Beyond Net Zero

A recent BBC news item about global climate change included the claim by a UK academic that we have only to stop emitting carbon and the temperature will start to fall. The speaker perhaps intended to provoke thought rather than to be taken literally; this post will consider some answers to the question of what would happen to earth’s temperature if greenhouse gas emissions could indeed be suddenly and completely stopped.

In 2007 NASA’s Earth Observatory Climate blog responded to the question: “Even if all emissions were to stop today, the Earth’s average surface temperature would climb another 0.6 degrees or so over the next several decades before temperatures stopped rising.” The single reason given was that “a great deal of the excess energy is stored in the ocean” and the thermal inertia associated with its huge heat capacity would result in a time lag of several decades before temperatures stopped rising. NASA pointed out that “If we wait until we feel the amount or impact of global warming has reached an intolerable level, we will not be able to “hold the line” at that point; some further warming will be unavoidable” (NASA, 2007).

Ten years later an updated article by Professor Richard Rood gave a fuller account of what might be expected if GHG emissions could be suddenly stopped. He outlines the basics of atmospheric heating and points out that we would not “return to the climate of our elders” after ceasing to emit greenhouse gasses. The carbon dioxide released through the burning of fossil fuels “will remain in the atmosphere for thousands of years”. Rood describes one of the natural processes that reduce atmospheric carbon dioxide through the formation of calcium carbonate and points out that this takes millennia. To reduce the greenhouse effect quickly after ceasing to emit carbon dioxide we would need to reverse deforestation and reform agriculture, as well as dealing with other GHGs such as methane and nitrous oxide. Like the NASA view outlined above Rood stresses the importance of the heat stored in the oceans: “After maybe 40 more years, scientists hypothesize the climate will stabilize at a temperature higher than what was normal for previous generations.” (Rood, 2017).

The Royal Society (2021) addressed the same question about a sudden halting of GHG emissions and supports and augments Rood’s statements. “If emissions of CO2 stopped altogether, it would take many thousands of years for atmospheric CO2 to return to “pre-industrial” levels due to its very slow transfer to the deep ocean and ultimate burial in ocean sediments.” Sea levels would probably continue to rise for centuries after temperatures stopped increasing, and “CO2-induced warming of Earth is therefore essentially irreversible on human timescales.” However, future scenarios “increasingly assume the use of technologies that can remove greenhouse gases from the atmosphere … thereby starting to reverse CO2-driven warming on longer timescales.” This brief article is supported by three graphs showing possible values of atmospheric CO2, surface air temperature and ocean thermal expansion for the years 2000 to 3000 depending on actions taken ranging from business as usual to aggressive emission reduction.

Rosane (2022) outlines some of the main points of a study which attempted to “answer how much global warming is already guaranteed by past greenhouse gas emissions.” In addition to carbon dioxide, the research considered emissions including methane, nitrogen oxide, sulphur, and soot. Not all polluting emissions cause atmospheric warming; sulphur can be a component of particles which reflect sunlight away from the Earth. Different GHGs also have different lifetimes in the atmosphere. Rosane placed the research in the context of the COP26 in 2021, and of the 2022 IPCC report. The study itself, by Dvorak et al. (2022) uses an emissions-based climate model to estimate temperature change following cessation of emissions in 2021 and in every year thereafter until 2080 following eight different pathways. The authors focus on two temperatures, the peak temperature reached in the decades following emissions cessation, and the eventual temperature reached in the year 2100. They note that some components of the climate system, such as sea ice, the hydrological cycle, hurricanes, and agriculture, are more sensitive to peak temperatures, while others such as glaciers, ice sheets, and sea level are more sensitive to long term warming. The paper gives details of the climate model used in the study (FaIR, 2018) and the methods and assumptions of the study. One of the findings is that “there is now a 42% probability that the world is committed to peak global warming … of at least 1.5°C based on past emissions alone”. The eight pathways used in modelling are the Shared Socioeconomic Pathways defined in the IPCC Sixth Assessment Report. Temperature predictions are shown graphically against time and against cumulative emissions.

Herring and Lindsey (2022) ask whether we can slow or even reverse global warming and refer to the methods known collectively as climate engineering or geoengineering, which have not been treated in detail in the above papers. Proposals include “injecting reflective particles into the upper atmosphere to scatter and reflect sunlight back to space” and drawing down carbon dioxide out of the atmosphere by seeding the oceans with iron. The authors point out that there may be unknown side effects and that there are “unresolved legal and ethical issues surrounding geoengineering.”

Capturing carbon dioxide from the air may be described as a form of climate change mitigation rather than as geoengineering, and is the subject of an IEA article, Direct Air Capture. It is described as expensive and at an early stage of development, with only 27 commissioned plants worldwide, and 130 more in development, but might have a significant future role (IEA, 2023).

References

 

Dvorak, M.T., et al., Estimating the timing of geophysical commitment to 1.5 and 2.0° C of global warming, Nature Climate Change, 12, June 2022, online, accessed 27 Jan 2024

https://www.nature.com/articles/s41558-022-01372-y.epdf

and at

https://eprints.whiterose.ac.uk/188408/6/Dvorak_Armouretal_revised_wtables_accepted.pdf

FaIR, 2018, FAIR v1.3: a simple emissions-based impulse response and carbon cycle model, EGU Geoscientific Model Development, online, accessed 27 Jan 2024

https://gmd.copernicus.org/articles/11/2273/2018/

NASA, 2007, If we immediately stopped emitting greenhouses gases, would global warming stop? NASA, Climate Q&A, 2007, online, accessed 27 Jan 2024

https://earthobservatory.nasa.gov/blogs/climateqa/would-gw-stop-with-greenhouse-gases/

Rood, R., 2017, If we stopped emitting greenhouse gases right now, would we stop climate change? The Conversation, 2017, online, accessed 27 Jan 2024

https://theconversation.com/if-we-stopped-emitting-greenhouse-gases-right-now-would-we-stop-climate-change-78882

The Royal Society, 2021, If emissions of greenhouse gases were stopped, would the climate return to the conditions of 200 years ago? The Royal Society (Question 20, published 2021), online, accessed 27 Jan 2024

https://royalsociety.org/topics-policy/projects/climate-change-evidence-causes/question-20/

Rosane, O., 2022, World has 42% chance of breaching 1.5 degrees if emissions stopped today, World Economic Forum, 2022, online, accessed 27 Jan 2024

https://www.weforum.org/agenda/2022/06/world-has-42-chance-of-breaching-1-5-degrees-if-emissions-stopped-today-new-study-finds/

Herring, D., and Lindsey, R., 2022, Can we slow or even reverse global warming? National Oceanic and Atmospheric Administration (NOAA), online, accessed 27 Jan 2024

https://www.climate.gov/news-features/climate-qa/can-we-slow-or-even-reverse-global-warming

IEA, 2023, Direct Air Capture, IEA, online, accessed 27 Jan 2024

https://www.iea.org/energy-system/carbon-capture-utilisation-and-storage/direct-air-capture 

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