CO2: turning the knob on climate

CO2 is termed the Earth's biggest control knob. It hadn't been until now, because a knob implies something that someone can turn to control things. In a normal, natural world and on relatively short timescales, say tens of thousands of years, carbon dioxide is interlocked with global mean temperature and other variables. Temperatures can drive carbon dioxide levels up or down, which in turn drive temperatures further up or down.

Carbon dioxide acts as a feedback that enhances temperature changes.

This is most obvious during the transitions between glacial and interglacial periods, when temperatures rise or drop and CO2 seems to follow along like a happy puppy. What is not obvious when looking at the readings is that while orbital forcings cause the initial change in temperatures, and CO2 levels rise or fall in accordance with that initial change, the subsequent temperatures themselves also rise and fall in accordance with the changing CO2 levels.

The basic formula behind a glacial termination is that something (orbital forcings) starts the increase in temperature. Actually, what really starts it is a change in the length and severity of northern hemisphere summers, without changing the overall amount of radiation reaching the planet at all. That stays fairly constant.

These seasonal changes in turn cause the ice sheets covering the northern hemisphere land masses to begin to melt. This reflects less sunlight back into space, and that really does change the amount of energy that the planet receives from the sun, which leads to warming. It also results in the release of methane, another powerful greenhouse gas, which warms the planet even further.

Then CO2 kicks in. The oceans warm. Warmer water cannot hold as much dissolved carbon dioxide and so the oceans release some CO2 into the atmosphere. CO2 in the atmosphere causes warming. The increased warming causes the ice sheets to retreat further, and the oceans to warm further, and more CO2 to be released.

This continues, but with limits. There is (or had been) only so much CO2 that could make its way into the atmosphere. The system only pushes this cycle so far. The many previous glacial terminations in the past 2.5 million years (a period known as the Pleistocene Epoch) have seen lows of about 180 ppm of CO2, and highs between 250 ppm and 300 ppm.

The main point is that temperatures and CO2 are interlocked, or at least had been until now. Temperature changes had to get the ball rolling, so on a graph they will lead the way, but the two work in concert. One is not pulling a leash to drag the other along. They each push and pull the other, working their way from low to high, or high to low, as an integrated system.

CO2 does not "lag" temperature. That's a simplistic, inaccurate and indiscriminate view of a complex interaction.

Turning the Knob

Unfortunately, contrary to recent natural history, man has learned how to remove the regulator and to dial up a far higher level of CO2 in the atmosphere. CO2 has become the climate's biggest control knob in the last two centuries or so, in the sense that it is in fact a control that mankind can twist, turn, tweak and, sadly, overdo.

A glacial termination happens on very, very long timescales relative to man. What we have done in the past two centuries, however, applies a change to CO2 levels — implying an equivalent change in climate — that would otherwise take nature 10 to 12 thousand years.

CO2 was once interlocked with temperature. In the past 200 years we have instead taken 337 gigatonnes of carbon out of the ground and injected it into the atmosphere and the oceans. Nature spent the better part of several hundred million years converting that carbon into new forms (coal, oil, gas) and sequestering it deep under the surface of the earth.

Man will be able to undo in 200 years what took nature hundreds of millions of years to accomplish, and in so doing, in that same time frame, we are duplicating a feat that normally takes nature 10,000 years to accomplish (i.e. increasing atmospheric CO2 levels by two thirds).

And, as an important point, we have no idea if we are capable of duplicating nature's feat of again sequestering that carbon underground. We have far too easily turned the knob in one direction, but with no capacity whatsoever to turn it in the other.

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