Building on Paris: integrating nitrous oxide mitigation into future climate policy

Building on Paris: integrating nitrous oxide mitigation into future climate policy

David R Kanter

, Stephen M Ogle

and Wilfried Winiwarter

Nitrousoxide(N2O)isanimportantcontributortoclimatechange andstratosphericozonedepletionandyetitreceiveslittle attentionineithertheglobalclimateorozoneagreements.More concertedeffortstoaddressN2Ocouldbekeyinmeetingthe2C targetandasuiteofSustainableDevelopmentGoals.Thepast severalyearshasseenmajoradvancesinN2Oscienceand technology: ourabilityto estimate and simulate current and future N2Oemissionshasimproved,andmoreeffectivemitigation practicesandtechnologiescontinuetoarriveonthemarket.

Moreover,nitrogen’suniquechemistrymeansthatreducingN2O emissionscouldsimultaneouslyaddressanumberofother environmentalthreatsexacerbatedbyNlosses,further enhancingthecost-effectivenessofmitigation.Consequently, future NationalDetermined Contributionsunder theParisClimate Agreementcouldusethisnewknowledgetodevelopnational N2Otargetsthatwouldhelptheinternationalcommunitymeetits climateandsustainabledevelopmentcommitments.

Addresses

1DepartmentofEnvironmentalStudies,NewYorkUniversity,285Mer- cerStreet,7thFloor,NewYork,NY,10003,USA

2NaturalResourceEcologyLaboratory,ColoradoStateUniversity, CampusDelivery1499,FortCollins,CO,80523,USA

3DepartmentofEcosystemScienceandSustainability,ColoradoState University,FortCollins,CO,80523,USA

4InternationalInstituteforAppliedSystemsAnalysis,Schlossplatz1,A- 2361Laxenburg,Austria

5InstituteofEnvironmentalEngineering,UniversityofZielonaGo´ra, Licealna9,PL65-417ZielonaGo´ra,Poland

Correspondingauthor:Kanter,DavidR(david.kanter@nyu.edu)

CurrentOpinioninEnvironmentalSustainability2020,47:7–12 ThisreviewcomesfromathemedissueonClimatechange,reactive nitrogen,foodsecurityandsustainableagriculture

EditedbyClemensScheer,DavidEPelsterandKlausButterbach- Bahl

ForacompleteoverviewseetheIssueandtheEditorial Availableonline1stJune2020

Received:20January2020;Accepted:20April2020 https://doi.org/10.1016/j.cosust.2020.04.005

1877-3435/TheAuthors.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBY-NC-NDlicense(http://creativecom- mons.org/licenses/by-nc-nd/4.0/).

Introduction

Nitrousoxide(N2O)posesaseriousthreattotheclimate and the stratospheric ozone layer. It is the third most abundantly emittedgreenhousegas(GHG)aftercarbon dioxide(CO2)andmethane(CH4),responsiblefor6%of CO2-equivalent emissions in 2014 [1]. N2O is also the largestremainingthreatto thestratosphericozone layer giventheglobalphase-outofchlorofluorocarbons(CFCs) andotherozonedepletingsubstances[2].Itsatmospheric abundance has increased steadily since the turn of the centuryatapproximately0.25%peryear.Thekeydriver isariseinreactivenitrogen(N)inthebiosphere(anyform of N other than atmospheric dinitrogen – N2), largely fromtheapplicationofsyntheticfertilizerandmanurefor foodproduction[3].OtherN2Oemissionssourcesinclude industry,energy,transportandwastewater[4].Ambitious N2O mitigation could avoid greenhouse gas emissions equivalentto5%–10%oftheremainingcarbonbudgetfor a2Cworld,andozonelossescomparabletothedepletion potentialoftheglobalstockofCFCsinoldrefrigerators, airconditioners,insulationfoams andotherunits [5,6].

However, despite growing acknowledgement of N2O’s importantcontributionstothesecriticalissues,ithasbeen largelyignoredinpolicycircles.Severalreasonsareoften cited:thedifficultyofmonitoringagriculturalemissions, whicharethedominantsourceofanthropogenicN2O;the lack of mitigation practices and technologies that are effective across multiple land-use types, climates and cultures; the mitigation costs compared to other GHG emission sources; the political power of farmer lobbies oftenresistanttoenvironmentalprotectionmeasures;and theessentialrolethatnitrogen(N)playsinfoodproduc- tion[7].Asaresult,N2Oisrarelydiscussedinnationalor international climate and ozone negotiations. Indeed, whileN2OisoneofsixGHGstargetedundertheUnited Nations Framework Convention on Climate Change (UNFCCC), most country plans (or Nationally Deter- mined Contributions, NDCs) submitted to the Paris Climate Agreement (signed on December 15, 2015;

entered into force November 4, 2016) specify broad measures that only tangentially affect N2O emissions.

Some Parties to the UNFCCC, such as the European Union,haveconsideredN2OaspartoftheiroverallGHG targetdevelopment[8],but only onecountry,Uruguay, includesexplicitmitigationtargetsfor N2O.

$OECDDisclaimer:Theopinionsexpressedandargumentsemployedinthispublicationarethesoleresponsibilityoftheauthorsanddonot necessarilyreflectthoseoftheOECDorofthegovernmentsofitsMembercountries.

And yet, addressing N2O wouldnot only deliver direct ozoneandclimatebenefits–betternitrogen(N)manage- mentpracticescouldalsoinfluencethemitigationpotential ofimportantstrategiessuchasbioenergyproductionand soilcarbonsequestration[4,9].Furthermore,improvingthe efficiencyof N use would decrease demandfor Haber–

BoschN,theindustrialsynthesisofammoniaattheheartof modernfertilizerproduction,andcouldthusreduceGHG emissionsconsiderably,giventhattheHaber–Boschpro- cessiscurrentlyresponsiblefor1.4%ofglobalCO2emis- sionsand1%ofglobalenergyconsumption[10].Finally, andperhapsmostimportantly,reducingreactiveNlosses andtheassociatedcascadeeffectofNintheenvironment meansthataddressingN2Owithaholisticapproachcould deliver significant local co-benefits from air and water pollutionabatementthatvastlyoutweightheglobalben- efitsfromaneconomicperspective[6,11].

Scientific research on N2O – from better constraining sources, to more accurate emission rates and effective mitigationtechnologies–hasadvancedconsiderablyover thepastdecade.Thisarticledescribesthelatestpolicy- relevant advances, their implications for climate policy development,andwhatamorefocusedapproachtoN2O infutureNDCs couldhelpachieve.

Policy-relevantadvances inN2O research The policy-relevant advances in N2O research can be compiledintofourcategories:emissionfactors,modeling, mitigationmeasuresandassessments.

Emissionfactors

Emission factors (EFs) can indirectly estimate green- house gas emissions from a range of production and consumption data, usually at national or international scales.Theresultingemissionsdataarepartofnational GHGinventories,whichprovidethebasisforreporting and communication of emission reductions to the UNFCCC. In the case of agricultural N2O emissions, annual synthetic fertilizer and manure production and consumptiondata are multiplied byan EFto estimate annualemission fluxes. Themostwidely usedEFsfor N2OweredevelopedbytheIntergovernmentalPanelon ClimateChange(IPCC)andassumealinearrelationship between N application rates and N2O emissions [12].

Forexample,a1%EFisappliedtosyntheticNfertilizer ratestoestimatedirectemissions(i.e.forevery100kgN applied,1kgofN2Oisemitted).However,recentstud- ies are finding nonlinear relationships, implying that N2O emissions per hectare are lower than the IPCC EFs at low N application rates, and higher at high N applicationrates–likelydrivenbyexcessNnottakenup bycrops, which canthen be emitted asN2O [13].For example,applyingtheIPCCTier1EFtoa50kgNha ¹ reduction in N application rate would generate an estimated reduction in N2O emissions of 0.5kg N2O-Nha ¹, regardless of the initial application rate.

However,using a nonlinear EF forupland grain crops derivedviameta-analysis,areductionfrom50kgNha ¹ tozerowouldreduceemissionsby0.37kgN2O-Nha ¹, whileareductionfrom300kgNha ¹to250kgNha ¹ would reduce emissions by 0.84kg N2O-Nha ¹, sug- gesting greater mitigation potential in regions with higherNapplicationrates[14].Thisnotonlyhasimpli- cationsforhowagriculturalN2Oemissionsareestimated innationalandregionalinventories,italsosuggeststhat inregionsoftheworldwheremanyfarmsapplyNatvery lowrates,suchassub-SaharanAfricaandpartsofEastern Europe, increases in N fertilizer use would generate relativelysmallincreasesinagriculturalN2Oemissions dependingontheagronomicpracticesandassociatedN useefficiencyofthecrops[15].Similarly,evenmoderate decreases in highly fertilized regions could trigger sig- nificantemissionsreductions.Nevertheless,itshouldbe notedthataddressingtheeffectofNapplicationratesto soils on N2O emissions remains challenging because most countries do not have census or survey data on thismetric,particularlyindevelopingcountries.Instead, the national total N added to soils is often the only availableinformation.

Another recent advance in EF development regards indirect emissions – N2O formed from other N com- pounds lost to the environment, namely nitrate (NO3 ), nitrogen oxides (NOx) and ammonia (NH3).

RecentstudiessuggestthatIPCCEFsforindirectemis- sionsarelow,especiallythe0.75%EFforindirectN2O from leached NO3 . One study in the U.S. Corn Belt estimatesanEFcloserto2%whichcouldtranslatetoan underestimationoftotalagriculturalN2Oemissionsinthe regionof upto40%[16].Infact,theIPCChasrecently raised the global default value to 1.6% for synthetic fertilizer application in wet climates [17], implying greater emissions than previously considered and even larger mitigation potentials. However, the IPCC also lowered the default value for drier climates to 0.5%, suggesting lower emissions and likely less mitigation potential in semi-arid and arid regions. Finally, several countrieshavenowdevelopedcountry-specificemission factors. This is particularly important in capturing nationalclimatic,agronomicandotherconditions[18,19].

Modeling

A major recent development in N2O modeling is the combination of a multi-inversion approach with an ensemble of surface observations to better constrain theregionalandtemporaldistributionofN2Oemissions.

Recentestimatessuggesttotal global N2Oemissions of 15.3–17.3(bottom-up) and 15.9–17.7 Tg N (top-down), demonstrating relatively close agreement [4,20]. Inver- sionapproacheshavealsoenabledmoreaccurateregional quantification of N2O emissions, showing good agree- mentbetweenEuropeaninventoriesandmeasurements [21,22] while highlighting underestimates for North

Americaninventories[23,24].Arecent globalintercom- parison of inverse models confirms the utility of N2O emission factors as well as the continental-scale non- linear relationship between N application and N2O responsenotedin Section“Emission factors”[25].

Moredetailedterrestrialbiospheremodelsareabletogen- eratehighresolutionestimatesofland-basedN2Oemissions from anthropogenicandnaturalsources[26].In addition, advancesinsoilprocessmodellinghaveledtomoredetailed representationsofnitrificationanddenitrification,thebio- geochemical processes underpinningsoil N2O emissions, acrossmultipletemporalandspatialscales.Forexample,the UnitedStateshasreduceduncertaintyinnationalestimates ofagriculturalsoilN2Oemissionsfrom+184%/ 70%using IPCCdefaultemissionfactorsto+49%/ 33%byapplying process-basedmodels[27].Suchmodelshavebeenableto integrateeffectslikefreeze-thawcycles[28,29]–theomis- sion of whichcould leadto anunderestimationof global agriculturalN2Oemissions by17%–28%[30]–andnon- linearN2OemissionresponsestoNinputapplications[31].

Severalintegratedassessmentmodels–modelscombining biophysicalandeconomiccomponents–andcropmodels havebeenusedtoquantifyagriculturalN2Oemissionsfrom plottoglobalscales[32].However,bottom-upinventories formostnon-agriculturalsector-specificemissionsarestill basedonIPCCEFsduetolimiteddevelopmentofmore advancedmethodsforthesesources[33].

Modeling has also contributed towards a better under- standingof howfuturechangesin climatecouldimpact N2O emissions. Warmer and wetter conditions will enhance the conditions for soil N2O emissions, acting as apositivefeedbacktoclimatechange[34].Indeed, changes in precipitationalone are projectedto increase total N loadingto rivers by19% withinthecontinental UnitedStatesbytheendofthiscentury,withimportant implications for indirect N₂Oemissions. Offsetting this increasewouldrequirea33%reductioninNapplication rates [35]. Climate change is also expected to cause changesin land useand management,whichwill likely impact terrestrialbiogeochemical cycles. Anincrease in theareaofirrigatedagriculturallandcouldstimulateN2O emissions increases of 50%–150%, likely a result of increaseddenitrificationactivity[36,37].Studiesfocused on N cycling and CO2 fertilization estimate that the cumulative warmingeffectof methane(CH4)and N2O emissionsovertheperiod2001–2010wasafactoroftwo larger than the cooling effect that resulted from CO2

fertilization [38], suggesting that mitigation efforts should be as focused on reducing emissions of these non-CO2greenhousegasesas onincreasingcarbon stor- age capacity. Finally, a number of different emissions scenarios for N2O over the 21stcentury have provided policymakersandotherstakeholdersinsightintotherisks of no action and the potential benefits of ambitious mitigation[5].

Mitigationmeasures

N2Omitigationtechnologiesandpracticesexistacrossall sectors.Technologiesin non-agriculturalsectorssuchas transport,energy,andindustry(nitricandadipicproduc- tion) arealreadywell establishedand widely used, par- ticularly in OECDcountries,with mitigationpotentials greaterthan80%[5,39,40].Forexample,theEUEmis- sionsTradingSystemhasalreadyspurredthewide-scale adoption of N2O abatement technologies in nitric acid plants. The agricultural sector has traditionally lagged behind given its heterogeneity, high mitigation costs, more modest mitigation potential compared to other sectors, and a powerful political lobby [40,41]. For example,arecentanalysisofUScroplandestimatesthat a carbon price of 35 USD per tonne CO2 equivalent, which is relatively high compared to current market prices,wouldreduceN2Oemissionsbylessthan4%[42].

Andyet,recentstudiessuggesthowtheN2Omitigation potential of the agricultural sector can be unlocked. A numberofmeta-analysesevaluatingenhancedefficiency fertilizers (EEFs,whichinclude nitrificationand urease inhibitors as well as controlled-release fertilizers) have foundthemtoreducefield-levelN2Oemissionsbyupto 50%,whileboostingyieldsandN useefficiency(NUE) [7].Other mitigation options based on changesin farm practicesandtheadoptionof precisionagriculturetech- nologies,suchasapplyingfertilizersatoptimalrates,have thepotentialtoreducefield-levelN2Oemissionsbyupto 40% [5,40]. Furthermore, several disruptive technolo- gies couldfundamentally transformhowhumanitycon- tributes to the N cycle, including the development of

‘meatless’ meat and N-fixing cereals [43,44]. If widely adopted, these technologies could lead to significant reductionsinN2Oemissions[45].However,thereisstill much progress to be made in developing technologies uniquelyadaptedtospecificclimates,cropsandcultures.

Governments could therefore playan important role in technologydevelopment,akintowhattheU.S.,govern- menthasdonetospurthedevelopmentofmoreenviron- mentally friendly cars via fuel efficiency standards [7].

This could be particularly impactful in the fertilizer sector,giventheconservativeR&Dculturethatcurrently prevails.Indeed,oneofthemostimportantrippleeffects oftheParisClimateAgreementhasbeenthesignalsent to the marketplace that theinternational community is committedto addressingclimatechange[46].

Assessments

Since2011,fourmajorNassessmentshavebeenreleasedin Europe[47],theU.S.[48],California[49]andIndia[50], with the first international N assessment scheduled for release in 2022 under the auspices of the International NitrogenInitiative(INI)andtheInternational Nitrogen Management System (http://www.inms.international).

These assessmentsdiffer fromIPCC reportsgiventheir singular focus on N while providing an in-depth

examinationofitsparticularregion’s Nbudget,impacts, mitigation opportunities, future scenarios and policies.

Froma policy perspective,the EuropeanN assessment excelledinbeingoneofthefirsttoestimatedamagecosts foreachmajorNcompound[51],includingN2O,spurringa burgeoningliteratureonthistopic[52,53].Forexample,a recent cost–benefit analysis evaluating N2O mitigation options in the United States demonstrated that by 2030ifonlytheclimateandstratosphericozonebenefits fromavoidedN2Oemissionsareevaluated(estimatedtobe approximately $1.8 billion), the cost of action (approxi- mately $22 billion) would be prohibitively expensive.

However,ifthetotalavoided N pollutionisconsidered, thenthebenefits(approximately$91billion)wouldout- weighthecostsbyaratioofover4:1[54].Inadditiontothis neweconomicframing,theU.S.assessmentrecommended arangeof specificmanagementstrategies,andtheCali- forniaassessmenttookthisastepfurtherbydevelopinga policyevaluationframeworkuniquelysuitedtoN[55,48].

These assessments are not only an important informa- tionalresourcetopolicymakers;theyperformanimpor- tant political role by communicating a clear scientific consensus that can provide a basis and momentum for policy development [56]. Looking ahead, the Interna- tional Nitrogen Assessment will need to be genuinely multi-disciplinary, integrating social sciences into the framinganddevelopmentofthereportinordertoaddress the N issue in a way that is relevant to policymaker concerns–examiningtheobstaclestobetterNmanage- mentandabroaderpolicyapproachacrosstheentireagri- foodchain.ThisinturnwouldlikelyraiseN2O’sprofilein ozoneandclimatenegotiations.

Implicationsfor climatepolicy

Takentogether,thescientificadvances inemissionfac- tors,modeling,mitigationtechnologiesandassessments meanthatthenextroundofNDCs,aswellasotherfuture policy efforts to limit greenhouse gas emissions, could include targetsspecifically focusedonN2O. Thiscould helpmanycountries,particularlyoneswithlargeagricul- turalsectors,developmitigationpathwaysconsistentwith a 2C, and possibly a 1.5C world [57]. For example, countries with high agriculturalN surpluses like China couldsignificantlyreduceapplicationratesandthusN2O emissionswithlittletonoyieldpenalty[58].Inanother example,Uruguay’sfoodsectorisresponsibleforcloseto threequartersoftheirnationalGHGemissions,withN2O contributing one third of national emissions. Conse- quently, its first NDC set an economy-wide target of reducing N2O emissions intensity by 51%–57% below 1990levelsby2030,and37%–43%inthelivestocksector.

These targets are based on a technical evaluation of differentmitigationmeasures,whichincludetheimple- mentation of nutrient recovery technologies onat least 40% of dairy farms and the adoption of regenerative management and other N management techniques on

10% of grasslands [59]. Other approaches could adopt cost-benefitanalysis, backcasting (working backfrom a set target based on technical analysis and stakeholder consultationtodevelopapathwayforachievingit)and/or optimization tools such as the GAINS model [40,60].

Evenifagricultureisnotanespeciallyimportantcontrib- utortoacountry’stotalGHGemissions,targetsfornon- agriculturalN2Oemissionscouldbedevelopedbasedon the technical and economic information available for mitigationstrategies in the industry, energy,waste and transport sectors. N2O-specific national targets would enablemoretransparentcomparisonsofNDCs,andthus makeiteasiertoindependentlytracktheirprogressand potentiallyincentivizeincreasedambitionover time.

Finally, while it is understandable that countries may wish to maintain a basket approach and not set GHG- specifictargetsinordertomaintainflexibility,thecasefor N2Otargetsgoes beyondtheclimatebenefits, as noted above.Ifimplementedproperlywithaviewtowardsthe N cascade,N2Omitigation coulddeliver local environ- mentalbenefitswhoseeconomicvaluecouldvastlyout- weigh the climate benefits – a particularly important characteristic giventhecurrent politicalclimateof eco- nomicnationalismacrossmanymajorcountries[6].Fur- thermore,giventheextensivelinksbetweenNandthe Sustainable Development Goals – a set of social, eco- nomicandenvironmentaltargetsadoptedbytheUnited Nationsin2015–betterNmanagementassociatedwith N2Omitigationcouldhelpachieveseveralofthe17tar- gets,frommoreresponsibleproductionandconsumption toprotectinglifeonlandandin water[61].

Conclusion

Science and technology have advanced to a point that policymakerscouldincludespecificN2Omitigationstrat- egies in future climatepolicies. Emissions can be esti- matedrelativelyaccurately,particularlyatnationalscales, and targets can be developed and implemented cost- effectivelybasedontried-and-testedtechnologiesespe- ciallyifthelocalco-benefitsofmitigationaretakeninto account. Furthermore, the relationship between policy andscienceandtechnologyisatwo-waystreet: astrong signalfromthepolicycommunitythatitiscommittedto actingonN2Ocouldstimulatemore financialandintel- lectual resourcesbeing allocatedto this issue, and thus potentiallyacceleratethedevelopmentand deployment ofN2Omitigationoptions.Thiswouldbetruenotjustfor the scientific community, largely funded by public research programs, but also the R&D investments of theprivate sector, whosetechnologies could befurther tailored to specific crops, climates and cultures. Ulti- mately, humanity has an extremely small window in which to stay below a 2C global average temperature increase.More focusedaction onN2Ocouldhelp meet thesetargetswhileachievingarangeofothergoalsfrom biodiversityprotectiontoimprovedairandwaterquality.

Conflictof intereststatement Nothing declared.

Acknowledgement

Thisarticleevolvedfromaworkshoptitled“ClimateChange,Reactive Nitrogen,FoodSecurityandSustainableAgriculture”heldattheKarlsruhe InstituteofTechnologyinGarmisch-Partenkirchen,Germany,on15–16 April2019,andwhichwassponsoredbytheOECDCo-operativeResearch Programme:BiologicalResourceManagementforSustainableAgricultural Systemswhosefinancialsupportmadeitpossibleforoneormoreofthe authorstoparticipateintheworkshop.

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