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How Do We Know That Humans Are Responsible For Rising CO2?

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How do we know that we really need to act on climate change?

That's a high-stakes question for today's world, and it's the question addressed by this series of Hubs. In the first of the series, How Do We Know That CO2 Is Warming The Planet?, this big question was broken down into three sub-questions:

1) How do we know that CO2 (and other greenhouse gases) cause planetary warming?

2) How do we know that we are responsible for the measured increase in greenhouse gas concentrations?

3) How do we know that warming will be a bad thing if it continues?

Summing up the answer to the first question from the first Hub: we know from nearly two centuries of painstaking scientific work that rising atmospheric CO2 should—some might say "must"—warm the Earth. CO2 concentrations have been observed to increase, by about 40%. And, sure enough, temperature has risen, too, just as predicted.

As Roger Revelle and Hans Suess wrote in 1957:

Thus 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. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years. This experiment, if adequately documented, may yield a far-reaching insight into the processes determining weather and climate.

After running the 'experiment' for the best part of six decades, the result is pretty clear.

But, some folks have asked, are humans really responsible for these rising levels of CO2, and other greenhouse gases? Might not warming cause increased CO2, rather than the other way around?

It sounds reasonable, especially since it is known that warmer water holds less CO2 than colder water does. But the short answer is "No, humans are definitely causing the increase in CO2." In this hub, we'll answer the question:

How Do We Know That Humans Are Responsible For Rising CO2?

There are three independent lines of evidence showing this. We'll consider each in turn.

A Chinese coal-fired power plant.

A Chinese coal-fired power plant.


Steely-eyed Accounting

First is simple accounting. It is tedious, but simple in principle, to add up the fossil fuel that we burn and to compare that to the observed increase. Doing so reveals that our CO2 emissions amount to roughly twice the observed increase. That is because the oceans, forests and grasslands of our world are absorbing about half the CO2 we vent into the atmosphere.

It is true that natural CO2 ‘fluxes’—the exchanges between atmosphere and other parts of the Earth system—are far larger than the human contribution. For example, every year the growth and decay of new foliage in temperate forests creates an observable ‘wave’ in the CO2 record. But the relative stability of atmospheric CO2 prior to the Industrial era shows that these fluxes were in rough balance. Presumably, they still are—though they may not remain so indefinitely.

The CO2 ‘balance’ can be compared to a bank account balance. If income and spending are equal over time, the balance will not change. But take on even a small additional expense, and that can change: if the expense is consistent, like a car payment, the balance will decrease. Continue long enough, and you’ll go broke. Similarly, the human CO2 emissions have been small but consistent. Our planet’s carbon budget has been seriously disrupted as a result.

Trend in Carbon 13 ratio, ~1978-2013.  "SO" denotes South Pole Observatory, while "MLO" is Mauna Loa Observatory.

Trend in Carbon 13 ratio, ~1978-2013. "SO" denotes South Pole Observatory, while "MLO" is Mauna Loa Observatory.

A Nuclear Signature

A second line of evidence showing the human origin of increased atmospheric CO2 is its isotopic composition. Most people know about carbon dating, which is often used to date archaeological finds. It uses measurements of the abundance of different forms of carbon to estimate the age of organic remains, such as wooden artifacts or bone fragments.

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A similar analysis shows that atmospheric carbon is increasingly derived from fossil fuel sources. Fossil-fuel derived CO2 contains little Carbon 13, and no Carbon 14; it is almost all Carbon 12. Therefore, as large amounts of fossil CO2 are released, the atmospheric abundance of the two heavier types of carbon should decrease. In fact, that is just what has been observed, in what has become known as the “Suess Effect.” (Dr. Charles David Keeling, who created the Mauna Loa Observatory CO2 measurement program, investigated the Suess effect as related to atmospheric CO2 as far back as 1979.)

That's shown in the graph (above right), in which the green line traces the declining proportion of atmospheric C13. It's essentially a 'fingerprint' of fossil fuel burning.

CO2 increase, ~1957-2013, versus O2 decrease, ~1992-2013.  "CGO" is Cape Grim Observatory, (Tasmania, Australia); "ALT" is Alert Observatory (Alert, Nunavut, Canada).  Note higher O2 and lower CO2 for southern hemisphere.

CO2 increase, ~1957-2013, versus O2 decrease, ~1992-2013. "CGO" is Cape Grim Observatory, (Tasmania, Australia); "ALT" is Alert Observatory (Alert, Nunavut, Canada). Note higher O2 and lower CO2 for southern hemisphere.

Oxygen Loss

Third, when carbon-containing fuels are burnt, the resulting waste CO2 is formed from two sources: the carbon comes from the fuel, while the oxygen comes from the air. So as we burn fossil fuels, we will also deplete atmospheric oxygen.

'Oxygen loss' sounds alarming, but fortunately there is much, much more oxygen in our atmosphere than carbon dioxide. Contrast the 400 parts per million CO2—that’s just 0.04%--with the 21% concentration of oxygen! Still, though the decrease in oxygen is small, it has nonetheless been successfully measured, and is consistent with the evidence discussed above.

"Wally" Broecker, Newberry Professor of Geology, Lamont-Doherty Earth Observatory, Columbia University.

"Wally" Broecker, Newberry Professor of Geology, Lamont-Doherty Earth Observatory, Columbia University.

As geochemist Wallace Broecker wrote:

While no danger exists that our O2 reserve will be depleted, nevertheless the O2 content of our atmosphere is slowly declining--so slowly that a sufficiently accurate technique to measure this change wasn't developed until the late 1980s. Ralph Keeling, its developer, showed that between 1989 and 1994 the O2 content of the atmosphere decreased at an average annual rate of 2 parts per million. Considering that the atmosphere contains 210,000 parts per million, one can see why this measurement proved so difficult.

This drop was not unexpected, for the combustion of fossil fuels destroys O2.

Ralph Keeling, the son of David Keeling (mentioned above), has shown that atmospheric oxygen has declined by roughly 600 parts ‘per meg’—that is, per million oxygen molecules—since 1990. (If my math is to be trusted, that means a decline of about 126 ppm in terms of overall atmospheric abundance.)

AR5, WG I, Chapter 6, figure 6.13.  The X-axis (horizontal) plots the difference between NH and SH emissions, while the Y-axis (vertical) shows the difference between hemispheric CO2 measurements.  The line indicates strong correlation.

AR5, WG I, Chapter 6, figure 6.13. The X-axis (horizontal) plots the difference between NH and SH emissions, while the Y-axis (vertical) shows the difference between hemispheric CO2 measurements. The line indicates strong correlation.

It's About Time, It's About Space

A fourth line of evidence arises from the fact that industrial production differs between hemispheres. Although carbon dioxide mixes well in the atmosphere, mixing is not quite perfect and does take time.

So, if you compared the hemispheric sources of carbon dioxide with hemispheric observations, you should find that the two correlate as industrial production shifts over time. And that is just what is observed. (See graph, above.)

Taking all these lines of evidence together, there is little room for doubt that it is we humans who are increasing atmospheric carbon dioxide.

NYC smog, 1988, seen from the World Trade Center.  Image courtesy Wikimedia Commons.

NYC smog, 1988, seen from the World Trade Center. Image courtesy Wikimedia Commons.

Coda: What of the future?

Our question is answered, and so, properly, this part of our discussion is at an end, and we could go on to consider the final 'sub-question', which asks "Is there reason to worry?"

But while we're talking about atmospheric concentrations, let's consider the future. As mentioned, we've seen an increase in atmospheric CO2 of around 40% so far. What might the future hold? How much more of an increase might we expect to see as the century passes?

There's no definite answer, of course, since what will happen depends upon what we choose to do over coming years. But we can estimate possible ranges, and that is just what was done in the most recent report by the International Panel on Climate Change, or IPCC. In that report ("AR5" for short), the panel created 'pictures' of how greenhouse gas emissions might evolve this century. These "Representative Concentration Pathways" range from RCP 2.6, (smallest increase) to RCP 8.5 (greatest increase). Here's what they look like on a graph:


As you can see, there is a big difference between extremes: RCP 8.5 leads to concentrations of nearly 1250 parts per million by the end of the century, while RCP 2.6 peaks at just above 450 ppm around mid-century, then slowly declines.

If you read the first Hub in this series, you may recall that climate sensitivity is thought to be about 3 degrees Celsius per 'doubling' of greenhouse gases. So 450 would represent less than one doubling from the pre-Industrial value of about 280 ppm, and 1250 would be more than 2 doublings. If the 3 C climate sensitivity value were to prove correct, then that would imply global warming of between 1.5 and 3 degrees Celsius.

AR5's projections are a bit more sophisticated than that, of course, and the warming would not be uniform around the globe. Here's what the projections give, mapped globally:

AR 5, WG I, Chapter 12, Figure 12-11.

AR 5, WG I, Chapter 12, Figure 12-11.

It should be mentioned, though, that RCP 2.6 seems extremely optimistic at present, as the decline in emissions initially is very steep, and after 2070 or so, humankind is projected not only to not be emitting any CO2, we are also projected to be actively removing CO2 from the atmosphere! By contrast, RCP 8.5 seems fairly close to what we are doing now—though with the agreement of the US and China, the world's largest emitters, to curb emissions, and with optimism in the air for international agreement in Paris at the end of 2015, there is some room at present to hope that we may, over the next few years, begin to bend the Keeling curve downward a bit.

But let's get back to our main topic, and the remaining question:

How do we know that warming will be a bad thing if it continues?

That's the topic for the next two Hubs in this series. The first is here now, just one click away! (Scroll down to the heading "Related Hubs." Just above that heading is a box including an icon for "Next"; that's where you'll click.)

Relevant Hubs by Doc Snow


Doc Snow (author) from Camden, South Carolina on April 15, 2019:

Yes, exactly. As an artsie myself, I have to say it's very much the scientific mindset on display with that graph, in my opinion; it's very logical once you get where they're coming from, but it's just not necessarily the way the rest of us would first think about it, or think to frame it!

Scott Belford from Keystone Heights, FL on April 15, 2019:

Thanks for the clarification, now it makes perfect sense.And it explains why I see the Blue area increasing over time which reflects the rapid increase in atmospheric CO2

Doc Snow (author) from Camden, South Carolina on April 15, 2019:

Thanks, Scott. Yes, that chart isn't the most intuitive presentation ever, is it?

My understanding of it is that the lower half of the graph isn't just sinks, as one would naturally think; it's a representation of how the total emissions shown in the upper portion are divided up. ("Partitioned.")

So the total emissions are 'partitioned' into 3 categories: land sinks, marine sinks, and what is retained in the atmosphere (that last being the "Measured Atmospheric Growth Rate" that you are asking about.)

Put another (and maybe better) way, the top half of the graph shows where emissions come from, while the bottom half shows were they end up. That's why the curves at top and bottom are exact mirror images of each other: the carbon budget has to balance on an ongoing basis.

Scott Belford from Keystone Heights, FL on April 15, 2019:

Great Hub, Doc. It added to my knowledge base, especially the part about the proportion of isotopes - a real bell-ringer that one.

Please explain, however, the blue area (second chart), which looks like a sink, but is titled Measured Atmospheric Growth Rate.

Doc Snow (author) from Camden, South Carolina on September 13, 2015:

The criteria for placing Mauna Loa are publicly available. It's remote, it's high, the vegetation signal is small, and despite your obsession with the volcano, it's quite simple to tell when volcanic CO2 gets blown that way.

I'm not sure which observatories you are talking about, since the six 'baseline' ones are:

Barrow, AK

Trinidad Head, CA

Mauna Loa, HI

American Samoa

South Pole

Summit, Greenland

Of these, only Mauna Loa is within 10 miles of a volcano. (Though the article linked below appears to be the source of your concerns. It puts the American Samoa observatory "150 miles downwind"--as if the wind blows in only one direction!--from a (then) new volcano--an undersea one. (CO2 is soluble in seawater--the oceans do indeed absorb considerable human-emitted CO2.))

So you must be talking about non-baseline observatories--in which case, 4--accepting your number--out of 166 isn't very impressive.

But put all that stuff aside. It's conceivable in theory that volcanoes could elevate CO2 levels. But how is it that they 'just happen' to synchronize their CO2 emissions rates in such a way as to create a rise in CO2 that's coherent with each other, with observatories that aren't near volcanos, and that are consistent with the overall "Keeling curve" as well as human emissions.

I'll tell you, in two words:

They can't.

Article link:

somethgblue from Shelbyville, Tennessee on September 13, 2015:

Oh for Crying Out Loud of laughing out loud which ever you prefer, the difference between volcanic CO2 and manmade CO2 is indistinguishable (that means you can't tell the difference), so why would you put a CO2 observation station right next to a volcano? Things that make you go Hmmm.

Oh wait why would you put not just one but four CO2 observatories within 10 miles of active volcanoes?

Yes, the ol' Earth System Research Laboratory established way back in 2006, gee less than 10 years old, hmmm can we conclude they have decades worth of data to base their conclusions on?

This is what the ESRL had to say about satellite-based COs sensing and I quote "Greenhouse gas abundances derived from optical absorption measurements from space can never be calibrated because one cannot control the abundance of the gases being estimated, nor can we control potential interfering factors in the optical path."

Oh no, I'm convinced LOL, folks that are in denial often are more concerned about appearing right to justify an sooth their egos, than they are about the truth.

None of that data could possibly be manipulated or altered to suit a vast money making machine, oh no your right, my humble apologies for wasting your time.

Doc Snow (author) from Camden, South Carolina on September 13, 2015:

Oh, for Pete's sake. Mauna Loa observatory is well above the volcanic vent. Occasionally, winds do loft the volcanic gases to the observatory. However, the staff is aware of that because, you know, they are measuring CO2, and they see the spike in the data.

Your criticisms of the siting of other CO2 observatories are strained, at best. I'll let readers make their own judgments, based on ESRL photos:

Moreover, there is a good deal more information available than just those stations. The ESRL Greenhouse Gas Reference Network comprises "166 sites in 46 countries."

As you will notice, there is also sampling from aircraft and balloons.

In addition, there is now satellite-based CO2 sensing:

And if all that is too subject to the sinister manipulations of 'the conspiracy', then you can splash out $229 and measure it yourself:

somethgblue from Shelbyville, Tennessee on September 13, 2015:

The real question you need to ask is where does all this information come from, what is all this 'science' based on?

There are about six or seven CO2 monitoring stations (observatories) located around the world that gathers this data, what can we learn from their locations?

Two are in Hawaii located downwind from two active erupting volcanoes, which we both know spew large amounts of CO2 into the atmosphere.

Two are located within the Arctic circle and are subject to CO2 emissions from the North Shore Oil industry and from Chinese and Soviet coal-burning plants that smelt nickel, copper and tin.

One is located at the South Pole right next to an electrical power plant that burns jet fuel 24/7-365 days a year.

One is in American Samoa downwind of three active erupting volcanoes and numerous submarine volcanic activity.

The Kermandec Island Observatory is located 10 km downwind of the active Raoul Island volcano.

Another one in New Zealand is located on a major fault zone with numerous submarine volcanoes and is on the trade winds for other volcanic activity.

So when I tell you that volcanoes are the major reason for the global Warming/Climate Change scenarios being touted by scientists (lobbyists) the world over, I was hoping you might do some due diligence and find out why?

This is the main reason I consider Climate Change a scam, because you cannot use this data tainted by volcanic activity and dismiss volcanic activity altogether, you can't have it both ways.

Now consider that their are over 100,000 active erupting volcanoes on the Oceans floors less than 4% of all the 3,000,000 volcanoes down under the waves and you begin to see how keeping the public and even researchers unaware is pretty easy.

The contention is not if manmade CO2 exist it's whether or not it is being reabsorbed into the natural recycling process. My contention is that it is and the data for this so-called science is based on flawed and skewed information designed specifically to show an increase.

The locations of these CO2 Observatories were not picked randomly.

Doc Snow (author) from Camden, South Carolina on March 12, 2015:

True enough, but a little short to make a Hub. ;-)

heinz57 on March 12, 2015:

How do we know that humans cause CO2? Because bears do not drive cars,Mountain Lions do not use oil,hawks do not use electricity,and wolves do not use spray cans.

Doc Snow (author) from Camden, South Carolina on March 02, 2015:

Yes, it's rather odd, isn't it?

And of course some of them seem to think that they are experts on reproductive biology as well, exceeding the level of knowledge possessed by the 'medical mainstream'!

It has been well said that one of the greatest assets of the Wright brothers, in creating the first airplane, was their intellectual humility. They knew what they did, and didn't, understand, and that was a great advantage in addressing what they *needed* to understand in order to accomplish their goals. Saying "I'm no scientist... [but I'm going to ignore the great bulk of those who are]" is pretty much the opposite. It's failing to ask the right questions to do the job. Faux humility versus the real thing.

But I shouldn't get into a full-fledged rant here. Thanks for your kind words about what you correctly call a summary!

And, by sheer chance, I just published the followup piece:

Just came back by to edit the last few words to reflect that!

Dr Billy Kidd from Sydney, Australia on March 02, 2015:

Great detailed summary touching so many of the basics.

It is interesting how U.S. politicians say they are not climate experts so they no nothing about this. Meanwhile, they all think they are expert economists!

Doc Snow (author) from Camden, South Carolina on March 02, 2015:

Great question. In the interest of keeping the Hub reasonably brief, I treated the isotope question pretty superficially, so let me expand the thought a bit here. (And I think I helped create your confusion by dragging in the bit about carbon dating. The 'signature' I'm talking about isn't carbon dating, but it does use the study of carbon isotope ratios, which is also what carbon dating does--and which people generally know about.)

Anyway, we *are* seeing more carbon in the atmosphere--that's directly measured with high precision since the late '50s, and with pretty good precision before that through a combination of older scientific measurements and measurements of CO2 in air bubbles trapped in ice cores. That ice core record now extends back 800,000 years, which is about 4x longer than modern humans have existed.

Where the isotope piece comes in is in determining that the carbon is from fossil fuels, and not from some natural source. Here's how physicist Spencer Weart explains it:

"Another marker of biological activity was the rare isotope carbon-13. Plants take less of it from the atmosphere than the lighter isotope carbon-12, so the latter is over-represented in coal and oil. The fraction of the lighter isotope in the air was increasing, proving (to a lingering band of skeptics) that the rise in CO2 came from humanity's use of fossil fuel, not from a mineral source such as volcanoes."

In other words, the atmospheric C13 ratio, and its change in comparison to human CO2 emissions, forms a kind of 'fingerprint.' Dropping C13 shows an increasing proportion of carbon from fossil sources.

(The C13 ratio is shown in the graph next to the heading "Nuclear Signature", above. It shows the decreasing C13 content, which is the 'flip side of the coin' for what Weart calls the increasing "fraction of the lighter isotope [ie., C12, 'normal' carbon.])

Does that make it clearer?

Thanks again for a great question, and I'll try to come up with a change that makes this a bit clearer in the body of the Hub, too.

James Packard from Columbia, Missouri on March 01, 2015:

Thanks for writing, Doc. I don't quite understand the carbon dating piece you've described under 'Nuclear Signature'. So, are we seeing evidence of more carbon in the atmosphere compared to years, decades past in rocks and organic material? What are you saying happens when fossil fuel is released into the atmosphere?

Doc Snow (author) from Camden, South Carolina on March 01, 2015:

Fascinating thought. But it's not true that "we haven't at all considered [water] in connection with climate change . . . except as an unfortunate aftermath." Water vapor provides the single largest greenhouse gas forcing, though it is dependent upon the stable CO2 forcing for its 'base level.' (I'm speaking very approximately here, in the interest of brevity.)

Another instance would be the process of 'evapotranspiration', in which the change of water into water vapor carries large amounts of heat into the atmosphere--it's a significant factor in the Earth's surface energy budget. See, for instance, this diagram:

So any quantitative study of climate unavoidably involves studying water, in all of its forms.

Susette Horspool from Pasadena CA on February 28, 2015:

I'm not looking for the earth to adapt so much, but I do believe that water plays a huge part in this scenario, and we haven't at all considered it in connection with climate change . . . except as an unfortunate aftermath. Rain and snow both capture carbon and take it down. But they aren't falling where they used to anymore, because of the way we've built our cities. I believe that the measures we're taking to remedy drought situations will also "correct" the rainfall, which will in turn start removing more carbon from the air, and THAT will have a remediating effect.

Doc Snow (author) from Camden, South Carolina on February 28, 2015:

The Earth isn't at risk. The current warming is remarkably fast, but the Earth has been through 'hothouse' and 'snowball' episodes in the deep past, and each time, life has survived. Though it was a pretty close call at the so-called "end-Permian wipeout", roughly 251 million years ago, when it appears that around 95% of all life was destroyed, taking nearly 50 million years to regenerate a comparable level of biodiversity. See summary here:

But traditional wisdom does not tell us that our Creator protects us from the worldly consequences of our own bad choices: in the Judeo-Christian tradition, Adam ate the forbidden fruit, and suffered death as a result. Nor does consideration of the world around us say that this wisdom is wrong: around the world, we see the innocent (and the not-so-innocent) suffer because of foolish or evil choices made by themselves or others. (I trust there is no need to cite specific examples.)

Why then would we assume that if we continue to choose to guide our actions by greed for short-term gain, rather than the intelligence and wisdom with which we have been gifted, that the natural consequences will not fall upon our heads?

Mark Johann from New Zealand on February 28, 2015:

With all these pollution, I still believe that earth has its own way of adapting to survive just like us humans. Our Creator made this earth and there is now way that He will leave us unprotected.

Doc Snow (author) from Camden, South Carolina on February 27, 2015:

Funny you should mention that. All I can say is, watch for the next two Hubs in the series!

P. Orin Zack on February 27, 2015:

Focussing on any aspect of a problem invites many people to just shrug and say, 'So what? Why is that important?' Spoken of in isolation, the increase in carbon dioxide is meaningless to large numbers of people. To get the point across to them, we have to speak in terms of how such a think affects them. A slow increase in global temperature, even if it causes sporadic changes to local climatic norms, is still a hazy thing to such people. So when you start speaking in terms of everyday effects, you'll get more of a rise out of them.

What's that? You say the shellfish will suffer? That their shells will be soft and they'll fall prey to other sea life? If the person eats shellfish, they might care, but not everyone does. Why should those people care? We need to point up all of the many ways in which the world around all of us will be harmed in order to make it real to the masses. What are the secondary effects of the change in the pH of the oceans? How will several inches of increased sea level along the north east US affect the economy of the country? What happens when it rises beyond that? What happens when the oceanic gyres stumble and fail? There are so many ways that people are affected by a rise in CO2, but little of it is obvious. Which sectors of the population will be affected first or most? And on and on.

Getting people motivated is going to be the hardest challenge.

Doc Snow (author) from Camden, South Carolina on February 27, 2015:

Thanks for reading my Hub… hope you learned something! (Or at least enjoyed the read.)

How do you see the story of human (and natural) greenhouse gas emissions? Where do you think we're headed?

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