Professor Colin Prentice, AXA Chair in Biosphere and Climate Impacts
“Carbon Dioxide: The Good News” – This is the title of a recent Global Warming Policy Foundation report (Goklany, 2015) that focuses on the benefits of CO2 for people. In a hard-hitting foreword, eminent physicist Freeman Dyson claims that the entire scientific and policy establishment has been suffering from a form of “tribal group-think” that involves systematically ignoring the “obvious” facts about CO2.
The reason I find this report dismaying is not that it misrepresents the science. In fact, much of it is quite correct and moreover, well-established in the scientific literature. But given the ever-increasing politicisation of climate science, its author evidently found it appropriate to set up a straw man hypothesis, according to which CO2 is said to be “evil and dangerous” (by whom?), and to write the report in a polemical way that tends to draw the reader to a conclusion that because this straw man can easily be burnt down, the current international interest in emissions reduction is entirely misguided. He accuses the IPCC (Working Group 2) of putting a particular, negative gloss on the impacts of climate change on crop yields, for example; but then he puts his own very particular gloss on several topics, including the celebrated warming “hiatus”, the subject of an extensive recent literature that he largely ignores, in favour of the popular pastime of rubbishing climate models.
What the science says
Let’s start with some of the many points where Goklany is quite right. I am paraphrasing what he says, but I think I have captured the essence; and added a few of my own asides.
- CO2 is indeed “plant food”, as well as being essential to a habitable climate! Both facts are universally known by plant and climate scientists.
- Rising CO2 levels in the atmosphere enhance crop yields. (For C3 crops including wheat, soybean and rice; but not maize and sugarcane. The latter group are already highly productive because they use the C4 photosynthetic pathway, whereby CO2 is concentrated to high levels around the chloroplasts. So in effect, they already have the advantage of a high CO2) Some experiments have shown that the gains can be less than standard plant physiology models predict; but that the gains are real is not in dispute. It follows that these increased yields owing to rising CO2 have undoubtedly diminished pressure to clear further land for agriculture.
- Correction of nutrient deficiencies is common practice already. The fact that CO2-induced productivity increases may not be fully realised due to nutrient limitations in natural ecosystems is largely immaterial for agriculture.
- Rising CO2 enhances productivity in natural ecosystems. The degree to which this effect is limited by nutrient availability is disputed, and seems to be different in different ecosystems (according to experiments). Nonetheless, this CO2 “fertilisation” effect can account (and is the only plausible mechanism) for the fact that terrestrial ecosystems are continuing to take up about a quarter of total anthropogenic emissions of CO2. There is no sign of any abatement of this “carbon sink” (see our blog: How much CO2 can trees take up?).
- Rising CO2 increases the water-use efficiency of plants. Measurable increases in green vegetation cover are occurring in semi-arid regions, because of this effect (see Ukkola et al. 2015).
- Aggregated indicators of human wellbeing continue to increase, confounding some early predictions of imminent environmental collapse and providing no evidence in support of the idea that CO2-induced climate change as far as it has occurred already represents a significant threat to human health or wealth.
- Today’s climate models still perform poorly when it comes to predicting local climate changes, or precipitation patterns generally. Comparing future projections from different models shows their predictions are inconsistent; they can’t all be right. More powerfully, predictions of past eras show that they have systematic biases, which are still not well understood (Harrison et al. 2015).
- There are some likely benefits of rising temperatures that should be weighed up against the hazards, including (up to a point) crop yields in mid- and high latitudes, and the reduction of excess winter deaths in many parts of the world.
- Some climate change impact assessments (especially some early studies on crop yields) failed to account for the beneficial effects of CO2; these should now be considered unrealistic, except as sensitivity tests.
- A fortiori, any human-health assessment that considers only the effects of climate change on pathogens or vectors, ignoring the positive effects of continuing investments in public health, should be considered completely unrealistic.
A mixed picture
However, these key points need to be tempered by some others that surely are also relevant to the discussion:
- The impressive improvements in crop yields over recent decades have not been primarily due to the CO2 They are simply too large to explain by CO2. Instead, technology has played a key role, and technological advances will continue to be required as agriculture has to adapt to an inevitably changing climate. A major international effort, the CGIAR Program on Climate Change, Agriculture and Food Security, exists for just this purpose.
This point actually seems to be closely in line with Goklany’s thinking. Just as climate change is not a major (negative) driver of human wellbeing, CO2 has not been a major (positive) driver, either. Human agency remains paramount.
- Rising CO2 is and will continue to be accompanied by a changing (warming) climate. (The true value of “climate sensitivity” – the long-term effect on global mean temperature of a CO2 doubling – continues to be controversial, but studies of past climates show that it is within the range predicted by climate models.) Along with the beneficial effects of CO2 on crops we can therefore expect more frequent heat stress events, which are a major cause of crop failure today. The most recent global assessment of crop yields in a changing environment found positive and negative effects for spring wheat and soybean – the positive effects mainly from rising CO2 and the negative effects mainly from heat stress. For maize, because it is already benefiting from internally concentrated CO2, the projection was all negative. Thus, the picture is mixed, and not all positive as Goklany’s report suggests.
On a couple of other very important points, Goklany is wrong.
- The brief treatment of sea level focuses on the differences among recent records (in particular, whether there is evidence for acceleration of sea level rise during recent decades), but ignores the fact that current, measured rates – whichever set of data you use – are much higher than rates recorded geologically for any period the 10,000 years or so, prior to 1700. He implies that the continuing sea level rise is not due to anthropogenic climate change but rather to some other, previous climate change. This is not consistent with the evidence. And sea level rise matters, because of the huge investments that will be needed to protect populous coastal cities. Some investment will be needed anyway, but it would be perverse in the extreme to make climate policy without taking account of the economic consequences of sea level rise.
- The report concludes that halting the CO2 rise “abruptly” would be a bad idea. However, no conceivable policy could do that; it would be a physical, economic and political impossibility, and no one is proposing it.
Now to be sure, certain lobby groups and NGOs, some media commentators, and a few advocates from the scientific community have used evidence selectively to support extreme environmentalist positions on climate change. In a previous blog post I commented on a report by three NGOs that claimed bioenergy was “dirtier than coal”, based on indefensible premises. The claim that 350 ppm represents an ideal CO2 concentration is highly questionable, as Goklany notes. I and several other plant and climate scientists recently commented on a published paper that used faulty logic to derive alarming, negative predictions of the effects of climate change on the future “suitability” of tropical lands for primary production (see the online discussion following Mora et al. 2015).
However, the various benefits of rising CO2 are actually well established in the scientific literature, even if sometime ignored. They are indeed “good news”. (This good news do not apply to other greenhouse gases, but CO2 is still the main story.) The good news should not blind us to the negative implications of continued unabated climate change, and the multidecadal lead times required for policies to have any discernible effect on CO2 and climate. These are the reasons propelling international pressure for long-term carbon neutrality, and nothing that Goklany says in his report invalidates them.
Finally, a comment about the unfortunate effects of political polarisation in science. I contend that Goklany, like previous contributors to the GWPF publication series, is in effect allowing the terms of discussion to be set by environmentalists! I do not think this is a good idea. It is peculiar that Goklany cites so much literature showing the beneficial effects of CO2 yet still appears to maintain that this work is ignored by scientists. It isn’t. We scientists produced this work, and we continue to use it and build on it. We do not claim that CO2 is “evil and dangerous”, nor do we imagine that models are truth! Polarisation makes for neither good science, nor sensible policy. Concern for human wellbeing is not the preserve of any one political tendency, and scientists are not lacking in it. We have a common interest; we should find a common cause.
Mora C, IR Caldwell, JM Caldwell, MR Fisher, BM Genco, SW Running (2015) Suitable days for plant growth disppear under projected climate change: potential human and biotic vulnerability. PLOS Biology 13: e1002167.
Harrison SP, PJ Bartlein, K Izumi, G Li, J Annan, J Hargreaves, P Braconnot, M Kageyama (2015) Evaluation of CMIP5 palaeo-simulations to improve climate projections. Nature Climate Change 5: 735-743.
Ukkola AM, IC Prentice, TF Keenan, AIJM van Dijk, NR Viney, RB Myneni, J Bi (2015) Reduced streamflow in water-stressed climates consistent with CO2 effects on vegetation. Nature Climate Change.