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Climate Hustle

Humans are greening the planet, but the implications are complicated

Posted on 15 July 2016 by John Abraham

The Earth’s climate is changing – in fact, it always changes. But in the current context of human influence, scientists try to decipher how much of the change is natural compared to human-induced.

One clear way humans influence the Earth is through the biosystem. For instance, farming changes the biosystem. By removing natural growth and planting annual crops that are harvested, we change the system in a way that could in turn affect other parts of the Earth system. In addition, the use of nitrogen based fertilizers can increase growth rate and lead to a greening of areas that are subject to fertilization. 

Another more indirect potential for humans to alter plant growth is through fertilization involving carbon dioxide. We know that humans have increased the amount of carbon dioxide in the atmosphere by approximately 40%. We also know that airborne carbon dioxide is a fertilizer for plants. So the obvious question is, “do our carbon emissions affect plant growth?”.

A new study, just out in Nature Climate Change, helps answer that question. This study focused on land areas in the northern hemisphere that were outside the tropical region. They obtained information from satellites to measure the greening of these lands areas to determine whether there was any significant change. The find that yes, in fact there is. Over approximately 30-year durations, this area has indeed gotten greener.

So, the next question is, what is causing the greening? To answer this question, the authors used computer simulations and ran them with and without human influences. When we say “human influences” we can mean many things, such as increase or decrease of farming, use of fertilizers, and airborne increase of carbon dioxide, just to name a few. The authors found that the only way the simulations matched the observations is when these human influences were included. That is, solely natural variations cannot be the cause. Not only that, but the match worked best when airborne carbon dioxide had a major role.


The spatial distribution of the linear trends in the growing season (April–October) leaf area index during the period 1982–2011 in the mean of satellite observations (upper figure), Earth system model (ESM) simulations with natural forcings alone (lower left figure), and ESM simulations with anthropogenic and natural forcings (lower right figure). Illustration: Oak Ridge National Laboratory

So what does this mean? Does it mean that greenhouse gas pollution is a good thing? Not really. Increased plant growth can have both positive and negative effects. The obvious effects are clear, such as potential improvements in farming or the increase in carbon sequestration by plants. However, increased growth occurs for both intended and unintended plants – including weeds. This could complicate farming, increase airborne allergens, and perhaps most importantly, change the flow of energy and water across the Earth’s surface in unintended ways. 

Furthermore, it isn’t clear what will happen to nutritional value of food crops – even if their growth rate increases. What is also very important to recognize is that a changing climate has many things changing at once. For instance, temperatures and rainfall patterns are being altered and these two affect the growth rates.

What is really nice about this new study is how they combined satellite images and computer calculations to tease out the separate effects of human and natural variations. I communicated with Peter Thornton, one of the authors. He told me:

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Comments 1 to 7:

  1. "Earth’s ecosystem is acting in a way to help mitigate our emissions by absorbing more of our annual release of heat-trapping gases"  Is that true of the ocean ecosystems?  It seems to me a variety of effects (warmer ocean, enhanced thermal stratification) might lead to the ocean being less effective at mitigation rather than more.

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  2. Pity that this article is a bit vague about the potential knock on effects!

    Warming has much wider implications for vegetation, especially trees.

    Warming has an impact on rainfall, drought, soil etc.
    These all have an impact on tree species survival and migration, that in turn has an impact on the species that depend on the trees.

    For instance English Oaks are a habitat for about 400 species, this includes a butterfly that only uses the oak as a habitat. That in turn has an impact on farming and other species that may prey on the 400 species that use the Oak.

    Trouble is the majority of people don't look at the wider picture, the get out clause is that it's to complicated for humans to understand.

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  3. I am a bit puzzled why the Greening trend is not discussed much more. It is a) factual and b) might have large effects on global warming. Some questions that arise are:

    - how much carbon could vegetation take out of the air? If terrestrial vegetation now has a total of about 600 gigaton of embodied carbon, can that grow to 800 gt, or maybe even double to 1.200 Gt? In that case a large part of human produced CO2 can be taken out of the air. Or is this a false assumption?

    - what happens with humidity and cloud development when forests become more dense? How does this effect cooling or heating?

    - as stomata have to be less open when co2 level increases plants become more h2o efficient. This would mean plants can more easily survive on dry and hot places. And how does this affect the temperature as formerly dry places (desserts) are reforrested again. It seems that dessers now cool off quicly and also have a sort of Albedo effect

    overall, this seems to be a very positive side-effect of raising co2 levels but it seems a bit ignored.

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  4. LinkeLau @3:

    1)  All coal and peat used to be plants as some time.  It follows that, at least in principle, all anthropogenic CO2 emissions could be drawn down by additional plant growth.  It is not clear, however, that that could be done without forests colonizing much, or all of current agricultural land, plus land opened up by global warming (ie, former areas of perma frost, or areas formerly lying under ice caps).

    Having said that, if you look at the last time the Earth approximated to those conditions, ie, the Carboniferous 300 million years ago, you had a situation where trees were able to colonize much of the land surface because of the complete absense of vertebrate grazers.  The energy stored as sugars by photosynthesis approximately balance the energy given up by respiration by the plants and their predators.  To a first approximation, a joule of energy stored as sugars can support either 10 tonnes of plant biomass, or a tonne of animal biomass.  That is significant  because the Earth currently supports what is probably the greatest level of animal biomass it has ever supported - which greatly limits how much carbon sequestration due CO2 fertilization is possible.  It is also significant because any growth in plant biomass for CO2 fertilization will drive a corresponding increase in plant grazers which will also significantly limit the possibilities of sequestration.

    2)  Two complex for me to say.  Given that stomata will shrink, that will to some extent compensate for the increased leaf area.  Once you also through increased predation into the mix the change in evapotranpiration is, I suspect, not predictable.

    3)  The increased resistance to aridity is the one clear benefit from the CO2 fertilization effect.  The effect of increased vegetation is to reduce temperature fluctuations (primarily because of the heat capacity of their retained water mass) but to warm the planet because plants have a lower albedo than do most soils (particularly arid soils) or sand.

    I agree that CO2 fertilization is likely a positive effect, but it is not straightforwardly so.  Remember that increased plant growth will also apply to weeds.  Further, increased leafy mass will increase the populations of insects that predate on plants, making it more difficult to protect crops. 

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  5. Dear Tom@4

    Thank you for your extensive answer and elaborations.

    1) In general I think it is a very hopefull message that additional plant growth could eventually drawn down all anthropogenic CO2 emissions, but I also understand that 'the price is high' for many plant and animal types. On the other hand - and maybe it is a form of my ignorance - I somehow trust that nature will find a new balance which will be positive for some plant and animal types and negative for others. Is it too bold to state that more vegetation in the long term means more biomass and also a wider variety of species? If we take a look at the dense rainforrest we are all happy about the variety of species that live there. Why couldn't that happen in large parts of lets say Australia, the Soviet Union, Canada or the US? With current agro technology development (agro towers, led light spectra, meat production based on stem cells, etc.) most of these lands will probably become obsolete for food production in the near future (30-100 years). So, reforrestry could at least sequester a lot of the access CO2 we pumped in that air. Lets 'built' forrests on agro ground that is no longer needed. It is already done in the Netherlands where obsolete grounds are 'given back' to nature.

    2. I don't have any expertise in this field and also based on logic reasonining it is hard to find an answer, but given the very cloudy atmospere in the rainforest I would guess that a lot of temperature will be trapped near surface, but as a compensation extra clouds reflect incoming solar. What the net effect will be? I can only guess. If I take a look at the maps that NASA produced it seems that the rainforrest could also have been a dessert without the forrest (assumption: the forrest keeps the forrest although solar irradiance is very high in some of those areas)

    3. Probably without these resistance to aridity the world would have been as it is now. Maybe this is one of the reasons (a miricle?) why the earth did not experience a runaway greenhouse effect before: plants adapt to a large extend if temperature as well as CO2 in combination increase to a high level by creating their own biotope? The cloudiness that follows (see reainforrest) helps re-radiating solar irradiance and that could start a new cooling phase?

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  6. LinkeLau, like you I would not care to quantify the degree of benefit coming from "increased greening".  However, not many years ago there was a study by agricultural scientists, indicating that another 1 degree rise in AGW would cause a roughly 5% drop in wheat/maize/rice production (per Hectare), and 2 degree rise would cause a 10% reduction.  If so, then this reduction in food for humans would override any benefit from increased leafy food for livestock.

    Alas, I have not managed to re-discover the reference for the study, but IIRC it involved 3 research centres, one of which was in Sri Lanka and one in USA or Europe, and one in Australia I think.

    In trying to quantify things, we must remember that the increased greening of the land would not apply to the 73% percent of the globe which is ocean ( and including Antarctica ).  And probably not apply to a further 3% which is already commited to conventional crops.

    I speculate that there would be no extra sequestration of carbon by seaweeds, since they are already suffering from a surfeit of "available carbon" courtesy of ocean acidification.

    Rainforest would be in the region of 2% [and falling] of global area, and that amount is small and unlikely to be permitted to increase.  So an increased "cloudiness" would likely not occur, to have any effect on cooling through more reflectiveness.

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  7. Being rather visual in my interpretations, one thing I notice from the image above is that a lot of the greening appears to be in the sub polar regions.  These are areas that for the most part have been either tundra or ice covered.

    We are already observing changes in these areas due to warming.  In addition, there has been concern voiced about increased emmissions of GHG's as these areas melt.  Finally, there are the obvious albedo changes.  As the title mentions, it's "complicated".

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