Nadine Unger

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.

Multimodel simulations of carbon monoxide: Comparison with observations and projected near-future changes

We analyze present-day and future carbon monoxide (CO) simulations in 26 state-of-the-art atmospheric chemistry models run to study future air quality and climate change. In comparison with near-global satellite observations from the MOPITT instrument and local surface measurements, the models show large underestimates of Northern Hemisphere (NH) extratropical CO, while typically performing reasonably well elsewhere. The results suggest that year-round emissions, probably from fossil fuel burning in east Asia and seasonal biomass burning emissions in south-central Africa, are greatly underestimated in current inventories such as IIASA and EDGAR3.2. Variability among models is large, likely resulting primarily from intermodel differences in representations and emissions of nonmethane volatile organic compounds (NMVOCs) and in hydrologic cycles, which affect OH and soluble hydrocarbon intermediates. Global mean projections of the 2030 CO response to emissions changes are quite robust. Global mean midtropospheric (500 hPa) CO increases by 12.6 ± 3.5 ppbv (16%) for the high-emissions (A2) scenario, by 1.7 ± 1.8 ppbv (2%) for the midrange (CLE) scenario, and decreases by 8.1 ± 2.3 ppbv (11%) for the low-emissions (MFR) scenario. Projected 2030 climate changes decrease global 500 hPa CO by 1.4 ± 1.4 ppbv. Local changes can be much larger. In response to climate change, substantial effects are seen in the tropics, but intermodel variability is quite large. The regional CO responses to emissions changes are robust across models, however. These range from decreases of 10–20 ppbv over much of the industrialized NH for the CLE scenario to CO increases worldwide and year-round under A2, with the largest changes over central Africa (20–30 ppbv), southern Brazil (20–35 ppbv) and south and east Asia (30–70 ppbv). The trajectory of future emissions thus has the potential to profoundly affect air quality over most of the worlds populated areas.

Climate simulations for 1880–2003 with GISS modelE

We carry out climate simulations for 1880–2003 with GISS modelE driven by ten measured or estimated climate forcings. An ensemble of climate model runs is carried out for each forcing acting individually and for all forcing mechanisms acting together. We compare side-by-side simulated climate change for each forcing, all forcings, observations, unforced variability among model ensemble members, and, if available, observed variability. Discrepancies between observations and simulations with all forcings are due to model deficiencies, inaccurate or incomplete forcings, and imperfect observations. Although there are notable discrepancies between model and observations, the fidelity is sufficient to encourage use of the model for simulations of future climate change. By using a fixed well-documented model and accurately defining the 1880–2003 forcings, we aim to provide a benchmark against which the effect of improvements in the model, climate forcings, and observations can be tested. Principal model deficiencies include unrealistically weak tropical El Nino-like variability and a poor distribution of sea ice, with too much sea ice in the Northern Hemisphere and too little in the Southern Hemisphere. Greatest uncertainties in the forcings are the temporal and spatial variations of anthropogenic aerosols and their indirect effects on clouds.

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.

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.

Clean the air, heat the planet?

Measures to control emissions of air pollutants may have unintended climatic consequences. The push toward cleaner air in Beijing before the 2008 Olympic Games was a vivid reminder of the need to control air pollution, not only in Asia but in many regions of the world (1). There is mounting evidence for particle- and ozone-related health effects (2, 3). Furthermore, ozone and aerosol particles affect Earths radiation balance (4, 5): Many aerosols cool the atmosphere (a negative forcing), whereas ozone and black carbon aerosol have a warming effect (a positive forcing). There is thus a strong motivation for treating air pollution control and climate change in common policy frameworks (5, 6). However, recent model studies (7–9) have shown that changes in pollutant and precursor emissions, atmospheric burden, and radiative forcing are not necessarily proportional. Furthermore, as Shindell et al. report on page 716 of this issue, current models do not capture many of the complex atmospheric processes involving aerosols and reactive trace gases (10).

CMIP5 historical simulations (1850–2012) with GISS ModelE2

Observations of climate change during the CMIP5 extended historical period (1850-2012) are compared to trends simulated by six versions of the NASA Goddard Institute for Space Studies ModelE2 Earth System Model. The six models are constructed from three versions of the ModelE2 atmospheric general circulation model, distinguished by their treatment of atmospheric composition and the aerosol indirect effect, combined with two ocean general circulation models, HYCOM and Russell. Forcings that perturb the model climate during the historical period are described. Five-member ensemble averages from each of the six versions of ModelE2 simulate trends of surface air temperature, atmospheric temperature, sea ice and ocean heat content that are in general agreement with observed trends, although simulated warming is slightly excessive within the past decade. Only simulations that include increasing concentrations of long-lived greenhouse gases match the warming observed during the twentieth century. Differences in twentieth-century warming among the six model versions can be attributed to differences in climate sensitivity, aerosol and ozone forcing, and heat uptake by the deep ocean. Coupled models with HYCOM export less heat to the deep ocean, associated with reduced surface warming in regions of deepwater formation, but greater warming elsewhere at high latitudes along with reduced sea ice. All ensembles show twentieth-century annular trends toward reduced surface pressure at southern high latitudes and a poleward shift of the midlatitude westerlies, consistent with observations.

Climate response to projected changes in short-lived species under an A1B scenario from 2000-2050 in the GISS climate model

Climate response to projected changes in short-lived species under the A1B scenario from 2000-2050 in the GISS climate model Drew T. Shindell 1,2 , Greg Faluvegi 1,2 , Susanne E. Bauer 1,2 , Dorothy M. Koch 1,2 , Nadine Unger 3 , Surabi Menon 4 , Ron L. Miller 1,5 , Gavin A. Schmidt 1,2 , David G. Streets 6 NASA Goddard Institute for Space Studies, New York, NY Center for Climate Systems Research, Columbia University, New York, NY Department of Atmospheric Sciences, University of Vermont, Burlington, VT Lawrence Berkeley Laboratory, Berkeley, CA Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY Argonne National Laboratory, Argonne, IL Abstract We investigate the climate forcing from and response to projected changes in short-lived species and methane under the A1B scenario from 2000-2050 in the GISS climate model. We present a meta-analysis of new simulations of the full evolution of gas and aerosol species and other existing experiments with variations of the same model. The comparison highlights the importance of several physical processes in determining radiative forcing, especially the effect of climate change on stratosphere-troposphere exchange, heterogeneous sulfate-nitrate-dust chemistry, and changes in methane oxidation and natural emissions. However, the impact of these fairly uncertain physical effects is substantially less than the difference between alternative emission scenarios for all short-lived species. The net global mean annual average direct radiative forcing from the short-lived species is .02 W/m 2 or less in our projections, as substantial positive ozone forcing is largely offset by negative aerosol direct forcing. Since aerosol reductions also lead to a reduced indirect effect, the global mean surface temperature warms by ~0.07°C by 2030 and ~0.13°C by 2050, adding 19% and 17%, respectively, to the warming induced by long-lived greenhouse gases. Regional direct forcings are large, up to 3.8 W/m 2 . The ensemble-mean climate response shows little regional correlation with the spatial pattern of the forcing, however, suggesting that oceanic and atmospheric mixing generally overwhelms the effect of even large localized forcings. Exceptions are the polar regions, where ozone and aerosols may induce substantial seasonal climate changes. 1. Introduction While well-mixed greenhouse gases (WMGHGs) dominate both the radiative forcing since the preindustrial (PI) and the debate over global warming, short-lived species also play an important role. Hence it important to better quantify their contribution to climate change. Additionally, mitigation of climate change via controls on short-lived species has several attractive features. Many of the short-lived species, ozone and the aerosols, are pollutants that cause substantial harm to humans, crops and natural ecosystems. Thus controls on these radiatively active pollutants could provide health benefits in addition to climate change mitigation. In some cases, controls may be beneficial for health but detrimental for climate, a trade-off that needs to be considered carefully. The effects of

Future climate change under RCP emission scenarios with GISS ModelE2

We examine the anthropogenically forced climate response for the 21st century representative concentration pathway (RCP) emission scenarios and their extensions for the period 2101–2500. The experiments were performed with ModelE2, a new version of the NASA Goddard Institute for Space Sciences (GISS) coupled general circulation model that includes three different versions for the atmospheric composition components: a noninteractive version (NINT) with prescribed composition and a tuned aerosol indirect effect (AIE), the TCAD version with fully interactive aerosols, whole-atmosphere chemistry, and the tuned AIE, and the TCADI version which further includes a parameterized first indirect aerosol effect on clouds. Each atmospheric version is coupled to two different ocean general circulation models: the Russell ocean model (GISS-E2-R) and HYCOM (GISS-E2-H). By 2100, global mean warming in the RCP scenarios ranges from 1.0 to 4.5°C relative to 1850–1860 mean temperature in the historical simulations. In the RCP2.6 scenario, the surface warming in all simulations stays below a 2°C threshold at the end of the 21st century. For RCP8.5, the range is 3.5–4.5°C at 2100. Decadally averaged sea ice area changes are highly correlated to global mean surface air temperature anomalies and show steep declines in both hemispheres, with a larger sensitivity during winter months. By the year 2500, there are complete recoveries of the globally averaged surface air temperature for all versions of the GISS climate model in the low-forcing scenario RCP2.6. TCADI simulations show enhanced warming due to greater sensitivity to CO2, aerosol effects, and greater methane feedbacks, and recovery is much slower in RCP2.6 than with the NINT and TCAD versions. All coupled models have decreases in the Atlantic overturning stream function by 2100. In RCP2.6, there is a complete recovery of the Atlantic overturning stream function by the year 2500 while with scenario RCP8.5, the E2-R climate model produces a complete shutdown of deep water formation in the North Atlantic.

Strong chemistry-climate feedbacks in the Pliocene

The Pliocene epoch was the last sustained interval when global climate was significantly warmer than today but has been difficult to explain fully based on the external forcings from atmospheric carbon dioxide and surface albedo. Here we use an Earth system model to simulate terrestrial ecosystem emissions and atmospheric chemical composition in the mid-Pliocene (about 3 million years ago) and the preindustrial (~1750s). Tropospheric ozone and aerosol precursors from vegetation and wildfire are ~50% and ~100% higher in the mid-Pliocene due to the spread of the tropical savanna and deciduous biomes. The chemistry-climate feedbacks contribute a net global warming that is +30–250% of the carbon dioxide effect and a net aerosol global cooling that masks 15–100% of the carbon dioxide effect. These large vegetation-mediated ozone and aerosol feedbacks operate on centennial to millennial timescales in the climate system and have not previously been included in paleoclimate sensitivity assessments.