Three practical and environmentally sound ways to sequester carbon
There has been a flurry of activity around carbon sequestration. For example, Elon Musk has offered a prize for a breakthrough method of carbon sequestration, emphasizing new technologies. Most proposals are based on high-tech means such as sequestration of carbon dioxide from power plants and or coal gasification facilities via chemical separation followed by pumping carbon dioxide into old gas or oil wells or possibly storing it as a liquid at the bottom of the ocean. Other fantastical suggestions propose removing carbon dioxide from the atmosphere using chemical engineering techniques.
It seems that the people making these proposals do not understand the concept of capital cost, and operating and maintenance costs versus results. This prejudice against low technology methods could really hurt us. We have to act now, and there are three methods that will actually work.
The three methods that I describe here are the best carbon sequestering methods from an environmental point of view.
Reforestation is fairly obvious, and it is already on the radar screen. It is possible to recreate biologically diverse forests that also provide food for humans. I’m going to set that idea aside not because it’s not a good one but because it is already understood by most people.
The two methods that are extremely important but aren’t discussed as much are biochar and rock weathering. Rock weathering currently removes about 0.3 billion tons per year of carbon from the atmosphere. A recent paper shows the potential to remove about a billion tons per year of carbon from the atmosphere through enhanced rock weathering by using rock dust. Mostly it is volcanic rocks which weather when they are exposed to carbon dioxide and water from the atmosphere. These rocks are produced by volcanoes which in the past blew off a huge amounts of carbon dioxide during volcanic eruptions. This is a part of the geochemical cycle of Earth.
Carbonate rocks are carried underground by plate tectonics and the carbon dioxide is blown off as the rock melts. In some cases there is a great deal of carbon dioxide and water dissolved in the magma as it rises, and depending on the nature of the lava flow these gases may escape quietly or explosively in volcanic eruptions. When magma escapes in a pyroclastic eruption, the igneous rock is in the form of ash with a high surface area.
It is obvious that it would be preferable to mine volcanic ash as opposed to grinding solid rock. There are numerous locations in the world where there are very deep deposits of volcanic ash from past supervolcanoes. For example, portions of Eastern Oregon have ash deposits that are about 100 meters thick. I do not know whether these big deposits have already equilibrated with atmospheric carbon dioxide, but I think they probably have not. Wherever such volcanic ash deposits lie in close proximity to navigable rivers, there is an opportunity to move many billions of tons of ash to land which needs help. This method could undo centuries of soil depletion while also removing carbon dioxide from the atmosphere.
Biochar is the other practical method to sequester carbon from the atmosphere. Any biological material can be partially gasified to produce a synthesis gas, liquid pyrolysis products, and a form of charcoal. You can think of biochar as the halfway point between wood and charcoal. Biochar retains desirable carboxylic acid groups that can bind with minerals in the soil and help to keep them from washing away. Biochar is not subject to biological decomposition as is compost for example. Carbon that goes into biochar is useful as a soil amendment, but unlike compost, biochar is stable against oxidation in the soil for thousands of years at least.
The carbon dioxide content of the atmosphere as measured at Mauna Loa in Hawaii has a sawtooth pattern.
The graph above is probably the most convincing evidence of the continuous increase of carbon dioxide in our atmosphere due to industrial emission of carbon dioxide. I want to draw your attention to the sawtooth nature of the plot. Every summer in the northern hemisphere, plants suck in carbon dioxide. Then in the winter the carbon dioxide level increases to a slightly higher point than it was in the previous cycle as the dead plant material decomposes during the winter.
If we can capture enough of this biomass and convert half of it’s carbon to biochar, we would be able to start reducing carbon dioxide. This would be a massive undertaking, but there is an obvious way to get started: conversion of crop residues to biochar on a massive scale. I have been looking for a co-author for a paper to quantify the potential of conversion of crop waste to biochar for carbon sequestration.
