Effects of CO₂ and Climate Change on Terrestrial Net Primary Productivity

Summary

This page compiles information on ways in which greenhouse-gas emissions and climate change will likely increase and likely decrease land-plant growth in the coming decades. The net impact is very unclear. I favor lower net primary productivity (NPP) because primary production gives rise to invertebrate suffering. Terrestrial NPP is just one dimension to consider when assessing all the impacts of climate change; effects on, e.g., marine NPP may be just as important.

Epigraph

This paper says:

On land, reports suggest a decline in the tropical sink^15,16, increased plant mortality^17,18 and decreased plant productivity due to droughts and extreme events^19,20. In contrast, others report that elevated CO₂ has led to increased photosynthesis⁸ and a greening of the biosphere^21,22. The relative contributions of the different processes involved in the terrestrial sink enhancement remain unquantified.

CO₂ fertilization

"In Defense of Carbon Dioxide" presents reasons to think more CO₂ would increase plant efficiency and resilience against drought. It notes that some greenhouse managers deliberately add CO₂ to their greenhouses in order to increase yields. (Note that from my perspective, this amounts to a condemnation of CO₂ rather than a defense.)

Increased CO₂ concentrations reduce photorespiration, which translates into greater plant productivity, although warmer temperatures counteract this effect by increasing photorespiration somewhat.

Ainsworth and Long (2005) include the following figure:

This study found that giving plants more CO₂ increased net primary productivity by 24% on average. Young trees and other small plants responded well to higher CO₂, but it remains undetermined how more mature trees would react.

This article explains: "Globally, vegetation is locking away more carbon as atmospheric CO₂ levels rise. Plants are growing faster, fuelled by a more fertile atmosphere. But the Amazon is eschewing this trend." However, by "eschewing this trend", I think what the article means is just that Amazonian plants are dying faster than they're growing. But if you look at the graph in that article, you can see that Amazon primary productivity is actually increasing, and it's just that death rates are increasing faster. Insofar as primary productivity is an approximation to suffering, this would suggest that suffering may be increasing even in the Amazon?

This page evaluates the size of the "carbon fertilization effect".

Zhu et al. (2016):

Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982–2009. We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO₂ fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO₂ fertilization effects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau.

Keep in mind that CO₂ fertilization is not a side effect of non-CO₂ greenhouse gases like methane (except via indirect temperature, humidity, etc. changes).

Complicating factors

This document says:

[Our model] results show a considerable increase in net primary production (NPP) over the last century, mainly due to the so called CO₂ fertilization effect. Many uncertainties, however, relate to this effect: In free air carbon dioxide enrichment (FACE) experiments, strong effects of NPP increase (in particular forests or coppice have been found (+20%)). [...] However, large uncertainties prevail, which prevent straightforward predictions of vegetation responses to elevated CO₂ in the current state of knowledge. These knowledge gaps relate to the allocation of carbon to various carbon pools with different turnover times, and to the impact of essential variables, such as nutrient availability, which have been found to be decisive for plant responses to elevated CO₂. In general fertilization by CO₂ (and also N) is expected to saturate at high levels. Additionally, there is evidence that crops grown under elevated CO₂ concentrations might be more susceptible to insect pests.

This post explains:

The effect of temperature change is generally positive to increase the productivity by enhancing the photosynthesis as long as the temperature is in a range of optimum level. When temperature exceeds the optimum level, it will increase the rate of respiration causing the NPP continuously declined. [...]

Experiments find that elevated CO₂ may increase plant productivity by increase carboxylation rates of RUBISCO in C3 Plants. Terrestrial Ecosystem Model (TEM) predicts that doubled CO₂ will increase 16.3% of the global NPP (Melillo et al. 1993). However, different response[s] occur in many northern and temperate ecosystems where elevated CO₂ may not much affect plant productivity because of lack of Nitrogen in the soil.

The effect of CO₂ to plant productivity may be limited by some factors, such as Nitrogen availability, plant acclimatization and water availability. Under low Nitrogen conditions, plants will have difficulties to transform elevated CO₂ into production. Moreover, in the long term, elevated CO₂ condition may cause the accumulation of carbohydrates in the plant tissues which may reduce the photosynthetic rates or decrease photosynthetic response to elevated CO₂.

This book says regarding CO₂ fertilization:

Under ideal conditions it can be a large effect; for C3 crops under doubled carbon dioxide, an average of +30%,⁴⁰ although grain and forage quality tends to decline with carbon dioxide enrichment and higher temperatures. However, under real conditions on the large scale where water and nutrient availability are also important factors influencing plant growth, experiments show increases under unstressed conditions in the range 10-25% for C3 crops and 0-10% for C4 crops. Enhanced growth has been observed for young tree stands but no significant response has been measured for mature forest stands. Ozone exposure limits carbon dioxide response in both crops and forests.⁴¹ [...]

[Studies of climate change's projected impacts on food crops] in general indicate that the benefit of increased carbon dioxide concentration on crop growth and yield does not always overcome the effects of excessive heat and drought. For cereal crops in mid latitudes, potential yields are projected to increase for small increases in temperature (2-3°C) but decrease for larger temperature rises [as shown in the below figure].⁴³ In most tropical and subtropical regions, potential yields are projected to decrease for most increases in temperature; this is because such crops are near their maximum temperature tolerance.

Higher latitudes more tropical

Climate change will in general make higher-latitude regions of the Earth more tropical, and growing seasons will be longer.

The Arctic in particular is expected to increase in temperature more than the global average, and warming of the Arctic seems very likely to increase wild-animal populations in that region by removing ice.

Rainforest loss

WWF reports:

In the absence of effective measures, global warming and deforestation could convert from 30 up to 60 per cent of the Amazon rain forest into a type of dry savanna, according to research carried out under the auspices of Brazil's National Space Research Institute (INPE).

This article reports on a study which

found that a 2°C rise above pre-industrial levels, widely considered the best case global warming scenario and the target for ambitious international plans to curb emissions, would still see 20-40% of the Amazon die off within 100 years. A 3°C rise would see 75% of the forest destroyed by drought over the following century, while a 4°C rise would kill 85%. [...]

the study showed that tree growth in high latitudes, such as Siberia, would increase, but would be unlikely to compensate for the carbon stocks lost from the Amazon.

Keep in mind that carbon stocks (i.e., standing plant production) are not the same thing as NPP (i.e., rate of growth of plant production), but presumably the two are somewhat correlated in this context (i.e., forests with higher carbon stocks probably have higher NPP).

Drought and desertification

This post says: "current study on the effect of drought on productivity shows that drought in the period of 2000-2009 has reduced NPP by 0.55 petagram Carbon globally. This is contradictory to the previous studies by Nemani et al. (2003) who suggest that global climate change increased NPP by 3.4 petagrams of carbon over 1982-1999 period."

This commentary highlights the expected increase in droughts in many regions due to climate change:

I first heard of the risks in a 2005 talk by climatologist Jonathan Overpeck of the University of Arizona in Tucson. He pointed to emerging evidence that temperature and annual precipitation were heading in opposite directions over many regions and raised the question of whether we are at the “dawn of the super-interglacial drought”. [...]

Recent studies have projected ‘extreme drought’ conditions by mid-century over some of the most populated areas on Earth — southern Europe, south-east Asia, Brazil, the US Southwest, and large parts of Australia and Africa⁸.

This 1996 article suggests that the net impact of climate change on desertification is unclear:

A possible increase of 1–3°C in arid lands over the next 50 years [...] would increase global potential evapo-transpiration (PET) by some 75–225 mm year^–1. The ratio of mean annual precipitation to PET would then decrease by about 4–5%, assuming that no substantial changes in rainfall took place in arid and semiarid lands. However, the impact of CO₂ on plants would boost photosynthesis and, therefore, primary productivity; it would also increase water-use efficiency via the reduction of stomatal conductance. It is therefore at present difficult to predict the net balance of these two opposite consequences or to prophesy which phenomenon would prevail: increased aridity or higher productivity and more efficient water use. At all events, the possible effect of a climatic fluctuation (or change) of the magnitude envisaged would have a trivial consequence on arid environments, as compared with the past and present impact of humans and their livestock.

This article reviews ways in which climate change seems to be greening desert areas, sometimes because of increased moisture in the air. As the article explains, various regions of the world are showing increased plant growth. This is unsurprising, since as Stefan Kröpelin says in the article: "There are always winners and losers if weather patterns change." A main reason that people make climate change out to be bad for the environment is that disruption of the status quo tends to cause more harm than benefit to humans in the short run. But near-term effects on human welfare aren't a good indicator of how climate change will affect total wild-animal populations in the longer term. As Ty Raterman notes: "Let us be slightly cautious [when making claims about environmental changes degrading ecosystem health], though, as ecosystem health is often understood in suspect anthropocentric terms."

Irregular rainfall

This page says: "Increased variability and intensity of rainfall as a result of climate change is expected to produce both more severe droughts and flooding". Presumably irregular precipitation is less conducive to plant growth than more steady amounts of rain.

That said, I would guess that human adaptation will somewhat mitigate the effect of flooding. An example of how flood control can work is described on this page, which says "Through careful creation and management of the Tennessee River watershed and its system of dams, [the Tennessee Valley Authority] TVA can virtually eliminate flooding in the Valley—no small feat." This page says: "A new generation of balancing dams are being developed to combat the possible consequences of climate change."

Plant nutritiousness

NPP affects invertebrate populations by regulating the numbers of invertebrates that have nutritious food with which to grow and reproduce. From this perspective, plant nutritiousness matters in addition to raw plant NPP.

This study explains:

The CO₂ level in Earth’s atmosphere is widely expected to double during this century (e.g., Falkowski et al. 2000; Houghton et al. 2001), substantially altering the nutritional quality of C₃ plants; levels of nonstructural carbohydrates will increase and protein (nitrogen) will decrease (Poorter 1993; Poorter et al. 1997; Wand et al. 1999). The effects of these changes on leaf-chewing insect herbivores have been examined primarily on caterpillars feeding on C₃ dicots (Bezemer and Jones 1998). The responses of caterpillars vary widely, ranging from effective compensation to reduced fitness.

That study itself found that grasshopper growth rates didn't decline with doubled CO₂ concentrations (p. 100, Table 2). The grasshoppers "maintained a high growth rate on the C₃ grass grown under elevated CO₂ despite significant changes in its foliar nutritional quality, suggesting that post-ingestive mechanisms enable these grasshoppers to compensate for variable nutritional quality in their host plants" (pp. 100-101).

My todo list

- Read the rest of Ainsworth and Long (2005) and Zhu et al. (2016), which are important papers.