Using negative emissions technologies to draw down carbon dioxide from the atmosphere will, over the long term, weaken and even reverse the planet’s two biggest carbon sinks—the oceans and forests, new modelling work has shown. This quirky effect ironically would require an even greater deployment of these same negative emissions techniques to get the atmospheric concentration of the greenhouse gas (GHG) down to safe levels.
A carbon sink is anything that absorbs more carbon than it releases as carbon dioxide. Most forests and the world’s oceans currently annually absorb about half of the CO2 that humans pump into the atmosphere via the combustion of fossil fuels, land-use change and cement production. This happens during photosynthesis in plants and phytoplankton (and to a lesser extent when some plankton and molluscs form their shells), and when CO2 reacts with seawater to form carbonic acid.
However, both climate change itself and an increase in atmospheric CO2 will effect how much carbon these sinks can take up. Likewise, drawing down CO2 via negative emissions technologies (NETs) will also impact the productivity of these sinks. Reducing the amount of CO2 in the atmosphere ironically will lower vegetation productivity, diminishing how much carbon that plants can take up. A similar phenomenon happens in the oceans. Whenever the pressure of an atmospheric gas is increased over a body of water, the gas will diffuse into that water until an equilibrium is reached. Drawing down atmospheric CO2 diminishes this pressure, in turn diminishing the uptake. Carbon sinks are not a constant.
Most modelled scenarios to keep global warming within 2°C above pre-industrial temperatures and all the scenarios that keep us within 1.5°C assume widespread and rapid adoption of NETs. Many scenarios assume net negative emissions—in other words, not just sucking CO2 out of the atmosphere, but sucking out more than we are pumping into it.
So researchers with some of the world’s leading climate research groups, from the Potsdam Institute for Climate Impact Research to Australia CSIRO’s Global Carbon Project, wanted to model for the first time what impact NETs would have on these carbon sinks as a result of such feedback effects.
The researchers focussed on one particular 1.5°C scenario in which global emissions peak in 2020 and then decline, becoming net negative in the 2080s via reduction in fossil fuel use and deployment of the NET known as bioenergy with carbon capture and storage, or BECCS introduced by 2020. They also took the long view, extending this analysis to 2300.
Their modelling showed that by the end of this century, natural carbon sinks will diminish somewhat but overall will still be drawing down CO2. Nevertheless, the weakening of these sinks will require greater adoption of NETs to achieve climate stabilization targets.
And by 2150, these sinks will have weakened significantly due to the rapid decrease in atmospheric CO2. By 2300, the land-based sink will have reversed and become a net source of CO2, albeit a modest one.
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