Why photosynthesis is so attractive

October 11, 2017 by Denis Pombriant

There are many suggestions for removing carbon dioxide from the air to help reduce the impact of global warming including various mechanical approaches to capturing CO2 from the air and moving it to underground storage. However, if you evaluate these ideas by how much pollution causing energy they still need to operate you might conclude that they solve one problem but create another that’s just as difficult.

There are numerous proposals for turning biomass into fuel that would be burned to generate energy, for instance. The thinking is that since biomass is of relatively recent origin that it is simply returning carbon to the air that was already there. In other words, it does not introduce carbon from underground petroleum reservoirs where it has been safely sequestered for millions of years.

That’s absolutely true but irrelevant for two good reasons. First, carbon is carbon and it doesn’t matter where the carbon is from because, second, there’s already too much carbon in the air and water and biomass schemes don’t remove carbon on a more or less permanent basis, they just manipulate it.

Then there’s energy. It takes energy to manipulate molecules. Some reactions, like burning carbon compounds in the presence of oxygen, liberate energy, which can be used for mechanical work. However, many reactions go in the opposite direction. It takes a good deal of energy to make a carbon compound like a sugar. In fact, it takes more energy to make an organic compound than it stores or gives back when it is burned. It is a disappointing fact of life in this universe with its laws of thermodynamics but there it is. The difference between the energy consumed in making chemical bonds like this and the energy actually captured and stored in those bonds is the efficiency of the reaction.

If you apply this knowledge to the carbon capture problem you get a much better picture of what we’re up against. If we capture CO2 and simply compress it and store it—some have suggested storing it underground in depleted oil wells—it is still a gas and it can leak from confinement in a thousand years or next week bringing back the problem. If instead you convert the captured carbon into something more stable you need a great deal of energy to make all of those chemical bonds. If you are aware that the energy available to us in the form of fossil fuels is diminishing then you can quickly conclude that it doesn’t make a lot of sense to use it to recapture CO2. So, neither of these approaches is very attractive.

But we need to do something about all the carbon in the air and water both because we need reduce global warming or face a catastrophe and, truth be told, because we’re running out of fossil fuels anyhow.

This is where photosynthesis comes in handy. As you know, photosynthesis is the process that all green plants use to capture carbon, liberate oxygen, and build organic molecules like sugars that the plants then use to make things like roots, stems, leaves, seeds and more. All life on this planet owes its existence to the sun and to green plants that make food for animals to eat.

The great thing about photosynthesis is that all of the energy needed to make organic molecules comes from the sun so you don’t need to think about an energy source, it’s already there. Annually green plants capture an estimated 130 terawatts of energy—quite a bit. For comparison, that’s about 6 times the power consumed by all of human civilization today. But more energy falls on deserts, the middle of the ocean, and other places where it doesn’t do much more than warm the surface. In those places there isn’t a lot of photosynthesis going on. Now remember that green plants support all life on the planet; they generate between 100 and 115 billion tons of biomass, from that 130 terawatts.

Right now, we’re thinking that if we could remove a trillion tons of carbon from the air and oceans, that it would make an appreciable difference to the global warming problem. So, what if we set a goal to double the amount of photosynthesis happening on the planet? That would mean going from generating 100 to 115 billion tons of biomass each year to generating 200 to 230 billion tons.

Could we really do this? If we could, in ten years we would have generated an additional 1 trillion tons of biomass. If we could do this the problem would change from carbon capture to simply what’s the best way to keep the captured carbon from disintegrating and becoming CO2 again? We’d probably conclude that some effort at encouraging photosynthesis is going to be needed for the foreseeable future. That would make climate change a chronic problem rather than the acute problem we see today. It’s an appealing idea because it would remove enough carbon from the environment to enable us to step back from the precipice and that’s worth doing.

Other questions that need answers include where would we be able to plant all of this greenery? The growing regions and forests of the earth are already pretty green. So how do we find the land? And while we’re at it where do we find all the water it will take to grow that much greenery? Maybe we don’t use land, perhaps we get more photosynthetic activity from the sea, or maybe we use renewables to make irrigation water, and maybe we do all this and more. We’ll explore the answers in future posts.


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