Why Future Net Negative Impacts of Global Warming Are Overestimated: Response to Conor Clarke, Part IV

This post responds to the last of Conor Clarke’s comments on my study, “What to Do About Global Warming,” published by Cato. This series started with the imaginatively titled, Response to Conor Clarke Part I, and continued with Cherry Picking Climate Catastrophes and  Do Industrialized Countries Have a Responsibility for the Well-Being of Developing Nations?

CONOR said:

I think Goklany is a bit picky and choosey with the evidence. … I also like the Goklany paper a lot. [THANK YOU!! I’ll take whatever I get.] But in this case it’s hard to resist. [Emphasis in original.]

To take one example (of several), Goklany’s hunger estimates rely heavily on those published by Global Environmental Change (GEC), which he uses to make the argument that “the world will be better off in 2085 with respect to hunger than it was in 1990 despite any increase in population.” But the GEC produced two estimates of hunger and climate change – one that assumes the benefits of CO2 fertilization and one that does not. Goklany picks the former estimate (I have no idea why), despite the fact the GEC says the effects of climate change “will fall somewhere between” the two. … [I}f you embrace anything other than the most Pollyanish CO2 fertilization estimate – the one that Goklany uses in his Cato paper – we will be living in a world in which climate change puts tens of millions of additional people at risk of starvation by 2085.

My RESPONSE:

First, let me elaborate on my selection of the set of studies that I used in my paper.  Essentially, the selected set of studies (published in Global Environmental Change) was the only one that had estimated global impacts using detailed process models in conjunction with the IPCC’s latest scenarios, and were peer reviewed.  Moreover, they come with a provenance that people who may be unhappy with my results cannot impugn. [This is important only because many people arguing about global warming seem to be more concerned about who did the study and whether the results bolster their predilections, than how the study was done.]  Specifically, virtually all the authors were intimately connected with the IPCC. The senior author of the hunger study was also the co-chairman of the IPCC’s Work Group II, which was responsible for compiling the portion of the IPCC’s latest assessment that dealt with impacts, vulnerability and adaptation. The authors of the water resource and coastal flooding studies were the lead authors of corresponding chapters in that IPCC report. An earlier version of the same set of impact studies was the basis for the claim by Sir David King, erstwhile science advisor to Her Majesty’s Government, that global warming was a more serious threat than terrorism (see here). The Stern Review also drew quite heavily from these studies (see below).

Let’s now turn to Conor’s comments on the hunger study and why I assumed that the benefits of carbon fertilization would be realized in the future. Indeed, the hunger study (Parry et al.) produced two separate estimates — one assuming that carbon fertilization is a reality, and the other assuming zero carbon fertilization.  But the two estimates are not equally likely. There are literally hundreds, if not thousands of experimental studies that show carbon fertilization is a reality (see also here), that higher CO2 not only increases the rate of photosynthesis, but also increases the efficiency of water use by plants (i.e., it confers a degree of immunity to drought), among the many other benefits CO2 bestows on plants and other carbon based life, including all creatures – big and small – in the biosphere that depend directly or indirectly on photosynthesis.  The probability that direct CO2 effects on crop growth are zero or negative is virtually non-existent (IPCC, 2001b: 254–256). Second, the positive effect of carbon fertilization was based on the average of experimental studies; it’s not an upper bound estimate. On the other hand, the notion of “zero fertilization” is an assumption not supported by the vast majority of empirical data. So averaging results from the two estimates makes no sense and would understate the average benefits that would likely result from carbon fertilization.

Notably, the Stern Review, invoked a study by Long et al. (subscription required) to estimate future levels of hunger based on “zero fertilization” using precisely the same study (Parry et al.) that I  – and Conor, in his comments – used. But Long et al.’s results have been disputed by other scientists (also see here), including some contributors to the IPCC’s assessment.  More importantly, Long et al. only suggested that under field conditions, carbon fertilization may be a third to less than half of what is indicated by experiments using growth chambers, not that it would be zero. It also noted that fertilization may be stronger under drought conditions or if sufficient nitrogen is employed. But drought is one of the bogeymen of global warming, and increased use of nitrogen is precisely the kind of adaptation that would become more affordable in the future as countries become wealthier, as they should if the IPCC’s scenarios are to be given any credence.  Indeed, that is one of the adaptations allowed in Parry et al. Also, the fact that crop yields are higher in richer countries is partly because they can more easily afford nitrogen fertilizers (see here, p. 78). In fact, China’s nitrogen use per hectare is already among the world’s highest. For all these reasons, even if one accepts the Long et al. study as gospel, it is reasonable to assume that the effect of carbon fertilization will be closer to the “higher” estimate from the Parry et al. study than to the “zero fertilization” case.

But, more importantly, the uncertainties related to the magnitude of the CO2 fertilization effect is most likely swamped by a major source of overestimation of hunger in Parry et al.’s estimates.

Although Parry et al. allows for some secular (time-dependent) increases in agricultural productivity, increases in crop yield with economic growth due to greater application of fertilizer and irrigation in richer countries, decreases in hunger due to economic growth, and for some adaptive responses at the farm level to deal with global warming, Parry et al. itself acknowledged that these adaptive responses are based on the “current range” of available technologies, not on technologies that would be available in the future or any technologies developed to specifically cope with the negative impacts of global warming (Parry et al., p. 57).  The potential for future technologies to cope with global warming is large, especially if one considers bioengineered crops (see here, chapter 9), which Parry et al. admittedly didn’t consider. Moreover, an examination of the sources cited in Parry et al. indicates that the “current range” of technology is actually based on 1990s or earlier technology. That is, it is not quite current.

The approach used in Parry et al. to estimate the impacts of global warming decades from now is, in essence, tantamount to estimating today’s level of hunger (and agricultural production) based on the technology of 50 years ago. In fact, the major reason why Paul Ehrlich’s Population Bomb turned out to be a dud was that it underestimated or ignored future developments in agricultural technology.

As noted in Part I of this series of responses, ignoring technological change can, over decades, lead to overestimating adverse impacts by orders of magnitude. Notably, due to a combination of technological change and increasing affluence, U.S. death rates due to various water related diseases – dysentery, typhoid, paratyphoid, other gastrointestinal disease, and malaria – declined by 99%–100% from 1900 to 1970.  For the same reasons, during the twentieth century, global death rates from extreme weather events declined by over 95%.

This basic methodological shortcoming, however, is not unique to Parry et al. It is common to ALL global warming impact studies that I have read – and I have read plenty of them.

For all these reasons, the adverse impacts of global warming for hunger (as well as other aspects of human well-being, e.g., due to malaria and coastal flooding) that I used in my paper are, more likely than not, substantially overestimated. And by the same token, ignoring technological change (and not fully accounting for increases in wealth) also assures that the positive impacts of global warming are likely to be underestimated, further overestimating the net negative impacts of global warming.

Therefore, far from being Pollyanish, the estimates used in my paper most likely substantially exaggerate the net negative impacts of global warming. Despite that, those estimates cannot justify emissions cuts that go beyond no-regret actions at this time or through the foreseeable future.