Sunday, October 15, 2006

Long-period astronomical forcing of mammal turnover

Climate forcingThe following Nature article, which has strong implications for legal responses to climate change and biodiversity loss, is attracting an unusually high amount of attention to The Scientific Lawyer. That's all the more reason to highlight it here at Jurisdynamics and BioLaw:

Jan A. van Dam et al., Long-period astronomical forcing of mammal turnover, 443 Nature 687-91 (Oct. 12, 2006) | doi:10.1038/nature05163; Received 16 January 2006; Accepted 11 August 2006:
Mammals are among the fastest-radiating groups, being characterized by a mean species lifespan of the order of 2.5 million years (Myr). The basis for this characteristic timescale of origination, extinction and turnover is not well understood. Various studies have invoked climate change to explain mammalian species turnover, but other studies have either challenged or only partly confirmed the climate–turnover hypothesis. Here we use an exceptionally long (24.5–2.5 Myr ago), dense, and well-dated terrestrial record of rodent lineages from central Spain, and show the existence of turnover cycles with periods of 2.4–2.5 and 1.0 Myr. We link these cycles to low-frequency modulations of Milankovitch oscillations, and show that pulses of turnover occur at minima of the 2.37-Myr eccentricity cycle and nodes of the 1.2-Myr obliquity cycle. Because obliquity nodes and eccentricity minima are associated with ice sheet expansion and cooling and affect regional precipitation, we infer that long-period astronomical climate forcing is a major determinant of species turnover in small mammals and probably other groups as well.
Milankovitch cyclesMilankovitch cycles or oscillations -- also known as Milanković cycles -- hold the key to this study. Astronomers such as Milutin Milanković have computed regular variations in the eccentricity, axial tilt, axial orientation, and orbital orientation of Earth's path around the sun. The predictability of these variations, see Ferenc Varadi, Bruce Runnegar & Michael Gil, Successive Refinements in Long-Term Integrations of Planetary Orbits, 592 Astrophysical J. 620–30 (2003), DOI: 10.1086/375560, underlies attempts to relate these variations to climate change.

In other words, as the earth's tilt and orbit change, so does its climate. As usual, NOAA has collected much of the data needed for this sort of sophisticated modeling.

Two caveats are in order. First, at least in the short run, anthropogenic effects are likely to outweigh astronomical forcing. Second, our inability to understand the mechanism by which astronomical forcing affects climate undermines our confidence in models purporting to relate climate change to Milankovitch cycles.


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