How Organic Life Makes Earth Livable

SCIENCE COMMENTARY

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SCIENTISTS now have looked at all the other planets and have found none that support life. It turns out that even Earth would be a tough place to live if it weren't for the fact that organic life itself makes our planet livable. This is yet another facet of the carbon dioxide (CO2) greenhouse warming story. It emphasizes the key role played by that process in Earth's habitability.

David W. Schwartzman of Howard University and Tyler Volk of New York University have taken a look at that warming effect, assuming our planet had no life. They find Earth probably would be intolerably hot. Chemical weathering of silicate rocks draws CO2 out of the atmosphere and locks the carbon in soils and sediments or in the sea. That extraction would proceed much more slowly without what scientists call ``biotic amplification of weathering.''

Reporting their study in the magazine Nature, Drs. Schwartzman and Volk conclude: ``If today's weathering is 10, 100, or 1,000 times the abiotic weathering rate, then an abiotic Earth would be, respectively, [about] 15, 30, or 45 degrees C warmer than today. The upper two temperatures are preferred estimates because of the probable almost complete absence of soil under abiotic conditions, suggesting that, without a biota that significantly enhances weathering rates, the Earth today would be uninhabitable for nearly all but the most primitive microbes.''

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Schwartzman and Volk think those hardy microbes made the rest of earthly life possible in the first place. They point out that, when life appeared on the early Earth 3.6 to 3.8 billion years ago, the air may have had so much CO2 that its sea level pressure was as much as 15 times higher than it now is. The planet's average surface temperature also was near 100 degrees C, water's present sea-level boiling point. Water would have remained liquid under the higher air pressure.

In other words, the two scientists suggest Earthly life arose in a pressure cooker with the evolution of heat-loving bacteria. These early organisms acted to speed up weathering, forming soils and lowering atmospheric CO2. This lessened the greenhouse effect, allowing temperatures to fall and higher life forms to evolve. Thus, the researchers say, ``Life may have been crucial in cooling early Earth and maintaining relatively cool conditions.'' They add, ``The Earth's earliest biota in this scenario did not optimize conditions for its existence, but was a crucial factor in creating conditions for low-temperature life to emerge and thrive.''

This work has more than academic interest. If scientists are to gain a better understanding of how much CO2 released by burning coal, destroying forests, and other human action may increase the present greenhouse warming, they need better knowledge of natural processes that take CO2 out of the air. And weathering is one of the main CO2 extractors.

Schwartzman and Volk base their conclusions partly on research with the weathering of Hawaiian basalt lavas and on laboratory experiments in chemical weathering. These demonstrate how strongly living organisms speed up that weathering either directly or indirectly through soil formation and other processes. This is one step in answering a major question in climatology: To what extent is life involved in stabilizing our planet's surface temperature? This research suggests that involvement is crucial.

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