Friday, October 29, 2010

Climate Change and Habitability continued

As a branching off point from my previous blog, another interesting comment I came across in the "Hot Zone" blog (about Climate Change) on the astrobiology magazine website was the following: Peter Ward, author of The Medea Hypothesis (2009), has taken the viewpoint (among other scientists) that life is rare in the universe because it is fundamentally harmful in its destabilizing affect on a planet’s climate. This statement is hard to refute in today's world as man's impact of Earth is becoming more and more obvious. I had never heard this theory before, but it is incredibly interesting. What is it about life that is so destabilizing?

This theory seems to go against a prevailing sentiment in society that the workings of the natural world have reached a dynamic equilibrium that is incredibly elegant and durable. Indeed, the notion of biomimetics in which nature is used as a template for man's engineering is based on this sentiment, as are the environmental laws that try to minimize the damaging influence of humans on land and the atmosphere.
This sentiment shows up in Henry David Thoreau's dictum, "In wildness is the preservation of the world." How could we have been so wrong, according to Peter Ward?

Peter Ward, a paleontologist at the University of Washington who specializes in mass extinctions, attacks no prevailing theory as strongly as he does the Gaia Hypothesis: the idea introduced in the 1970s that describes every living thing on Earth as part of one gigantic self-regulating organism. Indeed, in opposition to this hypothesis, Ward named his own hypothesis the Medea Hypothesis (for the Greek sorceress who killed her own children).

First off, what are some of Ward's reasons for this new conception of nature's role on Earth? In his view, the earth's history makes clear that, left to run its course, life is poisonous. He claims that long before humans came onto the scene, primitive life forms were trashing the planet. For example, around 3.7 billion years ago, they created a planet-girdling methane smog that threatened to extinguish every living thing; a little over a billion years later they pumped the atmosphere full of poison gas. (That gas, ironically, was oxygen, which later life forms adapted to use as fuel.)

Ward explains how he sees the story of life on earth as a long series of suicide attempts. Four of the five major mass extinctions since the rise of animals were caused by bacteria, and twice, he argues, the planet was transformed into a nearly total ball of ice thanks to the voracious appetites of plants. In other words, "it's not just human beings, with our chemical spills, nuclear arsenals, and tailpipe emissions, who are a menace. The main threat to life is life itself."

A main point Ward derives from his hypothesis is that the planet would not be better off without humans, rather that life would destroy itself anyways. He believes that environmentalism needs to restructure its underlying philosophy and he believes that humans are needed to save the planet from destruction. This is the start of quite a tangent so I won't go down that path, however, this hypothesis does have significant meaning for astrobiology as well.

This is incredibly important to the search for life on other planets. If life is indeed so toxic, life may be much less likely to exist than we have previously thought. Perhaps, it was not the origin of life that was so lucky on Earth, but rather the perpetuation of that life.

To further relate the Medea Hypothesis to climate change and astrobiology, Ward claims that the evidence of past extinctions - written in fossils and in the chemical makeup of deeply buried rock sediments - as well as the workings of today's oceans, atmosphere, and myriad food chains, shows that Earth tends not toward harmony but towards extremes. Stability appears to occur simply as respites between catastrophic boom-and-bust cycles. He attributes one of the largest extinctions in history to the out-of-control proliferation of plankton feeding on upwellings of nutrients from the ocean floor. Rather than being brought back to equilibrium, the tiny organisms reproduced until they choked off much of the life in the upper ocean. Exhausting their newfound food supply, they died en masse, and decaying by the trillions used up all the oxygen in the water, killing off everything else.

As for the earth's temperature control, more often than not positive feedback loops appear to be common--with warming triggering more warming, and cooling more cooling. In a process we're seeing today, as the planetary temperature rises, warming increases the rate at which soil releases greenhouse gases - not only carbon dioxide, but methane and nitrous oxide. It leads to more forest growth in places that formerly were barren tundra, even as more carbon dioxide in the air makes plants hardier and better able to grow in areas once given over to desert. More plants in more places means a darker earth, and therefore a more heat-absorbent and warmer one. It's an escalating feedback loop that becomes even more powerful as the planet's white, ice-covered poles give way to darker open water.

What does this mean for the search for planets with stable atmopsheres and temperature gradients? Ward's theory does not suggest that we should look for life on planets without stable atmospheres; it does suggest, however, that the origin of life may be relatively common, but that the existence of life may not be. Changes in climate may be driven by life on longer time scales than we are able to observe on other planets. Rather, we still need to look for relative stability to find life, because Ward supposes that life causes instability and thus ultimately condemns itself for extinction. Thus, though climate instability may be an consequence of life, it is not a criteria associated with habitability--it is quite the opposite.

It is also very important to realize that Ward's hypothesis, though incredibly provocative, is not as straight forward as he makes it seem. Though he emphasizes the occurance of positive feedback loops, there are plenty examples of negative feedback loops like those exemplified in the Gaia hypothesis. Indeed, these may still be more common; it may just be that since they are not as prone to catastrophism they are less obvious to observe.

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