by Judith Curry
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. – Zhu et al.
New from Nature Climate Change: Greening of the Earth and its Drivers
Abstract. Global environmental change is rapidly altering the dynamics of terrestrial vegetation, with consequences for the functioning of the Earth system and provision of ecosystem services. Yet how global vegetation is responding to the changing environment is not well established. 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. LCC contributed most to the regional greening observed in southeast China and the eastern United States. The regional effects of unexplained factors suggest that the next generation of ecosystem models will need to explore the impacts of forest demography, differences in regional management intensities for cropland and pastures, and other emerging productivity constraints such as phosphorus availability.
Phys.org has published the authors’ press release: CO2 Fertilization Greening the Earth. (Nearly) complete text:
An international team of 32 authors from 24 institutions in eight countries has just published a study titled “Greening of the Earth and its Drivers” in the journal Nature Climate Change showing significant greening of a quarter to one‑half of the Earth’s vegetated lands using data from the NASA‑MODIS and NOAA‑AVHRR satellite sensors of the past 33 years. The greening represents an increase in leaves on plants and trees. Green leaves produce sugars using energy in the sunlight to mix carbon dioxide (CO2) drawn in from the air with water and nutrients pumped in from the ground. These sugars are the source of food, fiber and fuel for life on Earth. More sugars are produced when there is more CO2 in the air, and this is called CO2 fertilization.
“We were able to tie the greening largely to the fertilizing effect of rising atmospheric CO2 concentration by tasking several computer models to mimic plant growth observed in the satellite data” says co-author Prof. Ranga Myneni of the Department of Earth and Environment at Boston University, USA. Burning oil, gas, coal and wood for energy releases CO2 in to the air. The amount of CO2 in the air has been increasing since the industrial age and currently stands at a level not seen in at least half-a-million years. It is the chief culprit of climate change.
About 85% of the Earth’s ice‑free lands is covered by vegetation. The area of all green leaves on Earth is equal to, on average, 32% of the Earth’s total surface area ‑ oceans, lands and permanent icesheets combined. “The greening over the past 33 years reported in this study is equivalent to adding a green continent about two‑times the size of mainland USA (18 million km2), and has the ability to fundamentally change the cycling of water and carbon in the climate system,” says lead author Dr. Zaichun Zhu.
Every year, about one‑half of the 10 billion tons of carbon emitted in to the atmosphere from human activities remains temporarily stored, in about equal parts, in the oceans and plants. “While our study did not address the connection between greening and carbon storage in plants, other studies have reported an increasing carbon sink on land since the 1980s, which is entirely consistent with the idea of a greening Earth,” says co‑author Prof. Shilong Piao.
The beneficial aspect of CO2 fertilization in promoting plant growth has been used by contrarians, notably Lord Ridley and Mr. Rupert Murdoch, to argue against cuts in carbon emissions to mitigate climate change, similar to those agreed at the 21st Conference of Parties (COP) meeting in Paris last year under the UN Framework on Climate Change (UNFCCC). “The fallacy of the contrarian argument is two‑fold. First, the many negative aspects of climate change, namely global warming, rising sea levels, melting glaciers and sea ice, more severe tropical storms, etc. are not acknowledged. Second, studies have shown that plants acclimatize, or adjust, to rising CO2 concentration and the fertilization effect diminishes over time,” says co‑author Dr. Philippe Ciais, Contributing Lead Author of the Carbon Chapter for the recent IPCC Assessment Report 5.
CO2 fertilization is only one, albeit a predominant, reason why the Earth is greening. The study also identified climate change, nitrogen fertilization and land management as other important reasons. “While the detection of greening is based on measurements, the attribution to various drivers is based on models, and these models have known deficiencies. Future works will undoubtedly question and refine our results,” says co‑author Dr. Josep Canadell, leader of the Global Carbon Project.
Roger Harrabin for the BBC
From Roger Harrabin’s article for the BBC, Rise in CO2 has greened planet Earth:
Climate sceptics argue the findings show that the extra CO2 is actually benefiting the planet. But the researchers say the fertilisation effect diminishes over time.
They warn the positives of CO2 are likely to be outweighed by the negatives.
The lead author, Prof Ranga Myneni from Boston University, told BBC News the extra tree growth would not compensate for global warming, rising sea levels, melting glaciers, ocean acidification, the loss of Arctic sea ice, and the prediction of more severe tropical storms.
This is in line with the Gaia thesis promoted by the maverick scientist James Lovelock who proposed that the atmosphere, rocks, seas and plants work together as a self-regulating organism.
“The greening reported in this study has the ability to fundamentally change the cycling of water and carbon in the climate system,” said a lead author Dr Zaichun Zhu.
A co-author Prof Pierre Friedlingstein told BBC News that carbon uptake from plants was factored into IPCC models, but was one of the main sources of uncertainty in future climate forecasts. Warming the Earth releases CO2 by increasing decomposition of soil organic matter, thawing of permafrost, drying of soils, and reduced photosynthesis – potentially leading to tropical vegetation dieback. He said: “Carbon sinks (such as forests, where carbon is stored) would become sources if carbon loss from warming becomes larger than carbon gain from fertilisation. “But we can’t be certain yet when that would happen. Hopefully, the world will follow the Paris agreement objectives and limit warming below 2C.”
Nic Lewis, an independent scientist often critical of the IPCC, told BBC News: “The magnitude of the increase in vegetation appears to be considerably larger than suggested by previous studies. This suggests that projected atmospheric CO2 levels in IPCC scenarios are significantly too high, which implies that global temperature rises projected by IPCC models are also too high, even if the climate is as sensitive to CO2 increases as the models imply.”
And Prof Judith Curry, the former chair of Earth and Atmospheric Sciences at the Georgia Institute of Technology, added: “It is inappropriate to dismiss the arguments of the so-called contrarians, since their disagreement with the consensus reflects conflicts of values and a preference for the empirical (i.e. what has been observed) versus the hypothetical (i.e. what is projected from climate models). These disagreements are at the heart of the public debate on climate change, and these issues should be debated, not dismissed.”
Richard Betts has a relevant article at Carbon Brief, Understanding CO2 fertilisation and climate change. Excerpts:
But while the general principles of CO2 fertilisation are known, there is still much to learn about how these processes will operate under future conditions that have not yet been experienced.
For example, how much does nutrient availability limit the response of the plants to increased CO2? How does this interact with changes in weather and climate? How do different species respond, and how does this affect the interactions between them?
The net result of these complex, interacting effects is likely to vary from place, and change over time. If we wish to assess the future consequences of our influences on the climate system, it is imperative that we understand the interaction between these process and the impacts of climate change.
Despite claims to the contrary, the conclusions of the IPCC take CO2 fertilisation properly into account in the assessment of climate change feedbacks involving the carbon cycle, and in the assessment of the impacts of climate change on ecosystems. They are also starting to account for this in the knock-on consequences for water resources, but that is more cutting-edge science and less advanced.
However, the IPCC assessments also acknowledge the uncertainties in the models. The current generation of models represent CO2 effects mostly based on understanding gained in the 1980s and 1990s, which is largely derived from smaller-scale studies in controlled conditions. So, while they are based on the best understanding available at the time of their development, they will require updating as new information becomes available.
While we are perhaps lucky that CO2 has this effect on plant physiology, in addition to being a greenhouse gas, it is not our ‘get out of jail free’ card when it comes to our ongoing emissions of CO2.
There are a wide range of consequences of these emissions. Some could be seen as positive, such as enhanced crop growth due to higher CO2, but most are generally viewed as detrimental – e.g. a warmer, drier climate leading to increased risk of forest fires. Even the enhanced growth by CO2 has its downsides, as faster-growing plants such as lianas seem to more able to out-compete larger, slower-growing species such as large trees. But whether the impacts are viewed as ‘good’ or ‘bad’, climate science is studying them all, both in models and in the real world. It’s exciting, groundbreaking science with profound consequences for society.
Comments from Nic Lewis
In response to Roger Harrabin’s query, here is the complete version of Nic’s comments:
Here are some brief preliminary comments.
1. The paper indicates that the primary cause of increasing plant productivity – which is good from a food production viewpoint as well as leading to increased land carbon uptake – is CO2 fertilisation. Estimating the sensitivity of land carbon uptake to CO2 level and to global temperature is tricky using just vegetation data, however.
2. This is not a new result, but the magnitude of the increase in vegetation, measured here by leaf area per unit land surface (LAI), appears to be considerably larger than suggested by previous studies.
3. There is some conflict between the probabilistic estimates of LAI trends (Figure 1.d). I am not comfortable with just taking (as they do) the average of the three observational dataset trend probability densities, when two of them peak at a fraction of the trend at which the third one peaks. Moreover, looking at the shapes of the probability density curves, I wonder if one or more of them has been badly distorted by use of an inappropriate prior distribution in the Bayesian statistical analysis involved. This problem has bedevilled observational estimation of climate sensitivity, typically pushing up sensitivity estimates and leading to great over-estimation of the risk of sensitivity being very high. In Figure 1.d, for two of the observational datasets their mean values are of the order of ten times as high as the trend at which the probability density peaks (the mode). Their modes may well be a better guide to the true trends.
4. I have difficulty in reconciling the LAI trend values implied by various statements and figures in the paper and the press release. And their estimate of an LAI trend increase of 0.068 m2 m-2 yr-1 over 1982-2009 seems to be 27x higher than that per Mao et al 2013 (their reference 7, which estimates 0.0025 m2 m-2 yr-1 over the same period, if I have read it right). That may be because I am misunderstanding something. It is difficult to properly understand papers from the Nature stable without reading their Supplementary Information, since Nature likes the main paper to be short. I will read the SI when it becomes available. However, it is also possible that there is some error in the paper; I have found significant errors in quite a few climate science papers.
5. The press release contains at least two scientific claims that are nothing to do with what is in the paper and that I consider highly dubious:
a) “Every year, about one‑half of the 10 billion tons of carbon emitted in to the atmosphere from human activities remains temporarily stored, in about equal parts, in the oceans and plants.” Storage in the ocean should be regarded as permanent, unless atmospheric CO2 levels fall. If they remain the same, the ocean will in fact take up more CO2 from the atmosphere, since it is a long way from chemically equilibrating to increased atmospheric CO2. Likewise, whilst carbon storage in individual plants is temporary, what matters is how much the total amount of carbon stored by the biosphere increases with increased atmospheric CO2 concentration. There is no reason to regard the resulting additional aggregate biosphere carbon storage to be temporary, unless atmospheric CO2 concentration falls.
b) “studies have shown that plants acclimatize, or adjust, to rising CO2 concentration and the fertilization effect diminishes over time,” says co–author Dr. Philippe Ciais”
I have not seen any good evidence of that being the case, and it seems to me unlikely. Dr Ciais was joint Coordinating Lead Author of the Carbon Cycle chapter (Ch. 6) of the IPCC assessment report 5 (not Contributing Lead Author as stated by the press release). Box 6.3: The Carbon Dioxide Fertilisation Effect in that chapter does not include such a claim. On the contrary, it states that “Since the AR4, new evidence is available from long-term Free-air CO2 Enrichment (FACE) experiments in temperate ecosystems showing the capacity of ecosystems exposed to elevated CO2 to sustain higher rates of carbon accumulation over multiple years “.
6. Land carbon uptake is poorly understood, but is projected to have a significant effect on the response of atmospheric CO2 levels to future carbon emissions. An excellent, very readable PNAS paper “Carbon cycle feedbacks and future climate change” last year by one of the best known carbon cycle experts, Pierre Friedlinstein, stated that the current generation CMIP5 Earth System Models simulated current global soil carbon between 500 and 3000 GtC (the observational estimate is ~1300 GtC). This extremely wide range shows the very poor level of understanding and modelling ability. He estimated, from observational data, that land carbon storage increased about twice as fast with increasing atmospheric CO2 as the per average CMIP5 model. The results in this new paper point in the same direction, but even further away from the CMIP5 mean. Friedlinstein also estimated that the decrease in land carbon storage with increasing GMST was too high by a factor of two in the average CMIP5 model. This all suggests that projected atmospheric CO2 levels in each of the RCP emission scenarios are significantly too high, which implies that global temperature rises projected by CMIP5 models are too high even were their climate sensitivities to be correct.
7. Bear in mind that this paper only covers land vegetation. The response of marine biota to increasing ocean CO2 concentration and temperature is also important, and is poorly understood.
UPDATE from Nic after the Supplementary Information was published:
Hi Judy, from studying the SI I’ve now worked out that Fig.1 in the article doesn’t show probability density for LAI trend at all: it is mislabelled. Rather, it shows the proportion of the land surface with each best-estimate trend. There is nothing probabilistic about it. So most of what I wrote in my 3. was based on a misunderstanding.
Also, I’ve now deduced from SI section S5 and Fig.S2 that their “growing season integrated LAI” isn’t what I thought it was based on their trend units of m2 m-2 yr-1: it depends on the time measurement unit and is many times higher than the mean growing season LAI index. They define it in terms of days, implying trend units of m2 day m-2 yr-1, but it looks to me as if the integration time unit is probably months, which would mean the correct units for all their LAI trends are m2 mth m-2 yr-1. If the paper had indicated the mean growing season integrated LAI from each dataset in the main paper (this information isn’t even given in the SI) than their units error would have been evident, and it would have been possible to interpret their LAI trends properly. I’m surprised peer reviewers didn’t require this; one can’t fully interpret the absolute trends and trend differences in Fig.1 without knowing what the global-mean absolute LAI values for the three datasets are. This difference in units almost certainly accounts for the most of the trend differences that I mentioned in my point 4. However, I am still mystified how the “global greening trend” – presumably the trend in their growing season integrated LAI – that they report can be 0.068 m2 m-2 yr-1 when the average of the trends shown in their Fig.1.d is well below that.
Here is the complete text of the statements that I emailed to Roger Harrabin:
“This article is arguably the most authoritative examination of the greening of the Earth that I’ve seen, although I’m sure it won’t be the last word on this topic. In evaluating the policy significance this, how to balance the beneficial effects of promoting plant growth for a rapidly increasing global population, versus the hypothesized rising sea levels and more extreme weather events, is not at all straightforward. It is inappropriate to dismiss the arguments of the so-called contrarians, since their disagreement with the consensus reflects conflicts of values and a preference for the empirical (i.e. what has been observed) versus the hypothetical (i.e. what is projected from climate models). These disagreements are at the heart of the public debate on climate change, and these issues should be debated, not dismissed.”
I began digging into this line of research around the time of the Minnesota trial on the social cost of carbon [link] (last October). At that time, an unpublished figure by co-author Myneni was hotly disputed. Apparently that diagram was an early version of this particular analysis (lead author was a postdoc with Myneni). The published version shows much more greening and a much higher attribution of the greening to human CO2 emissions.
This paper is arguably a ‘consensus’ statement of the primary global community of scientists that is working on this topic. I’m sure its not the last word on this, and I look forward to Nic Lewis’ further comments on the statistical methodology.
I was highly irritated by this statement in their press release:
The beneficial aspect of CO2 fertilization in promoting plant growth has been used by contrarians, notably Lord Ridley and Mr. Rupert Murdoch, to argue against cuts in carbon emissions to mitigate climate change, similar to those agreed at the 21st Conference of Parties (COP) meeting in Paris last year under the UN Framework on Climate Change (UNFCCC).
Why did IPCC Coordinating Lead Author Philippe Ciais feel the need to take this dig at Ridley and Murdoch? Ridley in particular has made public statements about greening several years ago, which were widely criticized at the time, that are supported by this new paper. It seems that Ciais is the only one of the authors to make such a derogatory public statement (at this point, anyways). What’s an IPCC author/advocate to do when science doesn’t play along with your policy narrative? Unfortunately, re-examining their premises doesn’t seem to be part of their playbook.
This paper highlights the very substantial uncertainties in our quantitative understanding of the global carbon budget. The significance of the new paper is this:
- As Nic Lewis points out, this paper alters the RCP scenarios in terms of resulting atmospheric CO2 content; i.e. the RCP scenarios are significantly too high
- This paper alters the dynamics of calculating the social cost of carbon, in the direction of a lower cost.