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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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