Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Saturday, March 5, 2011

The Evolution of Planet Earth

About 425 million years ago something quite extraordinary happened to this small rocky planet. A new type of living structure began to cover the surface of its dry landmasses. For a distant observer then  across the plains of the supercontinent Gondwana a peculiar green pigmentation would have appeared.

This was the dawn of vascular plants. A critical physiological characteristic of these lifeforms are the stomata - the pores on leaves that take in carbon dioxide, push out oxygen, and enable transpiration, the release of water vapor into the atmosphere. Transpiration is key to the uptake of water from the ground and as part of the cooling mechanism for plants.

How many stomata a plant needs is a function of species and growing environment. Some remarkable new research now indicates that we may be witnessing a fundamental shift in this parameter as global carbon-dioxide levels rise, as they are incontrovertibly doing. A pair of papers by Lammertsma and de Boer and colleagues in the Proceedings of the National Academies detail an extensive study of a diversity of plants in Florida. By tracking both changes in plant stomata over the past few decades as well as comparing modern plants to preserved samples from over a century ago, they find a clear trend. Today's plant life in Florida, and potentially elsewhere, have about 34% fewer stomata than they did more than a century in the past.

Cause and effect is tricky to understand. However, these data appear consistent with the rise in global CO2 over a hundred years. Higher CO2 concentration and plants need to breathe less, so they cut back the number of stomata. But this also impacts the rate at which they transpire, profoundly effecting the uptake of surface water and its release into the atmosphere. This process plays a central role in the Earth's hydrological cycle - the conveyor belt of water evaporation, precipitation and movement that links rivers, lakes, and oceans to the atmosphere. So what might happen if plants send less water vapor skywards? Although at first this would increase the reservoir of surface water it would also reduce atmospheric water content. Less water in the atmosphere and less precipitation. Dry regions might get drier. The entire cycle of freshwater in a region would alter. The upshot - adding CO2 to the atmosphere may actually help dry it out.

Plants are both adapting to changing environmental conditions, and effectively adapting the environmental conditions to their modified physiology. The Earth is evolving. Little wonder that debate is continuing on whether or not we are now within an episode of mass extinction - the end of the Holocene. A recent study of species diversity seems to suggest that we may not be quite there yet, at least in comparison to ancient events, but things are not particularly rosy either.

All of this brings me back to a theme that has come up before in these posts. Questions of planetary habitability and biological stability are extremely slippery. While some broad stroke assessments of whether a planet might be in a 'habitable zone' around its parent star can serve a purpose, I'm becoming less and less convinced that this is a very productive avenue of investigation. Witness the hoopla over recent planet detections. Yes, we absolutely want to learn more about how climate operates in different planetary configurations, and how geophysics and chemistry may effect the near surface environment of a world, but the equation that takes us to 'habitability' is a very murky one.

Perhaps ironically the very kinds of changes that are being wrought on the Earth by humans might also offer some vital clues about what we should be sniffing for on other worlds.  The sliding of ecosystems towards some new equilibrium, or even extinction, might actually be far more informative than a situation of perfect balance. Obviously this is looking quite a way into the future, but it could be critical to sort out some of these issues now since they will help determine whether we build instruments and telescopes to operate for decades or just years. The kinds of planetary environments we are so eager to find are also those that may require lifetimes to understand. Shifting the paradigm from discovery to study will require careful planning.

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