Four Revolutions in the Earth Sciences From Heresy to Truth

Try as we might, science is very much the work of human beings with all their foibles. As such, scientific advances aren't always straightforward and can run into opposition within scientific circles when new ideas run counter to currently established ones. In Four Revolutions in the Earth Sciences, American geologist James Lawrence Powell demonstrates this by taking the reader through the history of four ideas in the earth sciences that initially weren't accepted. This was a book I very much wanted to read.

The four ideas Powell tackles are the age of the Earth, continental drift and plate tectonics, meteorite impact, and the role of CO₂ in global warming. Nowadays all these ideas are thoroughly accepted and solidly supported by empirical evidence, but each had its own tortuous journey to get to that point. And each of these journeys was different in its own way.

The age of the earth was initially estimated at hundreds of millions of years. Thermodynamic calculations by Lord Kelvin, based on the rate at which the planet cooled since its formation, started adjusting that number downwards to ultimately some 20 million years only. Much too short for evolutionary processes Darwin had suggested. Around the 1900s this all changed when physicists discovered that constant levels of radioactive decay of atoms could be used to date, for example, rocks. For a fuller discussion on this, I refer the reader to Macdougall's Nature's Clocks: How Scientists Measure the Age of Almost Everything. These discoveries ultimately led to an age estimate for the earth of 4.6 billion years.

Continental drift and plate tectonics were put forward by German meteorologist Alfred Wegener in 1912. This is the idea that over geological time the continents have moved over the face of the earth. Wegener simply looked at maps and noticed how South America and Africa look like two puzzle pieces that fit neatly together. Until then geologists were convinced that the continents had always been where they are now. It took six decades for this idea to become accepted and it was so much discussed that Henry R. Frankel needed over 2000 pages spread over four books to give a complete overview of its history in his magisterial The Continental Drift Controversy. This section is a beautiful example of a paradigm shift as philosopher Thomas Kuhn described in The Structure of Scientific Revolutions. Evidence from various disciplines, including biogeography and geophysics, started to come together until scientists could no longer keep denying that the continental plates indeed move. The proposed mechanism, plate tectonics, holds that convection of deep layers in the planet moves the overlying plates around, causing seafloor spreading in some areas and subduction in others as one plate is pushed or dragged under another [edit: shame on me, as The Tectonic Plates are Moving! points out, this is a common misconception, even today. It is actually subduction that causes convection, not vice versa]. This has question marks around it, though, and Powell points out that we still haven't nailed down the exact mechanism. We may yet again see a paradigm shift here as observations accumulate to refute this idea. Or not.

Meteor strikes are another example of an explanation that was initially not accepted. Geology was ruled by uniformitarianism, the idea that changes were slow and gradual, and invocation of external influences was unnecessary. I have written about this before when I reviewed Cataclysms: A New Geology for the Twenty-First Century. The most famous example was, of course, the mounting evidence for a large meteor strike at the K-Pg boundary that led to the extinction of the dinosaurs. For more on this, see my review of The Ends of The World and Walter Alvarez's own account in T. rex and the Crater of Doom. But meteor strikes are also invoked to explain impact structures on earth and the moon (previously thought to be of volcanic origin), and even the formation of the moon (see The Big Splat: Or How Our Moon Came to Be for more on that).

Finally, and most familiar to readers, is the role of CO₂ in global warming, which was put forward by Guy Callendar (in all the current hubbub his name seems all but lost to memory, and readers would do well to turn to his biography The Callendar Effect: The Life and Work of Guy Stewart Callendar (1898-1964)). Initially, there was doubt whether adding carbon dioxide to the atmosphere would have much influence, as other gases were thought to be more important in absorbing heat radiation from the sun. It took an increased understanding of the fiendishly complicated interactions between land, water, air and the living world, together with increases in computing power, before we could really understand this. Uniquely, global warming has become subject of a huge public and political debate, and a powerful industry lobby denying climate change is real. Powell wrote about this in The Inquisition of Climate Science.

Powell's account of each of these four theories – and their rise from heresy to truth – makes for an absolutely fascinating read. This is very much a human story, and it's amazing to read the hubris, stubbornness and pride on display, especially where senior scientists were concerned. As mentioned above, science is a human endeavour, and for many people it's clearly hard to admit they are wrong and let go of cherished ideas. Even entertaining this notion and being willing to cooperate with others to try and falsify ideas was not in vogue in the past. Have we gotten any better at this? Maybe this is wishful thinking on my part, but I'd like to think science has become more collaborative, and scientists a bit wiser and more humble. Powell's riveting book serves as a powerful and well-written reminder why we should be. If you have any interest in the history of science or how science progresses this book comes highly recommended.