A review by Gert Korthof.   1 December 2007

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

The Emerald Planet David Beerling 2007

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:

1. a worldwide depletion of the ozone layer (at an altitude of 25 km) 251 million years ago at the end of the Permian
2. resulting in a lethal burst of ultraviolet-B radiation
3. which caused high mutation rates in plants across the world, destruction of forests
4. which caused the greatest mass extinction of all time 251 million years ago

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:

1. volcanic eruptions produce carbon dioxide (but these cannot be the only source of carbon dioxide)
2. 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
3. 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)
4. positive feedback: more climate warming, warmer oceans, lower oxygen in oceans, killing land and ocean life; collapse of ecosystems
5. spectacular rise and diversification of dinosaurs after extinction of primitive reptilian competitors

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.

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, 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.