COP21 Glossary of Terms Guiding the Long-term Emissions-Reduction Goal

The glossary of terms guiding the discussion on long-term signals for emissions reductions in the COP21 climate negotiations is complicated, to say the least. The international community’s goal of limiting warming below 2°C above preindustrial levels, if not 1.5°C,  to avoid the worst climate impacts does not provide much clarity for real-world actors, and, as a result, many countries and international stakeholders support including a complementary long-term goal to operationalize the 2°C goal in the Paris climate agreement.

The current negotiating text offers multiple options for such a complementary goal.  This glossary disentangles the various terms guiding long-term emissions reductions and defines their implications for an international agreement limiting warming to below 2°C, or 1.5°C.

• Decarbonization is framed around decreasing the ratio of carbon dioxide (CO2) or all greenhouse gas emissions related to primary energy production. While full decarbonization means zero unabated (not captured by carbon sequestration or storage) CO2 emissions from energy generation and industrial processes, decarbonization doesn’t imply zero emissions, as emissions can be balanced by carbon sequestration if adequate reductions or enhanced carbon sinks exist. To effectively communicate the scale of change needed, the term must be accompanied by a timeframe and rates of decarbonization.


• Net zero carbon emissions is considered a synonym for carbon neutrality. One key difference, however, is carbon neutrality can be achieved at the domestic level with offsets from other jurisdictions, while net zero emissions does not have the same connotation (though theoretically could be met via offsets). Both terms risk overshooting the carbon budget unless complemented by short-term emissions reduction targets. 


• Net zero GHG emissions can be confused with net-zero carbon emissions, but when accurately used, means all greenhouse gas emissions decline to zero, as opposed to just carbon dioxide. This is the same concept as net zero carbon emissions but conveys a net zero emissions target for CO2 and all non-CO2 gases. 


• Carbon neutrality means annual zero net anthropogenic (human caused or influenced) CO2 emissions by a certain date. By definition, carbon neutrality means every ton of anthropogenic CO2 emitted is compensated with an equivalent amount of CO2 removed (e.g. via carbon sequestration), but this term has been used differently on occasion. For instance, Costa Rica’s INDC would “achieve Carbon Neutrality by 2021 with total net emissions comparable to total emissions in 2005,” while Ethiopia’s goal is to “achieve carbon-neutral middle-income status before 2025” and at the same time “limit its net greenhouse gas (GHG) emissions in 2030 to 145 Mt CO2e or lower.” Instead of aiming at zero net emissions, some countries seem to misinterpret carbon neutrality as stabilizing emissions at a certain level.


• Climate neutrality is the same concept as carbon neutrality but rather than solely focusing on CO2 emissions, it extends to zero net anthropogenic greenhouse gas emissions (i.e. including emissions beyond carbon dioxide).


• Peaking emissions means emissions reach a specific maximum level by a specific date before declining afterwards. For example, the world could commit to peaking emissions at 44 gigatonnes CO2e in 2020 before reducing afterwards. Peaking emissions is critical to determining the global emissions trajectory timeline and ambition. Any delay of the peak date will require more aggressive emissions-reduction targets in later years and may make achieving the 2°C goal riskier if not altogether impossible.


• Percentage emissions reductions by a certain date is another way of communicating specific emissions reductions to be achieved by a specific date in a quantitative manner. For example, this goal could be framed as a 70 percent emissions reduction below 1990 levels by 2050.


• Low-carbon (or low-emissions) transformation conveys intent to transform our systems (e.g. technology, energy, political, infrastructure) and limit warming to 2°C.  However, without being complemented by quantified targets (such as emissions reductions and renewable generation capacity) with specific timelines, the term is too vague to send accurate signals to curb GHG emissions.


• Carbon budget is the amount of CO2 the world can emit while still having a likely chance of limiting warming to the 2°C target. The Intergovernmental Panel on Climate Change’s Fifth Assessment Report, issued in 2014, estimates the world has burned through two-thirds of the budget, and WRI calculates we could spend it entirely in two decades if emissions continue unabated.


• Sharing the carbon budget means sharing the estimated amount of cumulative CO2 emissions remaining for a likely chance (>66 percent) of achieving the 2°C goal. Unlike a goal reducing emissions a certain percentage by a certain date, a carbon budget goal limits cumulative emissions year after year, significantly reducing risks of overshooting the 2°C goal. Several countries like the United Kingdom and Norway have adopted a carbon budget approach, but it is not common practice. On its own, carbon budget sharing may not adequately create an emissions pathway with plausible decarbonization rates (e.g. we could quickly exhaust the carbon budget then require unprecedented annual emissions reduction rates in later decades).

Clear Communication Can Create a Clear and Effective Emissions-Reduction Timeline

Clearly defining complementary long-term goals to the 2°C target are critical to communicating necessary emissions reductions and a set timeline to countries, cities and the private sector. While these individual goals are not mutually exclusive, it is possible that there may be a combination in the final wording of the agreement. Defined clearly, such goals are part of the package that will catalyze additional action leading up to 2030 and keep that 2 degrees C goal still within reach.

To be effective, complementary long-term goals should:

• Be consistent with the most recent climate science;
• Limit long-term emissions, as well as cumulative emissions;
• Communicate clear dates, emissions levels and greenhouse gases covered; and
• Communicate a plausible emissions-reduction pathway that allows for a smooth transformation.