Tuesday, October 23, 2018

Waving at Volcanoes

Talk to climate change skeptics and you will notice a couple of themes that arise with some regularity, notions to which they reliably but confusedly turn to advance their case: ice ages and volcanoes.

Despite not understanding them, skeptics presume these phenomena somehow demonstrate that humans can't or don't materially affect the climate; that human effects are swamped by natural ones. Yet the reasoning behind such presumption is so inchoate and ill informed that the skeptic can do little more than offer it as a "handwave," as if the case were made implicitly by mere mention, without any need to explain more.

In the first instance, ice ages are imagined to be imponderably immense forces of nature that demonstrate natural climactic variability far beyond any effect mere humans might have. A planet subjected to periodic ice ages needn't worry about the trifling things people are doing. To the climate change skeptic, this is self-evident. One acquaintance of mine is convinced we don't know what causes ice ages, and so by implication we don't understand climate generally. Although he might not, "we" most certainly do, on both counts. Which suggests that the first order of business is to learn some things.

Not only do we know what causes ice ages, but we also know the relative strength of the natural climate "forcings" that instigate them. And hard as it may be to believe without a little elucidation, we know that natural forcings that instigate ice ages are extremely weak compared to the anthropogenic (human-caused) forcings that are now heating the planet. Not only do ice ages not refute anthropogenic climate change, but studying ice ages has considerable explanatory value in understanding what the climate is doing in the present, and why we are changing it so dramatically.

So we should stop waving at ice ages and actually learn something about them. Same for volcanoes.

The handwave at volcanoes usually goes something like this. Anthropogenic climate change is said to be caused by emissions of greenhouse gasses—particularly carbon dioxide—through the burning of fossil fuels. But volcanoes, which are completely natural, emit far more CO2 than humans. So humans can't be causing global warming.

Of course you see the fallacy in this formulation. A moment of critical reflection informs you that what matters is not whether humans emit more or less CO2 than volcanoes, but whether the increment added by humans—whatever volcanoes also emit—is altering the climate. If the climate is stable with volcanoes, but unstable with volcanoes plus humans, then humans cause the instability.

Volcanoes, after all, have been a geological feature of the planet for billions of years, erupting and (presumably) emitting CO2 all that time. It would be pretty dumb to blame volcanoes for very recent developments unless they've become unusually active. Have they? The climate change skeptic who mentions volcanoes has no idea.

So if volcanoes emit CO2, and CO2 causes warming, and to the extent the climate has been stable, there must be other physical processes that offset or counteract emissions from volcanoes and keep the planet from becoming ever hotter. We might, for example, hypothesize that some physical process removes CO2 from the atmosphere, even as volcanoes are adding it, with the two operating concurrently to keep atmospheric CO2 concentrations (and thus climate) fairly constant. And indeed, the carbon cycle consists of processes that move CO2 between various reservoirs, including the atmosphere, the ocean, and deep earth reservoirs. Rock weathering is an important mechanism for moving CO2 from the atmosphere to deep reservoirs over geologic time scales.

Now it turns out that the climate has not been stable over very long periods of tens to hundreds of millions of years. But in the present context, and for the current discussion, we can presume stability. The longer term variations are interesting and reasonably well understood, but worrying about them at the moment would just confuse our discussion. So we won't. For our purposes we will note that except for a few minor blips, the climate has in fact been quite stable since the end of the last ice age, which is to say for the past 10,000 years or so. This is the geologic epoch known as the Holocene, in which human civilization emerged and flourished, in no small part due to that climactic stability. That's the period we are presently considering.

During this recent period of climactic stability, volcanoes have been erupting and emitting CO2, as they always have. But the planet has not been warming. Not, at least, until very recently. Whatever is now happening, volcanoes are not the cause.

In fact, volcanic activity over the past couple of centuries has been a bit less than normal. Yet over that period atmospheric CO2 concentrations have increased by almost half, and the planet is heating up rapidly. Don't blame volcanoes: something else must be going on.

What could that something be? Well, these past couple of centuries of reduced volcanic activity and increasing CO2 and, most recently, increasing temperature, coincides with the Industrial Revolution, and the large scale burning of fossil fuels. First coal. Then coal and oil. Then coal, oil, and gas.

Given what you already knew about the long term presence of volcanoes on the planet, you probably recognized immediately that the handwave at volcanoes as formulated above is a red herring—even without knowing the relative emission amounts. But as a factual matter, do volcanoes actually emit more CO2 than humans? No, they don't. It isn't even close.

The U.S. Geological Survey says that volcanoes emit around 260 million tons of CO2 into the atmosphere per year. Human emissions amount to 32 billion tons per year. People emit 120 times as much CO2 as volcanoes. So the handwave isn't just a red herring; it's a canard. And yet such misconceptions that climate change skeptics tell each other have a way of circulating pretty much forever.

There's more. Volcanoes are indeed an important natural but very slow (relative to humans) emitter of CO2 over very long geologic time scales. (But don't forget we also posited processes that remove CO2 from the atmosphere, keeping things more or less in balance.) But on human time scales, volcanoes actually promote cooling, not warming! That's because volcanic eruptions spew reflective aerosols such as sulfur dioxide into the upper atmosphere. Those aerosols block sunlight from reaching the earth and warming it.

The most prominent recent example was the eruption of Mount Pinatubo in the Philippines in 1991. Pinatubo was the second largest volcanic eruption of the 20th century. Fifteen months after the Pinatubo eruption, scientists measured a global temperature reduction of around 1 degree Fahrenheit. The effect is quite significant.

A more astounding example was the 1815 eruption of Mount Tambora in the Dutch East Indies (present day Indonesia), the most powerful volcanic eruption in recorded history. The following year, 1816, came to be known as the "Year Without a Summer," due to the massive cooling effect from Tambora. The effect was likely exacerbated by the eruption of Mayon in the Philippines in 1814. The "volcanic winter" caused by Tambora resulted in massive crop failures and food shortages across the Northern Hemisphere. At that time humanity had no understanding that the frightfully bizarre weather of 1816 was a consequence of volcanic eruptions. Not only was the scientific basis for volcanic cooling not yet developed, but the vast majority of humans had no idea that there had even been a massive volcanic eruption somewhere on the planet.

By now the science behind such associations, and other physical processes that affect the climate, is well understood. Scientists refer to a large variety of natural and anthropogenic factors that cause the climate to warm or cool as "forcings," and they are reliably quantified. Positive forcings (such as CO2) promote warming, and negative forcings promote cooling. Volcanic eruptions actually have a net negative climate forcing.

Here's a subtle exception. Scientists have assigned a slight positive forcing value to volcanoes over the past two centuries to account for the fact that volcanic activity over that period has been somewhat less than normal. In other words, volcanoes haven't been erupting as much lately as they have over the longer term, and so their usual expected cooling effect is diminished a bit due to reduced activity. This small positive contribution by volcanoes goes into the larger mix of forcings that scientists combine to calculate the net result of all the factors driving the climate.

So, yes. Volcanoes are warming the planet a tiny bit. By not erupting! But the effect is minuscule, with volcanoes accounting for a small fraction (less than three percent) of the positive climate forcings presently operating.

Over geologic time scales volcanoes have been a (mostly) gradual contributor to atmospheric CO2, which in the proper amount is a very good thing, and not just because plants require it for photosynthesis. Some small amount of atmospheric CO2 is necessary to keep the climate comfortable and amenable to life. The earth would be far colder without carbon dioxide in the atmosphere.

But it doesn't take much. A scant 4 hundredths of 1 percent (0.0004) of  the mix of atmospheric gasses is CO2—only one molecule out of every 2,400. Compare that with 21 percent oxygen (0.21) and 78 percent nitrogen (0.78). Do not allow personal incredulity to keep you from accepting that such a tiny amount of CO2 has such a large effect on global temperature. It does, and the physics, which has been well understood since the 19th century, is quite straightforward.

Unfortunately, we now have too much of a good thing, thanks overwhelmingly to human emissions. The human CO2 contribution is a novel development in the history of the earth. Never before, over billions of years, has deeply sequestered carbon been dug up and intentionally burned.

The CO2 (and temperature) increase has been remarkably rapid compared to the rate of change that typically occurs during natural cycles, such as the progression in and out of ice ages. The long term average CO2 concentration over the Holocene was about 280 parts per million (ppm). That's the level that prevailed prior to the beginning of the Industrial Revolution, and is a good baseline against which to compare current levels.

Atmospheric CO2 concentration began increasing steadily when humanity started burning fossil fuels, and it has marched ever upward over the past couple of centuries as the rate of burning intensified. It recently hit 410 ppm—a 46 percent increase over preindustrial levels. CO2 concentrations continue to rise, and correlate closely with calculated emissions from burning fossil fuels, and also with an unambiguous and now rapid trend of planetary warming.

Scientists are able to accurately estimate atmospheric CO2 concentrations over the geologic past, going back millions of years. Concentrations over millennial time frames can be readily measured from ice cores. Exquisitely precise direct atmospheric measurements began in 1957. The graph of those emissions is called the "Keeling Curve". As an fascinating aside, the graph's sawtooth appearance is a consequence of annual variations correlated to the Northern Hemisphere's growing season.

Since 1957 atmospheric CO2 has increased by 100 ppm—around a third. That's a heck of a lot of CO2 added over a partial human lifetime. Mine, for example. Despite recent pledges by the world's nations to reduce their CO2 output, the steady increase continues. So does the temperature of the planet.

A sobering new special report by a working group of the IPCC (Intergovernmental Panel on Climate Change) warns that the planet is warming faster than previously believed; that total warming of 2 degrees Celsius (the upper limit of what was previously thought to be "safe") will have significantly worse effects than would 1.5 degrees; and that limiting warming to 1.5 degrees would require an immediate and dramatic transformation of the world's energy systems at a scale and speed that's hard to imagine. If you are tempted to throw up your hands and say it's all impossible, understand that these temperature targets are in no sense limits: things can and will get much worse if nothing is done.

The report says that limiting warming to 1.5 degrees Celsius would require net anthropogenic CO2 emissions to decline 45 percent from 2010 levels by 2030, and to reach net zero by 2050. Limiting warming to 2 degrees requires a decline of 20 percent by 2030 and a reduction to zero by 2075. 2030 is only 11 years away.

Regardless of what humanity chooses to do—and the prospects are not encouraging—volcanoes will remain a mute and uncaring presence on this planet we call home, doing what they have always done since time immemorial. They have neither agency nor any meaningful role to play in what comes next. Our climate, and our future, is entirely up to us. We need to stop waving at volcanoes and get to work.

Copyright (C) 2018 James Michael Brennan, All Rights Reserved

The latest from Does It Hurt To Think? is here.

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