Posted on 26 February 2013 by gws

A new paper by Hemming et al. presents physical, best case limits on the (opposing) effects of warming and CO₂ on the “greening” of the biosphere. Using the Hadley Center’s general circulation model in connection with an interactive plant response, they show that a doubling of atmospheric CO₂ in the model under no other limitations increases global terrestrial net primary productivity (NPP - overall plant growth) on average by 57%, spatially dominant in the tropics. While warming alone decreases NPP, the physiological effect of higher CO₂, on average, more than compensates for the losses globally. The new study uses an innovative “perturbed physics ensemble” (PPE), similar to ensemble weather forecasting, to study how quantitatively and spatially relevant the results are. However, as critically important nutrient limitations and potentially important biome shifts were excluded from this first study, the results are of limited relevance as they represent only a best case scenario.

Global Warming and the Carbon Cycle

As carbon dioxide levels in our atmosphere keep rising, they do not only affect climate, they also alter physical and biological processes that factor in atmospheric CO₂ concentrations as a significant component of carbon cycling. The two most important such processes are the exchange of CO₂ between the atmosphere and the ocean, and between the atmosphere and the terrestrial biosphere.

The latter process, known as a major part of the the terrestrial carbon cycle, consists of many individual steps and sub-processes of various complexity, which makes the field of terrestrial carbon cycle research one of the most heavily researched fields of the geosciences. It comes, therefore, maybe as no great surprise that the general public has only a limited understanding of the importance of carbon cycling, and that contrarians seeking to confuse and sow doubt among the public frequently cherry-pick only one aspect of the carbon cycle, the “carbon fertilization effect”.

“CO₂ is plant food”

The effect may have been most prominently introduced by the CO2-is-plant-food-meme in a congressional hearing in March 2009, which served to demonstrate again how important accurate scientific information is. The contrarian argument goes like this:

CO2 is needed for plant growth, thus more CO₂ will lead to more plant growth, and hence to a “greening” of the Earth. Ergo, more CO₂ is good for Earth.

There are multiple reasons why such a gross simplification of the role of atmospheric CO₂ in the terrestrial carbon cycle is short-sighted and strongly misleading. First of all, there is no real-world situation, in which only CO₂ increases, all else remaining equal. And second, those other effects of increased atmospheric CO₂, namely increased temperatures and altered moisture regimes, generally have adverse effects on growth.

SkS has addressed the meme at various points, prominently here, and recently here. However, the importance of the terrestrial biosphere’s reaction and any possible feedbacks to the atmospheric CO₂ increase is part of much ongoing research. Rightfully so, because humanity critically depends on the terrestrial biosphere, such as for food production and clean water just to name two obvious aspects. How our biosphere reacts to warming and higher atmospheric CO₂ will ultimately define how catastrophic we can expect our global CO₂-experiment with the climate to be. So wouldn’t it be good to know more about how much “greening” to expect and what it means?

NPP and CO₂ fertilization

All land plants carry out green leaf photosynthesis, the process of acquiring and converting atmospheric CO2 initially into simple sugars and from there into all other carbon-containing plant matter. Next time you look at that bush or tree in your yard, realize that about 50% of its dry mass is carbon, once part of the atmosphere in the form of CO₂. Over the course of each growing season, i.e. when ambient temperatures allow for active photosynthesis, plants accumulate carbon out of the atmosphere and store it, they grow. The net growth over a year is called the Net Primary Productivity, or NPP. Globally, it is estimated to have been around 60 Petagram (Pg, 10¹⁵ g) carbon in the 1990s, balanced by an equal amount or carbon returned to the atmosphere via (heterotrophic) respiration. Much of that respiration is occurring from degrading plant material in soils, meaning respired carbon entering the atmosphere is “older” than the newly stored carbon, and the biosphere (plants and soils) itself acts as an intermediate storage reservoir for carbon cycling in and out of the atmosphere. But as humans also appropriate a large amount of global annual NPP for food and products, some carbon returns faster.

Because the first step of photosynthesis is the diffusion and (biological) absorption of atmospheric CO₂ into a leaf, increasing CO₂ surrounding that leaf will initially speed up the photosynthetic uptake. In other words, the initial photosynthesis steps can be treated like a first-order process, in which the rate of uptake of CO₂ is a function of CO₂ abundance itself. Thus the fertilizer analogy: You feed it more, it grows more. However, and this is the first major culprit, this dependence of uptake on abundance is not linear but drops rapidly with increasing CO₂, ultimately flatlining above 1000 ppm CO₂. The second major culprit, or better say group of culprits, is that atmospheric CO₂ is not the only factor affecting photosynthesis and plant growth. Plants need many nutrients, and water, for optimal growth. Unless these nutrients, such as nitrogen or phosphorous acquired out of the soil, are provided in amounts increasing proportionally to the supply of CO₂, no continued fertilization effect of CO₂ can be sustained.

Calculating NPP changes

It is impracticable to measure carbon cycling rates on a global basis accurately. While the global atmospheric CO₂ monitoring network allows us to estimate how much atmospheric CO₂ is removed by the terrestrial biosphere annually, that amount only represents the difference between NPP and respiration, and does not show a correlation with atmospheric CO₂ (cf. discussion by the Global Carbon Project).

Because there are so many factors affecting annual terrestrial NPP, atmospheric CO₂, air temperature, nutrient and water availability, season length, seasonal temperature development, air pollution and other anthropogenic interferences to name a few, it is very complicated to provide a reliable estimate from knowledge of all relevant processes and their dependencies on physical and biological parameters. Nevertheless, the last decade has shown a flurry of global modeling efforts that combined climate modeling with terrestrial biosphere modeling. The present paper makes an important step forward as it tries to address why past findings have at times identified a larger or smaller fertilization effect, and sometimes even net carbon losses from the biosphere.

Similar to ensemble forecasting in weather research, where a base set of input parameters to the forecast is varied within physically reasonable boundaries to create an ensemble of forecasts that most often describes reality more accurately then each single forecast itself, the researchers used a base set of physical variables in their climate model to create a series of globally gridded ensemble (equilibrium) climate and associated NPP projections, with or without allowing for the physiological effect of CO₂ on photosynthesis. The latter allowed them to distinguish between the effects of warming itself on NPP (adverse) and that of warming and atmospheric CO₂ on NPP (favorable). The perturbed physics ensemble, PPE, was used to gauge how robust the NPP results were among the two drivers, climate and CO₂ in pre- and post-industrial [CO₂] worlds, meaning whether the effect, if any, could have arisen through random uncertainties in the climate model parameters themselves represented by the PPE spread.

Figure 1 (also in original): Ensemble average NPP (kg C m⁻² yr⁻¹) from the RadPhys simulations with both the effects of plant physiological forcing and radiative forcing/physical climate feedbacks, simulated with pre-industrial atmospheric [CO2]. Note: The authors erroneously used "per square centimeter" instead of the correct "per square meter" in their graphs and text; 1 kg m⁻² yr⁻¹ is equal to 10 metric tons (t) ha^-1 yr^-1 or 4 t per acre per year.

Mixed news for the tropics

Most of global NPP is accumulated in the tropics, Figure 1, a result consistent with all prior work on global NPP. The tropics are “lush” and became more so under doubled CO₂ in the model. While warming caused NPP losses throughout the tropics in the model (driven by temperature effects on autotrophic respiration), the physiological effect of CO₂ dominated the net, Figure 2.

Figure 2 (original Fig. 3a): Average changes in NPP (red: −0.2 to −0.3, dark green: 0.6 to 0.7 kg C m⁻² yr⁻¹) between pre-industrial and doubled [CO2] for the RadPhys (both warming and physiology) sub-ensemble.

However, the standard deviation of the PPE was also high in several regions of the tropics, Figure 3, prompting the authors to state

“In these locations, NPP is particularly sensitive to the specific choice of model parameterisations, such that even the sign of the NPP response could change according to the parameterisations adopted.”

So while there appears to be a robust signal that tropical NPP will increase as a result of increasing atmospheric CO₂, the spatial forecast is mixed.

Less good news in mid- and high latitudes

Outside the tropics, in the major food production regions of the northern hemisphere, the model's verdict is less encouraging. The model’s NPP increases were of similar magnitude than the PPE standard deviation, Figure 3, and the authors highlighted larger uncertainties of warming than CO₂ on expected NPP changes.

Figure 3 (Fig. 7a in original): Spatial differences between the average NPP changes and the standard deviations of the RadPhys (both warming and physiology) sub-ensemble. Yellow to light green represent neutral, i.e. −0.2 to 0.2 kg C m⁻² yr⁻¹, blueish colors represent positive (i.e. significant NPP increases), orange to red colors negative values (i.e. insignificant NPP increases).

Ultimately, the authors concluded that

“… these results indicate that the direction of the global average NPP response to doubled [CO₂] is likely to be positive, regardless of the values of the model parameterisations perturbed in this study.”

Insofar, one can hope that, and there is broad scientific consensus, that there will indeed be a small amount of “greening” from increased atmospheric [CO₂]. However, stressed at several points in the manuscript, the above scenario is for a situation where the basic composition and structure of the global terrestrial biosphere does not change under doubled [CO₂] and where nutrient limitations do not matter. As the primary nutrients nitrogen and phosphorous are key limiting factors in many ecosystems, particularly in the tropics, we can expect that real-world NPP increases as a result of doubling [CO₂] will be much lower than the ensemble average of 57% the authors calculated. And we expect that similar studies addressing this issue will be published in the coming years.

An additional word of caution

Model NPP in GCMs is generally an equilibrium value. It does not tell us anything about transient carbon fluxes, such as those triggered by fires, floods, heat waves or droughts, extreme events that are expected to increase in frequency as warming progresses. As the (devasting) effects of the US drought since 2011 are becoming more and more apparent, it is clear that even potentially large increases in crop yields cannot be sustained in an increasingly extreme weather world. Especially heat waves, e.g. the Europe-wide heat wave in 2003, can cause widespread plant mortality and crop yield drops that are expected to cause regular food security issues in the second half of the century.

Summary

• Doubling atmospheric CO₂ is likely going to cause some "greening" of the terrestrial biosphere globally under equilibrium conditions, especially if factors such as water and nutrient availability do not become limiting
• Likely, much of the greening will manifest itself in the tropics, not in mid-latitudes; warming negates much of the CO₂ effect
• Limiting growth factors are common throughout the terrestrial biosphere, thus the calculated "CO₂-only" effects represent a best case scenario
• Equilibrium conditions are a model construct not accounting for transient developments, particularly short-term weather extremes with the potential to eliminate long-term gains

Deborah Hemming, Richard Betts, Matthew Collins:

Sensitivity and uncertainty of modelled terrestrial net primary productivity to doubled CO₂ and associated climate change for a relatively large perturbed physics ensemble,

Agricultural and Forest Meteorology, Volume 170, 15 March 2013, Pages 79–88, http://dx.doi.org/10.1016/j.agrformet.2011.10.016