What does “net zero CO2 emissions” mean?

Net zero carbon dioxide (CO2) emissions are achieved when anthropogenic CO2 emissions are balanced globally by anthropogenic CO2 removals over a specified period.

Example: The second of Extinction Rebellion’s three demands states that “we must enact legally binding policy measures to reduce carbon emissions to net zero CO2 emissions by 2025 and reduce consumption levels.”

Some carbon terms defined: carbon sequestration, carbon sink, carbon offset, carbon neutrality

If you are just learning about climate change, you will want to be familiar with the following terms related to carbon:

Carbon Sequestration: Carbon sequestration is the process involved in carbon capture and the long-term storage of atmospheric carbon dioxide or other forms of carbon to mitigate or defer global warming. This may happen naturally or deliberately. As a natural ongoing process, it is the biogeochemical cycling between the atmosphere and various reservoirs, such as by chemical weathering of rocks. When carried out deliberately, the process may be referred to as carbon dioxide removal, which is a form of geo-engineering (Wikipedia).

Carbon Sink: A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period. The process by which carbon sinks remove carbon dioxide (CO2) from the atmosphere is known as carbon sequestration. Trees serve as carbon sinks during growing seasons. The oceans are also natural sinks as they absorb of carbon dioxide  via physicochemical and biological processes. Terrestrial plants are natural carbon sinks through the process of photosynthesis. Whilst the creation of artificial sinks has been discussed, no major artificial systems remove carbon from the atmosphere on a material scale (Wikipedia).

Carbon Neutrality: Carbon neutrality, or having a net zero carbon footprint, refers to achieving net zero carbon emissions by balancing a measured amount of carbon released into the atmosphere with an equivalent amount sequestered or offset, or buying enough carbon credits to make up the difference. It is used in the context of carbon dioxide releasing processes associated with transportation, energy production, and industrial processes such as production of carbon neutral fuel (Wikipedia).

Carbon Offset: A carbon offset is a reduction in emissions of carbon dioxide or greenhouse gases made in order to compensate for or to offset an emission made elsewhere. There are two markets for carbon offsets. In the larger, compliance market, companies, governments, or other entities buy carbon offsets in order to comply with caps on the total amount of carbon dioxide they are allowed to emit. In the much smaller, voluntary market, individuals, companies, or governments purchase carbon offsets to mitigate their own greenhouse gas emissions from transportation, electricity use, and other sources (Wikipedia).

Water Vapour

Water vapour is an extremely important gas found in the atmosphere. It can vary from 4% in the steamy tropics to nearly nonexistent in the cold dry regions of the Antarctic. Water vapor is a good absorber of Earth’s outgoing radiation and thus is considered a greenhouse gas. When water vapor is converted to a liquid during condensation, clouds are formed. Clouds are good absorbers of radiation given off by the Earth’s surface. The absorption of this energy raises the temperature of the air. But clouds are generally light-colored and hence reflect incoming solar radiation off their tops. The reflected light is sent back to space, never reaching the ground to warm the Earth. Thus clouds can have either a warming or a cooling effect on air temperature. For a more details discussion, read more . . .

Radiative Forcing

Radiative forcing or climate forcing is the difference between insolation (sunlight) absorbed by the Earth and energy radiated back to space. The influences that cause changes to the Earth’s climate system altering Earth’s radiative equilibrium, forcing temperatures to rise or fall, are called climate forcings. Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space. This net gain of energy will cause warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the sun, which produces cooling.

For more on Radiative Forcing, see HERE.

Permafrost

In geology, permafrost is ground, including rock or (cryotic) soil, at or below the freezing point of water 0 °C (32 °F) for two or more years. Permafrost is globally important because around 1.6 trillion tons of carbon is stored in these soils, which could be released if the ground thaws. Such thawing would effectively be irreversible on relevant time scales.

This has the potential to unleash a significant positive feedback mechanism. Rising temperatures will thaw permafrost regions most (as warming is greatest in the Arctic). This in turn will result in the rapid decomposition of carbon stores – releasing carbon dioxide and methane to the atmosphere. This thickens the blanket effect, warming Earth further and leading to more permafrost thaw.

To read more about why permafrost matters relative to climate change, see HERE and HERE.

And thanks to Nick Garland for this article on how long-dormant bacteria and viruses, trapped in ice and permafrost for centuries, are reviving as Earth’s climate warms.

Greenhouse Effect

The greenhouse effect is the trapping of the sun’s heat in the atmosphere of a planet by gases in that atmosphere. It’s called the greenhouse effect because it has a lot in common with how the glass of a greenhouse traps heat inside: heat can get into the greenhouse, but has more trouble leaving.

The greenhouse effect happens because of so-called greenhouse gases, which include carbon dioxide, methane, water vapor, and others. The biggest components of the atmosphere, nitrogen and oxygen, are not greenhouses gases. More details HERE.

Earth Blanket Effect would be more accurate but, because ‘greenhouse effect’ has been used so long and is so ubiquitous, I include it in this glossary. More details HERE.