Global warming discount expires soon

Caroline Alden, BURN Contributor

A recent BURN Journal post on the global carbon cycle and the fate of fossil fuel CO2 emissions – Carbon Cycle 101 – discussed how land plants and the world’s oceans slurp about half of fossil fuel CO2 emissions out of the atmosphere each year.

In other words, 50% of fossil fuel CO2 emissions are naturally sequestered by nature in land plants and ocean waters. The important corollary is that only half of the CO2 we emit each year remains in the atmosphere to trap heat and warm the globe.

Scientists have been waiting worriedly for these sinks to “saturate,” or quit taking so much extra CO2 out of the atmosphere. Models predict that land plants will soon become satisfied with the level of fertilization that extra atmospheric CO2 provides, and that ocean chemistry will soon lose its capacity to accommodate extra CO2.

A paper published last summer in Nature by researchers in Boulder, Colorado compared the growth rate of the concentration of CO2 in the atmosphere each year with the amount of CO2 put in the atmosphere by fossil fuel combustion each year since 1959.

What they found was that not only are land and ocean sinks still taking up excess CO2 from the atmosphere, but that the rate of uptake has grown steadily stronger for the last 5 decades!

sinks for caroline

Panel a shows the annual growth rate of CO2 in the atmosphere. Panel b shows emissions from fossil fuel combustion (in red) as well as land-use changes (gold). Panel c shows the difference between panels a and b, or the annual global net uptake of carbon by land and ocean sinks. The dark shaded areas represent 1-sigma uncertainties, and the light shaded areas represent 2-sigma uncertainties. (Source: Ballantyne and others, published in the journal Nature in August 2012)

This finding is both good news and bad news.

The good news is that, to date, climate change has probably been attenuated by strong natural sinks; if more of our emissions had remained in the atmosphere, the globe would have warmed more than it already has.

The bad news is that climate change is already happening, in spite of the 50% climate discount on emissions that the Earth’s natural sinks currently offer us.

That is troubling. What happens when natural sinks stop and that discount disappears?

Furthermore, these natural carbon sinks – the land sink in particular – are not permanent storage places for COand are vulnerable to extreme weather events and to climate change itself. Droughts and fires can release land carbon stores back to the atmosphere within a season.

When the land and ocean sinks saturate – and all signs say they very will soon – the impacts of each watt of electricity produced by a coal-fired power plant, of each mile driven by a gasoline-powered vehicle, and of each lawn mower lap around the back yard will be felt in full by greenhouse gas warming of the planet.

Caroline Alden is a graduate student at the Institute of Arctic and Alpine Research in the Department of Geology at the University of Colorado at Boulder.

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Party like it’s 399 (ppm)

By Caroline Alden, BURN Contributor

Does it matter when nature offers up round numbers? Maybe not, but for the same reasons that we attach special significance to anniversaries and birthdays ending in zero, humans treat big tick marks and bold milestones with gravitas.

For Earth’s climate, a very significant round number milestone was reached last week, when NOAA measured an atmospheric concentration of CO2 of 400 ppm at the Mauna Loa Observatory in Hawaii for the first time in modern history.

PPM – or parts per million – is a measure of concentration. 400 ppm means that for every one million parts dry air in the atmosphere (water is excluded because its concentration is variable), 400 of those parts are CO2. These ‘parts’ are moles: a chemist’s unit of measurement to keep track of molecules.

Think of the atmosphere as a big pot of soup with lots of finely chopped vegetables. Carbon dioxide is the carrots. Prior to the Industrial Revolution, if you filled a ladle with 1,000,000 really finely chopped vegetables, then you’d have found that 280 of those veggies in any given ladle-full of soup that you scooped would be carrots, or carbon dioxide.

Now, today, after we have been dumping extra chopped carrots into the soup (i.e. burning fossil fuels) for a couple hundred years, a ladle-full of 1,000,o00 veggie bits would include 400 carrot chunks (carbon dioxide). For the last few years, we have diluted the soup by about 4-5 carrot bits every year.

There are many places across the globe that measure atmospheric concentrations of carbon dioxide, but the measure of atmospheric COon Mauna Loa (Long Mountain in Hawaiian) is an important and historically significant indicator for two reasons.

First, because of the remote and high altitude location (measurements take place at a height of 2 miles above sea level), measurements of atmospheric CO2 at Mauna Loa generally come very close to representing the global mean concentration of that gas.

Keeling measuring CO2 at Mauna Loa in 1988. Photo: Scripps Institution of Oceanography/UCSD

Second, the record of CO2 at Mauna Loa represents the longest, continuous monitoring of carbon dioxide on Earth. In 1958, Charles Keeling, a scientist employed by the Scripps Institution of Oceanography in La Jolla, California, began regularly collecting samples of air from the atmosphere and measuring the concentration of CO2.

Within a few years, Keeling not only observed remarkable seasonal variability in CO(from large swaths of northern hemisphere plants breathing CO2 in and out, summer to winter), he also clearly showed – for the first time – that atmospheric CO2 was steadily increasing each year.

The canonical time history of Mauna Loa atmospheric CO2 concentrations, which scientists have relied on for 50 years, is, as a result, called the Keeling Curve.

Now. How big of an impact does a change from 280 ppm to 400 ppm have on the Earth’s climate?

To answer this question, it is best to peer back into Earth history to see what the world looked like the last time the atmosphere had 400 little carrots pieces for every million-chopped-veggie ladle full. Scientists have tried to do just that by looking at various types of ancient rocks and sediments, and even bubbles in ancient ice.

One good estimate of when atmospheric CO2 was last 400 ppm was produced by Yale researcher Mark Pagani and fellow scientists, who looked at the chemical properties of ancient ocean sediment.

What these scientists found is that the last time atmospheric CO2 reached 400 ppm was likely somewhere around 4.5 million years ago.

At that time, temperatures on the planet were an average of 4° Celsius (7.2° Fahrenheit) higher than today and sea level was about 22 meters (72 feet) higher. Because of a phenomenon known as Arctic Amplification, northern climes were even warmer – likely 19° Celsius (34.2° Fahrenheit) warmer than today.

Since the Earth’s climate system takes a little bit of time to adjust to atmospheric greenhouse gas concentrations, perhaps these are changes we might expect to see coming down the climate pipeline.

Caroline Alden is a graduate student at the Institute of Arctic and Alpine Research in the Department of Geology at the University of Colorado at Boulder.

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Carbon Cycle 101

Caroline Alden, BURN Contributor

Americans burn fossil fuels doing most of what we do every day – using electricity, driving to work, and buying food and goods. You probably know that the burning of fossil fuels results in the release of greenhouse gases, such as carbon dioxide (CO2), to the atmosphere. Maybe you’ve even calculated your carbon footprint.

But what about the big picture of carbon dioxide on Earth? How much CO2 is in the atmosphere already? Does it stay there forever? Leak out into outer space? (No.) Fall out of the sky as rain? (Also no.)

Carbon is everywhere, and the planet’s dynamic natural forces are continuously moving it from place to place. There are four major reservoirs, or stocks, of carbon on Earth: 1) in rocks (this includes fossil fuels), 2) dissolved in ocean water, 3) as plants, sticks, animals, and soil (which can be lumped together and called the land biosphere), and 4) as a climate-warming gas in the atmosphere.  (Check out the diagram below. Everyone loves dioramas, so it will henceforth be referred to as a diorama. You can do your best to envision it in 3D.)

carbonn cycle graphic

The Carbon Cycle (adapted from “Earth’s Climate: Past and Future,” by William F. Ruddiman)


In the carbon cycle diorama, the size of each reservoir  is expressed in GtC, and the transfer of carbon between reservoirs are written as GtC/yr. GtC stands for “Gigatons of Carbon”, which is the same as one billion tons of carbon.

One GtC/yr means one billion metric tons of carbon moved between reservoirs in one year. 

You can think of the carbon in each reservoir as a tiny building block – carbon is, after all, an atom. Under foot, carbon is a building block that helps create the structure of rocks and minerals. All around us, organic carbon forms the building blocks of life. In oceans and rivers, carbon is a building block of various molecules that exist together with H2O in all but the purest water. In the atmosphere, carbon is the central building block of several greenhouse gases, including carbon dioxide (CO2) and methane (CH4).


The biggest carbon reservoir on earth is in rocks, weighing in at some 66 billion metric tons of carbon. In very rare instances (as in roughly .004%), carbon in rock is in the form of coal, oil or natural gas. Most of the time it occurs as a chemical component of plain old granite, sandstone or limestone.

Carbon can leave rocks and enter the atmosphere. And, it can leave the atmosphere and go back into rocks.

Here’s how the first part works: rock-bound carbon enters the atmosphere via volcanoes, as shown by the yellow arrow in the diorama. (Apologies for not having drawn in a volcano; every good diorama should have one.)

For the second part: as wind and rain break down rocks over eons, CO2 is taken out of the atmosphere and put back into “rock” form as sediment (brown arrow).

The amount of carbon that enters and exits the atmosphere from volcanoes and into sediment each year is tiny compared to the amount that we emit by fossil fuel burning. Tiny, as in volcanoes typically emit less than 1/100th the amount of CO2 that humans emit every year.

For the record, human fossil fuel emissions may have  hit 9.7 billion metric tons of carbon in 2012 (emissions are still being tallied).

When fossil fuels are burned, the CO2 released enters far less stable reservoirs: first the atmosphere, and from there the trees and plants around us, and the ocean. Let’s look at how those reservoirs function, and what happens when they can’t handle any more carbon.


The next-biggest reservoir for carbon on Earth is the ocean. Scientists tend to split the ocean into two ‘pools,’ like a two-layer cake. The top layer goes from the surface to 100 meters down. Wind sloshes the water around, allowing CO2 gas to exchange with the atmosphere.

The bottom layer – or deep ocean – is bigger and less exposed to the atmosphere, and is therefore a good long-term storage place for large quantities of carbon.

Carbon moves between the ocean and atmosphere by diffusion. When the level of CO2 in the atmosphere increases, some of it dissolves into ocean water.

Now, back to our diorama. Notice that the value of the white “into-the-ocean” arrow is slightly bigger than that of the blue “out-of-the-ocean” arrow. This indicates that the ocean is sucking up excess carbon from the atmosphere.

It is fabulously useful that the ocean absorbs some of the excess CO2 in the atmosphere. Due to this imbalance, the oceans have been offering us a major (25%) global warming discount every year. In other words, 25% of fossil fuel carbon we emit gets drawn into the ocean for good. If the ocean weren’t such a sink for CO2, more would remain in the atmosphere, and more global warming would be happening.

There is a very bad downside to this discount, however. When ocean water absorbs carbon, it becomes more acidic. Hence the current degradation of the world’s coral reefs.

Furthermore, this discount won’t last for too much longer. The ocean’s chemistry will soon hit a threshold where it will stop absorbing CO2. When that day comes, we’ll have to reckon with much more global warming impact from each coal reserve and tank of gas that we burn.


The final reservoirs for carbon are the atmosphere and the terrestrial biosphere. As you can see in the diorama, they hold roughly equal amounts of carbon – a quantity close to that of the surface ocean.

Plants draw CO2 out of the atmosphere during photosynthesis. CO2 is plant food. During the night, some of that CO2 is returned to the atmosphere. When plants die and decompose, all of the rest of that CO2 is either returned to the atmosphere or turns to organic matter in soil.

Before the industrial revolution, the atmosphere contained roughly 600 GtC. As of March 2013, that number had risen to 843 GtC: a 40% increase. If the world’s oceans and plants hadn’t been sucking excess CO2 out of the atmosphere all these years, the increased burden of CO2 in the atmosphere could be something closer to 60 or 70%.

You’re rereading that sentence, aren’t you? Yes, that’s right. It’s not just the oceans; land plants are also sucking more CO2 out of the atmosphere than they are emitting back to the atmosphere. In fact, plants and the sea combined provide a 50% discount on emissions… as in, if these natural systems weren’t absorbing CO2, global warming would be twice as bad. This is an unbelievable stroke of good fortune for humans today.

The plant half of the discount is occurring because plants like a little bit of extra CO2 in the air – they use CO2 like we use food.

Sadly, though, we are close to reaching a level of atmospheric CO2 where plants will stop absorbing excess carbon from the atmosphere. Like a kid in a candy store, even plants hit the wall at some point and can eat no more.

As you can see, once carbon is unlocked from long-term storage as fossil fuels, that carbon goes into the atmosphere, land plants, and the surface ocean. One small forest fire, and all of the carbon stored in land plants returns to the atmosphere again to increase global warming. One Gigaton too many into the oceans and their waters will stop absorbing CO2.

The carbon cycle represents a vast and delicate balance. It seems clear that the safest option is for fossil fuels to stay deep underground where nature stored them millions of years ago.

Caroline Alden is a graduate student at the Institute of Arctic and Alpine Research in the Department of Geology at the University of Colorado at Boulder.

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