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Will Plants Save the World From Global Warming?

Updated on January 14, 2018
Jorge Cruz99 profile image

Jorge Cruz earned a doctorate in Plant Physiology and studied the effect of environmental conditions on carbon metabolism in wheat and rice.

In the latest argumentative twist of climate change deniers, there is a glimpse of recognition that the World is, after all, warming. But, "it is not all that bad". One of the disbelievers claim is that the rise in atmospheric carbon dioxide or CO2 might provide an increase in plant productivity which in turn will absorb the excess greenhouse gas (https://www.scientificamerican.com/article/climate-skeptics-want-more-co2/ ).

Are plants passive recipients of climate events or can they mitigate the effects of climate changes?

Will plants help be sufficient to mitigate the effects of Global Warming?
Will plants help be sufficient to mitigate the effects of Global Warming? | Source

Plants engender life

Carbon dioxide gives rise, through photosynthesis, to the ingredients for the genesis of all biomass on earth. Plants assimilate inorganic carbon dioxide and transform it into organic molecules essential for life with the aid of an enzyme called Ribulose Biphosphate Carboxylase-Oxygenase or Rubisco. Rubisco is the most abundant protein on the planet and it has been almost unchanged since it first evolved with cyanobacteria from methane-generating microorganisms around 2.5 billion years ago in an oxygen-depleted World.

Plants use sunlight energy to capture carbon dioxide and build the ingredients of life.
Plants use sunlight energy to capture carbon dioxide and build the ingredients of life. | Source

Your cat, mushrooms and the great majority of living creatures including ourselves owe our existence to Rubiscos's success; we all once were Rubisco's substrate: CO2. Short-term experiments have shown that when some plants are exposed to a higher concentration of CO2 their overall biomass increase. There goes the myth then that an increasing concentration of CO2 in the atmosphere will spur crop's yield and vegetation mass, which in turn will mitigate the effects of global warming.

Will then an overabundant supply of carbon dioxide lead plant's to self-indulgent gluttony?

It depends.

Plants go on a diet

In contrast to speedy animals that simply move away under unfavourable environmental conditions, plants possess outstanding mechanisms to tolerate stress—in situ. With a significant proportion of their body underground, plants comprise some of the most robust organisms on earth. This exceptional capacity is based on their ability to regulate nutrition, respiration and, putatively, World-saving photosynthesis.

The pickle is that when plants are exposed to environmental stress they stop capturing CO2, in other words, they stop eating, they go on a diet. Because their stomata (few-micrometers long orifices scattered over the surface of leaves) play the double function of capturing CO2 and releasing water.

Fluorescence microscope picture of stomata in a citrus leaf.
Fluorescence microscope picture of stomata in a citrus leaf. | Source

Under most stresses water is less available so plants close stomata to not lose vital water which leads to less carbon dioxide being captured and reduced growth. In such a situation, they welcome a higher concentration of CO2 because—as a compact nutritional bar—facilitates their carbon nutrition with smaller stomata aperture and helps them to survive environmental stress. Therefore, stressed plants boast the highest response to overriding diet under increasing atmospheric carbon dioxide concentration.

The personalities of plants

The green world exhibits an extraordinary diversity of species, varieties, and individuals. We are used to regarding plants as unanimated objects, as mere extensions of the landscape. They are, however, dynamic beings with their own individualities (see "The Cabaret of Plants: Botany and the Imagination", by Richard Mabey). Some generalizations though can be drawn while considering that numerous exceptions exist.


Crop plants have been engineered for centuries to maximize the yield of harvestable organs. Grains, fruits and roots typically grow to the top of their genetic capacity. A synchronized system of enzymes, hormones and other messengers regulate when and how much to photosynthesise and how to use newly photosynthesized products. Also, crop plants grow under relatively favourable conditions: they receive water and fertilizers, their competing weeds are removed, and they are planted during an optimum time of the year. Those conditions enable them not to have to close their stomata and not be limited by available CO2 in the atmosphere. Crop plants do not grow more because they are at the limit of their genetic potential (http://onlinelibrary.wiley.com/doi/10.1111/nph.12614/full). Most crop plants then, will not do much to capture the extra CO2 that we will introduce in the atmosphere.

Wild plants on the other side, are mostly under-grown because of environmental stresses such as drought, salinity, very high or very low seasonal temperatures. In those situations, plants tend to close their stomata to retain water and stay hydrated, which leads to a lesser capacity to capture carbon dioxide. An increasing atmospheric concentration of CO2 will offset some of the stress because it impacts stomata conductance and makes water use more effective as was pointed out in a 2016 article published in PNAS by scientists from the University of Washington in Seattle ( http://www.pnas.org/content/113/36/10019.full.pdf ). Therefore, some wild plants are likely to benefit from a rise in the concentration of carbon dioxide and produce more mass while sequestering this gas.

The same rationale might apply to crops such as rice and wheat grown under unfavourable conditions, in regions where salinity or drought hinder their growth and yield. Those plants exhibit a positive mass growth response to an increase in carbon dioxide concentration; however, under those conditions, the ratio of carbon over nitrogen in the grains is higher, which means that the nutritional quality of the harvest is lower because the grains contain less protein. The simultaneous increase in temperature might, in contrast, have a detrimental effect on wheat yield which was found to decline as much as 6% for every °C of further temperature increase (https://www.nature.com/articles/nclimate2470 ). Rice, from other perspective, is an important producer of greenhouse gases such as methane. Therefore, this crop produces an overall negative impact on a changing World climate under any scenario.

Tropical weeds and crops such as sugarcane and corn deserve a different consideration. They are only about 3% of all plant species but comprise close to 20% of all plant biomass. Evolved under the harsh conditions of tropical climates those plants developed a mechanism (called C4 photosynthesis) to concentrate CO2 in their body and they do not respond as much if we put more in the atmosphere. Argentinean scientists assert that while the response of most plants to rising CO2 under natural conditions could be up to 40-45%, in C4 plants would be only 10-20% ( https://cdn.intechopen.com/pdfs-wm/18412.pdf ).

And then there is phytoplankton, ubiquitous microscopic plants present in all the oceans. Phytoplankton might capture a significant quantity of CO2 and sink it down to the bottom of the ocean as it dies. A great potential solution to capture CO2, if not were because phytoplankton also needs iron and other minerals to grow and multiply, and therefore it might be limited to absorb additional CO2 by minerals availability.

Consequently, the capacity of plants to absorb the increasing concentration of CO2 differ dramatically between species and environments.

All in all, a recent study published in Nature Communications (https://www.nature.com/articles/ncomms13428 ) indicates that the rise in atmospheric carbon dioxide has slowed down during the last decade likely due to an increase in global carbon uptake, which putatively is due to both the greening of the earth (because higher temperatures enabling plant growth in temperate regions) and CO2 fertilization. The slowdown, however, is expected to be temporary, the biosphere removes only about 45% of the total carbon emitted by human activity—a figure that changes from year to year. It might drop in the future since many factors remain unexplored, including future generational changes in plant physiology and metabolism as plants adapt to new environments and increased human emissions.

Beyond CO2 capture: How else plants interact with climate?

One study suggests that, as the planet warms, plants will release more sunlight-blocking aerosols, a process that will reduce the warming effect of the sunlight by reflecting it back to space and by contributing to form cloud droplets that lead to cooler temperatures. The extent and impact of this phenomenon remain under scrutiny.

In contrast, warmer temperatures might increase decomposition of dead plant material in the soil that eventually contributes to atmospheric carbon dioxide mass and accelerate further global warming. A related phenomenon is the melting of permafrost in Siberia, North of Canada and other temperate regions. Decomposing organic matter trapped for millennia in the permafrost might be exposed and become an additional supply of CO2, methane, and other greenhouse gases.

Additionally, while rising CO2 concentration might increase mass production in some plant species, the overall environmental changes in temperature, wind, and humidity will be catastrophic for others. For example, rising temperatures might lead to an explosive proliferation of microbes, fungi, and insects some of which are harmful to plants. Those pests represent a challenge for agriculture and might drive the disappearance of some wild plant species leading to a less diverse flora, which in turn increases the vulnerability of remaining species to those pests.

Some plant species might be able to generate physiological changes to adapt to the new conditions or gradually move their territory to higher latitudes or elevations, others might not. A sensitive example is the giant Sequoias of western North America. Those colossal trees grew for thousands of years under peculiar environmental conditions not found anywhere else on Earth. It is an enigma if those emblematic organisms face extinction in a new, unrecognizable World.

The emblematic Giant Sequoias might face extinction with Global Warming
The emblematic Giant Sequoias might face extinction with Global Warming | Source

The influence of additional factors, including changes in light and ozone concentrations and global oscillations such as El Niño and La Niña are more difficult to assess.

As everything balances out it is estimated that plants might contribute to mitigating global warming in about 1% only.

It is a misconception then that plants might save or help to save the World from the grim effects of anthropogenic warming. Even when vegetation sequestration of carbon dioxide has contributed to slow down the speedy concentration rise of this gas current human release significantly outweighs the capacity of the flora to uptake CO2. Human activities, in particular those related to burning of fossil fuels, cement production, and use of land will need to be re-evaluated under a new perspective to give our World a chance to avoid an environmental cataclysm.

References
1. Harvey, C. 2017. Climate Skeptics Want more CO2. Scientific American, October.
2. Fatichi, S. et al. 2014. Moving beyond photosynthesis: from carbon source to sink-driven vegetation modeling. New Phytologist, 201:1086-1095.
3. Swann, A.L. et al. 2016. Plant responses to increasing CO2 reduce estimates of climate impacts on drought severity. PNAS. 113:10019-10024.
4. Asseng, S. et al. 2015. Rising temperatures reduce global wheat production. Nature Climate Change. 5:143-147.
5. Valeria Lara, M. et al. 2011. C4 Plants Adaptation to High Levels of CO2 and to Drought Environments, Abiotic Stress in Plants - Mechanisms and Adaptations, Prof. Arun Shanker (Ed.), ISBN: 978-953-307-394-1, In Tech, Available from: http://www.intechopen.com/books/abiotic-stress-in-plantsmechanisms-and-adaptations/c4-plants-adaptation-to-high-levels-of-co2-and-to-drought-environments.
6. Keenan, T.F. 2016. Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake. Nature Communications. 7:13428 doi

Comments

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    • Jorge Cruz99 profile imageAUTHOR

      Jorge 

      10 months ago from Canada

      Mary thank you for your comment. Indeed, it might seem that plants do more than 1%, but the calculations balancing our rate of atmospheric pollution show otherwise.

    • aesta1 profile image

      Mary Norton 

      10 months ago from Ontario, Canada

      That is interesting data. I always held the belief that plants do much more than just 1%.

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