From the Sierra Club

When they discuss removing carbon from the atmosphere climate scientists and others refer to carbon sinks which are as varied as they are numerous and some work better than others. In this first of several posts on carbon sinks we will look at the issue and hopefully educate our readers about how carbon sinks work and impact climate change. Even if you know about carbon sinks some of this information might surprise you.

The primary use of a carbon sink is to remove carbon from the atmosphere and render it harmless form a global warming perspective. In practical terms this means chemically changing CO₂ into something else that is stable at ambient temperatures (i.e. not prone to breaking down into CO₂ again) and that can ideally preserve the stabilized carbon for a long time. But lately the idea has also come to mean an approach to keeping unmodified carbon dioxide out of the atmosphere, a tricky proposition.

Green plants usually come to mind when we think about carbon sinks. In fact there’s an inverse correlation between atmospheric carbon and green plant activity in the northern hemisphere—where most of the land is. In the northern hemisphere’s growing season green plants absorb enough CO₂ to slightly reduce atmospheric levels. But since human activity contributes between 35 gigatons and 45 gigatons of CO₂ to the atmosphere each year, that much CO₂ and then some ends up back in the air over the course of the year.

Currently green plants remove between 100 billion tons and 115 billion tons of CO₂ from the air each year. That biomass makes up all of the plant material we see and a lot that we can’t. We’re aware of the food that green plants make but we often forget or don’t consider the roots, stems, leaves, husks, rinds, skins and other plant parts that are considered waste made by plants on the way to providing the things we eat. Also, green plants make things we have no intention of eating, like grasses that herbivores eat, oftentimes we eat the herbivores though. Forests, mosses, common weeds and almost anything else that’s living and green, but not frogs, make up part of the carbon sink.

In the oceans, microscopic green plants called phytoplankton—to distinguish them from microscopic animals that are called zooplankton—capture sunlight and make carbohydrates just like their terrestrial plant cousins. In the ocean, phytoplankton is the bottom of the food chain. Big fish might eat smaller fish but at the end of the day the smallest fist eat phytoplankton. Even zooplankton eats phytoplankton and so do baleen whales, some of the biggest animals on the planet.

Very often when we think about removing carbon from the air, we think about natural carbon sinks but natural sinks have drawbacks. For example, trees might live for 100 years but then they’ll decompose releasing their carbon back to the atmosphere; or they’ll burn either in forest fires or as fuel used in poorer parts of the world for cooking and heating. Carbon in the plants that we eat as food often makes the round trip form the atmosphere to food and back again in a matter of months. Much the same happens in the sea as phytoplankton and algae serve as a food source. So any natural carbon capture scheme would have cyclicality built into it. That’s not bad but it requires a certain amount of effort and resources.

A greater problem is finding places to grow more green plants. There’s plenty of empty land on the planet but if it’s not already under cultivation you can bet that it has one or more attributes that make it unsuitable for agriculture today—it’s a mountain, it’s a desert, it has inadequate sunlight or a short growing season, or it’s in a place where other conditions are inhospitable. For instance, Antarctica and Greenland are massive expanses of land that are covered by glaciers and don’t get enough sunlight to grow very much and they’re cold. Australia is a continent that doesn’t get very much rainfall and consequently, though it gets plenty of sunlight, it’s mostly not a great place to grow green things.

The there are places like the Amazon rainforest, which is being cut down for wood -or grazing cattle or just burned for charcoal and mined mineral resources and. The soils left behind when a rainforest is clear cut are often weak and not good for growing plants because their nutrients were primarily stored in the living things that are no longer there. Also, the process of clearing rainforest often leads to desertification.

So to summarize, some land will not easily come under cultivation to help with removing carbon from the air. Other land might be usable but it will be costly and require irrigation which really begets another problem, where to get enough fresh water.

We could also consider finding ways to increase the photosynthetic activity in the oceans and there are some, but that takes a separate post to unravel and we’ll do that soon.

Understandably then, when people think of climate change they think of it as an emergency that needs a man made solution and they tend to look for approaches that are quick, ambitious, and tend to be quite expensive. Unfortunately, all of that quickly leads to questions of what processes to use and which machines are best but most important it also causes us to ask how much the effort costs. It also drives questions about where the energy will come from to run the big machines. Once we get to costs it’s understandable that people and governments kick the can down the road. Man-made solutions are ruinously expensive. They have some other disqualifiers too and we’ll get to them in the next post.