When you study the climate of the deep past, time is relative. What one scientist may consider a long time — say, a decade — is only a short span of time to someone who routinely thinks in millions of years. Climate change is affected by processes operating at hours, millions of years, and everything in between. To complicate matters, some of the driving forces of climate operate in different directions on different timescales, so keeping everything straight isn’t trivial. This is the first in a series of posts explaining the timescales of climate change, from the scale of plate tectonics (millions of years) to sunspots (decades). Much of it borrows heavily from what I learned in a graduate course on Climates of the Past, which used William Ruddiman’s excellent book, Earth’s Climates, Past and Future. If you’re interested in reading beyond the simplified explanations here, I urge you to check it out.
Forget global warming for a moment: over the last 55 million years, the earth has been cooling. Antarctica was once lush and ice-free, and early crocodiles ambled beneath palm-like trees north of the Arctic circle. By the beginning of the Oligocene 34 million years ago, the earth was well on its way to “Icehouse” conditions, marked in the geologic record by the presence of permanent ice at the poles and the spread of temperate and cold-adapted plants and animals. This may seem counterintuitive as we break historic temperature records, but we’re in a very different climate regime than the one the dinosaurs experienced (237 to 66 million years ago).
To understand how the earth’s temperatures have cooled over very long timescales, you have to look at a key player in modern climate change: CO2. Carbon is constantly cycling through the earth system, released from the earth’s interior in gaseous form, and recycled back into the earth’s crust in sedimentary rock (more on this in a minute). To cool the planet on tectonic timescales, you must either 1) add less CO2 to the atmosphere through time, or 2) take more of it out.
If you want to add more CO2 to the atmosphere in deep time, you do it with plate tectonics. CO2 gas is transferred from deep within the earth to the atmosphere via two main channels: volcanoes formed at convergent (meeting) plate boundaries, and from magma released by seafloor spreading at divergent (separating) plates — the latter is where new crust is formed. By increasing the rate of plate tectonics, you increase the rate at which CO2 is added to the atmosphere. CO2 in the atmosphere traps solar energy (remember the greenhouse effect!), so higher concentrations of CO2 means you get warmer temperatures.
Taking CO2 out of the atmosphere is more complicated. The most common process is chemical weathering, which acts as something of a regulatory thermostat for the planet. Basically, rain and atmospheric CO2 form carbonic acid in the soil, which dissolves silica-rich rocks. This reaction produces bicarbonate, which is eventually transported to the oceans, where it ultimately ends up in the shells of marine organisms. When the little critters die, they fall to the ocean floor and eventually make up carbon-rich sedimentary rocks like limestone. In this process, carbon in the atmosphere becomes sequestered at the bottom of ocean, slowly, via the weathering of rocks and the efforts of tiny shell-building marine critters. If tectonic activity results in more uplift, you get more rocks exposed, which increases the rates of weathering, so more carbon ends up at the bottom of the ocean and out of the atmosphere. Chemical weathering essentially creates a negative feedback loop, dampening the effects of warmer or colder temperatures by changing the rate at which CO2 is drawn down for the atmosphere; warmer and wetter climates increase rates of weathering, which is like the AC kicking in at hotter temperatures, while colder temperatures slow weathering, which is like the AC turning off when the room gets too cold.
Tectonic activity can also affect the climate system depending on the position of the continents; having land masses at or near the poles can certainly help with polar ice cap formation, although we know from the geologic record that polar position isn’t sufficient to produce ice caps alone (it turns out that you still need to reduce CO2 concentrations for that). Tectonics also also affects the circulation of the earth’s oceans, which are like a great temperature conveyor belt. As critical oceanic gateways open or close, ocean currents can move heat around more or less effectively, which can warm or cool the planet and change the atmosphere’s moisture balance.
We understand these proceses because of the natural experiments of the past, from when continents moved, mountains were built, and seaways opened or closed. While we’ve been in an Icehouse Earth for around 55 million years, it’s only in the last 3 million years that we’ve seen regular cycles of ice ages and permanent ice in the Northern Hemisphere. This is thought to be due to several events in the recent geologic past. First, the uplift of the Tibetan Plateau over the last 55 million years has exposed a massive amount of young rock for weathering, which has helped reduce the amount of CO2 in the atmosphere. Secondly, the opening of the Drake Passage 25 to 20 million years ago created an uninterrupted belt of cold water around Antarctica, which may have intensified glaciation at the South Pole. Third, the closing of the Isthmus of Panama 10 to 4 million years ago separated the Atlantic and Pacific oceans, likely setting off chain reactions in the climate system that allowed ice sheets to start forming in the northern hemisphere.
What about the climate of the next 50 million years? Ruddiman points to the evidence that the earth was moving even deeper into an icehouse climate before humans started adding CO2 to the atmosphere; ice ages have increased in severity in the last million years, and northeast Canada has only been ice-free for about 10% of the last 500,000 years. Ruddiman is quick to point out, though, that predicting climate change on geologic timescales is impossible, largely because we’re still not sure how much past cooling has been driven by increased uplift (i.e., more weathering), slower seafloor spreading (less CO2 released from the earth’s crust), or reduced ocean heat transport (due to changes in plate tectonics). The driving forces behind plate tectonics are poorly understood, and rates are unpredictable. “If we cannot accurately predict these tectonic processes,” Ruddiman argues, “we cannot predict their climatic effects.”
Climate change at tectonic scales is an incredibly slow processes– we’re talking millions, or tens of millions of years for the effects to manifest. It took millions of years to cool the earth system to the point where ice started forming at the poles, and for Earth to make the transition from a Greenhouse (during the age of the dinosaurs) to an Icehouse (55 million years ago to today). Contrast that with how rapidly we’re adding CO2 to the atmosphere today, when we’ve just surpassed carbon dioxide concentrations of 400 ppm for the first time in 3 million years. And, as I’ll address in subsequent posts, tectonic factors are by no means the only timescales of climate change; while we may be living in an icehouse earth tectonically speaking, we’re a in a greenhouse at all other timescales.
Coming up next: Posts on orbital, millennial, and decadal-scale drivers of climate change.