am 30. Juli 2007
For many years, as fossil plants emerged from the rocks, it was believed that these records reflected changes in climate. Plants, it was assumed, had to adapt to variations in weather and other conditions. According to Beerling, plant life was instead the major prompter of climate change. The balance of atmospheric gases was determined by the micro-organisms floating in the seas. The ability to absorb carbon dioxide, coupled with the use of sunlight to convert that into nutrients gives plants the power to shift gas quantities. During the early days, plants exhaled oxygen. It was poison to most organisms, but those capable of using it began the drive leading to today's life. In this useful survey of all the forces forming today's world, Beerling traces how plants "changed Earth's history". Following his thesis requires the reader's close attention, since the organisation of the material is necessarily loose - not fixed chronology nor subject. The many topics to cover cannot be neatly niched.
To the author, the biggest mystery lies in the long delay between plants colonising the land and the formation of the first leaves. Leaf structure reflects how the plant is using energy. That, in turn, becomes a signal of how the atmosphere is composed at any given time. This knowledge was assembled over many years through the work of many researchers. Beerling traces the building of data resources and how the information was interpreted. Images of leaves and stems, analysis of the rock chemistry, field observations and laboratory experiments all contributed to the picture of plant evolution. Numerous surprises emerged, sometimes leading scholars to doubt the data and even their methodology. Looking at the life of plants down the ages is, as he puts it, looking "Through a glass darkly". Pervading his presentation is what the implications are for what is occurring in today's atmosphere - on which our life and those of our children, depends.
Beerling deems investigations into ancient atmospheres a form of "breathalyser", such as the police apply to suspected impaired drivers. In this case, however, it's not alcohol fumes that are measured, but carbon dioxide. Other gases are also sought, but they don't often leave sufficient clues. The information must be derived indirectly. Again, it's the plant's leaves that are used as the pointers to how ancient atmospheres fluctuated. Underlying the variations is the mighty force of plate tectonics. The shifting of land masses and changes in surface configuration leads plants to shift their survival strategies. Acting far more rapidly than creeping continents, the ability of plants to accelerate or impair rock weathering shifts the presence of gas quantities. Carbon dioxide quantities have varied markedly, leading to most of the world's history being warm times. Only recently - in geologic terms - has the planet experienced a cool era, which led to the "ice age" that scoured the Northern Hemisphere with massive glaciers.
As with so much in science, the revelation that plants drive climate instead of passively responding to it has produced at least as many questions as answers. There are anomalous circumstances that must be unravelled. The knowledge gained has led to the formation of "Earth system analysis" techniques using various forms of computer modelling. Many details, however, remain to be worked out. Atmostpheric studies are particularly impaired by lack of knowledge of cloud formation and distribution. Carbon itself, both as a greenhouse gas and as a component of plant growth, remains enigmatic. Beerling traces the selectivity of plants in choosing which carbon isotope will be utilised. That choice has impact on which plants will become dominant in a given area, which also has implications for the animal life living from them. There are no simple nor ready answers to what plants have meant in tracing life's development. Yet, as he emphasises frequently, these are questions that must be addressed further, and that, soon. Understanding our atmosphere is essential to our future. [stephen a. haines - Ottawa, Canada]