Feasibility of controlling atmospheric CO2 by large-scale biospheric fixation with durable storage of harvested biomass (Dyson, 1977)

Dyson (1977) asked whether the atmospheric CO2 concentration could be controlled by an emergency program of worldwide cultivation of fast-growing plants, with the fixed carbon placed in long-lived storage — long-lived trees, humus, or artificial peat bogs engineered so that harvested biomass does not digest to methane — arguing that no law of physics or ecology forbids halting or reversing the atmospheric increase within a few years, while identifying five unresolved questions on which the program depended: the partitioning of anthropogenic carbon between ocean and biosphere; the feasibility of semi-permanent, non-methanogenic storage at scale; the supply of fertilizer (about $2 \times 10^{7}$ tons N, $7 \times 10^{6}$ tons $P_{2}O_{5}$, and $1 \times 10^{7}$ tons $K_{2}O$ per year at an illustrative $5 \times 10^{9}$ tons C per year target); the land base of marginal lands, wetlands, and forests; and the climatic and ecological consequences of planting at that scale. Global carbon budgets, remote-sensing land assessments, nutrient-limitation studies, and coupled carbon-climate models have since addressed individual components of this program, but no analysis resolves all five jointly. Whether harvested biomass can be stored durably, verifiably, economically, and ecologically safely at the multi-gigatonne-per-year scale, and whether the coupled land, nutrient, water, and albedo constraints permit biological sequestration to reliably control atmospheric CO2 — the modern land-based carbon dioxide removal question — remains open.

Background

By the mid-1970s, direct atmospheric measurements had established that the carbon dioxide ($CO_2$) concentration of the air was rising, that fossil-fuel combustion released roughly $5 \times 10^{9}$ tons of carbon per year, and that only about half of this carbon stayed in the atmosphere — the remainder was being absorbed by the ocean and the land biosphere, but the split between those two sinks was unknown (the contemporary missing-sink problem). Working at the Institute for Energy Analysis in Oak Ridge, the physicist Freeman J. Dyson asked in a 1977 essay in the journal Energy whether the atmosphere's $CO_2$ content could be actively controlled by exploiting plant growth on a very large scale. Because photosynthesis (the process by which green plants convert atmospheric $CO_2$ into organic matter) already moves roughly an order of magnitude more carbon each year than fossil fuels release, Dyson argued that no law of physics or ecology forbids halting or reversing the atmospheric $CO_2$ increase within a few years by worldwide cultivation of fast-growing plants, with the fixed carbon placed in long-lived storage; he described the scheme as a temporary carbon bank that could hold $CO_2$ down while the world converted to non-fossil energy, not a permanent solution. Two capture routes were quantified from experimental data: short-rotation tree plantations (trees grown and harvested like an agricultural crop; American sycamore yielding about $750$ tons of carbon per $km^2$ per year, so roughly $7 \times 10^{6}\ km^2$ — about 5% of the world's land area — would be needed for an illustrative withdrawal rate of $5 \times 10^{9}$ tons of carbon per year), and aquatic plants such as water hyacinth (a fast-growing floating freshwater weed) yielding about $6000$ tons of carbon per $km^2$ per year on flooded land. For storage, long-lived trees left unharvested could hold carbon for decades, while harvested soft biomass would have to be kept from decomposing: Dyson proposed reproducing artificially the acidity and oxygen-poor sterility of natural peat bogs (waterlogged wetlands where dead vegetation accumulates as peat instead of decaying), so that stored biomass becomes a peat-like solid rather than digesting anaerobically to methane, a potent greenhouse gas. Dyson explicitly listed what had to be resolved before such an emergency program could be treated as operational policy: (1) the partitioning of past and future anthropogenic carbon between ocean and biosphere, including the then-unquantified stimulation of plant growth by rising $CO_2$ itself ($CO_2$ fertilization), which sets the baseline the program must add to; (2) the feasibility, environmental impacts, and scalability of semi-permanent, non-methanogenic storage of harvested biomass; (3) fertilizer supply — about $2 \times 10^{7}$ tons of nitrogen, $7 \times 10^{6}$ tons of $P_{2}O_{5}$, and $1 \times 10^{7}$ tons of $K_{2}O$ per year at the illustrative target ($P_{2}O_{5}$ and $K_{2}O$ being the oxide notations in which fertilizer phosphorus and potassium are conventionally reported), then roughly half of world fertilizer consumption; (4) regional surveys of marginal lands, wetlands, and existing forests; and (5) the climatic and ecological consequences of planting at that scale, including hydrology, biodiversity, and agriculture. Dyson and Gregg Marland restated and extended the same analysis in a 1979 U.S. Department of Energy workshop paper, and Marland (1988) revisited it after a decade. The question has since been absorbed into — and is a recognized origin of — what is now called land-based carbon dioxide removal (CDR): deliberate human activities that remove $CO_2$ from the atmosphere and store it durably. Its modern descendants include afforestation and improved forest management, bioenergy with carbon capture and storage (BECCS, in which biomass is burned for energy and the released $CO_2$ captured), and biomass carbon removal and storage (BiCRS, which stores harvested biomass directly, for example by burial in trenches, wood vaults, or engineered biolandfills). The recurring evaluation criteria for all of these — permanence (storage durability over decades to centuries), measurement, reporting, and verification (MRV), nutrient and land requirements, and biophysical side effects such as albedo (the reflectivity of the land surface, which changes climate directly in addition to the carbon effect) — are precisely Dyson's five issues in modern form.

Problem Statement

Determine whether — and at what scale, with what reliability, and under what constraints — the carbon dioxide content of the atmosphere can be controlled by a global program of biospheric carbon fixation: large-scale cultivation of fast-growing plants (short-rotation tree plantations and aquatic or other soft biomass) whose harvested or accumulated carbon is placed in semi-permanent, non-methanogenic storage, at rates sufficient to halt or reverse the atmospheric increase (Dyson's illustrative target: $5 \times 10^{9}$ tons of carbon per year, comparable to the fossil-fuel emissions of the 1970s). The open question is the joint resolution, with quantified uncertainty, of the five coupled questions identified in the source: (i) the baseline and marginal capacity of the ocean and land-biosphere sinks for anthropogenic carbon, including $CO_2$-fertilization feedbacks, to which any program adds; (ii) whether semi-permanent storage of harvested biomass (artificial peat bogs or their engineered equivalents) can be designed, operated, verified, and maintained at the required scale without anaerobic decomposition to methane and without adverse ecological consequences; (iii) whether the incremental nutrient demand (on the order of $2 \times 10^{7}$ tons N, $7 \times 10^{6}$ tons $P_{2}O_{5}$, and $1 \times 10^{7}$ tons $K_{2}O$ per year at the illustrative target) can be supplied within fertilizer production capacity and without disruptive economic or food-system reallocation; (iv) the land base — marginal lands, wetlands, and existing forests — able to sustain the required growth rates; and (v) the net climatic and ecological consequences of planting and storage at that scale, including effects on surface albedo, hydrology, biodiversity, and agriculture, so that the program's net effect on atmospheric $CO_2$ and climate is known rather than assumed. The joint resolution therefore turns on, for each of (i)-(v), the quantitative estimate with its uncertainty and the evidence supporting it, and on the sign and approximate magnitude of the program's net effect on atmospheric CO2 after accounting for the interactions among (i)-(v). The objective is the feasibility of the program as a whole — whether it could reliably hold down atmospheric $CO_2$ and at what scale — not that of any single component in isolation.

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