How can climate scientists be so sure burning fossil fuels is responsible for global warming?
Perhaps the most reliable method is isotopic fractionation, but first of all, what are isotopes?
Let’s start with the simplest element, hydrogen. The basic form of hydrogen consists of one proton and one electron. Hydrogen with no neutrons is just basic vanilla hydrogen. Hydrogen with one proton plus one neutron or 2H is called deuterium; and hydrogen with one proton plus two neutrons or 3H is called tritium. Tritium is a little unstable and decays over time. These three forms of hydrogen have different weights but react exactly the same chemically. They are “isotopes” of hydrogen. Hydrogen 1H and 2H are stable isotopes and don’t change over time.
How This Connects to Climate Science
We have solid documentation that carbon dioxide (CO2) has been increasing in the atmosphere. Where is it coming from? We may actually learn more by looking at the individual elements atmospheric gases are made from. The fact that isotopes of the different elements can have different weights means that they require more or less energy to move around even if they don’t react differently chemically.
More heat in the atmosphere means there is more energy in the system. In a low energy system the lighter isotopes move more freely, while the heavier isotopes may be left behind. In a higher energy system the heavy isotopes will also move. Measuring the ratio of lighter to heavier isotopes will give us an idea whether the environment was warmer (had more energy) or cooler (had less energy). In a warmer environment there should be a higher ratio of heavy isotopes to lighter isotopes. In a cooler environment there should be a higher ratio or fraction of lighter isotopes to heavy isotopes.
Remember 1H (hydrogen) and 2H (Deuterium) are stable and don’t deteriorate over time. That means that it would be possible to look at the fraction of heavy to lighter isotopes and get a picture of the energy (heat) in past atmospheres. We can find samples of past atmospheres in many ways. Bubbles trapped in glacial ice is just one way.
The hydrogen doesn’t even have to appear as pure hydrogen. Any molecule that has hydrogen in it like H2O or even a mineral with a hydrogen atom can be used. The ratio of lighter to heavier isotopes will give us clues to the energy in the system when that molecule was formed.
Two other elements used to investigate ancient atmospheres are oxygen and carbon. The simplest isotope of oxygen has eight protons and eight neutrons. Add the neutrons and protons and we get O16. There are also forms with nine and ten neutrons (O17 and O18). Oxygen 17 is rare so it doesn’t factor into our ancient climate investigation.
Oxygen and hydrogen can combine to make H2O. We have loads of that on Earth. When we combine these two elements they form a molecule that can have different weights. Water (H2O) may have two 1H atoms, or a 1H and a Deuterium, or two deuterium (2H isotope). The oxygen atom in H2O could be O16 or O18. These H2O molecules have different weights but will also occur in different ratios depending on the energy in the environment. Water (H2O) is involved in many of the chemical reactions that take place on Earth. We find water in liquid reservoirs, in the soil, in rocks, glaciers and sediment strata.
Water changes state as it evaporates. On a cooler Earth the light water molecules tend to evaporate into the atmosphere. As it condenses it may fall as rain or snow. Cool Earth snow and ice will have a high ratio of light water molecules. Snow and ice can sublime to water vapor. This further concentrates lighter water molecules in the atmosphere and next generation snow and ice. The inverse is the case in a warmer Earth where we would expect to find a higher ratio of O18 to O16.
The fraction of Deuterium and other lighter, stable isotopes can give us information on the energy (heat) in the past environment. We determine when that time was from various “proxies” like strata, tree rings, fossil records, radioactive decay and many other indicators.
If we compare the ratio analysis of isotopes in our sample to a known standard, we can actually estimate the temperature range of an ancient atmosphere.
Plotting isotopic ratio analyses can also give us information on the rate of change over time.
This brings us to carbon. Isotopes of carbon occur as C12, C13 and C14. Carbon 14 is unstable and deteriorates over time. In the short term we can use the decay rate of C14 to determine age but it is not useful when trying to date things that are millions of years old.
What is really important about carbon is that it is in all living things. Life has a strong preference for lighter carbon or C12. When plants pull in CO2 from the atmosphere in photosynthesis, they prefer C12. The oxygen is released back to the atmosphere and C12 becomes concentrated in the plant tissue.
The ratios of isotopes in modern history since the last ice age have remained relatively constant until about 150 years ago. Then the ratio of C12 to C13 began to change. The CO2 molecules in the atmosphere not only increased from about 282 ppm to over 400 ppm today, but the ratio of C12 to C13 changed dramatically. The carbon isotope C12 began to increase. The change corresponded to the onset of the industrial revolution and increased use of fossil fuels.
Where Did All That Lighter C12 Come From?
Around 300,000,000 years ago (Carboniferous Epoch), dead and decaying plant and animal life was deposited in the earth and became modern fossil fuels. Fossil fuels are concentrated C12. Remember life prefers the lighter C12 carbon, so we find fossil fuel molecules are almost entirely made with C12.
If the C12 is increasing in the atmosphere when it should be constant, it must come from somewhere outside normal sources. Here is the smoking gun that is held firmly in our hand. The CO2 increase in the atmosphere (almost entirely C12) comes from burning fossil fuels. Decades of looking for other sources have found nothing capable of producing the amount of C12 we detect in today’s atmosphere. The conclusion reached by 97.2 percent of all climate scientists is that it can only be ancient C12 from the combustion of fossil fuels.
Isotopic fractionation firmly points to fossil fuels as the source of increasing CO2 in our atmosphere. So there you have it.