The Emerald Planet:
How Plants Changed Earth's History

David Beerling

Oxford University Press 2007
A book review by Danny Yee © 2007
Emerald Planet explores the role of plants in some of the key events in the history of the Earth, focusing on their role in determining atmospheric composition and climate. Its chapters are organised chronologically, with each focusing on one era and telling a single coherent story, though these are woven out of varied strands.

Beerling assumes no background in plant biochemistry, anatomy, physiology, or taxonomy, or in geochemistry, ecology, or the numerous other disciplines he draws on. And he provides just enough background for the reader, without getting bogged down. If Emerald Planet is aimed at a general audience, however, it has documentation for the scientists: some sixty pages of endnotes, with references into the scientific literature.

Plants began colonizing the land around 465 million years ago and vascular plants appeared around 425 million years ago, heralding a burst of diversification. Leaves were slow to develop even though genetics and developmental biology suggest the basic foundations for them are old. Beerling suggests that plants reduced atmospheric carbon dioxide levels by increasing weathering and the resulting carbon sequestration. Lowered levels of carbon dioxide increased stomatal densities, increasing respiration and cooling, and thus allowing larger leaves that absorb more heat.

There are two independent lines of evidence for an "oxygen spike" in the Carboniferous, rising to around 35% of the atmosphere. One is from analysis of rock abundances — "the burial of organic matter and pyrite in rocks adds oxygen to the atmosphere, while the uplift, exposure, and weathering of those same rocks removes it" — and the other is from isotope analysis. Experiments with modern insects suggest increased oxygen was key to the development of giant insects. And oxygen starvation may have had a role in the extinctions at the end of the Permian.

Our recent problems with a hole in the Antarctic ozone layer may have an ancient precedent. Malformed fossil spores resembling those damaged by high levels of ultraviolet-B radiation suggest widespread depletion of the ozone layer around the end of the Permian, some 250 million years ago. There are several possible explanations for the ozone depletion, but the strongest contender is the volcanism of the Siberian Traps.

Stomatal pore numbers in fossil plants from Greenland suggest an increase in carbon dioxide levels around the Triassic-Jurassic boundary; there is also an abrupt spike in Carbon 12. There is little evidence for an extraterrestrial impact event, and volcanism associated with the Central Atlantic Magmatic Province is also unlikely to have been the sole agent. Volcanism may, however, have unlocked frozen methane hydrates from the sea floor, producing greenhouse effects which raised temperatures and induced further methane releases.

The Eocene was a greenhouse period, when polar temperatures approached modern equatorial levels and there were arctic and antarctic forests. Beerling describes the debate over whether deciduous leaf trait was an adaptation to warmth combined with long periods of darkness. There is a complex ecological balance between deciduous and evergreen trees, involving nutrients, frost, and the carbon cycle.

The warmth of the Eocene can't be explained by carbon dioxide effects alone, and ice cores reveal a key role for greenhouses gases such as methane and nitrous oxide, produced by bacteria. The limits of our climate models here suggest that current greenhouse models are missing key elements, and that future warming could be considerably worse than they suggest.

The development of isotope analysis provided the tools for understanding photosynthesis, but it was some time before the discovery of an alternative C4 photosynthesis, where carbon dioxide is concentrated to improve the performance of the key Rubisco enzyme. The dramatic rise of C4 grasses can be dated to around 8 million years ago using herbivore fossil teeth; Beerling argues that this was not driven by changes in the carbon dioxide concentration but by the spread of new fire regimes, in positive feedback with climate change.

The final chapter touches on some additional topics in plant physiology: the relationship between leaf shape and temperature, and the weakening of frost tolerance with increasing carbon dioxide levels. Beerling reiterates the need for a multidisciplinary approach to paleobotany, with fundamental contributions from developmental evolutionary biology and the study of modern plants and ecosystems.

This bland summary does little justice to either the depth or breadth of Emerald Planet. Like most works of popular science, it covers many people and episodes from the history of science, sometimes digressing quite widely. There's a two page description of the Cambridge Mathematical Tripos, for example, with the tenuous link that the father of the 4th Lord Rayleigh, who studied ozone, was a Senior Wrangler. And there's an account of Scott's final polar expedition — amazingly, Scott's party was still carrying 16 kilograms of fossil plants when they perished!

Overall, Emerald Planet has a good balance of material, offering plenty of variety without losing focus. It maintains a high level of excitement but never resorts to dramatisation or exaggeration, and it can be read by scientists without embarrassment.

Emerald Planet covers material that hasn't had much popular science coverage: Beerling points out in an endnote that Richard Dawkin's The Ancestor's Tale devotes just 11 pages out of 528 to plants. If paleobotany has been relatively neglected, however, concerns about climate change make it increasingly topical, and Emerald Planet should command a broad audience.

October 2007

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%T The Emerald Planet
%S How Plants Changed Earth's History
%A Beerling, David
%I Oxford University Press
%D 2007
%O hardcover, notes, index
%G ISBN-13 9780192806024
%P 288pp