Is further warming already 'baked in' ?
Contrary to widespread belief, climate models point to a halt in warming once we reach net zero
I love being right and don’t want to be wrong for any longer than I have to. Recently, as I’ve gotten deeper into climate science, I realized I was wrong about climate change. More specifically, about the amount of global warming already committed to because of past and current emissions. Okay, maybe I’m nitpicking. But I have long believed that the planet would continue to heat up, with additional warming already ‘baked in’, even if we stopped all emissions today. The rationale being that CO2 stays up in the atmosphere for a long time, increasingly holding on to more heat. While there is some truth to that view, it is not the whole story.
The United Nations IPCC report from August 2021, and the established science years before it, make it clear that our best models show that temperatures will stop rising soon after we reach net zero.
It is a shame that that’s not more widely known because it is good news: if we do our homework and zero out emissions, we can stop warming on its tracks and hopefully get this climate change thing under control.
The bad news of course is that we are nowhere close to achieving net zero. In fact, global emissions are still going in the wrong direction. In an realistic yet optimistic scenario we will see between 2C to 3C of warming before we can stop emitting greenhouse gases. Depressingly, that will come with irreversible effects like sea level rise and ocean acidification. But that is subject for another day. The point is that we should all be more motivated than ever to cut emissions because we have more power to keep the planet from warming than many people think.
Why warming stops at net zero
The source of my confusion was in not recognizing the difference between a scenario where net CO2 emissions stop altogether (i.e. net zero), and one where CO2 concentration is held constant (currently near 420 parts per million and rising.)
In the latter scenario, where the atmosphere holds on to a steady amount of CO2, temperatures indeed continue to rise for some time to come. The reason is the energy imbalance Earth finds itself in: there is more energy being absorbed by the planet than being radiated out into space (this is called radiative forcing.) That surplus energy turns to heat and currently most of that heat is absorbed by the oceans. There is a limit to the amount of heat the waters can hold and, as they warm up, they absorb less and less of it which in turn leads to an increasingly warmer atmosphere (felt by us more directly than warmer oceans.)
The question then is: will the CO2 concentration remain constant in a net zero world? A reasonable assumption would be yes, since carbon can stay in the atmosphere for thousands of years. It would then follow that whatever the CO2 concentration is at the time we reach net zero would hold steady, right? Not quite. There are several carbon sinks in nature (like plants, soil, and the ocean itself) that will keep on absorbing the surplus carbon. If we wanted to keep the CO2 concentration constant we would actually need to continue emitting at around 30% of current levels. Get below that and CO2 levels will start to fall.
The land and ocean are absorbing about half of the CO2 that humans emit each year. If emissions go to zero, these “carbon sinks” continue to take up some of the extra CO2 that was emitted in the past – quickly at first and then more slowly over time as they move toward a new equilibrium.
While some warming is in fact ‘baked in’ due to the current energy imbalance, in reality that warming is cancelled out by cooling caused by the drop in CO2 levels, thanks to natural carbon sinks.
The plot thickens: CO2, Methane, Aerosols
Many caveats are warranted in this story, the main one being that the picture painted above is a simplification focusing on net zero carbon emissions, but other greenhouse gas emissions matter too.
The overwhelming impact is from methane gas: CH4 is a very potent GHG which quickly vanishes so the impact on warming is severe but short lived. Nitrous oxide (N2O) is even more potent and it lasts longer than methane, but thankfully we emit much less of it.
Then there are aerosols, a class of tiny particles of sulfur and other elements that are masking global warming because they reflect sunlight, having an overall cooling effect. If we weren’t emitting aerosols today, the climate would be getting even warmer by 0.4C or more (lots of uncertainty in those estimates.) Many of the geoengineering approaches being researched today actually involve dumping massive amounts of sulfur dioxide high up in the atmosphere to cause rapid cooling, mimicking the effects of a large volcanic eruption like that of Mount Pinatubo in the 1990s which cooled the climate by about 0.5C for a few years.
The following chart shows the projected effects of ceasing each type of greenhouse gas emission. What we will experience once emissions stop in the real world should be similar to the purple line which combines net zero for all greenhouse gases + aerosols. That would first result in a quick bump in warming (from less aerosols, also short-lived) followed by overall cooling that is at first rapid (from less methane) and then gradual (from carbon being absorbed by natural sinks) until reaching a point of equilibrium. If we want to lower that point of equilibrium, we will need to remove more carbon from the atmosphere ourselves via direct air capture.
Another important point: keeping temperatures from rising is not the same thing as solving climate change. Even after net zero there will be plenty of work to do, likely involving a massive effort to go net negative to mitigate some of the impacts of living in a +3C world. Any additional warming compared to where we are today is bad news. And even after emissions cease we will still be faced with dangerous climate feedback loops we don’t yet fully comprehend, like biodiversity loss, and significant downstream impacts that we do understand fairly well and that would take a very long time to play out:
Melting glaciers and ice sheets and rising sea levels all occur slowly and lag behind surface temperature warming. A zero-emissions world would still result in rising sea levels for many centuries to come.
If you want to see the science, here’s a lot more:
Thanks to this Carbon Brief explainer (from almost two years ago!) for making me realize that I was wrong about this topic. I quote and link to it multiple times in this post, but only because it is so great.
This meta-analysis of 18 Earth Systems Models (the most comprehensive class of climate models) concludes that “the most likely value of ZEC [Zero Emissions Commitment: the change in global mean temperature expected to occur following the cessation of net CO2 emissions] on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory.”
Immerse yourself into this in-depth Q&A, the only overview you’ll ever need of the climate science in the IPCC Sixth Assessment Report (AR6).
Nonsensical words, salad, bullshit. You are not accounting for the 400 ppm CO2 equivalent of the methane in the permafrost..... It’s melting you know.