Net Zero: Science, Origins, and Implications

Abstract

This review explains the science behind the drive for global net zero emissions and why this is needed to halt the ongoing rise in global temperatures. We document how the concept of net zero carbon dioxide (CO2) emissions emerged from an earlier focus on stabilization of atmospheric greenhouse gas concentrations. Using simple conceptual models of the coupled climate–carbon cycle system, we explain why approximately net zero CO2 emissions and declining net energy imbalance due to other climate drivers are required to halt global warming on multidecadal timescales, introducing important concepts, including the rate of adjustment to constant forcing and the rate of adjustment to zero emissions. The concept of net zero was taken up through the 5th Assessment Report of the Intergovernmental Panel on Climate Change and the United Nations Framework Convention on Climate Change (UNFCCC) Structured Expert Dialogue, culminating in Article 4 of the 2015 Paris Agreement. Increasing numbers of net zero targets have since been adopted by countries, cities, corporations, and investors. The degree to which any entity can claim to have achieved net zero while continuing to rely on distinct removals to compensate for ongoing emissions is at the heart of current debates over carbon markets and offsetting both inside and outside the UNFCCC. We argue that what matters here is not the precise makeup of a basket of emissions and removals at any given point in time, but the sustainability of a net zero strategy as a whole and its implications for global temperature over multidecadal timescales. Durable, climate-neutral net zero strategies require like-for-like balancing of anthropogenic greenhouse gases sources and sinks in terms of both origin (biogenic versus geological) and gas lifetime. Expected final online publication date for the Annual Review of Environment and Resources, Volume 47 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Multi‐Decadal Response to Net Zero CO2 Emissions and Implications for Emissions Policy

“…Heavy and sustained reliance on offsetting between long‐lived with short‐lived climate pollutants (SLCPs) present specific policy challenges (Allen, Dube, et al., 2018; Allen, Shine, et al., 2018; Allen et al., 2022; Cain et al., 2019). Fortunately, this is less of an issue in ambitious mitigation scenarios like SSP1‐19, where the SLCP contribution to non‐CO 2 RF has largely stabilized by 2050 (Meinshausen et al., 2020).…”

“…The RAZE defines the fractional rate at which the temperature adjust over multi‐decadal timescales in response to net zero CO 2 emissions following a period of positive CO 2 emissions. As the difference of two small quantities, $$\rho $$ ≤ 0.3%/yr (Allen, Dube, et al., 2018; Allen, Shine, et al., 2018) and $${{\tau }_{2}}^{-1}$$ ∼ 0.3%/yr (Joos et al., 2013), the RAZE can be positive or negative with typical values of order ±0.1%/yr (Allen, et al., 2022). Setting $${\Delta}G$$ to zero in Equation (i.e., considering a period of net zero additional CO 2 emissions) gives $${\Delta}T/{\Delta}t={\kappa }_{E}o\overline{G}={\kappa }_{E}oG\left(t={t}_{\mathrm{n}\mathrm{e}\mathrm{t}\,\mathrm{z}\mathrm{e}\mathrm{r}\mathrm{o}}\right)$$.…”

Processes Intensification for Sustainability: Prospects and Opportunities

“…It is because of this that the adoption of emerging measures to reduce CO 2 emissions is of great importance. Specialists, such as the Intergovernmental Panel on Climate Change (IPCC), predict that to counteract the steady rise in global average temperature, emissions must reach net zero by 2050 . In other words, various sectors must be decarbonized, so the creation of a net-zero economy should become a priory.…”

US power sector carbon capture and storage under the Inflation Reduction Act could be costly with limited or negative abatement potential

“…Decarbonization, and specifically a goal of eliminating net anthropogenic greenhouse gas (GHG) contributions to climate change (reaching "net zero") by mid-century in order to increase the likelihood of keeping climate change-driven global temperature increases at or below the Paris Agreement's target of well below 2℃, with a goal of 1.5℃, has become a major policy driver in the energy sector and elsewhere (Allen et al 2022, White House 2021, Pye et al 2017, Eyre et al 2018, Sun et al 2021, Waisman et al 2019. In the United States (US), pollution control policies have historically involved mainly command-and-control regulatory approaches aimed at limiting or eliminating pollution of a specific type from a particular source, or marketbased approaches aimed at limiting total pollution of a specific type from a group of sources by incentivizing the lowest-cost actions (Kraft 2000).…”