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In The Curve: Monitoring Rising Carbon Emissions

By Tom Yulsman

In winter, snowshoes are one good way to negotiate the steep, snow-covered road up Niwot Ridge in Colorado’s Front Range. But if you’re Duane Kitzis, and the job for the day is to haul metal cylinders up the mountain, what you really need is a snowcat.

On a blustery morning last December, I joined Kitzis for the climb up the ridge, sharing the snowcat’s flatbed with the cylinders, which he was taking to a scientific station at 9,973 feet up the mountain, and his trusted assistant, a 95-pound Airedale terrier named “Little Bear.”

Duane Kitzis on the road up Niwot Ridge in Colorado's Front Range. Kitzis takes regular measurements of CO2 levels in the atmosphere to see how much they are changing. Credit: Tom Yulsman.

With the clanking tank treads of the cat, and smelly exhaust spewing from the back, a passing snowshoer could be forgiven for not guessing that this is arguably one of the most important environmental jobs on the planet. The air samples Kitzis collects in cylinders like these help scientists around the world determine how much carbon dioxide (CO2) is accumulating in the atmosphere — and thus how the climate might change as a result.

Kitzis, a scientist with the National Oceanic and Atmospheric Administration’s (NOAA) Earth System Research Laboratory in Boulder, makes a crucial contribution to this effort.

“My personal stake in this is to create the best data possible,” he says.

Creating that data may seem straightforward. Capture samples of the atmosphere from an unpolluted site, measure their chemical makeup, and presto: you’ve figured out the concentration of CO2 in the atmosphere, right? And if you do it over time, you’ve got the trend.

Not so simple. There are more than 140 sites worldwide — from the South Pole to the Arctic, and many places in between — where atmospheric monitoring is done. In addition to this, airborne sampling is carried out at more than 30 locations. So how can we be confident that the job is being done accurately and to the same exacting standards everywhere?

Helping to ensure that accuracy is what Kitzis does for a living. He creates "standards": samples of the atmosphere, preserved in canisters, whose chemical makeup he determines with great accuracy. Each year, he sends 400 of these canisters to monitoring sites worldwide so everyone can make sure they are producing results to the same exacting standard. 

In part thanks to his efforts, scientists can say with great confidence that so much carbon dioxide has accumulated in the atmosphere that its concentration is now above 390 parts per million (ppm) — and that this is largely our doing. They’ve also been able to chart with increasing precision the specific CO2 sources: for example, how much from fossil-fuel burning versus deforestation? (The answer might surprise you. Read on…)

And, of course, without that scientific confidence that comes from the work done by Kitzis and his colleagues, it would be impossible to say that we humans are responsible for the changes in climate already being observed worldwide, from melting glaciers to the increasing likelihood of heavy precipitation events.

I hitched the ride on Duane's clattering snowcat to get a start on learning how the remarkable global monitoring network and its system of checks and double checks works. On the ride up, I was well aware that the ridge we were climbing has played a crucial role in the history of CO2 monitoring, and thus our understanding of climate change: Atmospheric CO2 has been monitored at a site near treeline since 1968, producing the second longest record of the gas’s steady increase.

A CO2 analyzer that Kitzis uses on Niwot Ridge. Credit: Tom Yulsman.

But when did scientists first suspect that CO2 from fossil fuel burning and other activities could cause global warming?

Turn the clock back to 1827. This is when Joseph Fourier examined how the atmosphere could retain heat from the sun — a forerunner to later research that would firmly establish the details of the greenhouse effect.

It fell to Swedish scientist Svante Arrhenius to connect the dots between coal-burning, which releases CO2 — by then a known greenhouse gas — and global warming. (For an excellent overview of the history of modern climate science, see Spencer Weart’s online treatise for the American Institute of Physics: “Discovery of Global Warming.”)

But back in Arrhenius’ day, no one had even proved that CO2 was accumulating in the atmosphere, let alone that it was warming the planet.

Enter Charles David Keeling

That began to change in 1953, when Charles David Keeling, then a post-doctoral researcher at the California Institute of Technology, took a CO2 analyzer he had constructed, to Big Sur in California. Measurements he took there, and subsequently at other locations (most famously on top of Mauna Loa in Hawaii), provided the first ever hints that despite the potentially confounding influences of plants breathing CO2 in and out, it might be possible to measure a background concentration of the gas in the atmosphere that would be virtually the same anywhere in the world.

“This is why he was confident that if he went to a place like Mauna Loa he would likely measure something that would be true for the whole world,” says Pieter Tans, director of NOAA’s Carbon Cycle and Greenhouse Gases group (and Duane’s supervisor).

The Mauna Loa volcano rises high above the Pacific on the Island of Hawaii. “Here, the background concentration of carbon dioxide should not be influenced by forests or soils, or an inversion or the weather,” Tans says. “All that is stripped away.”

In March of 1958, Keeling installed an infrared gas analyzer at a scientific station more than 11,000 feet atop Mauna Loa to measure CO2 in the atmosphere. Its first reading was 313 ppm.

Over time, interesting patterns showed up — one of which would profoundly alter our perspective on humanity’s role within the natural scheme of things. The now famous “Keeling curve,” as it later became known, charts changes in the concentration of carbon dioxide in the atmosphere.

Keeling curve illustrating increasing concentrations of atmospheric CO2 at the Mauna Loa Observatory in Hawaii.

After a year of monitoring, Keeling noticed that CO2 had risen in the fall and winter, and dropped in spring and summer. This, he realized, was the pattern of a living, breathing Earth, caused by plants in the Northern Hemisphere growing in the warm season, and thus taking in CO2, and then going dormant in the cold season, allowing the gas to build up again. (The Northern Hemisphere dominates the cycle because of its greater landmass and thus vegetation cover.)

It also didn’t take Keeling very long to notice a striking trend: Every year, the average concentration of CO2 in the atmosphere had risen just a bit from the year before it.

This stunning observation offered the first strong confirmation that humanity was changing the chemistry of the entire atmosphere by burning fossil fuels. 

Charles Keeling's boss at the Scripps Institution of Oceanography, Roger Revelle, described the implications in this way: "Human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future."

The "experiment" Revelle referred to consists of our emissions of greenhouse gases, and the experimental subject is the climate system of our planet.

“Keeling was the first one who actually observed the seasonal cycle of CO2,” notes Tans (who last year was awarded the prestigious Roger Revelle medal by the American Geophysical Union). “He was the first one who also observed the rise from year to year in CO2. And along the way, he made a third important discovery: that there is such a thing as a global background concentration that is almost the same everywhere. That was not known before.”

The monitoring site established by Keeling high on the slopes of Mauna Loa in 1958 has maintained the longest continuous record of atmospheric CO2 in the world. And rigorous methods he pioneered are now used throughout the global network, including by Kitzis in Colorado.

The Mechanics of CO2 Monitoring

Most monitoring stations today draw samples of atmospheric gas into cylinders, which are then taken to a laboratory to be analyzed. At about a dozen sites CO2 is monitored continuously on the spot. In both cases, the most common instrument used to analyze the samples depends on the defining characteristic of greenhouse gases themselves: the fact that they absorb infrared radiation.

Carbon dioxide is known to absorb a specific amount of infrared radiation. So by measuring the absorption in an air sample, infrared analyzers give a read-out of how much CO2 is present.

But instruments drift, canisters leak, and all manner of other errors can subtly influence results. So how can these problems be avoided, and everyone in the monitoring network be kept on exactly the same page?

That’s where Kitzis comes in, with his snowcat and canine assistant.

From a two-room structure in a clearing on the forested slopes of Niwot Ridge, Kitzis regularly fills cylinders with ambient air. Then he hauls them back down the mountain to a NOAA lab where infrared analyzers are used to determine with exquisite precision the air’s chemical composition.

Kitzis and his dog, Little Bear, at work on Niwot Ridge filling cylinders with air for CO2 testing. Credit: Tom Yulsman.

These samples are then sent out to the global network of monitoring sites. Since the makeup of each one has been determined to very fine precision, the different groups can use them to verify that their own instruments are providing accurate readings.

In this way, the air Kitzis captures and analyzes is used as the “standard” against which other samples are compared, enabling atmospheric monitoring of CO2 concentrations around the world to be done precisely and in coordination.

“I am an empiricist,” Kitzis says. “I like instruments and measuring things.”

But how can Kitzis and his colleagues make sure that everything is kosher in their own lab? Here, the system gets even more complicated, with multiple internal calibrations involving two different sets of cylinders, some of which were collected on Niwot Ridge 25 years ago. (For the gory details, see this publication from NOAA.)

Thanks to these meticulous efforts, there is no scientific doubt about what is happening to the atmosphere.

“We are very certain about the increase in CO2,” Tans says. “In fact, it is the thing that is most certain in our knowledge about climate change.

Finding the CO2 Source

There is also no doubt where that CO2 is coming from: us.

Consider that CO2 concentrations in the atmosphere have now risen higher they’ve been in at least a million years. Moreover, the rise in CO2 “really gets going in the late 19th century,” Tans notes.

“It so happens that this is the same time that we started to burn coal. And then it really took off after the Second World War. The rate of increase tracks approximately the rate at which we have been burning coal, and then oil and natural gas. It goes in parallel,” Tans says.

But couldn’t it be possible that what we’re seeing is “just a burp of inorganic CO2 that is naturally coming out of the oceans?” Tans asks. “Well, we have isotopic ratios to prove that this cannot be the case.”

A molecule of CO2 is made of one carbon atom bonded to two oxygen atoms. Simple enough. But the carbon actually comes in different forms, or isotopes, which are determined by the number of neutrons in the atom. And that means CO2 molecules come in different isotopic flavors, with some CO2 molecules being made of carbon-12, others of carbon-13, and still others are made of carbon-14.

It turns out that plants have a kind of sweet tooth for the carbon-12 flavor. So if the extra CO2 in the atmosphere is coming from plants, as opposed to an inorganic source like an oceanic burb, it should be enriched in carbon-12. And that is precisely what scientists are seeing.

So, the increasing carbon dioxide has an organic source. But how do scientists know that it is coming from the burning of ancient organic material — namely the remains of plants that were transformed by heat and pressure over millions of years into oil, coal and natural gas? Isn't it possible that the CO2 buildup is purely the result of plants growing and respiring today?

"We have isotopes to disprove that too," Tans says.

Here's where carbon-14 comes in. All plants contain some of this isotope in their tissues — including plants that are growing today and plants that were transformed into fossil fuels millions of years ago. But carbon-14 actually decays relatively quickly over time. As a result, CO2 that comes from the burning fossil fuels actually has no carbon-14 in it (all of it has since decayed and none is left). So if combustion of fossil fuels is the source of the rising levels of atmospheric CO2, the amount of carbon-14 in the atmosphere should be decreasing at a rate consistent with the rate of burning. 

And once again, that is precisely what scientists are observing.

Analysis of the isotopic makeup of CO2 in the atmosphere — which provides a kind of chemical fingerprinting that points to where the gas is coming from — also has produced what Tans describes as one of the biggest surprises of his career: Very little of the increasing concentration of CO2 in the atmosphere is actually coming from deforestation.

“Globally, the terrestrial biosphere is not losing carbon; it’s actually sequestering carbon, despite deforestation,” Tans says. That’s because there is so much re-growth of vegetation that it is outweighing the effects of deforestation.

Thanks to David Keeling’s pioneering work, the meticulous care with which instruments around the world are kept on the same standard, and the methodical way in which the isotopic composition of CO2 is tracked, scientists have virtually no doubt that the burning of fossil fuels is the primary cause of steadily increasing levels of CO2 in the atmosphere.

“We have really, really nailed this thing,” Tans says.

“And here’s another thing we know that is very unpleasant: We know that the CO2 we have released is not going to disappear for thousands of years… So on a human timescale, this means emissions are forever.”

This, Tans argues, has an unsettling implication: “If we seriously want to tackle climate change, we eventually have to drive carbon emissions down to zero. Zero.” 

Back on Niwot Ridge last December, I watched as Duane Kitzis unloaded the snowcat and made adjustments to the lines feeding air from the atmosphere into his cylinders. With Little Bear curled at his feet, we chatted about the impacts of the rising atmospheric tide of CO2.

He noted that so far, we’ve managed to raise CO2 in the atmosphere to just above 390 parts per million (a threshold we crossed last summer).  This is an increase of 40 percent from the preindustrial value of about 280 ppm.

“That means we are retaining more energy in our atmosphere,” Kitzis said. “Exactly where is that energy going? The climate system is very chaotic, so we don’t know precisely… We have no measuring stick for this, no example for the release of eight billion tons of greenhouse gases each year.

“That’s the fun of it,” he said facetiously, “because we’re going to find out.” 

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