Historically, humans have seen themselves as able to make improvements over nature. “Plants are less efficient at capturing the energy in sunlight than solar cells mostly because they have too much evolutionary baggage.” One article on the internet decries. “MIT’s artificial leaf is ten times more efficient than the real thing”, another headline claims.
The idea that we can compete with nature and win is essential to bolster up another idea, the idea of progress. If humans can do the things that nature does more efficiently, then we can end our dependence on the living world around us and set out on our own. However, as I will go on to show, our best hope in adressing climate change lies not in technological solutions, but rather in large scale reforestation of our planet.
Natural photosynthesis versus Artificial photosynthesis
You’ve probably seen the comparison multiple times by now without a doubt. A plant turns about 1-2% of sunlight into biomass. In comparison, a solar panel captures far more energy. A solar panel can typically capture around 20% of incoming sunlight into electricity.
It’s first of all important to point out that this is a poor comparison. Plants are less interested in capturing energy, than they are in storing energy. If a solar panel captures energy, it still needs a mechanism of storing energy, unless we want to use solar panels merely as a supplement to our fossil fuel based energy grid.
More importantly however, unlike humans, plants tend to live in symbiosis with other plants, contributing to the good of an ecosystem in ways that do not directly benefit an individual plant and does not express itself in the form of stored biomass. Suzanne Simard noted in a BBC article that larger plants donate fixed carbon to smaller plants that grow in their shade. “These plants are not really individuals in the sense that Darwin thought they were individuals competing for survival of the fittest. In fact they are interacting with each other, trying to help each other survive.” She was recorded as saying.
As you might have anticipated, if we start considering the symbiotic processes in which plants engage to help the broader plant community, their efficiency leaves us biting the dust! One of the most important activities that plants engage in is transpiration. Plants move water up from their roots to their top. The water then evaporates and increases the humidity of the atmosphere. This process reduces local temperatures near the plant. It also has the effect of generating rainfall further inland. The Amazon is estimated to generate anywhere between 50 and 80 percent of its own rainfall through this mechanism. This is also why the ongoing Amazon deforestation is so dangerous, as removing half the forest may mean we lose the other half as well.
The process of transpiration requires energy, lots of it. Most of this energy doesn’t come from light that is fixated through photosynthesis. Rather, it’s used directly, by heating the surface of the plant, made possible by their dark green leaves. According to one study, an estimated 80% of incoming sunlight is used in the process of transpiration, by heating the plant’s leaves. Another study estimates the energy used for evapotranspiration by a tropical forest at anywhere between 88 and 97% of total incoming net radiation. Add to this the 1% used in photosynthesis to generate biomass, then add the portion that’s passed through the leaf, as well as the portion reflected by the leaf and you find that pretty much all of the energy is used in a very efficient manner by plants.
Of course plants perform a variety of other services that a solar panel or most other conventional parts of our anthropogenic infrastructure are unable to perform. Consider the example of climate regulation. Snow that falls on the Earth’s surface has a high albedo. Thus, a landscape covered in snow has a cold temperature, as most of the sunlight is reflected back into space rather than being absorbed. Trees on the other hand through their irregular structure have a darker albedo than a flat surface covered in snow, thereby warming their local environment.
This can lead to positive feedback loops. An example is Iceland. Before the vikings arrived, a quarter of the land was covered with trees. These trees were removed, which allowed snow to accumulate on the surface, which reduced temperatures on Iceland further, thereby preventing the trees from returning. Winter temperatures throughout medieval Europe are believed to have been significantly reduced as a consequence of rampant deforestation.
Of course there exist a variety of other functions plants perform. Plants prevent dust storms and soil erosion by holding the soil together. They create environmental niches for bacteria and fungi that perform a variety of useful functions, like biosequestering uranium and mercury. They remove large amounts of carbon dioxide from the atmosphere. Estimates are that plants also remove up to 60% of particulate matter from the atmosphere and 40% of NO2, thereby keeping our cities habitable. Finally, it has to be pointed out that plants play by different rules than humans. They absorb only resources available in their local environment, rather than using finite mineral deposits.
Plants as climate regulators
Through the process of evapotranspiration, plants keep our world habitable by reducing temperatures. Plants lose more water to evapotranspiration when temperatures are higher, thus their air-cooling effect is most prominent during very warm days. One estimate for a typical deciduous forest is that peak cooling rates amount to 75-125 W m-2. One experiment done in Manchester and Amsterdam on the cooling effect of trees in the city found a temperature reduction of 5-7 degree Celsius.
This effect doesn’t just apply to our cities, it applies to our countryside as well. Large swathes of land are deforested to grow crops, which means that the leaf area index (a measure of the share of the land’s surface covered by leaves) is greatly reduced, thereby drastically reducing the potential for cooling through evapotranspiration.
What are some places with a very high leaf area index? As you might expect, the rainforests of Congo, South America and Indonesia have a very high leaf area index. Thus, it is of enormous importance that these rainforests are preserved and the damage that has so far been done to them is reversed.
In more Northern latitudes, coniferous forests typically have a very high leaf area index. One estimate places the leaf area index between 5.9 and 14.1, in a coastal redwood forest that was actively “managed”, to increase light penetration. Thus it’s clear that the potential leaf area index is much higher even than we currently find in tropical rainforests.
During the last time when CO2 concentration were above 400 parts per million, the Pliocene, Europe was covered in redwood trees. The massive redwood trees that one covered Europe gave rise to the browncoal deposits we are burning today. As CO2 concentrations declined, the redwood tree’s ecological niche declined with it, until eventually these majestic trees only survived in fringe areas of the American west coast, with one other related species surviving in China. It’s quite apparent that the redwood tree played an important role in regulating our climate.
We shouldn’t expect to see miracles when it comes to carbon sequestration, even with trees as enormous as the redwood tree. Some simple calculations will show that we have burned a lot more carbon over the past two centuries than we can reasonably expect ever to sequester in living forests.
The IPCC estimated that we could theoretically sequester 87 gigatonnes of carbon up to 2050, which is very little compared to the total amount of around 546 gigatonnes of carbon that have been released since the start of the industrial revolution, of which about 356 gigatonnes come from fossil fuels. The vast majority of the carbon that has now entered the atmosphere can’t be removed by trees, we will have to learn to live with its presence.
Reforestation won’t be capable of reversing climate change, but it will be essential in the process of adaptation to climate change. By now, reversing climate change looks impossible, while preventing further climate change looks very unlikely as well. Very few climatologists and economists think that the two degree Celsius target can still be reached, unless we have greatly overestimated how much carbon is economically viable for us to extract.
The more important potential of reforestation lies in its ability to prevent soil erosion, prevent droughts by increasing precipitation and reduce temperature extremes. Forests create the kind of climatic conditions that make the Earth habitable for other lifeforms. Until the Neolithic revolution, Europe was almost completely covered in forests. It should return to that state as soon as possible.
Although it has so far proved rather difficult to constrain our carbon emissions, there may in fact be quite a lot of potential for reforestation of our planet. A total of 40% of the world’s land surface is used to keep us fed. The vast majority of that land is not used to produce the grains, fruits and vegetables directly consumed by humans, but rather, is used to feed the animals that we eat. Transitioning to a plant-based diet is the most important and easiest step we can take to reforest the planet.
It’s also important that we abandon some of the “green” schemes we have embarked on. Producing fuel out of corn ethanol is a very inefficient way of using land. Similarly, we have to abandon the myth that industrial agriculture is needed to feed the world. Small scale farms are actually typically more efficient in regards to land use. In the United States, the smallest farms yield $15,104 per hectare, compared to $249 per hectare for large farms.
The main reason small farms have difficulty competing with industrial agriculture is because small farms typically produce food in a more labor intensive manner, without the reliance on harvesting machines that characterizes large scale industrial agriculture. Land redistribution, from large owners to small owners, could help us to drastically increase land-use efficiency, thereby enabling us to return large portions of land to nature. Studies in Turkey found that yield per acre is much higher in smaller farms, because labor inputs per acre are much higher. George Monbiot has noted other examples here. Farms smaller than one hectare seem to be twenty times more productive than those over ten hectare.
Thus, addressing climate change and deforestation will require cultural and economic changes as well. The mechanization of agriculture, that has led to less than 2% of the population in most Western countries being employed in agriculture, has to be reversed. We should instead aim to return to a model of society where most people own land and are involved in self-sufficient food production. This would allow us to reforest large swathes of land and thereby preserve a habitable planet.