Ranking the risk of CO2 emissions from seagrass soil carbon stocks under global change threats

Seagrass meadows are natural carbon storage hotspots at risk from global change threats, and their loss can result in the remineralization of soil carbon stocks and CO₂ emissions fueling climate change. Here we used expert elicitation and empirical evidence to assess the risk of CO₂ emissions from seagrass soils caused by multiple human-induced, biological and climate change threats. Judgments from 41 experts were synthesized into a seagrass CO₂ emission risk score based on vulnerability factors (i.e., spatial scale, frequency, magnitude, resistance and recovery) to seagrass soil organic carbon stocks. Experts perceived that climate change threats (e.g., gradual ocean warming and increased storminess) have the highest risk for CO₂ emissions at global spatial scales, while direct threats (i.e., dredging and building of a marina or jetty) have the largest CO₂ emission risks at local spatial scales. A review of existing peer-reviewed literature showed a scarcity of studies assessing CO₂ emissions following seagrass disturbance, but the limited empirical evidence partly confirmed the opinion of experts. The literature review indicated that direct and long-term disturbances have the greatest negative impact on soil carbon stocks per unit area, highlighting that immediate management actions after disturbances to recover the seagrass canopy can significantly reduce soil CO₂ emissions. We conclude that further empirical evidence assessing global change threats on the seagrass carbon sink capacity is required to aid broader uptake of seagrass into blue carbon policy frameworks. The preliminary findings from this study can be used to estimate the potential risk of CO₂ emissions from seagrass habitats under threat and guide nature-based solutions for climate change mitigation.     

Would forestation alleviate the burden of emission reduction?: An assessment of the future carbon sink from ARD activities

Atmospheric CO₂ concentration can be decreased not only by reducing fossil fuel burning but also by increasing the terrestrial ecosystems that serve as sinks for CO₂. The Kyoto Protocol allows countries that are burdened with emission reduction commitments to use carbon sequestration by terrestrial sinks. However, opinions differ widely on how the inclusion of terrestrial carbon sinks in the legally binding framework (Article 3.3) will affect the demand for emission reduction during the commitment period. We approach this issue by combining a simulation model of the carbon stock change with that of land-use change. The result of the simulation shows that the Annex I countries in total may potentially claim for a net carbon offset as high as 0.2 GtC per year by carrying out ARD (Afforestation, Reforestation and Deforestation) activities. In order to come up with an effective long-term climate regime, political decisions are necessary to realize an appropriate balance between the sink enhancement and the emission reduction. Sink activities should not be too large to eliminate the efforts for emission reduction, nor too small to discourage the efforts in enhancing sinks. Although prediction of sink activities is an extremely difficult venture, several estimates of the potential should be carefully considered before political decisions. Appropriate inclusion of sink activities is also crucial for ratifying the Kyoto Protocol.     

Would forestation alleviate the burden of emission reduction? An assessment of the future carbon sink from ARD activities

Abstract

Atmospheric CO₂ concentration can be decreased not only by reducing fossil fuel burning but also by increasing the terrestrial ecosystems that serve as sinks for CO₂. The Kyoto Protocol allows countries that are burdened with emission reduction commitments to use carbon sequestration by terrestrial sinks. However, opinions differ widely on how the inclusion of terrestrial carbon sinks in the legally binding framework (Article 3.3) will affect the demand for emission reduction during the commitment period. We approach this issue by combining a simulation model of the carbon stock change with that of land-use change. The result of the simulation shows that the Annex I countries in total may potentially claim for a net carbon offset as high as 0.2 GtC per year by carrying out ARD (Afforestation, Reforestation and Deforestation) activities. In order to come up with an effective long-term climate regime, political decisions are necessary to realize an appropriate balance between the sink enhancement and the emission reduction. Sink activities should not be too large to eliminate the efforts for emission reduction, nor too small to discourage the efforts in enhancing sinks. Although prediction of sink activities is an extremely difficult venture, several estimates of the potential should be carefully considered before political decisions. Appropriate inclusion of sink activities is also crucial for ratifying the Kyoto Protocol.     

Carbon budget of managed forests in the Russian Federation in 1990–2050: Post-evaluation and forecasting

Considered are the contribution of managed forests in the Russian Federation to the climate change softening and the forecast of their carbon-depositing potential in the period till 2050 under different scenarios of the forest management. The sink of CO₂ to managed forests is estimated using the flow balance method. The CBM-CFS3 model worked out in the Canadian Forestry Service is used for predicting the carbon budget. It is found out that managed forests absorb 473.8 Mt of CO₂ per year. The carbon sink is caused by the reduction of the volume of the forest use and by the regeneration of cutover stands of previous years. Depending on the conditions of the forest use, by 2020 the CO₂ sink to managed forests will amount to 466–632 Mt/year and will be able to compensate from 21 to 29% of industrial emissions of greenhouse gases. The carbon absorption by managed forests will decrease to 105–235 Mt/year by 2050. To maintain and increase the carbon-depositing potential of managed forests, the Russian Federation needs the development of the system of purposeful activities on strengthening the protection against forest fires and on the intensification of forest reproduction.     

Temporary sequestration credits: an instrument for carbon bears

Abstract

Temporary crediting of carbon storage is an instrument that allows entities with emissions reductions obligations to defer some obligations for a fixed period of time. This instrument provides a means of guaranteeing the environmental integrity of a carbon sequestration project. But because the user of the temporary credit takes on the liability of renewing it, or replacing it with a permanent credit, the temporary credit must sell at a discount compared to a permanent credit. We show that this discount depends on the expected change in price of a permanent credit. Temporary credits have value only if restrictions on carbon emissions are not expected to tighten substantially. The intuition is illustrated by assessing the value of a hypothetical temporary sulfur dioxide sequestration credit, using historical data on actual SO₂ allowance prices.     

The leaky sink: persistent obstacles to a forest carbon sequestration program based on individual projects

Abstract

One strategy for mitigating the increase in atmospheric carbon dioxide is to expand the size of the terrestrial carbon sink, particularly forests, essentially using trees as biological scrubbers. Within relevant ranges of carbon abatement targets, augmenting carbon sequestration by protecting and expanding biomass sinks can potentially make large contributions at costs that are comparable or lower than for emission source controls. The Kyoto protocol to the framework convention on climate change includes many provisions for forest and land use carbon sequestration projects and activities in its signatories' overall greenhouse gas mitigation plans. In particular, the protocol provides a joint implementation provision and a clean development mechanism that would allow nations to claim credit for carbon sequestration projects undertaken in cooperation with other countries. However, there are many obstacles for implementing an effective program of land use change and forestry carbon credits, especially measurement challenges. This paper explains the difficulty that even impartial analysts have in assessing the carbon offset benefits of projects. When these measurement challenges are combined with self-interest, asymmetries of information, and large numbers, it prevents to a project-based forest and land use carbon credit program may be insurmountable.     

Uncertainty of forest carbon stock changes – implications to the total uncertainty of GHG inventory of Finland

Uncertainty analysis facilitates identification of the most important categories affecting greenhouse gas (GHG) inventory uncertainty and helps in prioritisation of the efforts needed for development of the inventory. This paper presents an uncertainty analysis of GHG emissions of all Kyoto sectors and gases for Finland consolidated with estimates of emissions/removals from LULUCF categories. In Finland, net GHG emissions in 2003 were around 69 Tg (±15 Tg) CO₂ equivalents. The uncertainties in forest carbon sink estimates in 2003 were larger than in most other emission categories, but of the same order of magnitude as in carbon stock change estimates in other land use, land-use change and forestry (LULUCF) categories, and in N₂O emissions from agricultural soils. Uncertainties in sink estimates of 1990 were lower, due to better availability of data. Results of this study indicate that inclusion of the forest carbon sink to GHG inventories reported to the UNFCCC increases uncertainties in net emissions notably. However, the decrease in precision is accompanied by an increase in the accuracy of the overall net GHG emissions due to improved completeness of the inventory. The results of this study can be utilised when planning future GHG mitigation protocols and emission trading schemes and when analysing environmental benefits of climate conventions.     

Ocean dynamics determine the response of oceanic CO2 uptake to climate change

The increase of atmospheric CO₂ concentrations due to anthropogenic activities is substantially damped by the ocean, whose CO₂ uptake is determined by the state of the ocean, which in turn is influenced by climate change. We investigate the mechanisms of the ocean’s carbon uptake within the feedback loop of atmospheric CO₂ concentration, climate change and atmosphere/ocean CO₂ flux. We evaluate two transient simulations from 1860 until 2100, performed with a version of the Max Planck Institute Earth System Model (MPI-ESM) with the carbon cycle included. In both experiments observed anthropogenic CO₂ emissions were prescribed until 2000, followed by the emissions according to the IPCC Scenario A2. In one simulation the radiative forcing of changing atmospheric CO₂ is taken into account (coupled), in the other it is suppressed (uncoupled). In both simulations, the oceanic carbon uptake increases from 1 GT C/year in 1960 to 4.5 GT C/year in 2070. Afterwards, this trend weakens in the coupled simulation, leading to a reduced uptake rate of 10% in 2100 compared to the uncoupled simulation. This includes a partial offset due to higher atmospheric CO₂ concentrations in the coupled simulation owing to reduced carbon uptake by the terrestrial biosphere. The difference of the oceanic carbon uptake between both simulations is primarily due to partial pressure difference and secondary to solubility changes. These contributions are widely offset by changes of gas transfer velocity due to sea ice melting and wind changes. The major differences appear in the Southern Ocean (?45%) and in the North Atlantic (?30%), related to reduced vertical mixing and North Atlantic meridional overturning circulation, respectively. In the polar areas, sea ice melting induces additional CO₂ uptake (+20%).     

Sensitivity of carbon budget to historical climate variability and atmospheric CO2 concentration in temperate grassland ecosystems in China

Chinese temperate grasslands play an important role in the terrestrial carbon cycle. Based on the parameterization and validation of Terrestrial Ecosystem Model (TEM, Version 5.0), we analyzed the carbon budgets of Chinese temperate grasslands and their responses to historical atmospheric CO₂ concentration and climate variability during 1951–2007. The results indicated that Chinese temperate grassland acted as a slight carbon sink with annual mean value of 7.3 T?g C, ranging from -80.5 to 79.6 T?g C yr^-1. Our sensitivity experiments further revealed that precipitation variability was the primary factor for decreasing carbon storage. CO₂ fertilization may increase the carbon storage (1.4 %) but cannot offset the proportion caused by climate variability (-15.3 %). Impacts of CO₂ concentration, temperature and precipitation variability on Chinese temperate grassland cannot be simply explained by the sum of the individual effects. Interactions among them increased total carbon storage of 56.6 T?g C which 14.2 T?g C was stored in vegetation and 42.4 T?g C was stored in soil. Besides, different grassland types had different responses to climate change and CO₂ concentration. NPP and R_H of the desert and forest steppes were more sensitive to precipitation variability than temperature variability while the typical steppe responded to temperature variability more sensitively than the desert and forest steppes.     

Importance of programme design for potential US domestic GHG offset supply and quality

Greenhouse gas (GHG) offsets are a central feature of most regional and national cap-and-trade systems. A greenhouse offset credit represents a tonne of carbon dioxide equivalent (CO₂e) reduced, avoided or sequestered by a project implemented specifically to compensate for emissions occurring elsewhere. Several existing modelling studies estimate the technically and economically achievable supply of GHG offsets from uncapped sources in the US. This analysis is among the few that consider how the design of offset protocols – and the corresponding rules for eligibility, measuring, verifying and awarding offsets – might impact actual offset crediting and the realization of GHG mitigation potential. The presented analysis demonstrates how rules for each of these factors could impact the supply of offset credits, as well as the emissions-reduction benefits of an offset programme. Findings indicate that although lenient offset rules and protocols may bring several times more credits to market than a conservative approach, these gains in offset supply would come at a significant cost to the effectiveness of the cap-and-trade system in achieving its central purpose: reducing overall GHG emissions. In particular, lenient rules and protocols could conceivably lead US emissions to exceed legislative targets by as much as 500 million tonnes CO₂e in 2020.     

Implications of delayed actions in addressing carbon dioxide emission reduction in the context of geo-engineering

Carbon dioxide emissions need to be reduced well below current emissions if atmospheric concentrations are to be stabilised at a level likely to avoid dangerous climate change. We investigate how delays in reducing CO₂ emissions affect stabilisation scenarios leading to overshooting of a target concentration pathway. We show that if geo-engineering alone is used to compensate for the delay in reducing CO₂ emissions, such an option needs to be sustained for centuries even though the period of overshooting emissions may only last for a few decades. If geo-engineering is used for a shorter period, it has to be associated with emission reductions significantly larger than those required to stabilise CO₂ without overshooting the target. In the presence of a strong climate–carbon cycle feedback the required emission reductions are even more drastic.     

Estimated supply of RED credits 2011–2035

Previous attempts to estimate the supply of greenhouse gas emission reductions from reduced emissions from deforestation (RED) have generally failed to incorporate policy developments, country-specific abilities and political willingness to supply offsets for developed countries’ emissions. To address this, we estimate policy-appropriate projections of creditable emission reductions from RED. Two global forest carbon models are used to examine major assumptions affecting the generation of credits. The results show that the estimated feasible supply of RED credits is significantly below the biophysical mitigation potential from deforestation. A literature review identified an annual RED emission reduction potential between 1.6 and 4.3 Gt CO₂e. Feasible RED supply estimates applying the OSIRIS model were 1.74 Gt CO₂e annually between 2011 and 2020, with a cumulative supply of 17.4 Gt CO₂e under an ‘own-efforts’ scenario. Estimates from the Forest Carbon Index were very low at $5/t CO₂e with 8 million tonne CO₂e annually, rising to 1.8 Gt CO₂e at $20/t CO₂e. Cumulative abatement between 2011 and 2020 was 9 billion Gt CO₂e ($20/t CO₂e). These volumes were lower, sometimes dramatically, at prices of $5/t CO₂e suggesting a non-linear supply of credits in relation to price at a low payment level. For policy makers, the results suggest that inclusion of RED in a climate framework increases abatement potential, although significant constraints are imposed by political and technical issues.     

1981—2019年全球陆地生态系统碳通量变化特征及其驱动因子

陆地生态系统碳汇显著降低大气CO₂浓度上升和全球变暖的速率,受人类活动和气候变化的影响,陆地生态系统碳通量具有强烈的时空变化,其估算结果仍存在较大的不确定性,不同因子的贡献尚不清晰。为此,利用遥感驱动的陆地生态系统过程模型BEPS模拟分析了1981—2019年全球陆地生态系统碳通量的时空变化特征,评价了大气CO₂浓度、叶面积指数（Leaf Area Index,LAI）、氮沉降、气候变化对全球陆地生态系统碳收支变化的贡献。1981—2019年全球陆地生态系统总初级生产力（Gross Primary Productivity,GPP）、净初级生产力（Net Primary Productivity,NPP）和净生态系统生产力（Net Ecosystem Productivity,NEP）的平均值分别为115.3、51.3和2.7 Pg·a^-1（以碳质量计,下同）,上升速率分别为0.47、0.21和0.06 Pg·a^-1。全球大部分区域GPP和NPP显著增加,NEP显著上升（p<0.05）的区域明显少于GPP和NPP。1981—2019年,全球NEP累积为105.2 Pg,森林、稀树草原及灌木、农田和草地的贡献分别为76.4、15.8、9.4和3.6 Pg。CO₂浓度、LAI、氮沉降和气候变化各自对NEP的累积贡献分别为58.4、20.6、0.7和-43.6 Pg,全部4个因子变化对NEP的累积贡献为39.8 Pg,其中CO₂浓度上升是近40 a全球陆地生态系统NEP上升的主要贡献因子,其次为LAI。     

The leaky sink: persistent obstacles to a forest carbon sequestration program based on individual projects

One strategy for mitigating the increase in atmospheric carbon dioxide is to expand the size of the terrestrial carbon sink, particularly forests, essentially using trees as biological scrubbers. Within relevant ranges of carbon abatement targets, augmenting carbon sequestration by protecting and expanding biomass sinks can potentially make large contributions at costs that are comparable or lower than for emission source controls. The Kyoto protocol to the framework convention on climate change includes many provisions for forest and land use carbon sequestration projects and activities in its signatories’ overall greenhouse gas mitigation plans. In particular, the protocol provides a joint implementation provision and a clean development mechanism that would allow nations to claim credit for carbon sequestration projects undertaken in cooperation with other countries. However, there are many obstacles for implementing an effective program of land use change and forestry carbon credits, especially measurement challenges. This paper explains the difficulty that even impartial analysts have in assessing the carbon offset benefits of projects. When these measurement challenges are combined with self-interest, asymmetries of information, and large numbers, it prevents to a project-based forest and land use carbon credit program may be insurmountable.     

Millennial timescale carbon cycle and climate change in an efficient Earth system model

A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8–3.3°C and predict atmospheric CO₂ close to present observations. Six idealized total fossil fuel CO₂ emissions scenarios are used to explore a range of 1,100–15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO₂ approaches equilibrium in year 3000 at 420–5,660 ppmv, giving 1.5–12.5°C global warming. The ocean is a robust carbon sink of up to 6.5 GtC year⁻¹. Under ‘business as usual’, the land becomes a carbon source around year 2100 which peaks at up to 2.5 GtC year⁻¹. Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6°C local warming is only briefly exceeded if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7 m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4–10 m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in two out of three model versions, but only under extreme emissions including exotic fossil fuel resources.     

Sink Potential of Canadian Agricultural Soils

Net greenhouse gas (GHG) emissions from Canadian crop and livestock production were estimated for 1990, 1996 and 2001 and projected to 2008. Net emissions were also estimated for three scenarios (low (L), medium (M) and high (H)) of adoption of sink enhancing practices above the projected 2008 level. Carbon sequestration estimates were based on four sink-enhancing activities: conversion from conventional to zero tillage (ZT), reduced frequency of summerfallow (SF), the conversion of cropland to permanent cover crops (PC), and improved grazing land management (GM). GHG emissions were estimated with the Canadian Economic and Emissions Model for Agriculture (CEEMA). CEEMA estimates levels of production activities within the Canadian agriculture sector and calculates the emissions and removals associated with those levels of activities. The estimates indicate a decline in net emissions from 54 Tg CO₂–Eq yr^–1 in1990 to 52 Tg CO₂–Eq yr^–1 in 2008. Adoption of thesink-enhancing practices above the level projected for 2008 resulted in further declines in emissions to 48 Tg CO₂–Eq yr^–1 (L), 42 TgCO₂–Eq yr^–1 (M) or 36 Tg CO₂–Eq yr^–1 (H). Among thesink-enhancing practices, the conversion from conventional tillage to ZT provided the largest C sequestration potential and net reduction in GHG emissions among the scenarios. Although rates of C sequestration were generally higher for conversion of cropland to PC and adoption of improved GM, those scenarios involved smaller areas of land and therefore less C sequestration. Also, increased areas of PC were associated with an increase in livestock numbers and CH₄ and N₂O emissions from enteric fermentation andmanure, which partially offset the carbon sink. The CEEMA estimates indicate that soil C sinks are a viable option for achieving the UNFCCC objective of protecting and enhancing GHG sinks and reservoirs as a means of reducing GHG emissions (UNFCCC, 1992).     

Competitiveness of terrestrial greenhouse gas offsets: are they a bridge to the future?

Activities to reduce net greenhouse gas emissions by biological soil or forest carbon sequestration predominantly utilize currently known, readily implementable technologies. Many other greenhouse gas emission reduction options require future technological development or must wait for turnover of capital stock. Carbon sequestration options in soils and forests, while ready to go now, generally have a finite life, allowing use until other strategies are developed. This paper reports on an investigation of the competitiveness of biological carbon sequestration from a dynamic and multiple strategy viewpoint. Key factors affecting the competitiveness of terrestrial mitigation options are land availability and cost effectiveness relative to other options including CO₂ capture and storage, energy efficiency improvements, fuel switching, and non-CO₂ greenhouse gas emission reductions. The analysis results show that, at lower CO₂ prices and in the near term, soil carbon and other agricultural/forestry options can be important bridges to the future, initially providing a substantial portion of attainable reductions in net greenhouse gas emissions, but with a limited role in later years. At higher CO₂ prices, afforestation and biofuels are more dominant among terrestrial options to offset greenhouse gas emissions. But in the longer run, allowing for capital stock turnover, options to reduce greenhouse gas emissions from the energy system and biofuels provide an increasing share of potential reductions in total US greenhouse gas emissions.     

The Consequences of CO2 Stabilisation for the Impacts of Climate Change

This paper reports the main results of an assessment of the global-scale implications of the stabilisation of atmospheric CO₂ concentrations at 750 ppm (by 2250) and 550 ppm (by 2150), in relationto a scenario of unmitigated emissions. The climate change scenarios were derived from simulation experiments conducted with the HadCM2 global climate model and forced with the IPCC IS92a, S750 and S550 emissions scenarios. The simulated changes in climate were applied to an observed global baseline climatology, and applied with impacts models to estimate impacts on natural vegetation, water resources, coastal flood risk and wetland loss, crop yield and food security, and malaria. The studies used a single set of population and socio-economic scenarios about the future that are similar to those adopted in the IS92a emissions scenario.An emissions pathway which stabilises CO₂ concentrations at 750 ppmby the 2230s delays the 2050 temperature increase under unmitigated emissions by around 50 years. The loss of tropical forest and grassland which occurs by the 2050s under unmitigated emissions is delayed to the 22nd century, and the switch from carbon sink to carbon source is delayed from the 2050s to the 2170s. Coastal wetland loss is slowed. Stabilisation at 750 ppm generally has relatively little effect on the impacts of climate change on water resource stress, and populations at risk of hunger or falciparum malaria until the 2080s.A pathway which stabilises CO₂ concentrations at 550 ppm by the 2170s delays the 2050 temperature increase under unmitigated emissions by around 100 years. There is no substantial loss of tropical forest or grassland, even by the 2230s, although the terrestrial carbon store ceases to act as a net carbon sink by around 2170 (this time because the vegetation has reached a new equilibrium with the atmosphere). Coastal wetland loss is slowed considerably, and the increase in coastal flood risk is considerably lower than under unmitigated emissions. CO₂ stabilisation at 550 ppm reduces substantially water resource stress, relative to unmitigated emissions, but has relatively little impact on populations at risk of falciparum malaria, and may even cause more people to be at risk of hunger. While this study shows that mitigation avoids many impacts, particularly in the longer-term (beyond the 2080s), stabilisation at 550 ppm appears to be necessary to avoid or significantly reduce most of the projected impacts in the unmitigated case.     

The contribution of forest carbon credit projects to addressing the climate change challenge

This article addresses the question of how forestry projects, given the recently improved standards for the accounting of carbon sequestration, can benefit from existing and emerging carbon markets in the world. For a long time, forestry projects have been set up for the purpose of generating carbon credits. They were surrounded by uncertainties about the permanence of carbon sequestration in trees, potential replacement of deforestation due to projects (leakage), and how and what to measure as sequestered carbon. Through experience with Joint Implementation (JI) and Clean Development Mechanism (CDM) forestry projects, albeit limited, and with forestry projects in voluntary carbon markets, considerable improvements have been made with accounting of carbon sequestration in forests, resulting in a more solid basis for carbon credit trading. The scope of selling these credits exists both in compliance markets, although currently with strong limitations, and in voluntary markets for offsetting emissions with carbon credits. Improved carbon accounting methods for forestry investments can also enhance the scope for forestry in the Nationally Determined Contributions (NDCs) that countries must prepare under the Paris Agreement.

POLICY RELEVANCE

This article identifies how forestry projects can contribute to climate change mitigation. Forestry projects have addressed a number of challenges, like reforestation, afforestation on degraded lands, and long-term sustainable forest management. An interesting new option for forestry carbon projects could be the NDCs under the Paris Agreement in December 2015. Initially, under CDM and JI, the number of forestry projects was far below that for renewable energy projects. With the adoption of the Paris Agreement, both developed and developing countries have agreed on NDCs for country-specific measures on climate change mitigation, and increased the need for investing in new measures. Over the years, considerable experience has been built up with forestry projects that fix CO₂ over a long-term period. Accounting rules are nowadays at a sufficient level for the large potential of forestry projects to deliver a reliable, additional contribution towards reducing or halting emissions from deforestation and forest degradation activities worldwide.     

A simple carbon cycle representation for economic and policy analyses

Integrated Assessment Models (IAMs) that couple the climate system and the economy require a representation of ocean CO₂ uptake to translate human-produced emissions to atmospheric concentrations and in turn to climate change. The simple linear carbon cycle representations in most IAMs are not however physical at long timescales, since ocean carbonate chemistry makes CO₂ uptake highly nonlinear. No linearized representation can capture the ocean’s dual-mode behavior, with initial rapid uptake and then slow equilibration over ∽10,000 years. In a business-as-usual scenario followed by cessation of emissions, the carbon cycle in the 2007 version of the most widely used IAM, DICE (Dynamic Integrated model of Climate and the Economy), produces errors of ∽2^°C by the year 2300 and ∽6^°C by the year 3500. We suggest here a simple alternative representation that captures the relevant physics and show that it reproduces carbon uptake in several more complex models to within the inter-model spread. The scheme involves little additional complexity over the DICE model, making it a useful tool for economic and policy analyses.