Quote:
Originally Posted by Peter Ward
Over geological time spans the balance of atmospheric composition has changed. I stress the change has been over *geological* times, so plants and organism adapt, over extended periods.
Sharp changes, as induced by volcanic, impact or greenhouse crises, lead to mass extinction of species (BTW, happening as we speak)
Agreed estimates put human activities pumping 300 billion tonnes of CO2 into the atmosphere during the last 200 or so years and CO2 *has* risen from 260 ppm to close to 380ppm with no seriously accepted "natural" mechanism
for this change.
Not bad for just 200 years.....and given geological time scales are in the *millions* of years one wonders what homo-sapiens can accomplish!
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Peter,
The idea the natural climate change only occurs over timescales of million of years is quite common and supports the suggestion that the present warming is so rapid that it must be anthropogenic. This a misconception. Firstly, most climate records lack the temporal resolution to resolve rapid change, had it occurred. Secondly, there are many climate events recorded in the relatively recent past (several 100ka) whos transition apparently took only decades.
The best reference I've found in a short search is:
Jonathan Adams, Mark Maslin and Ellen Thomas "Sudden climate transitions during the Quaternary" Progress in Physical Geography 23,1 (1999) pp. 1–36
If you can't access that an early version (minus diagrams and tables) is at
http://www.esd.ornl.gov/projects/qen/transit.html
I'll quote part of the abstract (my emphasis)
The time span of the past few million years has been punctuated by many rapid climate transitions, most of them on timescales of centuries to decades. The most detailed information is available for the Younger Dryas-to-Holocene stepwise change around 11 500 years ago, which seems to have occurred over a few decades. The speed of this change is probably representative of similar but less well studied climate transitions during the last few hundred thousand years. These include sudden cold events (Heinrich events/stadials), warm events (interstadials) and the beginning and ending of long warm phases, such as the Eemian interglacial. Detailed analysis of terrestrial and marine records of climate change will, however, be necessary before we can say confidently on what timescale these events occurred; they almost certainly did not take longer than a few centuries.
Various mechanisms, involving changes in ocean circulation and biotic productivity, changes in atmospheric concentrations of greenhouse gases and haze particles, and changes in snow and ice cover, have been invoked to explain sudden regional and global transitions. We do not know whether such changes could occur in the near future as a result of human effects on climate.
Phenomena such as the Younger Dryas and Heinrich events might only occur in a ‘glacial’ world with much larger ice sheets and more extensive sea-ice cover. A major sudden cold event, however, did probably occur under global climate conditions similar to those of the present, during the Eemian interglacial around 122 000 years ago. Less intensive, but significant rapid climate changes also occurred during the present (Holocene) interglacial, with cold and dry phases occurring on a 1500-year cycle, and with climate transitions on a decade-to-century timescale. In the past few centuries, smaller transitions (such as the ending of the Little Ice Age' at about AD 1650) probably occurred over only a few decades at most.
All evidence indicates
that long-term climate change occurs in sudden jumps rather than incremental changes.
Being 10 years old it's summary of climate change is obviously dated but none of the central tenents have changed. However its treatment of possible AGW is disappointing - but that isn't why you would read it. If I find a better ref I'll pass it on.
I'm not at all saying this shows that the present climate changes are purely natural, only that the speed of the change is not proof of AGW.
cheers,
David