434 Te European Nitrogen Assessment, ed. Mark A. Sutton, Clare M. Howard, Jan Willem Erisman, Gilles Billen, Albert Bleeker, Peringe Grennfelt, Hans van Grinsven and Bruna Grizzetti. Published by Cambridge University Press. © Cambridge University Press 2011, with sections © authors/European Union. Chapter Chapter Executive summary Nature of the problem Reactive nitrogen (N • r ) is of fundamental importance in biological and chemical processes in the atmosphere–biosphere system, altering the Earth’s climate balance in many ways. Tese include the direct and indirect emissions of nitrous oxide (N 2 O), atmospheric N r depos- ition and tropospheric ozone formation (O 3 ), both of which alter the biospheric CO 2 sink, N r supply efects on CH 4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NO x ) and ammonia (NH 3 ). Human production and release of N • r into the environment is thus expected to have been an important driver of European greenhouse bal- ance. Until now, no assessment has been made of how much of an efect European N r emissions are having on net warming or cooling. Approaches Tis chapter summarizes current knowledge of the role of N • r for global warming. Particular attention is given to the consequences of atmospheric N r emissions. Te chapter draws on inventory data and review of the literature to assess the contribution of anthropo- genic atmospheric N r emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe. Te use of N • r fertilizers has major additional efects on climate balance by allowing increased crop and feed production and larger popula- tions of livestock and humans, but these indirect efects are not assessed here. Key findings/state of knowledge Due to its multiple, complex efects on biospheric and atmospheric processes, the importance of N • r for the European greenhouse gas bal- ance has so far received insufcient attention. Te main warming efects of European anthropogenic N • r emissions are estimated to be from N 2 O (17 (15–19) mW/m 2 ) and from the reduction in the biospheric CO 2 sink by tropospheric O 3 (4.4 (2.3–6.6) mW/m 2 ). Te main cooling efects are estimated to be from increasing the biospheric CO 2 sink by atmospheric N r deposition at −19 (−30 to −8) mW/m 2 and by light scattering efects of N r contain- ing aerosol (−16.5 (−27.5 to −5.5) mW/m 2 ), in both cases resulting from emissions of NO x and NH 3 . Te production of O • 3 from European emissions of NO x is estimated to have a modest warming efect (2.9 (0.3–5.5) mW/m 2 ), which is largely ofset by the cooling efect of O 3 in reducing the atmospheric lifetime of CH 4 (−4.6 (−6.7 to −2.4) mW/m 2 ), giving an uncertain net warming of +1.7 (−6.4 to +3.1) mW/m 2 ). Overall, including all of these terms, European N • r emissions are estimated to have a net cooling efect, with the uncertainty bounds ranging from a substantial cooling efect to a small warming efect (−15.7 (−46.7 to +15.4) mW/m 2 ). Major uncertainties/challenges Te largest uncertainties concern the aerosol and N • r fertilization efects, and the estimation of the European contributions within the global context. Published estimates suggest that the default N • 2 O emission factor of 1% used by IPCC for indirect emissions from soils following N r depos- ition is too low by at least a factor of two. Te wider efects of fertilizer N • r , in allowing increased biospheric C cycling, food and feed production and populations of livestock and humans are a major uncertainty. Industrial production of N r can be considered as having permitted increased overall consumption (of food, feed and fuel) with major net warming efects. Tese interactions remain to be investigated. Chapter 19 Chapter Nitrogen as a threat to the European greenhouse balance Lead authors: Klaus Butterbach-Bahl, Eiko Nemitz and Sönke Zaehle Contrubuting authors: Gilles Billen, Pascal Boeckx, Jan Willem Erisman, Josette Garnier, Rob Upstill-Goddard, Michael Kreuzer, Oene Oenema, Stefan Reis, Martijn Schaap, David Simpson, Wim de Vries, Wilfried Winiwarter and Mark A. Sutton Executive summary Nature of the problem Reactive nitrogen (N • r ) is of fundamental importance in biological and chemical processes in the atmosphere–biosphere system, altering the Earth’s climate balance in many ways. Tese include the direct and indirect emissions of nitrous oxide (N 2 O), atmospheric N r depos- ition and tropospheric ozone formation (O 3 ), both of which alter the biospheric CO 2 sink, N r supply efects on CH 4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NO x ) and ammonia (NH 3 ). Human production and release of N • r into the environment is thus expected to have been an important driver of European greenhouse bal- ance. Until now, no assessment has been made of how much of an efect European N r emissions are having on net warming or cooling. Approaches Tis chapter summarizes current knowledge of the role of N • r for global warming. Particular attention is given to the consequences of atmospheric N r emissions. Te chapter draws on inventory data and review of the literature to assess the contribution of anthropo- genic atmospheric N r emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe. Te use of N • r fertilizers has major additional efects on climate balance by allowing increased crop and feed production and larger popula- tions of livestock and humans, but these indirect efects are not assessed here. Key findings/state of knowledge Due to its multiple, complex efects on biospheric and atmospheric processes, the importance of N • r for the European greenhouse gas bal- ance has so far received insufcient attention. Te main warming efects of European anthropogenic N • r emissions are estimated to be from N 2 O (17 (15–19) mW/m 2 ) and from the reduction in the biospheric CO 2 sink by tropospheric O 3 (4.4 (2.3–6.6) mW/m 2 ). Te main cooling efects are estimated to be from increasing the biospheric CO 2 sink by atmospheric N r deposition at −19 (−30 to −8) mW/m 2 and by light scattering efects of N r contain- ing aerosol (−16.5 (−27.5 to −5.5) mW/m 2 ), in both cases resulting from emissions of NO x and NH 3 . Te production of O • 3 from European emissions of NO x is estimated to have a modest warming efect (2.9 (0.3–5.5) mW/m 2 ), which is largely ofset by the cooling efect of O 3 in reducing the atmospheric lifetime of CH 4 (−4.6 (−6.7 to −2.4) mW/m 2 ), giving an uncertain net warming of +1.7 (−6.4 to +3.1) mW/m 2 ). Overall, including all of these terms, European N • r emissions are estimated to have a net cooling efect, with the uncertainty bounds ranging from a substantial cooling efect to a small warming efect (−15.7 (−46.7 to +15.4) mW/m 2 ). Major uncertainties/challenges Te largest uncertainties concern the aerosol and N • r fertilization efects, and the estimation of the European contributions within the global context. Published estimates suggest that the default N • 2 O emission factor of 1% used by IPCC for indirect emissions from soils following N r depos- ition is too low by at least a factor of two. Te wider efects of fertilizer N • r , in allowing increased biospheric C cycling, food and feed production and populations of livestock and r humans are a major uncertainty . Industrial production of N r can be considered as having permitted increased overall consumption (of food, feed and fuel) with major net warming efects. Tese interactions remain to be investigated.