Petrified leaves paint a scary picture of how quickly and severely our climate can change, says David Beerling
The past seizes upon us with its shadowy hand and holds us to listen to its tale," wrote Albert Seward, the celebrated Cambridge University palaeobotanist, at the end of his study of forests in the ancient Arctic. Little could he have imagined quite how tight the "shadowy hand" would seize us nearly a century after he concluded his study. As the threat of global warming takes centre stage in the political arena and looms large in the public consciousness, the grip of the past is being given new purchase thanks to a quiet revolution in the study of fossil plants, palaeobotany. A modern synthesis is emerging from the seamless integration of new knowledge of the living and the dead. It is forcing us to recognise that fossil plants are exquisite tachometers of past global change and that plants themselves are a geological force of nature. The renaissance in palaeobotany will inform debate on climate change.
The discovery 20 years ago that the density of microscopic pores (stomata) on the surface of trees' leaves is regulated by the amount of carbon dioxide in the atmosphere marked a critical moment in the revolution. As carbon dioxide levels have climbed since the pre-industrial era due to our combustion of fossil fuels and deforestation in the tropics, so trees have responded by producing leaves with fewer stomatal pores. This adjustment allows plants to absorb sufficient amounts of the gas for photosynthesis while conserving water by limiting its loss in transpiration. Palaeobotanists were quick to appreciate that this discovery handed them a biosensor - in the form of fossil leaves - for "breathalysing" the carbon dioxide content of the ancient atmosphere. It offered us a way to assess the link between the amount of carbon dioxide in the atmosphere and climatic change over millions of years: in other words, it allowed us to evaluate the greenhouse effect from a geological perspective.
Dramatic findings followed. Two hundred million-year-old fossil leaves excavated from rocks exposed along the eastern coastline of Greenland that span the Triassic-Jurassic boundary told a story of soaring atmospheric carbon dioxide levels just as global biodiversity crashed. Even though the interval was associated with massive volcanic activity, releasing carbon dioxide into the atmosphere as continents rifted apart to give birth to the Atlantic Ocean, this alone fails to account for soaring carbon dioxide levels. Other geochemical signatures in the leaves, and in marine sediments from localities around the world, suggest that the greenhouse gas methane must also have been involved.
Large amounts of methane are locked beneath the sea floor in a frozen crystalline form called gas hydrate, mainly around continental margins. It seems gradual warming towards the end of the Triassic period, perhaps caused by slowly rising carbon dioxide levels from the volcanic activity, spread into the deep ocean and melted the frozen hydrates locked into ocean sediments. On melting, the hydrates released massive amounts of methane into the ocean atmosphere system that oxidised to carbon dioxide and accelerated warming.
Studies of ocean cores offer even stronger evidence that a similar scenario happened again, 55 million years ago. This time, global temperatures increased by 5C in a few tens of thousands of years as an estimated 4.5 trillion tonnes of methane were released from the sea floor, a mass that is possibly equivalent to that of our entire fossil-fuel reservoir.
These "deep-time" global warming events raise fundamental questions. How much methane is stored in the form of gas hydrates on the sea floor of our present-day oceans? How stable is this gas capacitor in the face of rising ocean temperatures? What are the climatic consequences of a catastrophic release? The shadowy hand of the past points to some climatic surprises that may lie ahead as human-made greenhouse gases accumulate in our atmosphere.
So investigations on living plants allow us to extract greater environmental information from their fossil counterparts, nature's global change tachometers. By decoding their language, they are rewarding us with deeper insights into the Earth's history. But this is only one aspect of the revolution. Palaeobotany's resurgence is also being driven by the increasing realisation that vegetation is an active component of the climate system.
Seward gained fleeting fame outside regular scientific circles after examining and reporting on the precious haul of fossil plants recovered by Captain Scott's heroic polar party on its gruelling return from the South Pole. Scott's fossils from the Beardmore Glacier proved that forests had once grown on Antarctica within striking distance of the pole when the continent enjoyed a climate far warmer than it does today. The last polar forests became extinct in the Northern hemisphere about 40 million years ago as the Earth's climate switched from a "greenhouse mode" to its present "icehouse mode", but there are signs that polar forests are undergoing a resurrection, one that will have climatic consequences. The trees and shrubs of northern Alaska, previously held in check by the cold climate, are dancing to the tune of a warming climate and are migrating northwards.
Climatic change is likely to follow because, as vegetation in the high latitudes fills out, the stature of the plants masks the seasonal snow cover, replacing its white reflective surface with a darker one that absorbs the solar energy that warms the atmosphere. More warming could lead vegetation to expand northwards, thus generating a positive feedback loop that could drive further climate change.
Uncovering the ways in which vegetation has exerted its climatic influence in the past is a major challenge, but recent findings foreshadow our climatic future. And, with what has been dubbed the second Copernican revolution - our capacity to model the Earth as a system of its interacting components: the oceans, terrestrial and marine biospheres and the atmosphere - we can now predict future climate changes based on these findings.
For example, recent earth-system simulations of a warm climate interval known as the Eocene, about 50 million years ago, revealed that the extensive primordial wetlands pumped out sufficient methane to create a methane-rich "greenhouse". Climate warmth was further enhanced by the cocktail of gases emitted by forests and soils swathing the ancient continents that produced high levels of tropospheric ozone and nitrous oxide (potent greenhouse gases). As the atmospheric carbon dioxide concentration rises and our climate warms in coming decades, terrestrial ecosystems are likely to respond in a similar manner. By producing more of these gases or their precursors, vegetation looks set to exert another positive feedback on climate.
A meeting at the Royal Society in London this week called attention to this issue and reminded us that the global warming debate is not solely about carbon dioxide: other greenhouse gases present in trace amounts in the atmosphere can collectively act as a powerful lever on the climate system.
James Hansen, head of Nasa's Goddard Institute for Space Studies in New York, points out that today many of the so-called trace gases (or their precursors) are the by-products of industrial processes that are perhaps more readily controlled than carbon dioxide emissions. Hansen argues persuasively that if we want to buy ourselves time in the race to "defuse the global-warming time bomb", limiting the rising concentrations of these gases is a good place to start.
Seward published a seminal essay in 1892, Fossils as Tests of Climate , that marked a turning point in the development of palaeobotany. It opened people's eyes to the climatic possibilities fossil plants had to offer. The current revolution may yet be seen as another pivotal moment, this time marking the point when the dusty fossils in the basements of museums gained greater relevance than ever before to our contemporary world.
David Beerling is professor of animal and plant sciences at Sheffield University. His latest book, The Emerald Planet: How Plants Changed Earth's History , will be published by Oxford University Press in February 2007.
- Correction dated 22/29 December 2006
In the article "Fossils forecast a global scorcher" ( Times Higher , November 17), David Beerling’s sign-off should have been professor of palaeoclimatology in the animal and plant sciences department at Sheffield University. And "4.5 trillion tonnes of methane were released" should have read "4.5 billion tonnes of methane were released".