How do we know humans are responsible for climate change?

I write this a few days after David Attenborough’s program, “Climate Change – The Facts” appeared on the BBC. It received great acclaim but like so many accounts it explained the consequences, not the science behind how we know climate change is anthropogenic. It’s not surprising there are so few accounts of the science because it is rather complex and hence difficult to explain to the public. That complexity then leaves the door open for those who wish to mislead with over-simplifications. There are, in fact, multiple avenues of scientific evidence that humans are responsible for climate change, but I will explain just one of them.

Carbon dioxide is a greenhouse gas. It’s not the only greenhouse gas, but by far the most significant regarding rising temperatures since the industrial revolution. It gets its dubious reputation because it’s transparent to solar heat that passes through the atmosphere but absorbs infrared that irradiates back off the ground. The absorbed infrared then heats the troposphere leading to a rise in average surface temperatures worldwide. By analysing bubbles of gas trapped in ice cores we know that in 1790, at the start of the industrial revolution, atmospheric carbon dioxide concentrations were about 280 parts per million. At the time of writing (April 2019) this has increased to 412 parts per million which is the highest for 800,000 years.

The first reports linking carbon dioxide with global temperature were as early as 1896. Concerns that rising industrial outputs of carbon dioxide were warming the planet appeared in the 1950s. Nowadays 97% of climate scientists agree that the cause of the warming planet is human activity but there are still those who claim it isn’t real.

Nigel Lawson, who served in the government under Margaret Thatcher from 1981 to 1989 and was once the Secretary of State of Energy, wrote a book in 2008 called, An Appeal to Reason: A Cool Look at Global Warming. Here he says, “…. the great majority of those scientists who speak with such certainty and apparent authority about global warming and climate change, are not in fact climate scientists, or indeed scientists of any kind and thus have no special knowledge to contribute.” Of course, Nigel Lawson is no scientist either. If he was, then he might have known that climate change, like so many branches of science today, is multidisciplinary. My own area of expertise is with isotopes and isotope chemists have provided some strong evidence that climate change is indeed a result of human activity.

The periodic table lists 92 naturally occurring elements from hydrogen to uranium. Elements are defined as the fundamental building blocks of matter that cannot be broken down into simpler substances. Each element comprises its own type of atom that contains a unique number of protons; one proton for hydrogen, two protons for helium, three protons for lithium and so on, to 92 protons for uranium. Atoms also contain neutrons and while the number of protons defines the element, so the number of neutrons defines the isotope. Take, for example, the three principal isotopes of carbon called 12C, 13C and 14C. All carbon atoms have 6 protons but 12C has 6 neutrons, 13C has 7 neutrons and 14C has 8 neutrons (the sum of protons and neutrons is where the number in front of the C comes from).

All three isotopes of carbon will combine with oxygen to make carbon dioxide but the properties of each isotopic form are slightly different. We’ll come to 14C in a moment but let’s first consider 12C and 13C. When plants take in carbon dioxide during photosynthesis, they preferentially use 12C over 13C and so plant-based material is 13C-depleted. This phenomenon has utility in many scientific fields such as the study of food chains and to authenticate plant-based products. Largely formed from vegetation many millions of years ago, fossil fuels are depleted in 13C. If the increase in carbon dioxide comes from fossil fuel therefore, we should see atmospheric 13C concentrations becoming progressively lower over time. And indeed we do. In fact atmospheric 13C levels today are the lowest for 10,000 years. We know this from variations in tree ring 13C data obtained as an offshoot of archeological dendrochronology and from bubbles of gas trapped in ice cores.

14C is different to 12C and 13C because it is a radioactive isotope which decays away over a few thousand years. Constantly generated by cosmic ray bombardment, the atmosphere and biosphere contain small amounts of 14C. Because fossil fuels were formed over 100-million years ago however, any 14C would have long decayed away. As with 13C, if the increase in carbon dioxide comes from fossil fuel we should also see atmospheric 14C concentrations progressively decreasing over time. There is a glitch however, because of the atomic bomb tests in the 1950s, which injected large amounts of 14C into the atmosphere. Globally this doubled the atmospheric levels of 14C and therefore masked any potential dilution from fossil fuels. (A previous blog told the story of how the 14C from atomic bombs shows the age of cells in the body). We can nevertheless observe a decline in atmospheric 14C from the start of the industrial revolution up until 1950 by looking at those aforementioned tree rings and ice cores. There is, in fact, a direct relationship between the fall of 14C and the rise in total atmospheric carbon dioxide. Even with heightened bomb related 14C in the atmosphere it’s still possible to see depletion in areas of significant industrial activity, such as over China. All this clearly points towards fossil fuel being the source of rising atmospheric carbon dioxide, but what if this is coincidental and it comes from other places?

Is it possible that carbon dioxide is coming from geological sources? Because of its immense age geological carbon dioxide is free of 14C and so that would be consistent with the observed decrease of this isotope over time. The chances that geological seepage just happens to coincide with industrial centres is unlikely but is there more evidence to rule out this source of carbon? As it happens there is because geological sources of carbon dioxide have significantly larger amounts of 13C compared to fossil fuels. Indeed, if volcanic carbon dioxide was the source, then atmospheric 13C levels should increase not decrease.

What about the oceans? There is a common assertion amongst those denying anthropogenic climate change that rising carbon dioxide levels are not the cause of increased temperatures but instead arise from oceanic degassing as a result of the Earth warming by some other natural process. The problem with this is that gaseous exchange with the ocean occurs on the surface and surface carbon dioxide has the same levels of 13C and 14C as the atmosphere. If the oceans were the source of increasing atmospheric carbon dioxide, then there should be no change in the proportions of these isotopes.

When scientists proclaim a 97% confidence, then it’s never based on a single line of evidence. There are instead many independent lines of evidence all converging and pointing in the same direction. This is indeed the case with anthropogenic climate change but I have limited myself to one aspect of the evidence here. In fact I have simplified the isotopic evidence for clarity and there are many other aspects which also point in the same direction. No matter which way you look at it, the isotope data tell us that the source of rising atmospheric carbon dioxide is fossil fuel and human kind is the guilty party.

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