This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
I left California in the middle of a drought. The hills framing the 280 from the city to Palo Alto turned brown in the summer, as they always did, and then stayed brown through the winter. The pleasant seventy-degree air began to feel oppressive, the glorious blue sky a source of inchoate guilt. I lived in Oakland, by a cemetery on a hill. The winding paths past angel-topped mausolea and modest age-blacked gravestones were framed by a line of sweet gums, peppered with cypress, redwood, and olive. The smell—a peppery sweetness, pine without Christmas—is what I remember when I think of home.
I now live in New York City, where trees are considered suspicious, not allowed to congregate in groups outside the designated parks. The few on my street are framed by small cages to keep the local dogs away. The effect is pathetic, almost comic, a tiny prison for a tall defiant thing. The largest tree I have seen in New York is dead and in the natural history museum. It is an enormous stump of a California sequoia, each ring a record of a life that began twenty generations before the birth of the logger who ended it.
The trees in New York, and the city too, are fed by moisture carried from the Gulf of Mexico or the Atlantic. There is no dry season, just a scatter of rain and snow spread evenly on top of the year. California rainfall, though, beats with the erratic rhythm of the tropical Pacific. There, the long spine of South America dredges cold water from the depths of the ocean, upwelling to form a permanent pool of cold water that extends into the west, pushed there by the trade winds. The cold water sloshes back and forth, the winds that drive it pulse to their own beat, and the result of the dance is, in some years, a phenomenon of warmer-than-usual water: the Christ Child, El Niño. When he visits, California can expect a rainy winter. When the opposite conditions prevail—La Niña—it’s drought.
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There are other beautiful places that cycle through drought and floods with some regularity. In the eastern part of Australia, things are reversed. It is El Niño who dries, and La Niña who is associated with the rains. In the Mediterranean, it is a different cycle altogether. It has been like this for thousands of years.
For as long as we have been able to comprehensively observe the planet, we have been changing it. Satellites have recorded trends since the late 70s. But we have been increasing the level of atmospheric carbon dioxide since well before that. As scientists, we want to attribute the trends we measure to natural cycles like El Niño or externally imposed changes. But it can be difficult to tell if what we are seeing is unusual, because everything we have ever seen is unusual.
Trees have seen more than any of us. If you drill a very small bore into a tree, you can extract a thin rod striped with each year’s rings. If you do this again, to another tree, and then another, and then to thousands all over the world, you can weave them together into a drought atlas, a record of good and bad years, wet and dry soils. Scientists have built these for half the world: North America and Mexico, Europe and the Levant, monsoon Asia and eastern Australia. The map is a record of wet and dry years that stretches backward in time. The trees remember years before world wars and depressions, before industrial and political revolutions, before even the first contact between the invaders and the New World.
The collected rings of thousands of trees show a pattern of drying and moistening that ebbs and flows with time. El Niño and La Niña arrive with erratic regularity, air and water slosh back and forth, and the trees record the consequences. At the beginning of the twentieth century, though, a faint fixed pattern becomes discernible among the randomness, a quiet but strengthening note against a background symphony. Some regions—California, the Mediterranean, Australia—dry out. It is a small, almost imperceptible-to-humans drying, but it is a pattern that no natural cycle can reproduce.
This pattern is similar to one that I generate on my computer in a small office in New York. In climate models, one can remove the effects of randomness by re-starting time over and over again, creating a universe of alternate worlds that might have existed but did not. The average of these never-worlds smooths out the kinks and ripples of randomness. If something is left over, it must be something common to all the realities that could have been.
What is present in all of these worlds is carbon dioxide, which humans were even then slowly but steadily increasing. Carbon dioxide heats the planet, and warmer air is thirstier air. Even if there are no changes to rainfall, some areas will sink into drought under the atmosphere’s relentless demand for moisture.
And this is the pattern that emerges from the trees. It is unusual in the context of everything that has come before, recorded for long posterity in the tree rings. It is a signature of us.
There are few issues in science that interest me less than the question of when the Anthropocene began. I am not a rock, and I do not need to find proof of human activity in the geological record to be concerned about our impact. But our fingerprints stretch further back into the past than we might have realized. And the harmony between the predictions of my machine-world and the real world recorded in the flesh of tree gives me reason to listen to what those machines tell me for the future.
From my office in New York, I can look at these future projections and see California dancing from dry to wet and back again, until there is no again and it settles in to permanent drought. If the trees survive us, they may live to tell of a time where the grass turned brown, the map turned brown, and it was a long time before it was ever green again.