Climate Sensitivity Study Gives Reason for Caution and Hope

July 27, 2020

Been waiting for most useful take on this new paper.

You can hardly do better than Andy Dessler and Zeke Hausfather.

Andrew Dessler PhD and Zeke Hausfather in The Hill:

Climate change is real, caused by human activity and a serious problem that needs to be addressed. In a new study published last week, which one of us helped author, we’ve gotten a better understanding of exactly how serious. The good news is that some of the worst-case scenarios seem a bit less likely than before. The bad news is that it’s clearer than ever that warming will not be mild or modest in a world where we fail to cut emissions rapidly.

Climate scientists have been trying to narrow down how, exactly, the climate will respond to increasing levels of carbon dioxide (CO2) for well over a century. We call this “climate sensitivity” — in essence, how sensitive the climate is to our emissions. In 1979 a major report suggested that global surface temperatures would ultimately rise somewhere between 2.7 and 9.1 degrees Fahrenheit (1.5 and 4.5 degrees Celsius) if the amount of CO2 in the atmosphere was doubled. Thirty-five years later, the Intergovernmental Panel on Climate Change gave the same range in their most recent assessment report.

A four-year effort led by 25 experts in the field has finally been able to give us a better understanding of how sensitive the climate actually is. By combining lines of evidence from physics-based studies, historical temperature records and records from the Earth’s more distant past — such as during the last ice age — we find that if the amount of CO2 doubles in the atmosphere, the world will likely warm between 4.7F and 7F (2.6C and 3.9C).  

Such warming may not sound too bad — after all, what difference does a few degrees make? A lot, it turns out. If we go back to the depths of the last ice age, 20,000 years ago, we find that the global average temperature of Earth was only about 9F (5C) colder than today. That was enough of a difference to lower sea level by 300 feet and build up ice sheets thousands of feet thick covering much of North America. That Earth was basically a different planet.  

We now know that a worst-case scenario of high future emissions and high sensitivity risks warming comparable to the warming since the end of the last ice age, occurring over the next one or two centuries. That would impose extremely difficult challenges to human society and natural ecosystems.

It is not inevitable that we will double carbon dioxide in our atmosphere — we control whether we do better or worse than that. We can cut our emissions rapidly enough to avoid doubling CO2 this century, given the political will to do so, and still limit warming below the internationally agreed-upon guardrail value of 3.6F (2C). Alternatively, if our emissions continue increasing, we could end up tripling rather than just doubling atmospheric CO2 by the end of the 21st century. How much warming occurs is up to us, and by narrowing the range of climate sensitivity, we now have a better idea of the consequences of our actions.

On the emissions front we have some reason for modest optimism. Over the past decade, the world has made progress on bending down the emissions curve. Global carbon dioxide emissions are still rising, but only at half the rate they were in the 2000s. The world’s use of coal peaked in 2013 and has been declining. We succeeded in making clean energy less expensive, and carbon-free sources now provide more energy than coal worldwide. Nightmare scenarios where global emissions triple by the end of the century now seem far less likely, as we see them near a plateau.

At the same time, there is an ever-growing gap between where we are today and the emissions reductions needed to limit warming to globally agreed-upon targets. CO2 accumulates in the atmosphere over time, so simply stopping emissions from increasing does not stop the world from warming. Peak emissions — which we will hopefully see this decade — are only the first and easiest step on the long road to zero emissions.

At this point, our findings rule out scenarios where warming ends up being minor, but it also makes some truly catastrophic outcomes less likely. This means that how bad climate change will be rests firmly in our hands. To minimize the risk to future generations, it is crucial to control the one thing we can: our future emissions.

Dr Zeke Hausfather is the director of climate and energy at the Breakthrough Institute. Dr Andrew Dessler is a professor of Atmospheric Sciences at Texas A&M University.

24 Responses to “Climate Sensitivity Study Gives Reason for Caution and Hope”

  1. Sir Charles Says:

  2. neilrieck Says:

    With all due respect to the experts who spoke in the videos, please allow me to slap everyone in the face: For the last 10 million years, natural CO2 levels on average have oscillated between 180 and 280 ppm which is only a shift of 100. As soon as humanity pushed past 380 ppm we were already too high by 100. Any talk of going over 480 is insane.

    In Gretta Thunberg’s “How Dare You” speech she used the phrase “fairytales of eternal economic growth” which most people dismissed as an impossible concept to ponder even though the only thing that tries growing forever is cancer. Well, COVID-19 seems to have pushed the reset button on all economies which proved that she was right. We are all still alive while the crashing economy did not bring Armageddon.

    • Keith McClary Says:

      “fairytales of eternal economic growth”

      Raising the bar: on the type, size and timeline of a
      ‘successful’ decoupling
      24 Jun 2020

      “A global GDP growth of 2% until 2050 would necessitate a ‘successful absolute resource
      decoupling’ with 2.6 [times] more GDP produced by every ton of material use, for
      which no realistic scenario exists. Existing evidence does not support the
      conclusion that the right type of decoupling would be happening fast
      enough, and even a crude estimate of the needed timescale suggests that
      the expectation of success is unrealistic.”

      Click to access 10.1080@09644016.2020.1783951.pdf

    • rhymeswithgoalie Says:

      Well, COVID-19 seems to have pushed the reset button on all economies which proved that she was right. We are all still alive while the crashing economy did not bring Armageddon.

      This is rubbish. More and more households are being pushed beneath the poverty line, and the hurt is just starting. It may not seem like Armageddon to those of us sitting on a resilient income, but it is real, and it is bad.

  3. Sir Charles Says:

    So before we had ECS 3°C with a likelihood of 66%, and now it’s 3.25°C?

  4. jimbills Says:

    I’ve been frustrated by how the study has been portrayed in many media sources. A number of headlines have said, to the effect, that catastrophic warming will not happen. The Hill’s article linked above has the headline: “Global warming will not be catastrophic, but impacts won’t be minor”.

    Someone just browsing headlines would come away with the idea that warming isn’t as serious as it should be. The study itself says that the upper ends of climate sensitivity won’t happen at a CO2 doubling, but 1) we might not stop emissions at just a doubling, 2) we don’t yet know the full impact of warming from a doubling of CO2 on necessary things to maintain civilization like agriculture and water availability, and 3) the study doesn’t include other greenhouse gasses like methane and nitrous oxide, which are also increasing.

    Even at 2.6C to 3.9C, that could still very well be ‘catastrophic’, and people need to be aware of that.

    (Interesting, as I was writing this, The Hill just changed the title of the article to ‘Global warming’s impact: Not worst-case but still deadly, if we don’t act’. But it had the prior title for four days, well past the time it would get the most views.)

    • jimbills Says:

      Small detail – it appears I’m wrong about when The Hill article itself was posted. I could’ve sworn I saw it posted several days ago, but I’m probably mixing it up with another article with a similar headline. The Hill article linked above was posted this morning.

    • dumboldguy Says:

      Glad to see the “what about” mention of methane and nitrous oxide. Considering the heat waves and fires in Siberia and the weather that is breaking up the arctic sea ice, the specter of the “methane bomb” and runaway feedback mechanisms still looms over any attempt at really predicting what may happen.

  5. redskylite Says:

    Svante Arrhenius calculated something like a 5-6 degree Celsius rise back in 1896, so he wasn’t too far out, but he didn’t realize how quickly mankind could achieve the doubling.

    What we must realize is the temperatures quoted are for the doubling of CO2 and do not consider other GHG’s, and are the global average (it will be a lot hotter in some places).

    Fingers crossed for the magic industrial negative emissions devices (apart from the good old tree).

    Hang on it’s going to be a bumpy, desperate ride.


    With new models, scientists believe they can find out exactly how sequestration works. It could then be properly reflected in the next assessment of the United Nations Intergovernmental Panel on Climate Change (IPCC) – which likely will address drawing down atmospheric carbon.

    • redskylite Says:

      Carbon Brief 27th July:
      Guest post: Who should be responsible for removing CO2 from the atmosphere?

      “Nevertheless, carbon dioxide removal (CDR) – also known as “negative emissions” – in some form is needed to achieve the goals of the Paris Agreement. This means that solutions – whether natural or technological – and governance mechanisms will need to be ready for scale-up in the next couple of decades.

      But who should bear the responsibility for developing and deploying CDR? ”

  6. indy222 Says:

    I have not read the study yet, the peer-reviewed paper and not some media digestion. We’ve all been fooled over and over in the media with the promotion of Economic Growth Uber Alles by understating the consequences of rising CO2. Before I can feel any reassurance that ECS=5C is not the relevant number for our future, I will have to see how this new paper effectively debunks the CMIP6 models, the large majority of which show ECS=5C, as well as explicitly debunking the work of Friedrich et al. 2016 which studied paleo data going back through the many past Ice Ages, and finds that ECS is 1.5C during glacial phases, but rises to 4.9C during typical interglacials. But a typical interglacial has CO2 at 280ppm, not the 415ppm we already have today. So there’s every suspicion that if we were to break out our current interglacial, the relevant ECS would be even higher than 4.9C. Why would that be? Look at the paper on climate sensitivity which doubles as a review article, by von der Heydt et al. 2016 summarized by this graph – it shows that ECS is highly state-dependent, not a “constant” as simple modellers would assume. Every study shows ECS is higher in hotter climate states. If you’re tempted to dismiss the Friedrich et al work, remember that Michael Mann was impressed and found this work “sound” and should be taken seriously.

    Healthy skepticism is called for.

  7. redskylite Says:

    “A major new climate study has concluded that a doubling of carbon dioxide levels will lead to planetary warming of between 2.3 and 4.5 degrees Celsius, ruling out the lower end of previous estimates. However, discussing climate change as a single global average obscures a wealth of crucial detail. The planet is not warming evenly, and understanding the nuances of climate change at a granular level is critical for formulating an intelligent response.”

  8. grindupbaker Says:

    “9.1 degrees Fahrenheit” S.B. “8.1 degrees Fahrenheit”

  9. rhymeswithgoalie Says:

    Peter, could you please make it a point to ask these scientists about the difference between “global temperature rise” and “temperature rise where there is permafrost”?

    Once we pass the runaway permafrost melt threshold (producing more CO2 and CH4), human emission levels won’t mean so much.

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