Jorge Cruz earned a doctorate in Plant Physiology and studied the effect of environmental conditions on carbon metabolism in wheat and rice.
Do Plants Help Reduce Global Warming?
Some climate change deniers are now making the argument that, while the climate is indeed warming, the earth will adapt to stop any catastrophe. These disbelievers claim that the rise in atmospheric CO2 might provide an increase in plant productivity, which will absorb the excess greenhouse gas.
While this argument is misleading, it does pose an interesting question about whether or not plants are passive recipients of climate events or if they can mitigate the effects of climate change.
Why Plants Won't Save Us From Global Warming
It's not that planting trees to replace the tree we've uprooted or cut down won't have a relatively small but positive effect on the environment. It will. But, if we try to overhaul the environment with plants, we could change weather patterns again, causing even more negative consequences. It's true that planting trees is positive in many respects. But, planting too many too quickly would likely cause entire ecosystems to fall into chaos. So, if we can't slow down climate change enough by planting new trees, can the existing trees adapt to mitigate the effects of greenhouse gases?
Even though the vegetation sequestration of carbon dioxide has, throughout Earth's history, contributed to slowing down the rise of greenhouse gases, current man-made carbon emissions significantly outweigh the capacity of flora to absorb CO2. Human activities—those related to the burning of fossil fuels, cement production, and land use, in particular—will need to be re-evaluated. In short, replacing trees without decreasing human consumption will not have a significant enough effect on carbon levels for civilization to avoid future hardships.
How Trees and Plants Affect the Atmosphere and Life
Through photosynthesis, carbon dioxide gives rise to the ingredients for the genesis of all biomass on earth. With the aid of an enzyme called ribulose biphosphate carboxylase-oxygenase, known colloquially as RuBisCo, plants assimilate inorganic carbon dioxide and transform it into organic molecules essential for life. RuBisCo is the most abundant protein on the planet. It has been almost unchanged since it first evolved with cyanobacteria from methane-generating microorganisms around 2.5 billion years ago. At this point, the world was oxygen-depleted.
Short-term experiments have shown that when some plants are exposed to a higher concentration of CO2, their overall biomass increases. Well, there goes the myth 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. When these plants die, their increased mass decomposes, creating even more greenhouse gases.
That said, the opposing argument has less to do with biomass and more to do with a plant's capacity for "eating" (so to speak) CO2. Simply put, will an overabundant supply of carbon dioxide lead plants to become more gluttonous? Well, it depends.
Will an Overabundance of CO2 Cause Plants to "Eat More"?
In contrast to animals, plants cannot move away when environmental conditions become unfavorable. While plants do develop methods of seed distribution that help them compete in a changing environment, often having their seeds travel farther. Evolution takes thousands of years. Our current climate crisis is happening too quickly for any species of plant to evolve the means to escape or adapt to its changing environment in such a drastic way.
While plants possess outstanding mechanisms to tolerate stress—in situ—they cannot survive if the resources they depend on most are becoming more and more sparse. Take, for example, areas where desertification is happening. The amount of CO2 in the air is irrelevant because the problem isn't the availability of CO2. Rather, it's the availability of H2O.
Even though plants have an exceptional capacity to regulate nutrition, respiration, and putatively, world-saving photosynthesis, this is not enough to make them the cure-all for global climate change. With a significant proportion of their body underground, plants are some of the most robust organisms on earth. That said, plants are limited by their genes, so they can't simply turn into monstrous CO2 consumers without thousands of years of adaptation.
In fact, they aren't just limited by the slow pace of evolution but also by the effects of environmental stress. When plants are exposed to environmental stress, they stop capturing CO2. In other words, they stop eating, they go on a diet.
Their stomata (which are orifices scattered over the surface of leaves that are a few micrometers long) play the double function of capturing CO2 and releasing water.
When water is less available, plants close their stomata so that they don't lose vital water. Losing water leads to less carbon dioxide being captured and reduced growth. In such a situation, more CO2 is beneficial because it's already harder for the plant to capture the CO2.
Under certain stressful conditions, plants may override their diets in an attempt to survive environmental hardships. That said, it's not like the plants are suddenly consuming 10 times the amount they normally consume. And, if the stress is too much, then the plants simply die.
Temperature stress also creates another problem. While the plant might be "eating more," the plant is also less likely to be able to reproduce effectively. The reproductive (gametophytic) phase in flowering plants is often highly sensitive to hot or cold temperature stresses, with even a single hot day or cold night sometimes being fatal to reproductive success.
When considering all this information, plants' increased consumption of CO2 is not enough to mitigate the effects of human carbon emissions. So, while stressed plants boast the highest response to overriding diet under increasing atmospheric carbon dioxide concentration, they will most likely not be able to override the effects of climate change fast enough to avoid massive shifts in ecosystems.
How Different Plants Affect CO2 Levels
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. Even though there are over 300,000 species of plants, there are still some generalizations that can be drawn about how plants affect CO2 levels.
- Crop Plants: 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 photosynthesize 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 optimal 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 stop growing because they are at the limit of their genetic potential. Therefore, most crop plants will not do much to capture the extra CO2 that we will introduce into the atmosphere.
- Wild Plants: Wild plants are mostly under-grown due to environmental stressors, such as drought, salinity, and 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). 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.
- Rice and Wheat: 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 (as was found to decline as much as 6% for every °C of further temperature increase). Rice, from another perspective, is an important producer of greenhouse gases, such as methane. Therefore, this crop produces an overall negative impact on world climate.
- Tropical Weeds and Crops: Tropical weeds and crops, such as sugarcane and corn are only about 3% of all plant species, but they comprise close to 20% of all plant biomass. Having evolved under the harsh conditions of tropical climates, those plants developed a mechanism (called C4 photosynthesis) to concentrate CO2 in their bodies and they do not respond as much if we put more CO2 in the atmosphere. Argentinean scientists assert that the rate of CO2 absorption by most plants in the response to rising CO2 levels under natural conditions could be up to 40-45%, in C4 plants, that number would be only 10-20% (which is not enough to have a truly significant impact when compared to the rate at which humans produce greenhouse gases).
How Phytoplankton Affect CO2 Levels
And then there is phytoplankton, ubiquitous microscopic plants present in all the oceans. Phytoplankton show an incredible ability to capture a significant quantity of CO2 and sink it down to the bottom of the ocean as they die. While, at first glance, using phytoplankton to capture CO2 seems like a great solution to the problem, any attempt would raise even more problems. Aside from changing ecosystems, proliferating an abundance of phytoplankton would require a huge amount of resources. Phytoplankton don't just need CO2 to survive, they also need iron and other minerals to grow and multiply. Therefore, its ability to absorb additional CO2 is dependent on the availability of other minerals (minerals that other organisms need as well).
Consequently, the capacity of plants to absorb the increasing concentration of CO2 differs dramatically between species and environments. In fact, a recent study published in Nature Communications 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 is due to both the greening of the earth (because higher temperatures have enabled plant growth in temperate regions) and CO2 fertilization. The slowdown, however, is expected to be temporary. Earth's biosphere removes only about 45% of the total carbon emitted by human activity—a figure that changes from year to year. Eventually, this biomass reaches its limit. So, if greenhouse gas emission rates don't change, this adaptive effect will become irrelevant to the climate change conversation.
Beyond CO2 Capture, How Else Do Plants Interact With the Environment?
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, further accelerating global warming.
The melting of permafrost in Siberia, North of Canada, and other temperate regions exposes decomposing organic matter that has been trapped for millennia. This would likely become an additional supply of CO2, methane, and other greenhouse gases.
Additionally, while rising CO2 concentrations 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, but others will not.
For example, the giant Sequoias of western North America grew for thousands of years under peculiar environmental conditions not found anywhere else on Earth. If those conditions change too rapidly, it is possible that those emblematic organisms could face extinction in a new, unrecognizable world.
While it has become clear that some plants will survive global climate change (and some may even thrive), many other species could face extinction or depopulation. It's delusional to think that planting more plants will stop the negative effects of global climate change. And, plants will not magically adapt to heal the earth. Humans must adjust their priorities and take action to create new and better technologies that can reduce waste and offset carbon emissions. It will take more than plants to slow the rate of climate change enough for countries to plan for the shifts that will occur in everything from crop growth to global trade.
- Harvey, C. "Climate Skeptics Want More CO2." Scientific American. October 2017.
- Fatichi, S. "Moving Beyond Photosynthesis: From Carbon Source to Sink-Driven Vegetation Modeling." New Phytologist. 2014. 201:1086-1095.
- Swann, A.L. "Plant Responses to Increasing CO2 Reduce Estimates of Climate Impacts on Drought Severity." 2016. PNAS. 113:10019-10024.
- Asseng, S. "Rising Temperatures Reduce Global Wheat Production." Nature Climate Change. 2015. 5:143-147.
- Valeria Lara, "C4 Plants Adaptation to High Levels of CO2 and to Drought Environments, Abiotic Stress in Plants—Mechanisms and Adaptations." Prof. Arun Shanker (Ed.), 2011. ISBN: 978-953-307-394-1.
- Keenan, T.F. "Recent Pause in the Growth Rate of Atmospheric CO2 Due to Enhanced Terrestrial Carbon Uptake." Nature Communications. 2016.
This content reflects the personal opinions of the author. It is accurate and true to the best of the author’s knowledge and should not be substituted for impartial fact or advice in legal, political, or personal matters.