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The Emerald Planet
How plants changed Earth's History

A review by Gert Korthof.   1 December 2007

book The Emerald Planet. How plants changed Earth's History.
by David Beerling (2007), Oxford University Press, hardback 304 pages (info).


The Emerald Planet
The Emerald Planet
David Beerling
2007
   
Climate is hot. In 2007 Al Gore and the IPCC were rewarded with a Nobel Prize (1). The prize was rewarded "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change". In the same year two books appeared about the interaction of plants and climate (2). David Beerling has his own contribution to the understanding of our climate: The Emerald Planet. His book shows that ancient climates can be reconstructed from fossil plants in combination with plant physiology and so gives insight in the past history of the earth's climate. The subtitle suggests that the book is exclusively about how plants shape the climate of the earth. Indeed this is the most spectacular theme of the book. And from those issues an important conclusion emerges: our current climate models are still incomplete. However, the book is also about how the climate shapes the morphology and physiology of plants, that is: how plants adapt to climatic conditions. By virtue of those adaptations botanists can infer past climates. The story of palaeobotany fills a gap in the evolution literature. This is a welcome botanical supplement to an evolutionary biology dominated by zoology.

The puzzle Beerling discusses in Chapter 2, Leaves, genes and greenhouse gases, is why it took 40 million years for plants to evolve leaves –such seemingly simple innovation– and spread throughout the plant kingdom. This puzzle was new for me. I like these detective stories. From 400 to 360 million year ago plants colonized the land. From genetic research it appears that developing leaves should not be too difficult. So, this should not be a obstacle. His hypothesis is that plummeting carbon dioxide levels enabled leaf development. Fascinating are the effects on global climate, and geology and the evolution of terrestrial animals.

In Chapter 3, Oxygen and the lost world of giants, Beerling discusses the evidence for the remarkable hypothesis that 30%-35% oxygen levels of 300 million years ago enabled the evolution of giant dragonflies. A unique period in the history of the earth, because 50 million years later oxygen levels dropped to only 15%. Oxygen never reached that maximum again. Beerling explains on what evidence the historic oxygen levels are based.

Chapter 4, An ancient ozone catastrophe?, discusses a hypothesis I never heard of before. The elements of this hypothesis are:
  • a worldwide depletion of the ozone layer (at an altitude of 25 km) 251 million years ago at the end of the Permian
  • resulting in a lethal burst of ultraviolet-B radiation
  • which caused high mutation rates in plants across the world, destruction of forests
  • which caused the greatest mass extinction of all time 251 million years ago
Beerling discusses several geological factors that simultaneously caused the destruction of the ozone layer and the evidence for the 4 elements of the hypothesis. The most impressive thing I learned is the vulnerability of the ozone layer and consequently the natural vulnerability of life on earth. Without the ozone layer terrestrial plants and animals could not exist. Above that, the origin of the ozone layer is ultimately biological because it is continuously produced by the action of sunlight on oxygen, and oxygen itself is produced by plants.

Chapter 5, Global warming ushers in the dinosaur era, is about the explanation of mass extinctions 200 million years ago at the end of Triassic, begin Jurassic period. The summary is this: the supergreenhouse effect caused extinction. The Triassic/Jurassic mass extinction involved a fifth of all marine animal families and a quarter of all land animal families. (it was the third worst mass extinction of the big five). Key events are:
  • volcanic eruptions produce carbon dioxide (but these cannot be the only source of carbon dioxide)
  • carbon dioxide levels quickly increased threefold resulting in global warming, super-greenhouse conditions: 40-45 °C.
    * The evidence for high carbon dioxide levels: fewer pores in leaves of Greenland trees.
    * The evidence for the high temperatures: shape of leaves of Greenland plant fossils changed from large to small
  • greenhouse conditions caused warmer oceans which in turn released suddenly massive methane gas which is converted by oxidation to additional carbon dioxide
    * methanogens (Archaea) in the seafloor produce methane gas, which is trapped in methane hydrate (solid, frozen)
  • positive feedback: more climate warming, warmer oceans, lower oxygen in oceans, killing land and ocean life; collapse of ecosystems
  • spectacular rise and diversification of dinosaurs after extinction of primitive reptilian competitors
This is a speculative scenario but explains all of the available evidence. Beerling mentions criticism of the scenario and an alternative scenario such as the theory that a large asteroid or meteorite hit the earth with enough force to annihilate life at the end of the Triassic.
Beerling warns us that due to the current climate warming, there is increasing concern about the stability of the frozen gas hydrates in the ocean floor. They could deliver huge amounts of carbon dioxide to the atmosphere and then increase climate warming, increase ocean temperature, increase unfreezing of the frozen gas hydrates (positive feedback loop).

Chapter 6, The flourishing forests of Antartica. The polar forests had grown inside the polar circles. It is not true that they did grow in temperate climates and thereafter transported by plate tectonics. This tells us that conditions 50 million years ago (Eocene) were once considerably warmer than they are in the polar regions today. There was little difference between equatorial and artic temperatures. The evidence: the breadfruit tree in Greenland suggests to a subtropical climate within polar circle in Cretaceous! Additionally, fish-eating crocodile-like reptiles are found in the Canadian Archipelago. All this suggests warm summers with 25-30 ° and mild winters 0-5 °. Indeed, annual growth rings of fossil woods show productivity nowadays typical for temperate rainforests.

However, there is one unsolved puzzle: Why should the northern polar forests be seemingly composed of deciduous trees? ('deciduous' = trees that lose their leaves seasonally). A polemic lasting a century. According to Chaney (1946) trees without leaves might be expected to survive a dark but mild winter. The deciduous habit of trees is an adaptation for survival in warm dark polar winters. They do not need to keep useless leaves alive during dark winters (no photosynthesis possible) and are in a state of dormancy. Evergreens cannot live in those polar winters because too high respiration rates and are consuming their food reserves during dark winter by respiration. This hypothesis is more intuition than a falsifiable fact-based theory and has been attacked on several grounds (experimental data), as Beerling explains. The deciduous habit has not only benefits (lower wintertime respiration) but also costs (throw away valuable leaves) which should be compared quantitatively.

Unexpectedly, this knowledge can help us predict what will happen in our near future when climate starts to warm.

Chapter 7, Paradise lost. Remarkably, already in 1777 a fossil collector suggested that the fossil organisms he found in England are typical for tropical or subtropical regions. In 1840 a palaeobotanist noted the numerous remains of plants whose climatic preferences were overwhelmingly tropical or subtropical. Southern England in the Eocene (55 - 34 million years ago) was surrounded by warm tropical seas teeming with sharks, rays, and at the shore alligators, crocodiles, turtles: a paradise lost. These findings have been replicated in Canadian Artic, Tasmania (40 - 45 ° S). The puzzle of this chapter is: what caused this exceptionally warm episode 50 million years ago? How to explain the failure to reproduce the tropical climate when four times the present-day carbon dioxide content is fed into the climate computer models? In order to melt the snow at the poles one needs 8x the current level of carbon dioxide. An important conclusion follows: our current climate models are still incomplete (assuming Eocene climate data are correct) because methane, nitrous oxide, ozone, water vapour are missing from the models. The major greenhouse gases are water vapour and carbon dioxide. By themselves the minor agents of warming have small effects, but collectively they are a more potent climatic force. Furthermore, there are several positive feedback loops involved, a warmer or cooler climate raises or lowers their concentration in the atmosphere. The climate models evolve into 'Earth system models': the Earth is viewed as an integrated system: oceans + atmosphere + cryosphere + biosphere. This is nothing less then a Copernican revolution.
The next puzzle: how did the earth escape from a runaway greenhouse and turned into today's climate with frozen polar regions? It seems that declining carbon dioxide levels is the major culprit of cooling over the last 50 million years.
In trying to explain past climates, we discover much about factors that control climate.

Chapter 8, Nature's green revolution. There are two fundamentally different photosynthetic methods: C3 and C4 method. The C4 method is the most efficient method and is used by crops and weeds. Photosynthesis evolved 3 billion years ago when the atmosphere contained 100 times more carbon then now and the enzyme at the heart of photosynthesis, Rubisco (4), had a structural flaw which was solved by C4 photosynthesis about 30 million years ago (25 - 50 mya). The decrease of atmospheric carbon dioxide posed a serious problem for Rubisco. The C4 invention boosted plant life greatly, but was restricted to subtropical regions. The C4 method is an adaptation to dioxide starvation.
The 'carbon dioxide starvation hypothesis': atmospheric CO2 levels caused 6-8 million years ago a switch from C3 plants (forests) to C4 plants (savannas and grasslands) (2). However, the low carbon dioxide levels started 11 million years before the C4 grasslands expanded, and this is not easily explained away! An additional hypothesis proposes that a self-reinforcing fire cycle accelerated deforestation and promoted (C4) grassland expansion and that smoke from fires suppresses cloud formation and rainfall. Wildfire can influence climate. So there are interactions between fire, trees, grasslands, clouds, smoke, climate and carbon dioxide. Grasslands are flammable. The relevance for feeding the human world population: rice is a C3 plant which is inferior to C4, so needs improvement by genetic engineering.

Chapter 9, Through a glass darkly. The synthesis Beerling is trying to establish is the integration of knowledge of the physiological and ecological behaviour of living plants and ecosystems; palaebotany and evo-devo (genetic pathways). Plants are a significant geological force in the history of our earth, a critical feedback linking biology, chemistry, physics. First: one needs experimental knowledge of plant physiology to interpret plant fossils correctly. Secondly, the spread of forests across the landscape transformed the nutrient, water, energy circulation systems that maintain Earth's climate. Plants accelerate chemical weathering of rocks (I wished this was more clearly explained). Without plants the carbon dioxide content of the atmosphere would be 15x the present-day concentration. On short-term timescales plants can change the colour and hence reflectivity of the land surface, add the greenhouse gas water vapour to the atmosphere. Grasslands steered the rise of grazing animals (horses, rhinos, antelope + their predators). Surprising connection between the rise of grasses and diatoms in the ocean. Climate warming is counteracted by global dimming. All these interrelations makes this book a pleasure to read!

      The task to create Earth system models is hugely ambitious. In designing such systems, Beerling neglected -this may sound weird- the theory of evolution. What is the position of neo-Darwinism in Earth System Science? The book's subtitle claims that plants changed the history of the earth, but this is not discussed in the context of neo-Darwinism. I missed a clear discussion of the importance to distinguish between (1) plant physiology (very short-term changes), (2) short-term evolutionary phenomena (Darwinian adaptation of plants to climate) and (3) long-term phenomena such as the effects of large-scale plant activities on the climate. The third effect is absent from neo-Darwinism. To me Beerling's claim -plants changed the history of the earth- looks like a case of niche construction. Niche construction is absent from neo-Darwinism too. A good example of an extension of the standard evolutionary theory is: 'Niche Construction - The neglected process in evolution' by F. John Odling-Smee et al (2003). It claims that evolution depends not on one, but on two selective processes: natural selection and niche construction.

Apart from these remarks, The Emerald Planet is a highly successful book loaded with new insights and facts.

The Note system: Praise and a suggestion to the publisher.

An attractive feature of Beerling's book is that every chapter begins with a short summary of the hypothesis he is going to argue in the chapter. That is really helpful, because the reader is able to focus on the arguments and the evidence and is less easily distracted by asides. Furthermore, the idea to organize a chapter around a hypothesis prevents endless meandering in all directions, I guess. A hypothesis or a puzzle is a good organizing principle as well for the author as for the reader. Finally, and not unimportantly, the reader can skip a chapter when he is not interested in the specific hypothesis. This is respectful to a reader with limited time budgets and quickly wants to decide what to read and what not.

The Notes system: notes are placed in a large endnotes section (in total 654 notes, on average 72 notes per chapter, in 59 pages of small print, that is 21% of the book excluding the index). The notes are numbered discontinuously throughout the book. In principle this should cause no problems. Unfortunately, the notes system seems designed to hamper retrieval of the notes. Only the start of a new chapter section in the Note section reads 'Chapter 1', etc, (without the chapter titles) and the chapter number is absent from the next 6.5 pages (on average). Instead, the page header of the entire Notes section reads "Notes","Notes","Notes"... repeated 59 times. As if that were not enough to hamper retrieval, the publishers omitted chapter numbers from the page headers of the main text of the book (only the title of the chapter is printed). So it is frustratingly cumbersom to locate a note. A chapter number does not help to find a chapter title!
Simple solutions would be:
  • footnotes at the bottom of each page, or:
  • a continuous numbering 1 – 654 of all notes in the book, or:
  • a chapter prefix before each note in the Endnotes section: 1.1, 1.2, 1.3 ... 9.1, 9.2, 9.3 signifying: 'first note of chapter 1' ... 'first note of chapter 9'.
Each of these 3 solutions would solve the problem. The first solution is old-fashioned. The second solution is unusual, but why? Publishers do number pages continuously, so why not number notes continuously? I have never seen the third solution, but what could be more easy to do than to add a prefix? It is beyond my understanding why the publisher is unable to produce a decent notes system. It would be a user-friendly feature of any book. This is even the more surprising for a book which has such thoughtfully designed chapter openings.


       Notes  

  1. On 12 October 2007, Al Gore and the U.N.'s Intergovernmental Panel on Climate Change, IPCC, won The Nobel Peace Prize 2007.
  2. Oliver Morton (2007) Eating the Sun: How Plants Power the Planet (Fourth Estate, HarperCollins): hardback 412 pages of text (excluding index, bibliography, further reading). Compare with Beerling: 216 pages of text. In Amazon this book is titled: Eating the Sun: How Light Powers the Planet.
    - "those photosynthetic mechanisms are the basis of our lives twice over: the ultimate source of all our food and the ultimate source of all our breaths." (nice!)
    Although there is a large overlap with The Emerald Planet, there are topics such as Lovelock and Gaia which are extensively discussed by Morton but ignored by Beerling. Because Morton's book is bigger, he is able to discuss extras such as the human impact on the climate, global energy consumption and production, origins of photosynthesis. It is a pity that the Introduction is off-putting (poetic language, personal anecdotes, irrelevant stories) because the book as a whole is useful. The writing style strongly contrasts with Beerling's to-the-point writing style. Nice detail: Morton's book is printed on recycled paper (quite appropriate!). This is what Morton says about Lovelock: "The idea that life is content passively adapting to environments over which it has no sway – which really was the dominant paradigm just forty years ago- has gone for good, and Lovelock played a defining role in its demise." (p.256).
  3. Beerling does not mention that at that time humans started to diverge from the chimp lineage. Maybe this is not a coïncident?
  4. "Rubisco, the key enzyme for CO2 fixation into sugar, is inefficient because it cannot easily discriminate between oxygen and CO2 and so wastes energy by fixing O2. The enzyme evolved at a time when O2 levels in the atmosphere were much lower than they are today, and there was therefore little evolutionary pressure to select for an ability to discriminate between the two molecules. Photosynthetic organisms have evolved to circumvent the problem of rising atmospheric O2 levels in two ways: first, by making more of a slower-acting version of Rubisco with an improved ability to discriminate; or second, by using various 'add-ons', called CO2-concentrating mechanisms. .. CO2-concentrating mechanisms, CCMs, have evolved independently in cyanobacteria, microalgae and some plants (mostly those regarded by us as weeds). ... Cyanobacteria have retained an ancient form of Rubisco that is almost three times as efficient as that found in most crops." G. Dean Price, Susan M. Howitt (2014) Plant science: Towards turbocharged photosynthesis, Nature, 513, 497–498 (25 September 2014)

       Further Reading  

  • Oliver Morton: see Note 2.
  • David J. Beerling (2001) Vegetation and the Terrestrial Carbon Cycle: The First 400 Million Years
  • Richard A. Kerr (2007) 'GLOBAL WARMING: How Urgent Is Climate Change?', Science. Discussion of what to do, how to act, uncertainties. Having issued their fair and balanced consensus document, many climate scientists now cite oft-overlooked reasons for immediate and forceful action to curb global warming.
  • World Climate Research Program WCRP. The Amsterdam Declaration (2001) reads: "The Earth System behaves as a single, self-regulating system comprised of physical, chemical, biological and human components."
  • Gabrielle Walker & David King (2008) The Hot Topic. Reviewed in Nature: "In truth, I have never enjoyed reading a book on climate change more."
  • Patricia G. Gensel (Editor), Dianne Edwards (Editor) (2001) Plants Invade the Land (Paperback)
  • Karl J. Niklas (2001) (1997) The Evolutionary Biology of Plants (Paperback)
  • Thomas N. (2009) Paleobotany The Biology and Evolution of Fossil Plants. 2nd ed. (textbook on the evolution of plants). (review). ("the most accurate, useful, and well-illustrated comprehensive account of fossil plants now in print.")

       Reviews  

  • Review of The Emerald Planet: Jennifer C. McElwain (2008) in the American Scientist: "For the most part, however, The Emerald Planet is beautifully written, fresh and provocative. Beerling is a good teacher". Free access.
  • William A. DiMichel (2008) 'Plants as a force of nature', Trends in Ecology & Evolution, Volume 23, Issue 2, February 2008, Pages 60-61
  • Review of Oliver Morton (2007) Eating the Sun by Richard Fortey in Nature 449, 284-285 (20 September 2007).
  • Review of James Lovelock (2000) The Ages of Gaia. A Biography of our Living Earth

 
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Copyright ©G. Korthof 2007 First published: 1 Dec 2007 Updated: 2 Dec 07 F.R./N: 25 Sep 2014