Remembrance of Climates Past

As the climate heats up, we are heading back in time, very rapidly.

Climate change is the challenge of our times and of our planet. However attractive it is to not care, to ignore it, to hide in traditional ways of thinking, to let inertia have its way, inexorable change getting worse by the year. The American way of life can not go on, and will not go on as before. This year has been a remarkable demonstration of the range of catastrophe, from melting Arctic villages to Pacific Northwest heat waves, California wildfires, record draught on the Colorado river, hurricanes running out of letters, and catastrophic floods in Europe. Migration crises around the world point to another implication- that as the global South becomes unlivable, increasing hordes of people will be knocking on the borders of the Northern countries, who have authored the mess.

To get some perspective on the change, we can look backwards into the geological record to see where we are going, and how fast. Earth has had a very diverse climatic history, from its beginning in a Venus-like cloud of high CO2 and no oxygen, to "snowball earth" freezes, to torrid warm periods extending to the poles. Over the last few billion years, earth's climate has had a fundamentally, if slowly, self-correcting mechanism based on CO2 production and consumption. CO2, needless to say at this point, is the main variable in our atmosphere's tendency to retain or give up solar heat. Volcanoes liberate CO2 from geologic and organic buried carbon. Organic carbon can also be liberated by fires and decomposition of organic carbon, including exposed coal, methane, and oil deposits. On the other hand, the biosphere fixes and buries carbon, and on an even more vast scale the weathering of exposed rocks drives the formation of carbonate minerals that lock up atmospheric CO2. When conditions are warm, weathering of rocks accelerates, as can organic fixation and burial, drawing down CO2. When conditions are cold, ice sheets cover the land and inhibit both organic fixation and rock weathering, allowing CO2 to build up in the atmosphere.

These cycles mean that over a scale of millions of years, earth does not get caught irretrievably (as Venus has) in an inhospitable climate. Instead, our recent ice ages ebbed and flowed, back and forth as the CO2 balance in the atmosphere responded fitfully to geologic conditions. The dramatic snowball periods, which occurred just before the Cambrian period, came to an end even though the earth-wide snow cover dramatically reduced solar absorbance. But it also reduced weathering and organic fixation of CO2, so eventually, CO2 built up to the very high levels needed to overcome the snowball effect and the climate snapped back to very warm conditions.

A key point in all of this is that climate change over earth's history has been driven geologically, and thus has been slow. Slowness has critical effects in allowing the biosphere to adapt. The typical driver is a new spate of volcanic eruptions, which release lots of CO2. This takes thousands of years to happen, so while this can be fast in geologic terms (a prime example is the Paleocene-Eocene thermal maximum, which took maybe 20,000 years to drive the climate from very warm to quite torrid, roughly 55 million years ago). However, the homeostatic mechanisms kicked in, and this torrid phase only lasted  a couple of hundred thousand years. Another example has been the slow uplift of the Tibetan plateau, which exposed a great deal of rock to weathering, thus drawing down atmospheric CO2. This is thought to have driven the cooler temperatures and glaciations of the last few million years.

A notorious exception is the K-T boundary extinction, where an asteroid hit the earth and changed the climate overnight. And life suffered correspondingly, with all the dinosaurs wiped out. (Well, all except for birds). Whatever was not pre-adapted somehow for this instant crisis failed to make it through. The stress this put on the biosphere is obvious, catastrophic, took many millions of years to recover from, and changed the trajectory of evolution dramatically.

An extremely rich graph of the last 70 million years of earth's climate, from a recent benchmark paper. Temperatures are shown on top right, while the isotopic findings that undergird them are shown on top left (temperature proxy based on oxygen isotopes) and bottom left (carbon concentration proxy based on carbon isotopes). The overall trend is correlation between the two, with CO2 the primary driver of higher temperature, and subject to swings for various geologic and biological reasons. Temperature is also affected secondarily by orbital mechanics and other factors. Even the Eocene high temperatures were driven by CO2, though the correlation is not so clear here.

What does all this mean for our current trajectory? The graph above helpfully supplies the current IPCC scenarios of temperature change, under stringent, medium, and business as usual scenarios. The temperature today (green) is already equivalent to conditions of about five million years ago. So in time machine terms, we have travelled, in the span of a century, five million years of climate history, to before the recent ice ages. We are already beyond the stringent scenario, obviously, so the only possible futures we have to look forward to are the medium and no-action scenarios, which, within the next fifty to one hundred years, will put us, in time machine terms, fourteen and forty million years into the past, respectively. And what of the century after that? CO2 stays in the atmosphere for many thousands of years, so not only do we have to reduce emissions now, we will have to remove those that have already happened. Climate stewardship will be humanity's job whether we like it or not.

The biosphere can not cope with this rate of change. While we often think in narcissistic terms of how humans will suffer, we are the lucky ones, being the most adaptable creatures ever devised by evolution. Our problems are nothing compared to the rest of the biosphere. The ability of animals to migrate or shift their ranges is highly strained by the availability of the rest of their essential networks, mostly based on plants at the base of the ecological network. And plants are not going to have the ability to migrate at these speeds and generate new ecosytems in more northerly areas. To us, the speed of climate change is slow, barely discernible on a lifetime scale. But in earth history terms, it is blindingly fast, just a blip over an asteroid impact, and far faster than normal ecosystem dynamics, let alone evolution, can cope with. Uncounted species are falling by the wayside, victims of another great extinction in earth history in this, the anthropocene geological epoch.

Time machines are exciting tropes of science fiction, allowing amazing journeys and byzantine plot twists. But usually, the outcome is not good, since changing the time line has unpredictable and sometimes catastrophic effects. Typically, a ruse is employed to extricate the heroes from the twisted plot, and everyone sighs with relief at the end when the normal time line is restored. Our climate path is not heading for such a happy ending. We are gambling, now consciously and willfully, with not only our own civilizational existence, but with the progressive and rapid degradation of the entire biosphere, on this warp-speed trip into the geological past.

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