365 Days of Climate Awareness 348 – Net Zero

Net zero is the concept of balancing inputs of greenhouse gases into the atmosphere with their removal, resulting in no net gain, and therefore, no radiative forcing—no planetary heat gain. The terms “net zero” and “carbon neutral” are often used interchangeably. Their meaning is similar, but net zero includes the other greenhouse gases like methane, nitrous oxide, and the whole range of fluorocarbons and others. “Decarbonizing the economy” is a convenient shorthand for removing greenhouse gas emissions, since CO₂ is our dominant output and accounts for most of the planet’s warming.

Greenhouse gas emissions by type, 1990-2015.

The concept of net zero accounts for planetary sources and sinks. The concept of “gross zero”(or, less properly, “absolute zero”) refers to the complete elimination of all emissions, which is not realistic. There is no simple equation for net zero, because of the immensity and complexity of the world’s climate system. Attempts to quantify the carbon removal effects of, for example, oceanic plankton, tropical rainforests, and norther taiga, are ongoing efforts but have considerable margins for error. Furthermore, their roles are not static. Warming oceans and active deforestation in the tropics and throughout the north are changing those drawdowns annually (see how noisy the signals for land and atmosphere are in the diagram).

Global emissions and global sinks for carbon dioxide.

Our baseline check on net zero emissions is by measuring atmospheric concentrations of greenhouse gases. National inventories and goals—and goals by sub-national and non-state actors like states, cities and companies—are critical but are not the end in themselves. The global stocktakes, required every five years by the Paris Agreement (with 2021’s stocktake delayed to 2023 due to COVID) are the most visible check on nation-by-nation emissions. To the extent that nationally determined contributions (NDCs) are informed by science, they should represent reasonably accurate estimates of each country’s total emissions. Not all pledges, however, are adequate, and not all are even law.

Climatic adequacy of countries' NDCs (as of 2020).

Gross zero would result in declining concentrations of greenhouse gases, since we would not be replacing those drawn down naturally. (I don’t count sequestering, which has yet to show any realistic potential.)  Our strategies for achieving net zero center on three general spheres of activity (described in far more detail in the third volume of the IPCC’s 6^th Assessment Report):

1.      Replacement of fossil fuels with alternate (renewable and…nuclear?) energy sources;

2.      Increased efficiency where fossil fuels are still used;

3.      Restoration of natural carbon sink environments like forests and swampland.

Legal status of countries' NDCs around the world.

While our society remains a five-alarm fire in so many other respects, we certainly have an immense task in front of us to bring our global emissions into balance with nature’s capacity to absorb them. We have eight years until the first major checkpoint, and another twenty until the second. Every day counts.

Tomorrow: demand, services and social aspects of mitigation.

Be brave, be steadfast, and be well.