From Icebox to Hothouse, and Back Again

Better modeling, by including the biosphere, retrodicts more of Earth's dynamic climate history.

Climate change, while ignored by the current administration, is not ignoring us. The Earth is warming well past where it has been for millions of years. But before that? While the planet has generally had stable climates, they have varied substantially through time, and have gone though occasional catastrophes. There was a little ice age, in the middle of the last millenium, thought to have been caused in part by the depopulation of the Americas due to European diseases. The ensuing regrowth of forests covering the Americas drew down CO2 from the atmosphere and cooled the climate. But more to the point, there have been far more severe episodes, both of heat (the end-Permian extinction event) and cold (the Sturtian glaciation of the Precambrian). All of these arise from CO2 levels, as CO2 is the master controller of heat in the atmosphere, thanks to the greenhouse effect. (As it is on Venus as well.) 

For example, the end-Permian extinction is thought to have been caused by unusual volcanism in what is now Siberia. Over a mere 100,000 years, this poured an estimated 26,000 petagrams of CO2 into the atmosphere, causing its concentration to shoot up to about 2500 ppm (parts per million) and temperatures to shoot up as well, killing off 90% of all species. What we are doing now is much faster, though admittedly in early days. We are pouring roughly 11 petagrams of CO2 into the atmosphere yearly, which has raised CO2 concentrations from a preindustrial 280 ppm to 427 ppm today. It would take us another one to two thousand years to cause a 90% extinction event!

A bedrock of our climate thermostat is the silicate cycle. Since the vast majority of carbon on earth is locked up in rocks, (carbonates of silicon, magnesium, and calcium), not in the biophere, it is rocks that have a dominant effect. Volcanoes belch out CO2 in huge amounts. That CO2 slowly eats away at rocks that are exposed, re-forming carbonate compounds that are weathered off and back into the ocean. Where these compounds (with those built by shelled animals of all kinds) are gradually deposited on the sea floor and subducted back into the Earth's crust. Some of those carbonates are reduced at depth and brought forth again by volcanic activity. The more CO2 there is in the atmosphere, and the warmer it is, the more weathering happens and thus the faster CO2 levels are brought back down. That is the elegant thermostat that has kept Earth at mostly mild temperatures through its long history. 

However, this is a slow thermostat, taking hundreds of thousands of years to equilibrate. Unusual events, like an asteroid impact, prodigious volcanism, or the advent of human ingenuity, can make a mess of things way faster than the silicate cycle can deal with in its slow, grinding way. Many subtler influences can also come into play, like cycles in the tilt of the Earth towards the Sun, or continental arrangements that lead to particular patterns of ocean circulation, can create variations such as ice ages. A recent paper brought out peculiar influences from the biosphere that can also affect, and even destabilize, the thermostat on longer time horizons

The oceans are responsible for roughly half of photosynthetic productivity, and they are also where the carbonate minerals get buried. So how they react to changes in the atmosphere are very influential in the whole cycle. These authors ran half-million year simulations of climate perturbations while including not only the silicate cycle, but also reactions by the biosphere and especially the phosphorous cycle, which has a strong influence on biological productivity. It turns out that when the atmosphere has lower levels of oxygen than we do today, (as was the case during the Precambrain epoch), high CO2 levels cause long-term rises in biosphere productivity and also in phosphate recycling out of the ocean floor. The extra phosphate increases biological productivity even more, and thus causes CO2 drawdown to persist past where the silicate cycle would level out for the long term. The result can be a rebounding ice age after a hot phase. 

Model results over 500,000 years, showing rebound from an injection of high CO2 at year 10,000. A shows concentrations of CO2 over time, B shows O2 concentration, and C shows sea ice, which goes to zero at first, the rebounds sharply, especially under the blue condition of 0.6 times current oxygen concentration in the atmosphere. It shows how exquisitely sensitive the climate is to CO2.

These models make some sense of the Precambrian climate cycles, which had a few dramatic swings that went through so-called snowball Earth phases where the entire surface of the planet seems to have iced over. The silicate cycle naturally came to the rescue eventually, spewing enough CO2 from volcanoes to overcome the snow / albedo effects of all the ice and cause a rebound hot phase. Between the rising oxygen levels and the extreme climatic swings, the stage was somehow set for the rise of animal life, leading the so-called Cambrian explosion, though there was a fair amount of simpler precursor animal live in the Precambrian.dd

https://www.science.org/doi/10.1126/science.adh7730

A schematic of the proposed cycle, with CO2 coming in from vulcanism (red) and being disposed of by various means, first and foremost the silicate cycle (blue). OC = organic carbon, P = phosphorous/phosphate, OCpetro = organic carbon weathered out of sediments, coal, limestone, and other geologic formations. Thus the brown color shows this paper's additions to the classical silicate cycle.

While it is just a modeling paper, models are what we think and do in science. It is nice to have laboratory confirmation for areas of science (like molecular biology) that permit it, but historical sciences, especially those pertaining to whole planets as systems, have to be more forensic and speculative. This new model is a refinement on the basic silicate cycle, and thus seems a strong improvement on what has heretofore been a science of more or less back-of-the-envelope estimation. And judging from this new model, the authors propose that the next ice age is not being put off indefinitely by our profligate emissions, but rather that organic burial feedbacks will bring it closer (than 400k years away) with additional overcooling thereafter!

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