How Do We Know That Humans Are Responsible For Rising CO2?
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.
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.
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.
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 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.
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.
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.)
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, right.)
Taking all these lines of evidence together, there is little room for doubt that it is we humans who are increasing atmospheric carbon dioxide.
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:
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.)
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