The carbon dioxide hanging over our heads

Climate’s Big Picture, Part 3

There is a big heat-trapping blanket of greenhouse gases overhead. For carbon dioxide, it now amounts to a 50 percent excess atop the benign preindustrial concentration. Our most recent ambition, zero emissions annually, will not get rid of this legacy carbon dioxide, and it is what causes our climate problems.

In medical physiology, emissions is usually prefaced by nocturnal. (Perhaps that is how it came to apply to those fumes emerging from protruding tailpipes and tall smoke­stacks.) However, terming the excess CO2 as ‘emissions’ tends to mislead us, as the invisible annual additions of CO2 tend to stick around for far longer than mere fumes. Yet many people reason about CO2 in the same way they reason about visible air pollution — and that minimizes the future danger.

“Wait!” you may say in surprise. “Emissions reduction has worked for similar problems such as urban air pollution and the ozone problem. It should work for carbon dioxide as well. Explain that!”

Such is indeed the reasoning behind our leaders telling us, year after year, to double down on emissions reduction. But those are flawed analogies. Nature cleans up visible air pollution such as haze, smoke, pollen, fumes, and smog with the next good rain. That means that reducing the emissions rate really does reduce the highest air concen­tration attained by accumulating irritants before the next good rain arrives. Yet that is not how things work for the molecular-sized greenhouse gases; nature’s rate of removal can be 10,000 times slower than for haze and smoke.

Furthermore, as a matter of a common cognitive ineptitude, people often confuse flow (a rate) with accumulation (a quantity). For example, it is all too easy to confuse a balanced budget (zero flow) with the national debt (what we must pay interest on, via taxes). Many people struggling with climate concepts will confuse emissions (a rate) with the excess CO2 accumulation.

There are two more common confusions involving emissions. The same word ‘emissions’ is commonly used for both the flow rate and the accumulation, a conflation that forces you to guess what the speaker intended from the context. Try imagining a conversation where ‘miles’ could either mean a rate like speed, miles per hour — or it could mean a distance, the accumulation of miles traveled.

When mentioning ‘emissions’ without a qualifier, scientists usually have rates in mind. But even when we say ‘Annual emissions’ to make the rate connotation explicit, the phrase is often shortened by those who write graph labels and headlines.

Adding to this rate-accumulation confusion (economists call it the flow-stock confusion), input is often confused with net input, which is inflow minus the outflow of the same thing from a containment. After net zero is achieved by annually sinking as much extra CO2 as we add that year, how long will nature need to take most of the excess carbon dioxide out of circulation? Rough answer: at the current rates, it takes 28 years for nature to clean up half of the 140-ppm excess.

Imagine we fixed our emissions tomorrow. There would still be a 50% excess of carbon dioxide overhead. ‘Net zero’ would only take us down to 350 ppm 28 years later. While 350 ppm has been proposed as a backing-up goal, on the way up that concentration was reached about 1990, after the continents started warming four times faster than the ocean surface in the mid-1980s.

Thus 350 ppm is not a sufficient goal; it is still in the danger zone.

The 320 ppm of the mid-1960s, when the excess CO2 was only 40 ppm, was the last ‘possibly safe’ concentration before the warming ramp began in the mid-1970s. Assuming we could shut down annual emissions tomorrow, how long would it then take for nature’s processes to cool the earth’s surface back to the surface temperatures associated with 320 ppm?

The fastest way is probably via increasing the sunlight reflected back out into space, the way that sea ice, clouds and hazy skies do. That is not a long-term silver bullet because it does nothing to address the other effect of rising CO2: acidification of surface waters, which kills off the ocean’s food chain.

Volcanic eruptions reaching the stratosphere cool us for several years; “under a white sky” is a good description of 1991 after the Mount Pinatubo eruption.

Natural carbon-cycle processes cool by cleaning up the lingering CO2 excess: some sink CO2 and organic debris into the ocean depths, others weather limestone, and some goes into fertilizing additional plant and plankton growth.

How long does all of that take — and how vulnerable is the CO2 storage they utilize? The resulting CO2 decay curve is not exponential with a 15-year time constant, as one might assume from plotting the early years on a semi-log graph and fitting a straight line. Essentially, stopping annual emissions today might see a 50% decrease in the excess carbon dioxide accum­ul­ation over the next 28 years. The natural removal processes for CO2 will take a thousand years to draw the accumulation down to 20% remaining. Such a decay curve is said to have a “long, fat tail,” tapering more like an alligator’s than a dog’s.

Figure 3–2. Nature’s time course (orange) for getting rid of extra carbon dioxide once the annual additions stop; from Joos et al (1996). Blue line shows what an exponential fit to the first few years would have suggested about the future time course, down by half in a dozen years. But in reality, natural processes take 28 years to remove half of the excess. Another 28 years only reduces the excess by an additional 8%.

Now let us ask: How long, once sunk, will the excess carbon remain out of the carbon cycle’s circulation loop, what traps that extra heat? Decades? Centuries? Millennia? Forever?

It depends. New grass takes CO2 out of the air, but it decomposes or burns in several years, putting most of that CO2 back into the air. If leaves fall off and rot every winter, they don’t count, only the carbon in whatever wood growth remains alive.

A new tree reaches maturity in 30–50 years; that is when rot within the tree increases enough to release, via bacterial respiration, as much CO2 as new growth captures elsewhere as wood. At maturity, a tree ceases being a net carbon sink. Indeed, it continues to be an expense, needing protection from drought, fire, and bark beetle — all of which put the stored carbon back into the air as CO2 over the next year. There are many good reasons to plant more trees, but 21st-century climate relief may not be among them.

And, in the coming decades until we can remove enough CO2 annually to stave off slippery slopes, we need to keep the trees we already have. Because a tree’s replacement may take a half-century to achieve the same carbon storage, it may prove necessary to stop cutting down whole forests. Putting stored CO2 back into circulation now must be avoided for all storage sources, not merely the fossil fuels underground. Furniture or wooden buildings may have eighty-year lifetimes, but paper products often revert to CO2 or methane CH3 within a few years.

The ocean depths (the 98% below the wind-mixed surface layer) are a much better long-term sink than are forests. There is no fire or drought down there, not even beetles. Deep ocean already has so much suspended carbon debris that sinking all of our excess CO2 below the thermocline would only increase the total by several percent. It is often a thousand years (about thirty human generations) before deep waters begin to circulate back up to the surface, allowing excess CO2 to slowly creep back into the air overhead during the following few millennia.

The amount of annual carbon debris that reaches the bottom to become sediment is less than one percent of what stays suspended in the ocean depths, so the deep suspended debris and attendant CO2 is the big carbon sink on the millennial time scale, a suitable candidate for storing the excess carbon now in circulation. Below the thermo­cline is where much of the excess carbon will eventually go in any event; what we need to do is to speed up the natural processes.

“Carbon sink” is usually meant as a metaphor — but here sinking is to be taken literally.

William H. Calvin, Ph.D., is Affiliate Professor Emeritus at the University of Washington School of Medicine in Seattle.

Link to Part 4.



The focus here will be on 21st-century surges in global weirding occurring atop the creep in global warming. Indeed, extreme weather surge-and-stay may collapse our civilization before the creep up in temperature does. We must back out of the danger zone and its slippery slope.

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William H. Calvin

President, Professor emeritus, University of Washington School of Medicine in Seattle. Author, many books on brains, human evolution, climate