they form. We are running down our stock of fertile topsoil, the one thing we absolutely need to support civilization in the long run. Viewed globally, that is quite a feat.
In the 1990s, I read an influential paper arguing that human activity had become a significant global factor in moving soil and rock. This was news to me, because geologists regard exploding volcanoes and shifting tectonic plates as the forces shaping the planet, not people. Human activity doesn't figure into thinking about deep time and earth history because we arrived on the scene far too recently. And yet, as we remodel our world at a phenomenal pace geologically speaking, we are neglecting to conserve and protect the critical resource right beneath our feet. Instead, the topsoil lost from agricultural fields becomes soil out of place—in other words, dirt.
If Earth had come with an operating manual, the first chapter might be titled something like “How to Protect Soil, Mother of All Life.” It would portray soil as the dynamic interface between geology and biology, the bridge between the dead world of rock and the bustling realm of life.
When I considered the history of agriculture through a geological lens, I saw a boomerang effect—how we treat land determines how the land will treat us, and for how long. I also saw that we can avoid the common fate of ancient societies as long as we do not repeat their grand folly of stripping off fertile topsoil at an unsustainable rate. Unfortunately, that is exactly what we are doing, only this time on a global scale.
In researching Dirt I found that every few decades over the last 150 years, luminaries have sounded the alarm over soil erosion. But our memory and attention span are short. Efforts to reverse the course of soil erosion tend to get hijacked by other priorities, such as plowing the plains to sell grain to Europe during the First World War, or cashing in on crops like biofuels today.
Archaeologists have documented links between soil loss and the decline or collapse of ancient societies around the world. In the literature of environmental history one often finds the argument that deforestation caused the soil erosion that plagued past civilizations. But having cut my geomorphological teeth studying erosion in the steep, soil-mantled timberlands of the Pacific Northwest, I knew that such extensive, dramatic soil loss required not just clearing a forest, but keeping vegetation from coming back—and that this was actually pretty hard to do. I started to suspect that agriculture was the real culprit in long-term soil loss. Here lay a fundamental conundrum: did the agricultural practices that fueled the rise, growth, and expansion of civilizations also sow the seeds of societal decline through long-term soil degradation and loss?
Historians rightly maintain that every civilization has a unique story, with multiple factors influencing its rise and fall. But I came to suspect that the state of the land—the state of the soil—directly affects the health and resilience of societies. In this view, individual droughts, wars, and economic disruptions or dislocations may serve as the triggers for societal collapse, but the state of the land loads the gun. In other words, soil degradation and erosion help explain why particular events or circumstances take down societies. Of course geography matters, but this is not to say that geography is destiny. Far from it; what matters as much if not more, is how people treat the land through their cultural, economic, and social systems.
As I became convinced that agriculture caused damaging soil erosion time and again in the past, I wanted to assess how great a problem it remains today. There were no scientists measuring rates of soil erosion in the fields of ancient Greece or Rome. But we have such data now to address whether agricultural soil loss is a reasonable hypothesis for limiting the lifespans of civilizations. So I parked in the University of Washington's library for several weeks and gathered up all the data I could on modern rates of soil erosion from agricultural fields and long-term rates of erosion from landscapes around the world.1 I discovered that the world's farmlands erode as much as the high Himalaya.
Think about it: it's quite a trick to make flat-as-pancake land behave like the highest mountains in the world. We don't tend to farm steep terrain, but we've managed to transform Iowa and Kansas into places that erode like Nepal. That can't go on forever. With average rates of erosion off of plowed fields exceeding a millimeter a year, it would only take a few centuries to erode through the topsoil in most regions of the world. And while a millimeter a year sounds pretty darn slow, the natural pace of soil formation is nowhere near as fast. To a geologist a few centuries is the blink of an eye. It sounds like an eternity to a politician running for re-election, a corporate executive obsessed with next quarter's profits, or a subsistence farmer toiling to feed a family.
And yet anyone can appreciate the “problem of agriculture,” as Wes Jackson has so aptly put it, because plowing a field exposes the soil to erosion. Bare earth is rare in nature at elevations below timberline. Think about the grasslands and forests you may have walked in your life. How much bare soil do you see? Not much, if any. Outside of arid environments, plants typically blanket the ground surface, their leaves intercepting rainfall and their roots binding the soil. Now think about the brown color of the water running off a freshly plowed field or the exposed soil of a construction site during a sudden downpour. Leaving the soil bare for some part of the year makes it vulnerable to erosion, whether by wind or rain, resulting in a rate of soil loss tens to hundreds of times faster than nature makes it. And yet, plow-based agriculture strips the soil from the land at such a slow pace few bother to make soil, let alone soil conservation, a top priority.
Life makes soil. Soil makes more life. Put simply, that is the story of the past half billion years. The evolution of plants and the rise of life on land fed the soil and the soil, in turn, fed more and bigger plants that nourished increasingly complex communities of animals. Life and soil were partners until modern agriculture changed the game. How long can modern agriculture keep us alive by breaking the soil-life bond? Viewed over any geologically meaningful time scale, an agricultural civilization that degrades the soil will be transient—it cannot last if is destroys its own foundation.
In the 1990s, researchers reported that since the Second World War, soil erosion caused farmers to abandon an area equivalent to about one-third of all present cropland. Needless to say, the loss of agricultural production in an area larger than India won't help us feed the world later this century, when it is projected that we'll need to feed another several billion people. The estimated rate of world soil erosion now exceeds new soil production by as much as 23 billion tons per year, an annual loss of not quite one percent of the world's agricultural soil inventory. At this pace, the world would literally run out of topsoil in little more than a century. It's like a bank account from which one spends and spends, but never deposits.
We can be sure that the disruptive effects of soil depletion and degradation will become apparent well before we actually run out of fertile soil, just as dwindling supplies and rising oil prices are already disrupting oil-addicted societies even though we may never literally run out of crude. The key question remains when we will run out of enough fertile soil to feed everybody? Before we reach that point, we would be wise to implement agricultural reforms to head off a crisis and build a sustainable foundation for a global civilization.
It would not be quite right to speak of peak soil in the same sense as peak oil because, viewed globally, soil was at its peak back when humanity invented the plow. Nevertheless, we've been drawing down the world's natural capital ever since. Today we are at another turning point, because now we are depleting our most fundamental resource—the one that feeds us—everywhere all at once. And with today's globally interconnected society, it would be naïve to think that disruption of regional food supplies will not influence global security.
A common thread relating to natural resource management runs through both this book and King of Fish, my previous book on the history of humanity's interaction with salmon. It seems that the slower the emergency, the less motivated we are to do anything about it. In both cases—soil and salmon—degradation has occurred over extended time spans that mask the severity of the extended problem and prevent it from becoming a priority that compels effective action. And yet, the current global state
