There is a question that worries climate scientists a lot: will global warming stop when we reach zero greenhouse gas emissions?
There are two schools of thought on this matter: the “optimistic” one claims that when (and if) we reach net zero emissions of greenhouse gases (CO2, methane, N2O, CFC, …), the long-term increase in the earth’s temperature will stop; the other side is the “pessimistic” one, which states that our emissions risk of increasing the concentration of greenhouse gases in the atmosphere so much to create a domino effect difficult to stop. The labels “optimist” and “pessimist” here have no positive or negative meaning, but only serve to convey how bad things could get for human civilization. After all, reality is neither “optimistic” or “pessimistic”, it simply “is”.
Anyway, there are great scientists supporting either of those sides. The climate is an extremely complex system, so it is not surprising that some aspects of climate change are still under debate. On the causes, instead, the debate is closed. Increasing the atmospheric concentration of greenhouse gases is in fact one of the simplest ways to change the climate; it’s already happened many other times in the past, even when the cause was an external factor as with the ice age. Think of a house of cards; only certain cards can be moved and only in certain positions, or the castle falls. Climate works the same way: if a key component is removed or altered, the entire system can collapse.
Now, the one million euros/pounds/dollars question is: have we already messed too much with those components? The simple answer is that we don’t know yet. On one hand, the climate has a strong inertia to changes, even when they occur suddenly; just think that in less than 200 years humans have altered the atmosphere more than it has changed in the last million years [1]. The climate is also very resilient, so it needs a good jolt to make it move from its lethargy. Nonetheless, the climate has been changing for several decades now. The increase in global average temperature is breaking all records: 2023 was the hottest year ever recorded globally and February 2024 was the warmest month on record [2]. The main consequence of all this excess energy in the climate system is that extreme events are becoming more frequent and violent around the world [3].
The problem of global warming will certainly not be solved any time soon. Human emissions of greenhouse gas are still increasing, albeit at a slower rate than in the past, but we still need to ask ourselves what will happen to climate change once we get to net zero emissions. Let’s see some arguments, starting from the pessimistic side.
Global warming will not stop
There are two big factors supporting this hypothesis. The first is that greenhouse gases emitted into the atmosphere have a residence time before being removed; for example, CO2 takes between 50 and 200 years to be removed from the atmosphere, while for other substances such as fluorinated gases the residence time varies from decades to thousands of years. Greenhouse gases therefore remain for a long time in the atmosphere to cause warming, even after emissions have gone down to zero. The alternative is to create a way to capture that stuff and artificially remove it from the atmosphere [4]; however, those techniques are not at a sufficiently mature stage at the moment.
The second factor is feedback mechanisms, which I have already talked about in a previous article. Those are loops that amplify small changes to make them eventually become big, just as a snowball rolling down a snowy mountain slope can become an avalanche. Having significantly increased the concentration of greenhouse gases in the atmosphere (CO2 now amounts to 50% more than during the pre-industrial period), there is the possibility that some of those loops have been set in motion irreversibly until they become points of no return, accelerating global warming.
As claimed by one recent study, climate change is in fact accelerating [5], also maintaining that the threshold of 1.5°C of global warming compared to the pre-industrial period will be certainly exceeded within the next 6 years. According to the authors, the climate would be much more sensitive to the increase in the concentration of greenhouse gases than previously thought; it’s a concept called climate sensitivity, which I’ll deal with in another article. The gist is that global temperature increases nonlinearly as greenhouse gas concentrations increase, precisely because of feedbacks. Everything is mediated by the various components of the climate system (atmosphere, ocean, ice, soil, …). In particular, lately the ocean has stored much of the excess heat caused by greenhouse gases; however, as it gradually gets hotter, its ability to absorb heat decreases, accelerating the long-term increase in atmospheric temperature.
The study is based on paleoclimatic data (i.e. from the Earth’s remote past), from 1 million to 66 million years ago. This is needed to incorporate very slow climate processes into the analysis, which cannot be included in climate models for computational time and cost reasons. The models therefore have a lot of uncertainty about those long-term processes (thousands of years or longer), which may be important for future climate change but about which we still know little. We must therefore take into account that there may be global warming in the pipeline, even after our emissions go to zero.
Global warming will stop
The more “optimistic” vision is based on the fact that, once greenhouse gas emissions are stopped the concentration of these gases in the atmosphere slowly begins to decrease, even without CO2 capture and storage. In fact, it is estimated that biological cycles based on carbon are capable of consuming enough greenhouse gases to balance the warming effect. As a result, global warming would stop. In the long run, more and more greenhouse gas is removed and the global-mean temperature begins to decrease [6]. This is not just optimism, but it is also the most widespread opinion in the scientific community. In fact, climate models suggest that zero emissions means zero warming, unless the concentration of greenhouse gases in the atmosphere becomes more than 4 times the current level. At that point we’re screwed.
That said, there is still a lot of research to be done on this topic. Models are becoming increasingly complex (up to tens of millions of lines long) and generating petabyte (millions of gigabytes) of data. Supercomputers are also becoming more powerful, able to manage climate simulations up to a few tens of thousands of years long. It makes me dizzy to think about what the Earth’s climate might be like in 10,000 years, knowing that in the meantime it could become similar to Tatooine or the planet Arrakis. After all, who wouldn’t want to ride one of Dune sandworms?
Jokes aside, by the time we reach net zero emissions the global temperature will be too high compared to the pre-industrial period. We will certainly exceed the 1.5°C threshold by then, and most likely the 2°C threshold as well; according to national determined contributions (NDCs) of emissions reductions, we will not hit zero emissions by the end of this century. If this seems like a very distant goal, remember that it is only 76 years away from today, so a child born this year will probably be alive in 2100. This image by Climate Action Tracker explains the global state of climate action. With current policies there is even the possibility of exceeding 3°C of global warming by 2100.
There is too much CO2 in the atmosphere and it must be removed. Leaving all the work to natural carbon cycles is not a good idea, so we are devising systems to capture greenhouse gases from the atmosphere and store them safely underground [8]. Unfortunately, this technology is still in its infancy, but it is already being used as greenwashing to justify further greenhouse gas emissions .Until we have a more precise idea of the future trajectory of climate change, the best solution is not to rely on immature technology, but to act incisively and effectively to bring us to zero emissions as soon as possible [9].
Note: The original version of this article can be found here: https://www.noidiminerva.it/quando-finira-il-riscaldamento-globale/.
Image Credits: Global Warming by Nick Youngson CC BY-SA 3.0 Alpha Stock Images
SOURCES
[1] https://climate.nasa.gov/vital-signs/carbon-dioxide/
[2] https://www.washingtonpost.com/weather/2024/03/06/earth-hottest-february-winter-climate/
[3] https://climate.nasa.gov/extreme-weather/
[4] https://www.bbc.co.uk/news/science-environment-64723497
[5] https://academic.oup.com/oocc/article/3/1/kgad010/7335888
[7] https://climateactiontracker.org/global/cat-thermometer/
[8] https://www.iea.org/energy-system/carbon-capture-utilisation-and-storage/direct-air-capture
[9] https://www.imperial.ac.uk/news/249475/emissions-plans-must-consider-risk-global/
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