October 27, 2016

Will Global Greening Save Us All?

Last week, at the Royal Society in London, author and optimist Matt Ridley explained how he believes the risks of dangerous global warming have been exaggerated by activists and scientific institutions. Ridley’s read of the science is that higher CO2 levels in the atmosphere have spurred plant growth (a good), while warming has been moderate (a limited bad). He thinks this balance will probably continue, thus invalidating alarm about climate change and reducing the case for reducing greenhouse gas emissions.

Ridley’s perspective is common with many libertarians and others on the Right, so it is worth thinking about why it is unconvincing. Ridley’s speech fails to convince me, because I think he is selectively reasoning with the scientific evidence and posing the wrong questions. I do not think that the evidence is so strong that (1) the benefits of CO2 fertilization will outpace the harms of climate change or (2) we can bank on mild warming in the future.

Greening the Earth

Ridley spends considerable time showing the latest evidence that plants thrive in a high CO2 world. CO2 is an important ingredient in photosynthesis. So when there is more CO2, plants do better (as long as some other ingredient, like nitrogen or water, is not also limiting their growth).

Over the last 30 years, satellites orbiting the Earth have observed a general greening of the land surface. You can use a climate modelone which represents how plants respond to changes in climate, land use practices, and CO2 concentrationsto calculate how much of that greening is due to increased CO2. In the paper Ridley cites, it’s about 70 percent.  

I agree that the CO2 fertilization effect is for the good. It will probably help agricultural yields and spur growth in forests. Accelerated growth has allowed the terrestrial biosphere to capture excess CO2 from the atmosphere and has thus lessened the fraction of human emissions that remain in the atmosphere and cause warming.

But the greening effect of CO2 is at best a salve, not a cure. The simple question comes, at what point will the marginal benefits of CO2 fertilization be outpaced by the harms associated with changing temperature, precipitation, sea level, and other climate phenomena? Here, I don’t think Ridley makes a considerate case.

Firstly, I don’t think that it is at all clear that elevated CO2 will win out over temperature and precipitation changes when it comes to plant growth. Even for highly managed systems like croplands, it is not clear that CO2 fertilization has counteracted climate damages in cereal grains to date. And work that endeavours to investigate the dual effects of climate change and CO2 fertilization on global agriculture finds a future of loss rather than gain. For plants and forests worldwide, the case is even less clear.

Meanwhile, many of the damages that come from climate change are unrelated to more efficient photosynthesis. Think sea level rise or heat impacts on economic development. Models that embark on the brave task of calculating the net benefits and costs of climate change find that damages increase as temperatures go up, and while CO2 fertilization can ease the costs associated with CO2 emissions, we are already past the point where marginal emissions cause net loss. When it comes down to the arithmetic, much of the benefit that Ridley claims is already sunk.

However, those analyses do rely on temperatures actually going up, because the damages they model come from temperature change not excess CO2 (while excess CO2 does cause ocean acidification, the costs of that phenomena are not known). Here Ridley does offer some response, namely that we should expect less temperature increase from CO2 emissions than models or the IPCC would have us believe.

This is ground that we have tread before on our blog.

  • Ridley argues that recent data-based methods of estimating the sensitivity of the climate show that CO2 emissions will lead to much less warming than models predict. However, we have learned in the last year that these methods are not directly comparable because of biases in temperature records and in how they account for non-CO2 climate drivers. When both of these are taken into account, models and data do not look so different.
  • Ridley also states, but does not show, that climate science has serially been wrong in its predictions of the warming effect of CO2. While this is a line that plays well in the skeptic blogosphere, the peer-reviewed literature tells a different story. Comparing models with observations over short periods is hazardous, and comparisons of the last 30 years are complicated by the warming slowdown of the early 21st centurywhich we now know had much to do with wobbles in the tropical Pacific and small volcanic eruptionsand is not reason to believe that temperature increases will be drastically less than expected going forward.

Greening of tropical forests is cold comfort if Ridley is wrong about climate sensitivity. Which is why we should think about reasonable policies in the face of an uncertain future.

But I acknowledge that CO2 sensitivity may be lower than the average climate model. What will that mean in practice? We’ve shown before that it hardly absolves CO2 emissions from causing damages (though would moderate them); nor does it release the burden of stabilizing atmospheric CO2 to halt climate change.

Say atmospheric CO2 increases and stabilizes at about double the pre-industrial concentrationholding somewhere around 550 parts per million by the end of the centurythen the final temperature increase will be closer to 2 °C than 3 °C. That would be a huge relief, as it is hardly controversial that damages grow with temperature increases.

But make no mistake that stabilizing atmospheric CO2 at double the preindustrial values asks us to stabilize and then cut global emissions through 2050 (c.f. Figure 6.11, IPCC AR5 WG3 Chapter 6). As the poorest build modern economies, this implies emissions cuts in the developed world.

Neither the garden of Eden nor the Book of Revelations

Ridley’s case against climate alarmism emphasizes the benefits of CO2 fertilization over the costs of climate change, which is fine editorializing, but weakly addresses how climate damages will progress in the future. The evidence for a very low climate sensitivity that he offers is not compelling.

But there is fair reason to ask how ambitious climate policies should be over this centuryespecially when it comes to finding a fair balance between growing CO2 emissions in the developing world and establishing ambitious targets for stabilizing the climate. With activists looking to restore atmospheric CO2 to 350 ppm and international efforts likely to miss their target of something like 450 ppm, asking what the world would look like at 550 ppm is no sin.

Indeed, Ridley imagines a world where atmospheric CO2 is 600 ppmnot too far off from 550 ppmand the result is not too bad. He is probably right: should that world come to be, it probably won’t be the book of revelations. But I am skeptical it will restore Eden.