REVIEW
CLAIM: there's this myth that's developed around carbon dioxide that it's a pollutant […] Carbon dioxide is a perfectly natural gas, it’s just like water vapor, it’s something that plants love. They grow better with more carbon dioxide, and you can see the greening of the Earth already from the additional carbon dioxide in the atmosphere.
Sara Vicca, Postdoctoral research fellow, University of Antwerp:
That claim is correct, and it is generally assumed the elevated CO2 concentrations have contributed considerably to the land carbon sink (ca. 30% of our CO2 emissions have been absorbed by land ecosystems). The claim may still be misleading, though. Plants don’t only need CO2 and water, but also nutrients like nitrogen and phosphorus. If the latter are not sufficiently available, plants may not respond to elevated CO2 at all. Moreover, climatic changes (and particularly extreme events) are an important threat to ecosystems and to the land carbon sink. It is expected that nutrient limitations and extreme events will reduce the land carbon sink and may eventually turn the land into a source of CO2.
I didn’t mention soil (for simplicity), but that’s an important part of the land carbon sink, too.
- Zhu et al (2016) Greening of the Earth and its drivers, Nature Climate Change
- Terrer et al (2016) Mycorrhizal association as a primary control of the CO2 fertilization effect, Science
- De Graaff et al (2006) Interactions between plant growth and soil nutrient cycling under elevated CO2: a meta-analysis, Global Change Biology
- Zaehle et al (2015) Nitrogen Availability Reduces CMIP5 Projections of Twenty-First-Century Land Carbon Uptake, Journal of Climate
- Zhang et al (2014) Nitrogen and phosphorous limitations significantly reduce future allowable CO2 emissions, Geophysical Research Letters
- Reichstein et al (2013) Climate extremes and the carbon cycle, Nature
Alexis Berg, Research Associate, Harvard University:
Yes, we are seeing an increase in vegetation around the world, with satellites showing up to 50% of the land surface greening over the last 30 years, most of it being indeed attributed to increased atmospheric CO2*. This is not a surprise. This vegetation increase is consistent with the fact that we know that about a quarter of our CO2 emissions is being taken up by the land biosphere (another quarter going into the oceans, and the remainder staying in the atmosphere). This greening is taken into account in climate models. The real question is how long this greening is going to go on (there are already indications that it is slowing down), as we expect that as climate warms further, adverse impacts on ecosystems may start to offset the positive impact of increased atmospheric CO2—particularly in places where regional climate is moving away from the “comfort zone” of current ecosystems.
- Zhu et al (2016) Greening of the Earth and its drivers, Nature Climate Change
Jean-François Exbrayat, Post-doctoral Research Fellow, The University of Edinburgh:
Carbon dioxide is a gas that is naturally present in the atmosphere. Plants grow by taking up CO2 from the atmosphere through the process of photosynthesis. Terrestrial ecosystems have taken up about 20-30% of fossil-fuel emissions since the 1960s[1]. Through this so-called “CO2-fertilization”, plants help reduce the growth of atmospheric CO2 responsible for climate change, and this might be what William Happer refers to when talking about greening.
A global “greening”, (i.e., longer growing seasons) of the Earth has also been detected in satellite observations of leaf area index from the 1980s to the present day, and modelling experiments have attributed 70% of this greening to CO2 fertilization[2].
However, plants grow better with more CO2 only if they also have access to nutrients like nitrogen and phosphorus to fix the photosynthesised carbon. For example, results from Free-Air CO2 Enrichment (FACE) experiments[3] have shown that the initial enhancement of plant productivity under CO2-fertilization was declining through time. This enhancement could, however, be restored and maintained through an additional nitrogen treatment. Results from another FACE experiment in Australia[4] have recently shown that, under current phosphorus limitations, no additional growth was recorded under increased CO2.
These results from FACE experiments raise uncertainty on the sustainability of the CO2-fertilization effect in mostly nitrogen-limited temperate and boreal forests and mostly phosphorus-limited tropical ecosystems. Accordingly, adding nitrogen and phosphorus limitations on plant growth in Earth system models reduces the projected capacity of the land surface to continue offsetting fossil-fuel emissions during the 21st century[5].
- [1] Le Quéré et al (2016) Global Carbon Budget 2016, Earth System Science Data
- [2] Zhu et al (2016) Greening of the Earth and its drivers, Nature Climate Change
- [3] Norby et al (2010) CO2 enhancement of forest productivity constrained by limited nitrogen availability, PNAS
- [4] Ellsworth et al (2017) Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil, Nature Climate Change
- [5] Wieder et al (2015) Future productivity and carbon storage limited by terrestrial nutrient availability, Nature Geoscience
Pierre Friedlingstein, Professor, University of Exeter:
This is the usual misleading argument that if CO2 is good for plants, it cannot be bad for the climate.
Happer’s statement is correct—CO2 is needed for plant growth (along with water, nutrients, and energy from the sun)—but it does not change the fact that CO2 is a greenhouse gas. It increases the radiative forcing of the planet and leads to warming, as observed over the last century.
What he says is like saying, “Sun cannot cause skin cancer. The sun is natural, it’s something plants love, they grow better with more sunshine!”
