Are carbon cycle responses reversible after an overshoot?

Many of the CO2 emission pathways that could limit global temperature rise to 1.5 or 2°C are based on the assumption that we can remove some CO2 from the atmosphere after it has been emitted and climate change will reverse.  In principle, this is technologically feasible, but how will the carbon cycle respond to this situation?

A recent study, led by CONSTRAIN researcher Kasia Tokarska and published in Earth’s Future, looks at how both the climate and the carbon cycle will respond to such “overshoot scenarios”, where a given temperature limit is initially passed but then returned to by artificially removing carbon dioxide from the atmosphere (i.e. “negative emissions”).

The results show that carbon cycle responses depend on the CO2 emission pathway, as well as how big the overshoot is. For example, for low to moderate levels of overshoot, Global mean temperature change can be reversed by CO2 removal, and is independent of the CO2 emission pathway. However, after an overshoot, more carbon is stored in the ocean and less on land.  

This has important implications for how we use the carbon budget concept (the total amount of CO2 emissions that can be emitted whilst keeping global mean temperature rise below a given level). The good news is that no corrections are needed for carbon budgets based on ambitious mitigation scenarios with low levels of overshoot presented in the IPCC Special Report on 1.5 °C, as global mean temperature is reversible in that case.

However, even though temperature change can be reversed, the state of the carbon cycle differs after an overshoot. It is important to remember that environmental changes other than global mean temperature rise, such as ocean acidification, are not easily reversible.