So, stop me if you’ve heard this one before, but it is often said that we know more about the moon than we do about our own oceans. However, palaeo-oceanographer and climate scientist Eelco J. Rohling points out we know more than you might think. His new book, The Oceans: A Deep History, takes the reader through a 4.4-billion-year history of Earth’s oceans. Much more than just a book about water, this is foremost a book about the intimate link between our planet’s climate and its oceans, as they are far more intertwined than you might give them credit for.

I will come right out and say that I found this book a challenging read. Partly this is because my background is not in geochemistry, and there is quite a bit of that here, partly this is because, whether you like it or not, the Earth system is complex. Fiendishly complex. Trying to put into words the many interlocking feedback mechanisms playing out on different timescales will, therefore, make for dense reading in places.

The Oceans starts off with three introductory chapters that are quite technical, featuring a lot of oceanographic, geochemical, and atmospheric chemistry details necessary to understand the rest of the book. There are a number of helpful illustrations in this section that the reader will be referred to time and again. It is not until page 83 that Rohling starts his journey through the deep history of our oceans. To structure the book, he has chosen three key topics important to our understanding of past and current climate fluctuations.

“[…] the Earth system is complex. Fiendishly complex.”

The first thing most people will think of when you mention climate change is temperature, and Rohling walks the reader through reconstructions of Earth’s thermal record. There were times where our planet was frozen solid (periods appropriately known as Snowball Earth), such as the Huronian glaciation between 2.4 and 2.1 billion years ago, and more “recently” in the Neoproterozoic Era between 750 and 580 million years ago. The latter was followed shortly by the famous Cambrian explosion, an episode in Earth’s history where life flourished. During the reign of the dinosaurs, the world has been much hotter, with average temperatures some to 10°C to 15°C higher than they are today. This chapter is a good example of the challenge of putting into words the complex interplay between the organic and inorganic carbon cycles, as fluctuations in atmospheric carbon levels are responsible for these long-term temperature swings.

The second topic, of much relevance today as well, is ocean acidification. As Rohling explains throughout the book, the oceans are an enormous buffer for carbon. But just as the oceans aren’t a featureless bathtub (see the introduction to Deep-Sea Fishes: Biology, Diversity, Ecology and Fisheries for a good overview of ocean topography), they are also not a chemically inert bathtub. Levels of dissolved carbon dioxide change the acidity of water, which affects shell-bearing organisms large and small. One episode where strong ocean acidification is implied is the end-Permian mass extinction some 252 million years ago, which was so vividly described in Brannen’s book The Ends of The World: Volcanic Apocalypses, Lethal Oceans and Our Quest to Understand Earth’s Past Mass Extinctions, and is the subject of Benton’s When Life Nearly Died: The Greatest Mass Extinction of All Time. A more recent, and therefore better documented, episode was the Paleocene-Eocene Thermal Maximum some 56 million years ago. During both these events, large amounts of carbon dioxide were released relatively quickly (several tens to hundreds of thousands of years is quick geologically speaking), leading to drops in oceanic acidity. The Ammonoids, the now-extinct group of cephalopods that we met in Staaf’s Squid Empire: The Rise and Fall of the Cephalopods, was one group that was heavily hit during the end-Permian extinction episode.

The final topic that is hugely influential for life is water’s oxygen content. Lack of oxygen in water is also known as anoxia. Ocean Anoxic Events are periods of prolonged (hundreds of thousands of years) and widespread (possibly global) lack of oxygen. This, too, has relevance today as water pollution is once again leading to persistent “dead zones”; regions of the deep sea that remain devoid of oxygen for months or years at a time, choking most life forms to death. Ironically, the layers of putrefying organic material that build up during these historic Ocean Anoxic Events were slowly transformed into black shales, of much interest to the fossil fuel industry today.

“Our inability to conceptualise deep time is a huge problem […]”

Throughout the book, Rohling in passing mentions the important messages deep history has for our current times, but a book like this of course has to have a chapter explicitly dealing with current climate change. Looking at deep history, what makes the last 200 years different is not necessarily the amount of carbon we have so far released through the burning of fossil fuels, it is the speed. Conditions on our planet are changing faster than any time before, based on our reconstructions from Earth’s deep history, with the rate of change rivalling or exceeding even that seen during the end-Permian mass extinction. The two hundred years since the Industrial Revolution may seem like ages to humans, but they are a blip in deep history. Our inability to conceptualise deep time is a huge problem, something that is discussed in-depth in Bjornerud’s book Timefulness: How Thinking Like a Geologist Can Help Save the World.

And that is where this book shines. By surveying what the palaeoclimatological record reveals, by describing all the feedback mechanisms we have uncovered, it becomes abundantly clear that denying that current climate change is happening is absurd. Denialists both love to argue the quantitative details and grossly simplify them. Why can we not predict how much sea levels will rise exactly? Why is it not getting hotter everywhere? And yes, the finer details and timing of what awaits us are subject to a certain amount of uncertainty and are based on forecasts and modelling studies, subject to revision (guys, for f**** sake, this is how science works). But by attacking the quantitative details they are missing the bigger picture. Qualitatively speaking none of the changes that are forecasted are based on conjecture: Earth’s deep history, as explained here so in-depth, tells us what happens when our atmosphere and oceans change. People want to shrug this off and say “but the planet recovered, right? We are here today!” But this again painfully highlights our inability to conceptualise deep time. The natural geochemical processes that can undo rising temperatures or increased ocean acidity play out over hundreds of thousands of years. So, yes, the planet will recover from us, but we will not like the ride there.

The Oceans, then, is a book with many relevant lessons to understanding climate change today. Whether the dense material will reach the people who need to see it most is something I am doubtful of. But if you want to understand how our oceans work, and how they have influenced life as we know it, this book is mandatory reading that thoroughly covers the subject. Be prepared to engage your brain while reading it though.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

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