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SV: Mystery of Mass Extinctions Is No Longer Murky



2) Is extremely unlikely, given that we are here. It has been a lot
warmer in the past (as a matter of fact as recently as 125,000 years
ago), but no runaway occurred.

On the other hand there is some evidence that runaway cooling *can*
occur, though it hasn't happened since the Proterozoic when insolation
was about 6% lower than now, so it may not be possible any longer.

Tommy Tyrberg


-----Ursprungligt meddelande-----
Från: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] För don
ohmes
Skickat: den 2 juli 2008 17:17
Till: dinosaur@usc.edu
Ämne: Re: Mystery of Mass Extinctions Is No Longer Murky


If: 1) the recent rise in global temps is caused by increased partial
pressures of greenhouse gases.
    2) our current climate system is such that a "run-away" greenhouse
can occur.
    3) current increased levels of greenhouse gases are driven by human
activity (very little reasonable doubt about this last).

Then: we may set the bar of mass extinction to a level impossible to
exceed.

Don

--- On Tue, 7/1/08, Michael Habib <mhabib5@jhmi.edu> wrote:

> From: Michael Habib <mhabib5@jhmi.edu>
> Subject: Re: Mystery of Mass Extinctions Is No Longer Murky
> To: "'DML Dinosaur Mailing List'" <dinosaur@usc.edu>
> Date: Tuesday, July 1, 2008, 10:53 PM
> > If we want to rank ourselves amongst mass extinction
> causes, then 
> > we'll really have to lift our game. Extinction
> events caused by direct 
> > or indirect human activity are nothing compared to
> those in the 
> > geologic past. The Permian/Triassic extinctions in
> particular really 
> > raised the bar.
> 
> Our "ranking" as a mass extinction cause depends
> a great deal on how 
> you deal with the differing temporal scales.  The total
> diversity loss 
> due to humans, to date, is modest compared to any of the
> mass 
> extinctions of the past (especially the Permian/Triassic,
> as 
> mentioned).  However, the *rate* of extinction may be
> comparable.  
> Thus, if we assume that the current rate holds for tens of
> thousands of 
> years, the total percentage loss could be comparable to at
> least some 
> of the mass extinctions.
> 
> It depends a great deal on how fast the mass extinctions of
> the 
> geologic past really were, which is hard to get a precise
> measure on.  
> Some were clearly very fast, geologically speaking, but to
> make 
> comparisons to the modern rate, we need even better
> precision than is 
> currently available.  The comparison is also made difficult
> by the 
> issues of taxon sampling.  And, of course, there is the
> issue of 
> extrapolation: do we make the comparison assuming that all
> living 
> species that are threatened bite the dust, or some
> fraction?  Can we 
> take the rate over the last 100 years and extrapolate to
> get the 
> expected numbers in 1 million years, or should we expect
> the rate to 
> taper after those species most susceptible to human
> mediated effects 
> perish?
> 
> The upshot is that one can get numbers that are comparable
> between 
> "expected" human-mediated extinctions, and the
> estimated mass 
> extinctions of the past, but only with a given set of
> assumptions.  
> Other sets (which are not objectively better or worse) give
> very 
> different numbers.
> 
> Cheers,
> 
> --Mike
> 
> 
> Michael Habib, M.S.
> PhD. Candidate
> Center for Functional Anatomy and Evolution
> Johns Hopkins School of Medicine
> 1830 E. Monument Street
> Baltimore, MD 21205
> (443) 280 0181
> habib@jhmi.edu