Direct Radiative Forcing Of Climate Research Articles

Impact of present and future aircraft NOxand aerosol emissions on atmospheric composition and associated direct radiative forcing of climate

Abstract. Aviation NOx emissions not only have an impact on global climate by changing ozone and methane levels but also contribute to the deterioration of local air quality. A new version of the LMDZ-INCA global model, including chemistry of both the troposphere and the stratosphere and the sulfate-nitrate-ammonium cycle, is applied to re-evaluate the impact of aircraft NOx and aerosol emissions on climate. The results confirm that the efficiency of NOx to produce ozone is very much dependent on the injection height; it increases with the background methane and NOx concentrations and with decreasing aircraft NOx emissions. The methane lifetime variation is less sensitive to the location of aircraft NOx emissions than the ozone change. The net NOx radiative forcing (RF) (O3+CH4) is largely affected by the revised CH4 RF formula. The ozone positive forcing and the methane negative forcing largely offset each other, resulting in a slightly positive forcing for the present day. However, in the future, the net forcing turns to negative, essentially due to higher methane background concentrations. Additional RFs involving particle formation arise from aircraft NOx emissions since the increased hydroxyl radical (OH) concentrations are responsible for an enhanced conversion of SO2 to sulfate particles. Aircraft NOx emissions also increase the formation of nitrate particles in the lower troposphere. However, in the upper troposphere, increased sulfate concentrations favour the titration of ammonia leading to lower ammonium nitrate concentrations. The climate forcing of aircraft NOx emissions is likely to be small or even switch to negative (cooling), depending on atmospheric NOx or CH4 future background concentrations, or when the NOx impact on sulfate and nitrate particles is considered. However, large uncertainties remain for the NOx net impact on climate and in particular on the indirect forcings associated with aerosols, which are even more uncertain than the other forcings from gaseous species. Hence, additional studies with a range of models are needed to provide a more consolidated view. Nevertheless, our results suggest that reducing aircraft NOx emissions is primarily beneficial for improving air quality.

A global model simulation of present and future nitrate aerosols and their direct radiative forcing of climate

Abstract. The ammonia cycle and nitrate particle formation are introduced into the LMDz-INCA (Laboratoire de Météorologie Dynamique, version 4 – INteraction with Chemistry and Aerosols, version 3) global model. An important aspect of this new model is that both fine nitrate particle formation in the accumulation mode and coarse nitrate forming on existing dust and sea-salt particles are considered. The model simulates distributions of nitrates and related species in agreement with previous studies and observations. The calculated present-day total nitrate direct radiative forcing since the pre-industrial is −0.056 W m−2. This forcing corresponds to 18% of the sulfate forcing. Fine particles largely dominate the nitrate forcing, representing close to 90% of this value. The model has been used to investigate the future changes in nitrates and direct radiative forcing of climate based on snapshot simulations for the four representative concentration pathway (RCP) scenarios and for the 2030, 2050, and 2100 time horizons. Due to a decrease in fossil fuel emissions in the future, the concentration of most of the species involved in the nitrate–ammonium–sulfate system drop by 2100 except for ammonia, which originates from agricultural practices and for which emissions significantly increase in the future. Despite the decrease of nitrate surface levels in Europe and North America, the global burden of accumulation mode nitrates increases by up to a factor of 2.6 in 2100. This increase in ammonium nitrate in the future arises despite decreasing NOx emissions due to increased availability of ammonia to form ammonium nitrate. The total aerosol direct forcing decreases from its present-day value of −0.234 W m−2 to a range of −0.070 to −0.130 W m−2 in 2100 based on the considered scenario. The direct forcing decreases for all aerosols except for nitrates, for which the direct negative forcing increases to a range of −0.060 to −0.115 W m−2 in 2100. Including nitrates in the radiative forcing calculations increases the total direct forcing of aerosols by a factor of 1.3 in 2000, by a factor of 1.7–2.6 in 2030, by 1.9–4.8 in 2050, and by 6.4–8.6 in 2100. These results show that the agricultural emissions of ammonia will play a key role in the future mitigation of climate change, with nitrates becoming the dominant contributor to the anthropogenic aerosol optical depth during the second half of the 21st century and significantly increasing the calculated aerosol direct forcing. This significant increase in the influence that nitrate exerts on climate in the future will at the same time affect regional air quality and nitrogen deposition to the ecosystem.

Negligible direct radiative forcing of basin‐scale climate by coccolithophore blooms

The water‐leaving radiance, defined as radiation from the sun reflected off particles in water and exiting the ocean surface back into the atmosphere and space, is often used to derive ocean‐colour information from remotely sensed data. However, it is in itself a measure of the amount of solar irradiance reflected by oceanic constituents and, therefore, not available to the Earth's heat reservoir (changes in which can affect the Earth's energy balance and climate). A strong influence on the water‐leaving radiance is observed from coccolithophore blooms, owing to the highly reflective calcareous platelets or coccoliths covering these marine algae. We analysed remotely sensed water‐leaving radiances (1998–1999) over the N. Atlantic, where the blooms are spatially and temporally most abundant, and found that the direct radiative forcing of climate between 402–565 nm (the major range of optical influence) by coccolithophores in this ocean is negligible (∼0.05 W m−2 mean annually). This is in contrast to what in situ or laboratory measurements on the immense local intensity of coccolithophore visible light scatter in the past two decades have led us to believe.

Challenges add up in quantifying radiative impact of mineral dust

Predicting the diverse radiative impact of mineral dust has several significant, related challenges. One is to quantify the complex spatial and temporal variations of dust burden. Another is to quantitatively relate the properties of dust and its radiative effects to dust life cycle in the atmosphere.The basic physics behind the processes governing the production, transport, and evolution of mineral aerosols are understood. But the state‐of‐the‐art climate and chemistry model predictions rely heavily on oversimplified parameterizations of some of these processes while ignoring others. As a result the magnitude and even the sign of dust net (solar plus IR) direct radiative forcing of climate remains unclear. Interdisciplinary research efforts to integrate modeling with the laboratory, in situ, and satellite measurements could provide a new perspective on improving quantification.

A Comparison of Climate Forcings Due to Chlorofluorocarbons and Carbon Monoxide

The direct radiative forcing of climate by carbon monoxide (CO) is generally considered to be negligible. However, a recent study of localised clear‐sky surface irradiances asserts that the forcing by CO may be comparable to that by CFC‐11. Nevertheless no detailed comparison of CO and CFC climate forcings has yet been made. Thus the present study estimates the radiative impact of the increases in CO, CFC‐11 and CFC‐12 that have occurred since industrialisation. A radiative transfer model is used to reproduce the results of the earlier study. Clouds are then added, and the stratosphere‐adjusted forcing at the tropopause (the “climate forcing”) calculated. Global‐mean anthropogenic climate forcing by CO is determined to be 32% of that by CFC‐11, 12% of that by CFC‐12, and 9% of that by the CFCs combined. Even if the contemporary global‐mean CO concentration is increased by a factor of three, the climate forcing by CO is still only 75% of that due to CFC‐11. Regarding instantaneous clear‐sky forcings, further analysis shows that surface measurements can give a misleading impression of effects at the tropopause. While the indirect effects of CO on climate change are not yet properly quantified, the direct radiative effects appear, as previously thought, to be of only minor significance.