D. Koch

Present-Day Atmospheric Simulations Using GISS ModelE: Comparison to In Situ, Satellite, and Reanalysis Data

Abstract A full description of the ModelE version of the Goddard Institute for Space Studies (GISS) atmospheric general circulation model (GCM) and results are presented for present-day climate simulations (ca. 1979). This version is a complete rewrite of previous models incorporating numerous improvements in basic physics, the stratospheric circulation, and forcing fields. Notable changes include the following: the model top is now above the stratopause, the number of vertical layers has increased, a new cloud microphysical scheme is used, vegetation biophysics now incorporates a sensitivity to humidity, atmospheric turbulence is calculated over the whole column, and new land snow and lake schemes are introduced. The performance of the model using three configurations with different horizontal and vertical resolutions is compared to quality-controlled in situ data, remotely sensed and reanalysis products. Overall, significant improvements over previous models are seen, particularly in upper-atmosphere te...

Improved Attribution of Climate Forcing to Emissions

All Together Now Deciding how to change emissions of polluting gases that affect climate through their radiative forcing properties requires that the quantitative impact of these emissions be understood. Most past calculations of this type have considered only the radiative forcing of the specific emission and its atmospheric lifetime. Shindell et al. (p. 716; see the Perspectives by Arneth et al. and by Parrish and Zhu) use sophisticated atmospheric chemical and climate modeling to determine how gas-aerosol interactions affect the radiative properties of the atmosphere, finding significant departures from the standard method for emissions of methane, carbon monoxide, and nitrogen oxides. These findings should help to optimize strategies for mitigating global warming by reducing anthropogenic emissions. Chemical interactions between atmospheric gases and aerosols modify the global warming impacts of emissions. Evaluating multicomponent climate change mitigation strategies requires knowledge of the diverse direct and indirect effects of emissions. Methane, ozone, and aerosols are linked through atmospheric chemistry so that emissions of a single pollutant can affect several species. We calculated atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions in a coupled composition-climate model. We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol. Thus, assessments of multigas mitigation policies, as well as any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions.

Tropospheric sulfur simulation and sulfate direct radiative forcing in the Goddard Institute for Space Studies general circulation model

Global simulations of tropospheric sulfur are performed in the Goddard Institute for Space Studies (GISS) general circulation model (GCM) and used to calculate anthropogenic sulfate direct radiative forcing. Prognostic species are in-cloud oxidant H2O2, dimethylsulfide (DMS), methanesulfonic acid (MSA), SO2 and sulfate. Compared with most previous models (except others with prognostic H2O2), this model has relatively high anthropogenic SO2 and sulfate burden. We show that this is due partly to the depletion of the prognostic H2O2 and that moist convection delivers significant levels of SO2 to the free troposphere in polluted regions. Model agreement with surface observations is not remarkably different from previous studies. Following some previous studies, we propose that an additional in-cloud or heterogeneous oxidant is likely to improve the simulation near the surface. Our DMS source is lower than sources in previous studies, and sulfur values in remote regions are generally lower than those observed. Because of the high flux of SO2 to the free troposphere and the relatively low natural source, our model indicates a larger global anthropogenic contribution to the sulfate burden (77%) than was estimated by previous global models. Additional high-altitude observations of the sulfur species are needed for model validation and resolution of this issue. Direct radiative forcing calculations give an annual average anthropogenic sulfate forcing of −0.67 W/m2. We compare the radiative forcings due to online (hourly varying) versus offline (monthly average) sulfate and find little difference on a global average, but we do find differences as great as 10% in some regions. Thus, for example, over some polluted continental regions the forcing due to offline sulfate exceeds that of online sulfate, while over some oceanic regions the online sulfate forcing is larger. We show that these patterns are probably related to the correlation between clouds and sulfate, with positive correlations occuring over some polluted continental regions and negative correlations over high-latitude oceanic regions.

Global concentrations of tropospheric sulfate, nitrate, and ammonium aerosol simulated in a general circulation model

Global sulfate aerosol composition is simulated online in the Goddard Institute for Space Studies general circulation model II′ (GISS GCM II-prime). Four sulfur species, hydrogen peroxide, gas phase ammonia, and particulate ammonium are the prognostic tracer species, the emissions, transport, and deposition of which are explicitly simulated. Nitric acid fields are prescribed based on a global chemical transport model. An online thermodynamic equilibrium calculation determines the partitioning of ammonia and nitrate between gas and aerosol phases, and the quantity of aerosol water based on the temperature, relative humidity, and sulfate concentration in each GCM grid cell. The total global burden of sulfate, nitrate, ammonium, and aerosol water is 7.5 Tg and is most sensitive to changes in sulfur emissions. Tropospheric lifetimes for ammonium and ammonia are 4.2 and 0.9 days, respectively; the tropospheric ammonium burden is 0.30 Tg N, compared with 0.14 Tg N for ammonia. Simulated ammonium concentrations are generally within a factor of 2 of observations. Subgrid variability in measured concentrations hinders comparison of observations to predictions. Ammonium nitrate aerosol plays an important role in determining total aerosol mass in polluted continental areas. In the upper troposphere and near the poles, cold temperatures allow unneutralized nitric acid to condense into the aerosol phase. Acidic aerosol species tend to be neutralized by ammonia to a greater degree over continents than over oceans. The aerosol is most basic and gas phase ammonia concentrations are highest over India. Water uptake per mole of sulfate aerosol varies by two orders of magnitude because of changes in relative humidity and aerosol composition. Spatial variations in aerosol composition and water uptake have implications for direct and indirect aerosol radiative forcing.

General circulation model assessment of direct radiative forcing by the sulfate-nitrate-ammonium-water inorganic aerosol system

An on-line simulation of aerosol sulfate, nitrate, ammonium, and water in the Goddard Institute for Space Studies general circulation model (GCM II-prime) has been used to estimate direct aerosol radiative forcing for the years 1800, 2000, and 2100. This is the first direct forcing estimate based on the equilibrium water content of a changing SO42−-NO3−-NH4+ mixture and the first estimate of nitrate forcing based on a global model of nitrate aerosol. Present-day global and annual average anthropogenic direct forcing is estimated to be −0.95 and −0.19 W/m2 for sulfate and nitrate, respectively. Simulations with a future emissions scenario indicate that nitrate forcing could increase to −1.28 W/m2 by 2100, while sulfate forcing declines to −0.85 W/m2. This result shows that future estimates of aerosol forcing based solely on predicted sulfate concentrations may be misleading and that the potential for significant concentrations of ammonium nitrate needs to be considered in estimates of future climate change. Calculated direct aerosol forcing is highly sensitive to the model treatment of water uptake. By calculating the equilibrium water content of a SO42−-NH4+ aerosol mixture and the optical properties of the wet aerosol, we estimate a forcing that is almost 35% greater than that derived from correcting a low relative humidity scattering coefficient with an empirical f(RH) factor. The discrepancy stems from the failure of the empirical parameterization to adequately account for water uptake above about 90% relative humidity. These results suggest that water uptake above 90% RH may make a substantial contribution to average direct forcing, although subgrid-scale variability makes it difficult to represent humid areas in a GCM.

Transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM

We simulate the major anthropogenic aerosols, sulfate, organic carbon and black carbon, in the Goddard Institute for Space Studies General Circulation Model (GISS GCM), and examine their transport, relative abundances, and direct radiative forcing. Both present-day and projected future emissions are used, as provided by the IPCC SRES (A2) scenarios for 2030 and 2100. We consider the sensitivity of the black carbon distribution to the treatment of its solubility and allow solubility to depend upon exposure to gas phase production of sulfuric acid (case S), or time (case A), or a fixed rate (case C). We show that all three approaches can be tuned to give reasonable agreement with present-day observations. However, case S has higher black carbon in the arctic winter, owing to reduced SO2 oxidation and black carbon solubility. This improves upon the arctic deficiency in previous models, though may be somewhat excessive in this model. We also show that with a different ratio of sulfur/carbonaceous emissions, the case S mechanism can give significantly different results compared to the other mechanisms. Thus, in the 2100 simulation, with reduced sulfur and increased black carbon emissions, the black carbon burden is ∼13% higher and the direct radiative forcing is ∼40% higher in case S compared to the other cases. We consider the relative abundances of carbonaceous and sulfate aerosols in different regions. In the current simulations, carbonaceous aerosols exceed sulfate at the surface in Asia and much of Europe and throughout the column in biomass burning regions. We show that the model ratio of carbonaceous to sulfate aerosols increases with altitude over many oceanic regions, especially in summertime, as was observed during the Tropospheric Aerosol Radiative Forcing Observational Experiment campaign; however, over land and during other seasons the ratio generally decreases with altitude. The (present day) direct radiative forcings for externally mixed (case A) black carbon, organic carbon, and sulfate are calculated to be 0.35, −0.30, and −0.65 W/m2, respectively. In the 2100 simulation these forcings are 0.89, −0.64, and −0.54 W/m2, respectively. The net anthropogenic aerosol global average forcing seasonality inverts between the current and future simulations: the forcing is most negative in (Northern Hemisphere) summertime in 2000 but is least negative or even positive during (NH) summer in 2100; this inversion is more extreme in the Northern Hemisphere.

Global atmospheric black carbon inferred from AERONET

AERONET, a network of well calibrated sunphotometers, provides data on aerosol optical depth and absorption optical depth at >250 sites around the world. The spectral range of AERONET allows discrimination between constituents that absorb most strongly in the UV region, such as soil dust and organic carbon, and the more ubiquitously absorbing black carbon (BC). AERONET locations, primarily continental, are not representative of the global mean, but they can be used to calibrate global aerosol climatologies produced by tracer transport models. We find that the amount of BC in current climatologies must be increased by a factor of 2–4 to yield best agreement with AERONET, in the approximation in which BC is externally mixed with other aerosols. The inferred climate forcing by BC, regardless of whether it is internally or externally mixed, is ≈1 W/m2, most of which is probably anthropogenic. This positive forcing (warming) by BC must substantially counterbalance cooling by anthropogenic reflective aerosols. Thus, especially if reflective aerosols such as sulfates are reduced, it is important to reduce BC to minimize global warming.

Attribution of climate forcing to economic sectors

A much-cited bar chart provided by the Intergovernmental Panel on Climate Change displays the climate impact, as expressed by radiative forcing in watts per meter squared, of individual chemical species. The organization of the chart reflects the history of atmospheric chemistry, in which investigators typically focused on a single species of interest. However, changes in pollutant emissions and concentrations are a symptom, not a cause, of the primary driver of anthropogenic climate change: human activity. In this paper, we suggest organizing the bar chart according to drivers of change—that is, by economic sector. Climate impacts of tropospheric ozone, fine aerosols, aerosol-cloud interactions, methane, and long-lived greenhouse gases are considered. We quantify the future evolution of the total radiative forcing due to perpetual constant year 2000 emissions by sector, most relevant for the development of climate policy now, and focus on two specific time points, near-term at 2020 and long-term at 2100. Because sector profiles differ greatly, this approach fosters the development of smart climate policy and is useful to identify effective opportunities for rapid mitigation of anthropogenic radiative forcing.

Vertical transport of tropospheric aerosols as indicated by 7Be and 210Pb in a chemical tracer model

We use the natural radionuclides 7 Be and 210 Pb as aerosol tracers in a three-dimensional chemical tracer model (based on the Goddard Institute for Space Studies general circulation model (GCM) 2) in order to study aerosol transport and removal in the troposphere. Beryllium 7, produced in the upper troposphere and stratosphere by cosmic rays, and 210 Pb, a decay product of soil-derived 222 Rn, are tracers of upper and lower tropospheric aerosols, respectively. Their source regions make them particularly suitable for the study of vertical transport processes. Both tracers are removed from the troposphere primarily by precipitation and are useful for testing scavenging parameterizations. In particular, model convection must properly transport and scavenge both ascending 210 Pb and descending 7 Be. The ratio 7 Be/ 210 Pb cancels most model errors associated with precipitation and serves as an indicator of vertical transport. We show that over land the annual average 7 Be/ 210 Pb ratio for surface concentrations and deposition fluxes vary little globally. In contrast, the seasonal variability of the 7 Be/ 210 Pb concentration ratio over continents is quite large ; the ratio peaks in summer when convective activity is maximum. The model overestimates 7 Be in the tropics, a problem which we relate to flaws in the GCM parameterization of wet convection (excessive convective mass fluxes and no allowance for entrainment). The residence time of tropospheric 7 Be calculated by the model is 23 days, in contrast with a value of about 9 days calculated for 210 Pb, reflecting the high-altitude versus low-altitude source regions of these two tracers.

Global impacts of aerosols from particular source regions and sectors

Calculated direct anthropogenic radiative forcings are � 0.29, � 0.06, and 0.24 W m � 2 for sulfate, organic, and black carbon, respectively. The largest BC radiative forcings are from residential (0.09 W m � 2 ) and transport (0.06 W m � 2 ) sectors, making these potential targets to counter global warming. However, scattering components within these sectors reduce these to 0.04 and 0.03 W m � 2 , respectively. Most anthropogenic sulfate comes from power and industry sectors, and these sectors are responsible for the large negative aerosol forcings over the central Northern Hemisphere.