Projects that better manage, protect, and restore ecosystems are widely viewed as win-win strategies to address two of the biggest global challenges of this century: climate change and biodiversity loss. Yet the potential contribution of such nature-based solutions (NbS) to mitigating climate change remains controversial.As the race to net zero gains momentum, decision makers urgently need to know: what is nature's place in the race? Analyses of nature-based solutions often focus on how much carbon they can remove from the atmosphere. Here, we provide a new perspective by modelling how that carbon removal will impact global temperatures-a critical metric as humanity attempts to limit global warming.Our analysis shows that NbS can have a powerful role in pulling down temperatures in the long term: land use changes will keep on acting long past the time of peak warming, and have an important role to play in planetary cooling in the second half of this century. Before midcentury, NbS can provide real but limited mitigation benefits. Critically, the more ambitious the climate target, the shorter the timeframe for NbS to have an effect on peak warming.If the greatest value of NbS lies in the long-term, these projects must be properly designed to ensure longevity. This means paying closer attention to the long-term carbon sink benefits of NbS projects, as well as their impacts on biodiversity, equity, and sustainable development goals. It also means continuing to limit global warming through other means, from decarbonization to geological storage of CO2.Our model reinforces the conclusion that an ambitious scaling of NbS needs to be implemented quickly and thoughtfully-but not at the expense of other necessary solutions.

Abstract. Here we present an update to the FaIR model for use in probabilistic future climate and scenario exploration, integrated assessment, policy analysis, and education. In this update we have focussed on identifying a minimum level of structural complexity in the model. The result is a set of six equations, five of which correspond to the standard impulse response model used for greenhouse gas (GHG) metric calculations in the IPCC's Fifth Assessment Report, plus one additional physically motivated equation to represent state-dependent feedbacks on the response timescales of each greenhouse gas cycle. This additional equation is necessary to reproduce non-linearities in the carbon cycle apparent in both Earth system models and observations. These six equations are transparent and sufficiently simple that the model is able to be ported into standard tabular data analysis packages, such as Excel, increasing the potential user base considerably. However, we demonstrate that the equations are flexible enough to be tuned to emulate the behaviour of several key processes within more complex models from CMIP6. The model is exceptionally quick to run, making it ideal for integrating large probabilistic ensembles. We apply a constraint based on the current estimates of the global warming trend to a million-member ensemble, using the constrained ensemble to make scenario-dependent projections and infer ranges for properties of the climate system. Through these analyses, we reaffirm that simple climate models (unlike more complex models) are not themselves intrinsically biased “hot” or “cold”: it is the choice of parameters and how those are selected that determines the model response, something that appears to have been misunderstood in the past. This updated FaIR model is able to reproduce the global climate system response to GHG and aerosol emissions with sufficient accuracy to be useful in a wide range of applications and therefore could be used as a lowest-common-denominator model to provide consistency in different contexts. The fact that FaIR can be written down in just six equations greatly aids transparency in such contexts.

The relationship between cumulative CO2 emissions and CO2‐induced warming is determined by the Transient Climate Response to Emissions (TCRE), but total anthropogenic warming also depends on non‐CO2 forcing, complicating the interpretation of emissions budgets based on CO2 alone. An alternative is to frame emissions budgets in terms of CO2‐forcing‐equivalent (CO2‐fe) emissions—the CO2 emissions that would yield a given total anthropogenic radiative forcing pathway. Unlike conventional “CO2‐equivalent” emissions, these are directly related to warming by the TCRE and need to fall to zero to stabilize warming: hence, CO2‐fe emissions generalize the concept of a cumulative carbon budget to multigas scenarios. Cumulative CO2‐fe emissions from 1870 to 2015 inclusive are found to be 2,900 ± 600 GtCO2‐fe, increasing at a rate of 67 ± 9.5 GtCO2‐fe/yr. A TCRE range of 0.8–2.5°C per 1,000 GtC implies a total budget for 0.6°C of additional warming above the present decade of 880–2,750 GtCO2‐fe, with 1,290 GtCO2‐fe implied by the Coupled Model Intercomparison Project Phase 5 median response, corresponding to 19 years' CO2‐fe emissions at the current rate.

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.

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