Historical temperatures: If necessary, review the Ice Cores section of the Isotope Ratio Mass Spectrometry Learning Tool. Because heavy water and light water contain different isotopes, they evaporate and condense at different rates. Also, because evaporation and condensation depend upon temperature, if the temperature changes, the ratio of heavy water to light water in ice cores will change accordingly. The change in the ratio between heavy water and light water is measured using mass spectrometry and calculations of the ¹⁸O/¹⁶O ratio and the ∂¹⁸O value. Therefore, knowledge of isotopes and how they affect a substance’s properties is vital when determining historical temperatures.

Carbon sourcing: By comparing the current isotope ratio of ¹³C/¹²C in atmospheric carbon dioxide molecules to historical ¹³C/¹²C ratios indicated by tree rings and ice cores, chemists are able to determine whether the increase in atmospheric carbon concentrations since the Industrial Revolution is due to natural activity (such as cellular respiration) or human activity.

Plants generally take in carbon-12 for photosynthesis more readily than carbon-13, which decreases the ¹³C/¹²C ratio in the plant but increases this ratio in the atmosphere. By contrast, humans produce atmospheric carbon by burning fossil fuels, which releases the carbon retained in ancient plants into the atmosphere. Because the ancient plants that are burned contain more carbon-12 than carbon-13, a greater proportion of carbon-12 is released into the atmosphere, lowering the ¹³C/¹²C ratio in the atmosphere. Climate scientists are also able to determine ¹³C/¹²C ratios that were present in past decades and centuries, because this ratio is reflected in the ¹³C/¹²C ratio of compounds found in tree rings and ice cores. By comparing the current ¹³C/¹²C ratio in atmospheric carbon dioxide to historical ratios, climate scientists have determined that today’s ¹³C/¹²C ratio has decreased significantly since humans began burning fossil fuels at the beginning of the Industrial Revolution. The current ¹³C/¹²C ratio is actually the lowest it has been in the last 10,000 years, strongly suggesting that the increase in atmospheric CO₂ is due to anthropogenic sources.

In addition, because the isotope ratios of carbon atoms differ depending on the origin of the carbon, it may be possible to use isotope ratios to determine the region of the world, even the country, that atmospheric carbon has originated from. This analysis could become a useful tool to monitor treaties or agreements that require countries to meet specific carbon emission reduction targets. Isotopic analysis could be used to determine whether or not certain countries are emitting more carbon than they specified in the agreement.