"As earth's climate warms, due to the sun's natural increase in luminosity, says Kasting, weathering of continental silicate rocks will speed up; causing CO2 to be removed from the atmosphere and, in turn, put back into the earth in the form of carbonate sediments."
Greg http://www.chicagotribune.com/business/technology/chi-nsc-life-on-earth-to-hit-brick-wall-in-500-million-20131023,0,4003446.story Life On Earth To Hit Brick Wall In 500 Million YearsBruce Dorminey Forbes 2:05 a.m. CDT, October 23, 2013 Complex life here on earth will hit a habitability wall in only 500 million years time; not in an almost languorous 1.75 billion years, as reported in a recent global media flap. The flap - spurred by a paper in the journal Astrobiology - failed to cover earth's future carbon dioxide (CO2) "compensation limit," says James Kasting, a prominent planetary scientist at Penn State University, whose own models were used by the paper's authors. The CO2 compensation point, says Kasting, is the crucial limit at which the net rate of plant respiration exceeds that of oxygenic photosynthesis. Once this limit is crossed, its immediate effect would be to essentially render as much as 95 percent of earth's plant life with an inability to grow. Kasting says the Astrobiology paper's lead author Andrew Rushby, a doctoral candidate in environmental science at the University of East Anglia in the U.K., and colleagues simply didn't account for this expected drawdown of atmospheric CO2 which would be caused by longterm increases in earth's surface temperatures. "That's an integral part of the lifetime of the biosphere calculation," said Kasting. As earth's climate warms, due to the sun's natural increase in luminosity, says Kasting, weathering of continental silicate rocks will speed up; causing CO2 to be removed from the atmosphere and, in turn, put back into the earth in the form of carbonate sediments. This atmospheric Rubicon will be brought on by the sun's natural increase in luminosity as it expands and ages - a continuing process by which our own star's brightness spikes upward by roughly ten percent every one billion years. Thus, as the sun's luminosity grows and earth's CO2 concentrations fall towards 150 parts per million (ppm), says Kasting, most of the world's plants and trees will likely disappear. He says it's possible that some of the biotic slack might be taken up by plants - such as corn, sugar cane and tropical grasses - that are able to function under such low CO2 concentrations. "But it will be a very different planet," said Kasting. Kasting's models point to the remaining plants going extinct 900 million years from now when CO2 levels falls below 10 parts per million (ppm). "Despite all the press coverage," said Rushby, "our main intention was to determine how extrasolar planets stacked up in terms of habitability [timescales] when compared to earth." As Rushby and colleagues point out in their Astrobiology paper, the solar system's circumstellar habitable zone - roughly defined as an orbital "Goldilocks" region at which an earthlike planet can harbor liquid water at its surface - is hardly "static in time or space." The authors correctly note that such habitable zones are "proportional" to increases in luminosity over the lifetime of a given star. But, in fact, Kasting says the inner edge of the habitable zone is "actually not that easy to find," since it depends on clouds and relative humidity, neither of which, he says, can be easily calculated in a one-dimensional climate model. Yet, in any case, he notes this precipitous drop in earth's atmospheric CO2 should occur at about the same projected rate. "So, the lifespan of our biosphere will not change, and this new [Astrobiology] paper is simply misleading on this question," said Kasting. Although the sun won't envelope earth for at least another five billion years, or long after our star turns into an expanding Red Giant, Kasting says the "punchline" is that earth won't remain habitable through to the sun's own end. "Bad things start to happen much earlier than that," said Kasting. Kasting suggests one alternative would be to geo-engineer our way around our sun's luminosity increase by constructing space-based solar shield. Would earthlike planets around the locally ubiquitous Red M dwarf stars have a different habitable zone lifetime? Kasting says "because M stars age so slowly and brighten at a very slow rate," earthlike planets in their midst would likely remain unaffected by any such brightening phenomenon. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/groups/opt_out.
