Wednesday, September 1, 2021

September 2021 science summary

Climate emission curves


This month is all about climate change (the photo above is from mini golf course in Brooklyn, where the three paths represent emissions scenarios).

First - I want to clarify something from last month's update. I mentioned that big old trees are especially important for carbon sequestration, and that increasing the length of time between forests being logged and/or leaving the biggest trees can be helpful to retain that carbon. But the term "proforestation" that I used is apparently commonly used to mean no forest management at all. That means eliminating any tree cutting at all (not just commercially, but even for pest management, fire control, or ecological goals). While logging can have environmental downsides (including for carbon), wood products are also relatively sustainable, and cutting trees is one of several important forest management tools that may be useful even in lands managed primarily for conservation. In cases where not cutting trees increases the risk of severe fire, that could even lead to worse outcomes for carbon sequestration and storage. Apologies for the poor choice of language.

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You probably have heard that the 2021 IPCC report came out this month: Don't feel like reading 1800 pages or even the 90 page summary or 215 page FAQ? Check out the 2 page 'headline statements' here:
The one thing to highlight for me is that limiting warming to 1.5C is now harder but not impossible (requiring net zero by 2055 w/ 90% gross emissions reductions). Current policies agreed to under Paris would still lead to 3C warming by 2100. One finding that I read about in this New York Times article ( really struck me: a bad heat wave that used to happen every ~50 years is now happening every ~10 years, and will happen every ~5 years at 1.5C or nearly annually at 4C. Don't miss this interactive tool to explore the results (and note you can download spatial data via the "share" icon):

Want some good news about climate? Costa et al. 2021 have a brief overview of the use of GWP* as a way to calculate the global warming potential of methane (CH4) more accurately than GWP100 (100 year warming). This is super-tricky and took me a while to wrap my head around, but the basic issue is that because methane is naturally breaking down over time, GWP* looks at emissions 20 years ago (which are now breaking down into CO2) to understand net changes in CH4 (as opposed to GWP100 which treats it as constant over 100 years). It means that relatively small reductions in biogenic methane (mostly from livestock and soil) emissions could get us to a stable point where globally methane is emitted and broken down at the same rate. Since the CO2 that feeds into biogenic CH4 is pulled out of the atmosphere via feed, that means it's arguably easier than we thought earlier to get to the point where globally biogenic methane isn't causing warming. Two big caveats: 1) this doesn't apply to fossil methane (where the end product of CO2 is pulled out of the ground, leading to a net increase), 2) this only makes sense when estimating global methane levels, and NOT in thinking about reductions at a local level. The latter point is important - a company or country could use this metric to make small reductions in livestock emissions and "take credit" for the breakdown of high past levels, and argue they are carbon negative (they are not). But if you take away the attribution issue, and focus on estimating warming, this offers an improvement. Let me know if this is still confusing to you, and/or you think I got this wrong!

Green et al. 2019 quantify the well known issue of climate and soil moisture having a big impact on carbon sequestration. Using climate projections (now out of date w/ the 2021 IPCC report) they find that as climate gets more variable, carbon sequestration and storage by plants will strongly decline (offsetting CO2 fertilization by 2060 under conservative assumptions). By 2085 carbon sequestration will have fallen to half of what it would have been without the moisture variability. The potential  carbon lost through drought and fire exceeds potential carbon gains from unusually wet conditions. This is mostly b/c plants get a lot less productive under drought, while too much water doesn't spur lots of growth (and can even be harmful with floods). Surprisingly they focused on plant biomass rather than modeling soil carbon changes, nor did they look at the impact of this changing soil moisture on methane or nitrous oxide soil emissions. There's a summary of this paper at

Fox et al. 2018 looked at how common parasites in lambs impact methane production. I was intrigued by the title (which says they drive a 33% increase in 'methane yield') which seemed to indicate a huge potential to reduce GHGs from lamb through medical treatment. But that is a fairly artificial metric of methane emissions per mass of feed. A more meaningful metric is total methane emissions (fig 2), which were highest in healthy lambs eating a normal diet, moderately lower in parasitized lambs, and slightly lower than that in healthy lambs eating less to control for the weight loss impact of parasites. Usually the main metric is emissions per kg of meat; I estimated this from final body weight using table 1 and figure 2 and it looks like healthy lambs emitted ~1.83 daily g CH4 / kg final body weight, parasitized lambs emitted ~1.67 daily g CH4 / kg final body weight, and healthy lambs on a restricted diet emitted ~1.62 daily g CH4 / kg final body weight. My take away from this paper is that eliminating parasites is unlikely to deliver much climate mitigation (in this case without dietary adjustment would INCREASE GHGs), although it may be desirable from an animal welfare or efficiency perspective.


Costa Jr, C., Wironen, M., Racette, K., & Wollenberg, E. (2021). Global Warming Potential* (GWP*): Understanding the implications for mitigating methane emissions in agriculture. CCAFS Info Note. Wageningen, The Netherlands

Fox, N. J., Smith, L. A., Houdijk, J. G. M., Athanasiadou, S., & Hutchings, M. R. (2018). Ubiquitous parasites drive a 33% increase in methane yield from livestock. International Journal for Parasitology, 48(13), 1017–1021.

Green, J. K., Seneviratne, S. I., Berg, A. M., Findell, K. L., Hagemann, S., Lawrence, D. M., & Gentine, P. (2019). Large influence of soil moisture on long-term terrestrial carbon uptake. Nature, 565(7740), 476–479.

IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.

p.s. Here's one more picture from the climate change mini golf course, showing a polar bear on melting ice floes