The Earth After Us

Title: The Earth After Us – What Legacy Will Humans Leave In The Rocks?

Author: Jan Zalasiewicz

Publisher: Oxford University Press 2008 (First)

ISBN: 978-0-19-921497-6

Pages: 241

Dr. Jan Zalasiewicz is a lecturer in geology at the University of Leicester, and was formerly with the British Geological Survey. A field geologist, palaentologist and stratigrapher, he teaches various aspects of geology and Earth history and researches fossil ecosystems and environments spanning more than half a billion years of geological time. He has published over a hundred papers in scientific journals. This book hinges on a hypothetical scenario which unfolds 100 million years in the future when our Earth is visited upon by a group of intergalactic travellers. These ‘creatures’ may find the earth to be devoid of human life, as humans would have long gone extinct by their own foolish actions of modifying the climate, inadvertently, that is. Perhaps the future visitors would still find intelligent life forms like some form of rodents which had evolved during this time unmolested and ready in a position to assume wider control of the planet they virtually own among themselves. The would be explorers might experience the same difficulties our own palaentologists faced during the last two-three centuries, excavating the hidden treasures from rocks, mud, sediments and other fossil-preserving areas. The author puts forward the question that whether the visitors would be able to deduce the presence of a bi-pedal, super-intelligent life-form which ruled the earth like no species did before or after for a brief span of about 10,000 years, which is nothing but a wink in geological time. He explaints the standard practises in geology, goes on to explain how the information bearing strata are formed underground, plate tectonics, how earth’s crust is undergoing upheavals and downfalls, what the people who aspire to become fossils should do to ensure that their remains turn up under the microscopes of a future society.

Plate tectonics happen only on earth, as compared to other planets in the solar system due to its liquid core consisting of molten iron. This liquidity causes the continents placed on the crust to move in different directions which depend on the subduction movements of undersea plates. The idea was first proposed by Alfred Wegener, but was largely ignored by the scientific community as the claims were outlandish and the proofs, speculative at best. Later geologists’ consistent research proved it to be beyond doubt and many questions which were left unanswered now succumbed to human intuition. The crust, which is the outermost layer of earth goes up and down in tectonic escalators. Sea floor may become mountain tops and cities may go down under sea, for which we have ample evidences. When the sea floor goes up as mountains and is subjected to erosion by wind and water, the fossils contained in it become visible to naked eyes. An area which is going down will be further deposited with sediments and the extra weight further pushes it down. This downgoing areas are the places best suited for preservation as they will form strata which are immune to erosion, but nevertheless subjected to immense pressures which may make them out-of-shape and hard to reconstruct.

The earth has experienced several ice ages and we are now living in an interglacial period (between two ice ages) which is warm, but started only about 10,000 years back. At that time, the earth was a really cold place, with most of North America and Europe under glaciers several kilometers thick and a sea level which was hundreds of meters below than at present. The reason behind the ice ages were not clear to scientists. James Croll and Milutin Milankovitch, who were astronomers predicted that the variations in the amount of sunlight collected by earth due to changes in the tilt of earth’s axis, wobbles of the axis and the changes in the ellipticity of earth’s orbit caused the ice ages and other climate cycles. Stratigraphic evidence collected from deep sea samples around the globe accurately confirmed the predictions made in the 19^th century and now such Milankovitch cycles, consisting of sub-cycles of 100000, 40000 and 20000 years are now an accepted fact. The cycling between global warmth and ice ages are asymmetric – the onset of cold is gradual, while the warming is relatively faster. Geologically, we are on the way to another ice age. “All things being equal, the Earth should now be sliding slowly back towards the grip of ice. We have had almost exactly 10,000 years of warmth, which is already longer than any other warm spell over at least the last 400,000 years. This climatic gift has allowed our civilization to develop to its current extent, where it colonizes virtually every agriculturally productive part of the globe” (p.75). But human induced global warming is upsetting all adjustments on earth’s regulatory mechanism. Carbon dioxide levels in the atmosphere now stands at 380 ppm, which is the highest in a span of one million years! The pre-industrial level was 280 ppm and the levels during the last glacial period was 180 ppm. The carbon dioxide level is increasing by 0.5% or 1.5 ppm every year. The level of methane, which is a much more potent green house gas than CO₂, is twice that of one million year ago. We seem to be sliding into a full blown warmth with increased sea levels and most of the cities getting flooded. However, this flooding may preserve them for examination by future explorers, which is little consolation for them!

The study of fossils in strata are called biostratigraphy and was invented by a civil engineer named William Smith in England. By the turn of the 19^th century he noticed the remains of marine fauna such as ammonites, sea-urchins and others in strata and prepared the first geological map of England. Dating of these fossils took place later, when radioactive decay was discovered. In addition to radiometric dating, a sort of time reckoning based on the astronomical cycles of earth (Milankovitch cycles) are also in vogue which is much more precise than radiometric dating. The would be researchers would find that there was a mass extinction (almost 95% of the species) by the end of paleozoic era and is known as the Permian extinction. At the end of the mesozoic era, there was another extinction, this time triggered by a meteorite impact which destroyed – among others – the different species of dinosaurs which roamed the earth at that time.

The human species, though it controlled the planet like no other life-form previously would lay very few materials to be recovered as fossils in the future. Almost all of our present day articles like plastic, machinery, computers and such things would be quickly recycled to base elements and other compounds by natural activities once the humans disappear from earth. Also, our rich culture has stood on the face of the earth for only 200 years, right after the industrial revolution which boosted the numbers and economic well being of humans in every part of the world. (It cannot be said every part but surely there are some ripple effects). A span of 200 years is virtually indistinguishable in the strata as the thickness of such a duration in it would be nearly 1 mm. But determined explorers would still be able to deduce a lot of things from such scant evidence. The paleobiology can be deduced from the analysis of pollen grains embedded in these layers. Studies in Europe show the growth of tundra grass, then birch forest, then pine, then oaks etc from the study of pollen, mirroring the removal of ice from most of the continent. Such difference in temperatures depend directly on the CO₂ content. Increased carbon dioxide in the atmosphere causes the seas to become acidic, thereby slowing down the growth of coral reefs which lock away excess CO₂ in the form of calcium carbonate, but in the short term, when this is formed from two bicarbonate ions, a carbon dioxide molecule is released into the atmosphere. But this will be small – some tens of millions of tons of carbon every year, as compared to 7 billion tons emitted by humans. The mechanisms for emission and remission of carbon dioxide are so numerous and interlinked that a true understanding of how they will respond is still poorly understood. Increase in CO₂ will produce global warming due to positive feedback like release of absorbed CO₂ in ocean and land and clathrates. On the other hand, there is a mechanism by which the increased weathering of rocks will reduce carbon dioxide levels. The positive feedback happens over the scale of hundreds or thousands of years while the negative feedback (rock weathering) works over hundreds of thousands of years thereby negating the chances of humans to take advantage of it. By the time the earth regulates the amount of carbon dioxide back to manageable levels, the human race would long have gone extinct.

The author gives a few suggestions to tide over the crisis. “A clear prospect of direct action, perhaps, and directed application of substantial resources (a modest proportion of the world’s military budget would go a long way) to smooth out the bumpy ride that looks to be in store for the human race. The adoption of softer energy paths. A re-foresting of the world. Finding contentment without the compulsive overuse of resources. Perhaps most importantly, curbing population growth – though without the natural selection of the strongest and most ruthless. To achieve substainable numbers while still curing the ill and succouring the weak would be our civilization’s greatest triumph” (p.239).

A clear book written by an expert geologist with an empathetic heart for the society’s well being and preservation. The author is obviously at ease with the subject matter and presents the details with the relish of a master craftsman. The arguments are outright logical and the sharp conclusions which follow are natural and convincing. The book is excellently divided to chapters preserving the modularity and comprehensiveness. Rather than attempting to see the earth through the eyes of alien visitors a hundred-million years from now, we should focus on how our own geologists unearthed the secrets of ages long past from the meagre samples they had at their disposal. This book helps us do so.

There are very few improvements which could be suggested, but the total absence of illustrations or diagrams is the most prominent. A book of such nature, covering a time span of billions of years would do well with a set of nice colour plates. The technical language of the geologist author sometime goes over the heads of ordinary readers like myself who have no initiation to the wonders of geology. This makes some of the passages tough for laymen. Also, the pessimistic view represented in many chapters, like the human society is doomed, whatever we may do etc is disheartening. We are not fully knowledgable about the resilience of our species for such a long haul, but still we might expect to find some humans to live somewhere on the planets, which the author claims to be full of teeming life even then!

The book is highly recommended.

Rating: 3 Star