A clarion call for “net zero” greenhouse gas emissions has been embraced by nearly everyone — environmentalists, politicians, corporations, and nations. More than 130 countries, including the world’s biggest oil exporter, Saudi Arabia, have established, or are developing, net-zero emissions targets. Adding to that, at least a fifth of the world’s largest corporations, representing some $14 trillion in sales, have announced net-zero emissions targets by midcentury. Even airline companies, collectively responsible for five percent of global warming, have publicized net-zero policies. These include United Airlines, American Airlines, Jet Blue, Delta, as well as other major U.S. and international airline companies.
At first glance, the idea seems eminently reasonable: Offset greenhouse gas emissions by removing equal quantities of greenhouse gases from the atmosphere, to be permanently sequestered in soils, plants, oceans, and possibly artificial carbon capture and storage systems. In that way the atmospheric concentration of greenhouse gases is stabilized. But is net-zero really a sufficient response to the climate crisis?
A closer look at the dynamics of the climate system, including the way the carbon cycle works, reveals that “net zero” can be only a temporary, transitory step, if we are to restrain the worst consequences. Global emissions must be rapidly reduced to a level as close to zero — “near zero” as opposed to “net-zero” — as possible. Net-zero and near zero are not the same, even though some well-informed environmentalists conflate the two.
Existing so-called net-zero policies are making things worse, not better. Despite a plethora of exposés (see for example here, here, and here), net-zero promises are still rife with fraud. Many net-zero pledges are just public relations ploys that enable corporations to continue emitting high volumes of greenhouse gases while “owning” undervalued carbon-absorbing forests or mangroves elsewhere in the world. And in some cases, those offsets burn down or are destroyed in other ways.
Particularly egregious was the deliberate production of potent greenhouse gases a decade ago for the sole purpose of chemically destroying those gases immediately after their production. The “manufacturers” then sold carbon permits earned from the destruction of their greenhouse gases to other corporations, which were then free to pollute. A more recent example of such devil-may-care environmental destruction involves the global aviation group Lufthansa, a corporation that boasts a net-zero pledge, with hundreds of subsidiaries. Lufthansa confirmed in early 2022 that it had sent empty or near-empty planes that winter into the air 18,000 times to hold onto landing slots at major airports — thereby releasing immense quantities of greenhouse gases to the atmosphere.
At the level of nations, a Washington Post investigation found that 196 countries underreported their annual emissions by between 8.5 billion and 13.3 billion metric tons of greenhouse gases. (The lower bound exceeds the annual emissions of the United States, and the higher estimate nearly matches those of China.) For example, Malaysia’s most recent report on its greenhouse gas emissions implies that its trees absorb carbon dioxide four times faster than those in neighboring Indonesia. That accounting trick enabled Malaysia to eliminate more than 243 million tons of carbon dioxide emissions from its 2016 inventory, a triumph for greenwashing.
Malaysia is hardly alone. Other countries, including Mexico and the USA, underreport emissions of nitrous oxide, a greenhouse gas released from the use of fertilizers, that is 265 times more powerful over a 100 year period than carbon dioxide in heating the atmosphere. Direct measurements of greenhouse gas concentrations in the atmosphere offer a far better assessment of the climate system than do suspect national emissions reports.
The seemingly unending litany of fraudulent net-zero policies, both corporate and governmental, surely calls into question the viability of such policies within the framework of capitalism. But are there also intrinsic problems with net-zero on a scientific level? Stripped of the greenwashing and corruption now extant, could rigorously implemented net-zero policies sufficiently address the climate crisis?
Some background information is helpful in understanding the issues.
Earth’s Energy Imbalance
When more energy radiates down to the planet than radiates away from Earth out to space, the planet heats up and temperatures rise. This energy imbalance, an effect of greenhouse gases, is an example of radiative forcing. Greenhouse gases in the atmosphere capture infrared radiation emanating from the surface (land and water) and re-radiate some of it back to Earth. As long as the net energy flux is directed toward the planet, average temperatures increase.
The global mean energy imbalance from 2010 to 2018 is estimated to be 0.87 ± 0.12 watts per square meter (W/m2) of Earth’s surface area, and satellite observations from 2001 to 2020 indicate that the globally averaged energy imbalance has been increasing at a rate of 0.38 ± 0.24 W/m2 per decade. So a reasonable estimate for the current global energy imbalance is roughly one watt per square meter. Adding up these contributions for the entire surface area of the planet yields a rate of energy transfer into Earth’s climate system of 510 terawatts. This is the rate at which energy is entering the climate system, energy that has not yet resulted in increased temperatures.
Think of a flame heating a pot of cold water. The temperature of the pot of water will increase as long as the flame transfers energy to it. Similarly, as long as there is a net flux of energy into Earth’s climate system, the planet will warm. As the planet warms, it radiates increasingly more energy out to space until its outbound energy matches the inbound energy from the sun, and then the surface temperature of Earth stabilizes.
The Carbon Cycle
To understand the limitations of any net-zero emissions policy, it is helpful to consider not only energy imbalances, but also the role of the planetary carbon cycle — an integral component of the climate system. There are four major planetary carbon depositories: sedimentary rocks and solid earth; the land surfaces; the oceans; and the atmosphere.
The carbon cycle moves carbon among these reservoirs on different time scales, ranging from annual cycles to changes over millions of years. Of these reservoirs, the atmosphere holds the least carbon. But atmospheric carbon is the root cause of the climate crisis because of the greenhouse effect.
Each greenhouse gas has a different lifetime in the atmosphere. The IPCC has estimated that if all emissions of greenhouse gases suddenly stopped, “methane concentration would return to values close to preindustrial level in about 50 years, nitrous oxide concentrations would need several centuries, while CO2 would essentially never come back to its preindustrial level on time scales relevant for our society.”
Because of its relative stability and abundance, in what follows we focus on carbon dioxide.
Trees and other plants remove carbon dioxide from the atmosphere through photosynthesis, but only temporarily, and much depends on what happens to the plants after they die. Nearly all of the organic carbon produced from photosynthesis is eventually respired, that is, returned to the atmosphere. Once a plant dies, its organic carbon is transformed back to carbon dioxide, whether through gradual decay, being ingested by animals, or conflagration in a forest fire.
Deforestation from human activities has resulted in a total of about 180 billion metric tons of carbon released to the atmosphere since 1750 (the approximate start of the industrial era). Plants and soils have reabsorbed about 80 billion tons since that time, with a net historical loss of about 100 billion metric tons of carbon to the atmosphere through deforestation.
There is a limit to the amount of carbon that plants and soils can hold before reaching saturation levels. Soil scientists have estimated this maximum holding capacity to be 100 billion metric tons or less, the amount that nature was able to sequester before the advent of the industrial revolution (for scientific reports on the maximum carbon capacity of Earth’s biomass, see for example here, here, here, here, and here).
There is 50 times as much carbon dissolved in the oceans as there is in the atmosphere. Most of this is inorganic carbon, and only a tiny portion, about one billion tons, is in the form of living carbon. Since oceans cover 71 percent of Earth’s surface, the interface between the ocean and atmosphere is enormous, and the two continually exchange vast amounts of carbon. Carbon is released from the ocean to the air in some parts of the world and, conversely, carbon dioxide from the air is dissolved in the ocean in other parts.
The direction of the flux depends on the relative partial pressures of CO2, with the flow of carbon dioxide into the more dilute of the two bodies (ocean and atmosphere). Colder ocean waters can hold more carbon than warmer waters. The rate of exchange between the oceans and the atmosphere is comparable to the rate of exchange of carbon dioxide back and forth between the atmosphere and the land. The net flow of carbon is from the atmosphere to the oceans, a process that leads to ocean acidification with devastating effects on ocean ecology. As with plants and soils, there is a limit to the amount of carbon that the oceans will absorb. That limit is reached when the concentrations of carbon in the atmosphere and ocean reach equilibrium.
Land plants and soils currently absorb about a quarter of human carbon dioxide emissions, with ocean waters absorbing another 25 percent, and this helps to reduce the greenhouse gas concentration in the atmosphere. All together, soils, plants, and the oceans absorb about half of humanity’s carbon dioxide emissions, but that could change.
The capacity of the land to absorb carbon is likely to decrease as the world heats up and tropical climate regions expand. Soils hold more carbon than plants and animals collectively, but there is great variation. Soils in tropical regions hold very little, whereas soils in the higher latitudes, especially permafrost soils, hold vast amounts. As the planet heats up, tropical regions are likely to expand, and the soils capable of holding more carbon will retreat toward the poles, where there is less land area. This feedback increases atmospheric greenhouse gas concentrations as global warming proceeds. Moreover, a 2021 study published in Nature found that when elevated carbon dioxide levels lead to increased plant growth, the effect on soils is usually a decrease in carbon storage.
Net Zero, Near Zero, and Net Negative
The 2015 Paris Agreement called on the world to
“achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century, on the basis of equity, and in the context of sustainable development and efforts to eradicate poverty.”
If a global net-zero policy were to follow this directive, and include the oceans among Earth’s carbon sinks (which they are), then a net-zero policy amounts to maintaining a constant concentration of greenhouse gases in the atmosphere. Humanity’s emissions would exactly match the carbon draw-downs of plants, soils, and oceans (and possibly artificial carbon capture and storage systems). To achieve this goal rapidly, the rate of carbon dioxide emissions would have to be cut in half, since the ocean and land sinks absorb only half of what we produce today.
Research with Earth system models suggests that land and ocean sinks would persist for several decades in such a scenario. As a result, oceans would become increasingly acidic during that time, endangering an already fragile ecosystem. In addition, radiative forcing (referenced above) would continue to heat the planet. Climate models indicate that with a constant concentration of atmospheric greenhouse gases, global warming would continue for several centuries with a net warming of perhaps half a degree Celsius (depending on the initial concentrations), though unanticipated tipping points could result in greater increases.
In an alternative scenario, if nations were to agree to equalize anthropogenic emissions with land sinks only (without including ocean CO2 drawdown in the accounting), then for a few decades the result would be a “net negative” global emissions policy. In such a scenario, humanity’s emissions would exactly match the carbon draw-down from plants, soils, and (possibly) artificial systems, while the draw-down from the oceans would continue with concomitant increases in ocean acidification. To achieve this goal rapidly, the rate of carbon dioxide emissions would therefore have to be cut by 75 percent, since land sinks absorb only a quarter of what we produce today.
In both of the above scenarios, net zero and net negative, the natural carbon sinks eventually weaken. Once the carbon concentration between the atmosphere and oceans equalizes, the oceans no longer act as carbon sinks. And increases in soil carbon and absorption by plants do not continue indefinitely: They move toward a new equilibrium value and then cease, with an upper bound (see above) of no more than a 100 billion metric ton capacity. Earth system model studies confirm significant weakening, or even reversal, of ocean and land sinks under future low-emission scenarios. Thus, after a few decades of either policy being implemented and maintained — global net zero, or net zero relative to land sinks only — emissions would be forced to decrease to near zero.
Earth system models indicate that stabilizing the climate at a given temperature requires that anthropogenic CO2 emissions be decreased to near zero, not merely net zero. Using an alternative approach, a comprehensive study of Earth’s energy imbalance found that (in 2019) “the amount of CO2 in the atmosphere would need to be reduced from 410 to 353 ppm to increase heat radiation to space by 0.87 Watts per square meter, bringing Earth back towards energy balance.”
This is consistent with the identification of 350 ppm as a maximum safe CO2 concentration, as argued by Columbia University climatologist James Hansen and promoted by the organization 350.org. Thus, stabilizing temperatures and stabilizing carbon atmospheric concentrations are not the same — and, indeed, are mutually exclusive. The former requires a “near zero” global emission policy plus draw-downs from all natural sinks.
According to Hansen, there is no longer a realistic possibility of keeping global warming to 1.5° C or less, and even the target of 2° C is in jeopardy. Half of the climate model studies based on RCP 2.6, which is a collection of emissions scenarios designed to keep Earth below 2° C of warming, conclude that the 2°C limit can be met only with large negative emissions beyond what natural sinks are able to provide.
It is imperative to reduce emissions rapidly and render them as close to zero as possible in order to limit the destruction of the climate crisis. The achievement of net-zero emissions must be only a near-term intermediary step toward near zero emissions worldwide.
_____________
Graphics credit: photo of factory smokestacks by Ella Ivanescu on Unsplash; green slime courtesy of pngall.com CC BY-NC 4.0
