The Suess Effect...

Examining Human Influence on Earth's Carbon Cycle

The Suess effect is a phenomenon that was named after the Austrian-American scientist Hans Suess, who first reported it in the 1950s. It refers to the disruption in the natural ratios of carbon isotopes in the atmosphere, caused by human activities.

Quick isotope refresher… Isotopes are versions of a particular chemical element that differ in neutron number while retaining the same number of protons in their atomic nuclei. This means that isotopes of an element have the same atomic number (which defines the element), but different mass numbers due to the variance in the number of neutrons. Despite these differences in mass, isotopes of the same element exhibit very similar chemical behavior because chemical properties are primarily determined by the number of protons.

Isotopes can be stable or radioactive. Stable isotopes do not change or decay over time, making them a constant part of the environment and useful for various scientific analyses, such as studying ancient climates and tracing sources of water. On the other hand, radioactive isotopes are unstable and decay over time, emitting radiation in the process. This property makes them valuable in various applications, from medical diagnostics and treatment (using isotopes like technetium-99 in imaging or iodine-131 in treating thyroid conditions) to geologic dating (using uranium-238 for dating ancient rocks). The rate of decay of radioactive isotopes, known as their half-life, is a key concept used to measure the age of materials and understand the dynamics of Earth's processes and the cosmos.

Carbon has three naturally occurring isotopes: carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C). These isotopes differ in the number of neutrons in their nuclei, with 12C and 13C being stable, and 14C being radioactive. 12C is the most abundant carbon isotope, making up about 98.93% of the carbon in the Earth's atmosphere. It is the standard isotope against which the atomic masses of all elements are measured. 13C is a stable isotope comprising about 1.07% of atmospheric carbon. It is used in paleoclimatology and ecology to study carbon cycle processes. 14C is a radioactive isotope produced in the upper atmosphere through the interaction of nitrogen-14 with cosmic rays and decays with a half-life of about 5,730 years (half of the radioactive isotopes will decay every 5,370 years). Its presence in organic materials is the basis for radiocarbon dating. 14C is present in the atmosphere in trace amounts, roughly one part per trillion of total carbon.

Fossil fuels, such as coal, oil, and natural gas, are derived from the ancient remains of plants and animals that lived millions of years ago. Since 14C has a half-life of about 5,730 years, all of it has decayed away in fossil fuels, meaning that these materials are essentially devoid of 14C. Fossil fuels contain predominantly 12C, with a small amount of 13C. The absence of 14C in fossil fuels is a key factor in the Suess effect, where the burning of fossil fuels dilutes the atmospheric concentration of 14C impacting radiocarbon dating and studies of the carbon cycle. Fossil fuels also tend to have a lower 13C/12C ratio compared to atmospheric CO2 because photosynthetic organisms preferentially absorb 12C, enriching the organic matter in this isotope. When fossil fuels are burned, they release CO2 that is depleted in 14C and 13C, altering the isotopic composition of atmospheric CO2.

CO2 from natural sources, such as respiration by plants and soil microbes, maintains a balance of carbon isotopes reflective of the contemporary atmosphere and biosphere. These sources include a mix of 12C, 13C, and 14C, with the presence of 14C indicating recent biological activity. CO2 released from volcanic activity can have a 14C signature similar to fossil fuels, depending on the source of the carbon within the Earth's mantle or crust. However, it is currently estimated that the amount of CO2 released by volcanoes is significantly smaller than that from burning fossil fuels. The thawing of permafrost can release ancient organic carbon that, when decomposed into CO2 or methane, has a depleted 14C signature. This carbon can contribute to atmospheric concentrations with isotopic signatures resembling those of fossil fuels.

By measuring the ratios of 13C/12C and the presence of 14C in atmospheric CO2 samples, and making certain assumptions, scientists can estimate the proportion of CO2 derived from fossil fuel combustion versus natural sources. This is important for understanding how human activities are altering the carbon cycle and contributing to the increase in atmospheric CO2 levels, claimed to be a major driver of global warming.