A negative carbon dioxide emission or negative emission or a process that is carbon negative gives a permanent removal of the greenhouse gas carbon dioxide from Earth's atmosphere. It is considered the direct opposite of carbon dioxide emission, hence its name. It is the result of carbon dioxide removal technologies, such as bio-energy with carbon capture and storage, biochar, direct air capture or enhanced weathering.
Negative emissions is different from reducing emissions, as the former produces an outlet of carbon dioxide from Earth's atmosphere, whereas the latter decreases the inlet of carbon dioxide to the atmosphere. Both have the same momentary net effect, but for achieving carbon dioxide concentration levels below present levels, such as 350 ppm, negative emissions are critical. Also for meeting higher concentration levels, negative emissions are increasingly considered to be crucial as they provide the only possibility to fill the gap between needed reductions to meet mitigation targets and global emission trends.
In the OECD Environmental Outlook to 2050 released at the 2011 United Nations Climate Change Conference, the authors commented on the need for negative emissions, stating "Achieving lower concentration targets (450 ppm) depends significantly on the use of BECCS".
Carbon dioxide sinks and negative carbon dioxide emissions
A carbon dioxide sink such as a concentrated group of plants or any other primary producer that binds carbon dioxide into biomass, such as within forests and kelp beds, is not carbon negative, as sinks are not permanent. A carbon dioxide sink of this type moves carbon, in the form of carbon dioxide, from the atmosphere or hydrosphere to the biosphere. This process could be undone, for example by wildfires or logging.
Carbon dioxide sinks that store carbon dioxide in the Earth's crust by injecting it into the subsurface, or in the form of insoluble carbonate salts (mineral sequestration), are considered carbon negative. This is because they are removing carbon from the atmosphere and sequestering it indefinitely and presumably for a considerable duration (thousands to millions of years). However, Carbon Capture technology remains, at best, theoretical and is yet to reach more than 33% efficiency. Furthermore this process could be rapidly undone, for example by earthquakes or mining.