Study of this period may therefore provide constraints on climate sensitivity, broadly defined as the global mean temperature response to radiative forcing, which are useful for climate change prediction (2, 3). On the other hand, analogies between early Paleogene and future climates are not straightforward because so many other variables apart from CO 2 have also changed (4), and because climate sensitivity may change across different climate states (3,5). Given the crucial role that paleoclimate data from past greenhouse climates play in informing the debates about future climate change (6, 7), an investigation of the strengths and limitations of using such data to make inferences about climate sensitivity is in order.Recent progress in reconstructing the early Paleogene has brought an across-the-board upward revision of surface temperature estimates. Tropical sea surface temperatures (SSTs), previously thought to be similar to or even cooler than modern SSTs, were 3-10°C warmer than today (8-12), although uncertainty on these values remains large (13). Extratropical surface temperature...