John G. Halstead

John Halstead - Research Fellow at the Forethought Foundation. Formerly Head of Applied Research at Founders Pledge and researcher at Centre for Effective Altruism DPhil in political philosophy from Oxford

Wiki Contributions


Noticing the skulls, longtermism edition

 that also applies to people working on global development as well, and to pretty much all philanthropy. So, there is nothing special about longtermism on this score

Betting on the best case: higher end warming is underrepresented in research

Hello, thanks for your response. 

On the Sherwood et al thing, another issue is that I don't understand why you would use the Weitzman estimate of ECS when you have already mentioned the Sherwood et al (2020) estimate. The Sherwood et al (2020) estimate is superior, and that is clear from reading their paper. The IPCC also now accepts that it is superior. So, I don't understand why you would mention "the most recent estimate" and then use one from 6 years ago instead. 

"Overall, your criticism mainly seems to be the fact that current estimates of climate sensitivity have a smaller range that the one we used." That is not my criticism. My criticism is that emissions look set to lower and the right tail of climate sensitivity has thinned which makes a clear material difference to your conclusions. 

On your last point:  this does make it look like the central claim of your paper is wrong.  I am going to paste some charts from the last impacts report below on some key climate impact areas to illustrate:

Sea level

Food yields


Species loss

Fire frequency 

Ocean net primary production


Maximum fish catch potential (A1B is between RCP6 and RCP8.5)

 On drought they say "Climate change is likely to increase the frequency of meteorological droughts (less rainfall) and agricultural droughts (less soil moisture) in presently dry regions by the end of the 21st century under the RCP8.5 scenario (medium confidence). {WGI AR5 Chapter 12} This is likely to increase the frequency of short hydrological droughts (less surface water and groundwater) in these regions (medium evidence, medium agreement)."

Ocean acidification

Good news on climate change

The new models account for potential feedbacks from permafrost carbon. I'm also not especially worried about that feedback or the one from methane clathrates. The world was about 4 degrees warmer a few million years ago, and we didn't get a rapid carbon input from these sources. And the models and basic physics suggest that these would be slow acting multi-centennial scale feedbacks.

The Sherwood et al (2020) paper accounts for evidence from the paleoclimate which should in principle pick up some tipping points from the past, though what we are doing now is not a perfect analogue for past climate change in various ways, and paleoclimate proxies are imperfect. Our confidence in the linear relationship between cumulative emissions and warming is lower the higher emissions get. The IPCC is less sure it holds once we get past 1,000 billion tonnes of carbon (on top of the 650 billion tonnes we have already emitted). The Sherwood et al (2020) paper only estimates ECS for up to two doublings of CO2 concentrations, so 1,100ppm. Beyond that, we have less of a clue, especially as CO2 concentrations wouldn't have been that high for tens of millions of years. 

I am  worried about feedbacks if emissions do get that high. Imo, the most worrying thing about climate change is the potential for unexpected surprises, especially from cloud feedbacks, eg here. That is the first time a fast feedback has shown up in the models.  But that is something we reach when we get to 1,300ppm, which is probably several centuries away. 

There is some stuff from the planetary boundaries people arguing that we are on the brink of massive and disastrous tipping points even at 2 degrees, eg this widely cited paper from 2018. That paper fits the planetary boundaries pattern of arguing that there is a potentially significant environmental tipping point close by, on the basis of limited or non-existent evidence and argument. 

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hi sasha, RCP is a 'representative concentration pathway'. The number refers to the radiative forcing from GHGs in 2100 measured in watts per square metre. So, on RCP8.5, the extra forcing would be 8.5 watts per square metre.

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Hi Ben, 

CO2 concentrations on the different shared socioeconomic pathways are shown in Table 5 here. On the most likely scenario - RCP4.5 - CO2 concentrations would double relative to pre-industrial by around 2060. 

I think this comes down to the difference between the transient climate response to cumulative emissions and equilibrium climate sensitivity. On the assumption that CO2 concentrations stabilise, ECS tells you the warming you get eventually once the climate system has reached equilibrium (not including ice sheet feedbacks). If CO2 concentrations stabilise, then it would take decades to centuries for the system to reach equilibrium.  Whereas the warming figures in the table is the warming you get at 2100. I have wrestled with trying to convert things to CO2 concentrations and then trying to infer warming from ECS, but it is unnecessary. CO2 concentrations will not stabilise, so the system will never truly be in equilibrium. The TCRE is much more informative. 

How cumulative emissions translate into CO2 concentrations is model-dependent. 1 ppm of atmospheric CO2 is equivalent to 2.13 gigatonnes of airborne carbon. However, the amount of carbon that we burn that remains in the atmosphere (the airborne fraction) changes with emissions - the airborne fraction increases the more we emit because land and ocean carbon sinks get exhausted, which you can see in the doughnut charts below. 

How can we make Our World in Data more useful to the EA community?

Just want to say as an EA researcher your website is an absolute godsend.

Betting on the best case: higher end warming is underrepresented in research

Thanks for sharing this paper. I disagree with most of the key claims you make here. 

Firstly,  you cite the recent updates on climate sensitivity, but without clarifying that this has narrowed uncertainty at both the left and right of the tail. "The most recent estimates of equilibrium climate sensitivity show a similar distribution, narrowing the range of outcomes to exclude rises below 2 °C but not ruling out warming above 4.5 °C (Sherwood et al 2020)." The first thing to say is that equilibrium climate sensitivity measures the warming conditional on an increasing in co2 concentrations - it doesn't tell us the unconditional probability of a particular level of warming. Moreover, this is misleading as a description of the Sherwood et al (2020) paper because it makes it sound like the right tail hasn't changed. The paper also excludes ECS above 5.5 degrees.

Secondly, you say "GHG concentrations of 700 ppm could produce a 10% chance of exceeding a temperature rise of 6 °C (Wagner and Weitzman 2015). Such concentrations would be passed by 2100 under six of the nine of the CMIP6 SSP-RF baseline and 6.0 W/m2 forcing scenarios (Riahi et al 2017, Gidden et al 2019)." As Johannes and I discuss in our recent post, this risk now seems a lot lower

Thirdly, this is not a criticism, but the Raftery estimates have since been updated by Liu and Raftery who find "The median forecast for 2100 is 2.8 °C, with likely range (90% prediction interval) [2.1, 3.9] °C. The median is 0.4 °C lower than that of Raftery et al.1, the upper bound is 1.0 °C lower, while the lower bound is 0.1 °C higher. The tighter interval reflects the additional 5 years of data and the improved model."

Fourthly, I'm not sure whether I have misunderstood your paper, but the entire impacts literature focuses on the impact of >3 degrees of warming. The reason for this is that  it focuses on the impacts of RCP8.5 at 2100, which implies warming of 4-5 degrees. It seems like in your search strategy, you have searched for mentions of particular levels of warming. But if you had searched for mentions of RCP8.5, you would have found that almost the entire impacts literature focuses on comparing that scenario to RCP2.6 or lower. 

Good news on climate change

agree on the first one - it is very good. I hadn't seen the second one thanks for sharing! 

Good news on climate change

Hi, thanks for this comment.  I agree with half of it. We definitely don't have momentum in decarbonising hard to decarbonise sectors like industry and aviation. But the solution there is to make cheap hydrogen, which we can do with super cheap renewables and nuclear hydrogen gigafactories.  Whether we will do this is the great unknown of climate policy. But the IEA estimates account for lack of progress in these sectors, so they don't affect the central point of our piece. Also, some parts of industry are easier to decarbonise. Davis et al (2018) estimate that the hard to decarbonise parts of industry account for 9% of emissions. 

On agriculture and land use, the first thing is that I'm not sure I agree on that IPCC assessment of their contribution to emissions. I think that estimate overweights the importance of short-lived pollutants like methane, as discussed here. When we're thinking about tail risks the real problem is CO2 because it is so long-lived. Second, forest area is increasing in temperate regions. Depending on the source, this started in temperate regions in 1990, or perhaps decades earlier. Indeed, some sources argue that once we account for fire management, plantations and replanting, forests hold more carbon now than they did in 1900 - I'm not sure how reliable this study is though. But what all of this shows is that the solution to forest loss is development - things seem to follow a kuznets curve type relationship. As before, this is also accounted for in estimates of likely emissions pathways.

Good news on climate change

Yes though I suppose it is still unclear whether they will get it through or not. China is going to spend a fortune on solar and nuclear over the next few decades, which is good. 

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