[Show abstract][Hide abstract] ABSTRACT: Fires are responsible for a range of gaseous and aerosol emissions. However, their influence on the interannual variability of atmospheric trace gases and aerosols has not been systematically investigated from a global perspective. We examine biomass burning emissions as a driver of interannual variability of large-scale abundances of short-lived constituents such as carbon monoxide (CO), hydroxyl radicals (OH), ozone and aerosols using the GISS ModelE composition-climate model and a range of observations, with an emphasis on satellite information. Our model captures the observed variability of the constituents examined in most cases, but with substantial underestimates in boreal regions. The strongest interannual variability on a global scale is found for carbon monoxide (~10% for its global annual burden), while the lowest is found for tropospheric ozone (~1% for its global annual burden). Regionally, aerosol optical depth shows variability that exceeds 50%. Areas of strong variability include the tropical land regions and northern high latitudes, while even regions in the northern mid-latitudes experience substantial inteannual variability of aerosols. Ozone variability peaks over Equatorial Asia in boreal autumn, partly due to varying biomass burning emissions, and over the western and central Pacific in the rest of the year, mainly due to meteorological fluctuations. We find that biomass burning emissions are almost entirely responsible for global CO interannual variability, and similarly important for OH variability. The same is true for global and regional aerosol variability, especially when not taking into account dust and sea-salt particles. We show that important implications can arise from such interannual influences for regional climate and air quality.
Journal of Geophysical Research Atmospheres 06/2015; DOI:10.1002/2014JD022926 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Examination of effective radiative forcing (ERF), a measure of changes in Earth's energy balance, facilitates understanding the role of various drivers of climate change. For short-lived compounds, ERF can be highly inhomogeneous geographically. The relationship between the spatial patterns of ERF and surface temperature response is poorly characterized, however. We examine that relationship in the latest generation of global climate models. We find that the uneven distribution of historical aerosol, ozone, and land use forcing leads to substantial differences compared to the well-mixed greenhouse gases (WMGHG). There is a stronger response per unit global mean forcing to historical inhomogeneous forcing than to WMGHG both globally and in much of the Northern Hemisphere (NH) extratropics, in fairly good agreement with results inferred from observations. Our results indicate that the enhanced global mean response is attributable to the concentration of inhomogeneous forcing in the NH extratropics, where there is strongest sensitivity to forcing, rather than to processes specific to the inhomogeneous forcers. In many regions, inclusion of inhomogeneous forcing greatly increases the spread in historical temperature changes simulated by the models, suggesting that better forcing characterization could play an important role in improving modeling of decadal-scale regional climate change. Finally, incorporating observed temperatures, the results provide estimates of global historical aerosol forcing (−1.0 ± 0.4 W m−2) consistent with other studies (though with narrower uncertainties) and also provide constraints on NH and NH extratropical historical aerosol forcing (−1.4 ± 0.6 and −1.2 ± 0.6 W m−2, respectively) and aerosol + ozone forcing.
Journal of Geophysical Research Atmospheres 05/2015; DOI:10.1002/2014JD022752 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We test the current generation of global chemistry-climate models in their ability to simulate observed, present-day surface ozone. Models are evaluated against hourly surface ozone from 4217 stations in North America and Europe that are averaged over 1° × 1° grid cells, allowing commensurate model-measurement comparison. Models are generally biased high during all hours of the day and in all regions. Most models simulate the shape of regional summertime diurnal and annual cycles well, correctly matching the timing of hourly (~ 15:00) and monthly (mid-June) peak surface ozone abundance. The amplitude of these cycles is less successfully matched. The observed summertime diurnal range (~ 25 ppb) is underestimated in all regions by about 7 ppb, and the observed seasonal range (~ 21 ppb) is underestimated by about 5 ppb except in the most polluted regions where it is overestimated by about 5 ppb. The models generally match the pattern of the observed summertime ozone enhancement, but they overestimate its magnitude in most regions. Most models capture the observed distribution of extreme episode sizes, correctly showing that about 80% of individual extreme events occur in large-scale, multi-day episodes of more than 100 grid cells. The observed linear relationship showing increases in ozone by up to 6 ppb for larger-sized episodes is also matched.
[Show abstract][Hide abstract] ABSTRACT: Concerns have been raised about the possible connections between the local and regional photochemical problem and global warming. The current study assesses the trend of ozone in Hong Kong and the Pearl River Delta (PRD) in South China and investigates the interannual changes of sensitivity of ozone to air temperature, as well as the trends in regional precursors. Results reveal, at the three monitoring sites from the mid-1990s to 2010, an increase in the mean ozone concentrations from 1.0 to 1.6 mg m (3 per year. The increase occurred in all seasons, with the highest rate in autumn. This is consistent with trends and temperature anomalies in the region. The increase in the sensitivity of ozone to temperature is clearly evident from the correlation between ozone (OMI [Ozone Monitoring Instrument] column amount) and surface air temperature (from the Atmospheric Infrared Sounder) displayed in the correlation maps for the PRD during the prominently high ozone period of JulyÁ September. It is observed to have increased from 2005 to 2010, the latter being the hottest year on record globally. To verify this temporal change in sensitivity, the ground-level trends of correlation coefficients/regression slopes are analysed. As expected, results reveal a statistically significant upward trend over a 14-year period (1997Á2010). While the correlation revealed in the correlation maps is in agreement with the corresponding OMI ozone maps when juxtaposed, temperature sensitivity of surface ozone also shows an association with ozone concentration, with R 00.5. These characteristics of ozone sensitivity are believed to have adverse implications for the region. As shown by ground measurements and/or satellite analyses, the decrease in nitrogen oxides (NO 2) and NO x in Hong Kong is not statistically significant while NO 2 of the PRD has only very slightly changed. However, carbon dioxide has remarkably declined in the whole region. While these observations concerning precursors do not seem to adequately support an increasing ozone trend, measured surface levels of formaldehyde, a proxy for volatile organic compound (VOC) emissions, have risen significantly in the PRD (2004Á2010). Hence, the reactive VOCs in the PRD are likely to be the main culprit for the increase of ozone, as far as precursors are concerned. Despite the prevailing problem, model simulations suggest prospects for improvement in the future.
Tellus B 07/2014; 66. DOI:10.3402/tellusb.v66.23455 · 3.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fire emissions associated with tropical land use change and maintenance influence atmospheric composition, air quality, and climate. In this study, we explore the effects of representing fire emissions at daily versus monthly resolution in a global composition-climate model. We find that simulations of aerosols are impacted more by the temporal resolution of fire emissions than trace gases such as carbon monoxide or ozone. Daily-resolved datasets concentrate emissions from fire events over shorter time periods and allow them to more realistically interact with model meteorology, reducing how often emissions are concurrently released with precipitation events and in turn increasing peak aerosol concentrations. The magnitude of this effect varies across tropical ecosystem types, ranging from smaller changes in modeling the low intensity, frequent burning typical of savanna ecosystems to larger differences when modeling the short-term, intense fires that characterize deforestation events. The utility of modeling fire emissions at a daily resolution also depends on the application, such as modeling exceedances of particulate matter concentrations over air quality guidelines or simulating regional atmospheric heating patterns.
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Advances in Modeling Earth Systems 06/2014; 6(2-2):441-477. DOI:10.1002/2013MS000266 · 5.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a description of the ModelE2 version of the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) and the configurations used in the simulations performed for the Coupled Model Intercomparison Project Phase 5 (CMIP5). We use six variations related to the treatment of the atmospheric composition, the calculation of aerosol indirect effects, and ocean model component. Specifically, we test the difference between atmospheric models that have noninteractive composition, where radiatively important aerosols and ozone are prescribed from precomputed decadal averages, and interactive versions where atmospheric chemistry and aerosols are calculated given decadally varying emissions. The impact of the first aerosol indirect effect on clouds is either specified using a simple tuning, or parameterized using a cloud microphysics scheme. We also use two dynamic ocean components: the Russell and HYbrid Coordinate Ocean Model (HYCOM) which differ significantly in their basic formulations and grid. Results are presented for the climatological means over the satellite era (1980-2004) taken from transient simulations starting from the preindustrial (1850) driven by estimates of appropriate forcings over the 20th Century. Differences in base climate and variability related to the choice of ocean model are large, indicating an important structural uncertainty. The impact of interactive atmospheric composition on the climatology is relatively small except in regions such as the lower stratosphere, where ozone plays an important role, and the tropics, where aerosol changes affect the hydrological cycle and cloud cover. While key improvements over previous versions of the model are evident, these are not uniform across all metrics. Key Points Description of the GISS ModelE2 contribution to CMIP5 Impact on evaluation of structural changes in composition and ocean treatment Ocean model choice is an important structural uncertainty
Journal of Advances in Modeling Earth Systems 03/2014; 6(1):141-184. DOI:10.1002/2013MS000265 · 5.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Increased concentrations of ozone and fine particulate matter (PM 2.5 ) since preindustrial times reflect increased emissions, but also contributions of past climate change. Here we use modeled concentrations from an ensemble of chemistry–climate models to estimate the global burden of anthropogenic outdoor air pollution on present-day premature human mortality, and the component of that burden attributable to past climate change. Using simulated concentrations for 2000 and 1850 and concentration–response functions (CRFs), we estimate that, at present, 470 000 (95% confidence interval, 140 000 to 900 000) premature respiratory deaths are associated globally and annually with anthropogenic ozone, and 2.1 (1.3 to 3.0) million deaths with anthropogenic PM 2.5 -related cardiopulmonary diseases (93%) and lung cancer (7%). These estimates are smaller than ones from previous studies because we use modeled 1850 air pollution rather than a counterfactual low concentration, and because of different emissions. Uncertainty in CRFs contributes more to overall uncertainty than the spread of model results. Mortality attributed to the effects of past climate change on air quality is considerably smaller than the global burden: 1500 (−20 000 to 27 000) deaths yr −1 due to ozone and 2200 (−350 000 to 140 000) due to PM 2.5 . The small multi-model means are coincidental, as there are larger ranges of results for individual models, reflected in the large uncertainties, with some models suggesting that past climate change has reduced air pollution mortality.
Environmental Research Letters 09/2013; 8(3-3):034005. DOI:10.1088/1748-9326/8/3/034005 · 4.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: the radiative forcing caused by anthropogenic aerosol sources is
essential for making effective emission control decisions to mitigate
climate change. We examined the net direct plus indirect radiative
forcing caused by carbonaceous aerosol and sulfur emissions in key
sectors of China and India using the GISS-E2 chemistry-climate model.
Diesel trucks and buses (67 mW m-2) and residential biofuel
combustion (52 mW m-2) in India have the largest global mean,
annual average forcings due mainly to the direct and indirect effects of
BC. Emissions from these two sectors in China have near-zero net global
forcings. Coal-fired power plants in both countries exert a negative
forcing of about -30 mW m-2 from production of sulfate.
Aerosol forcings are largest locally, with direct forcings due to
residential biofuel combustion of 580 mW m-2 over India and
416 mW m-2 over China, but they extend as far as North
America, Europe, and the Arctic.
Geophysical Research Letters 08/2013; 40(16):4409-4414. DOI:10.1002/grl.50805 · 4.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study, we utilize near-simultaneous observations from two sets
of multiple satellite sensors to segregate Tropospheric Emission
Spectrometer (TES) and Measurements of Pollution in the Troposphere
(MOPITT) CO observations over active fire sources from those made over
clear background. Hence, we obtain direct estimates of biomass burning
CO emissions without invoking inverse modeling as in traditional
top-down methods. We find considerable differences between Global Fire
Emissions Database (GFED) versions 2.1 and 3.1 and satellite-based
emission estimates in many regions. Both inventories appear to greatly
underestimate South and Southeast Asia emissions, for example. On global
scales, however, CO emissions in both inventories and in the
MOPITT-based analysis agree reasonably well, with the largest bias (30%)
found in the Northern Hemisphere spring. In the Southern Hemisphere,
there is a one-month shift between the GFED and MOPITT-based fire
emissions peak. Afternoon tropical fire emissions retrieved from TES are
about two times higher than the morning MOPITT retrievals. This appears
to be both a real difference due to the diurnal fire activity
variations, and a bias due to the scarcity of TES data.
Journal of Geophysical Research Atmospheres 07/2013; 118(14):8054-8066. DOI:10.1002/jgrd.50624 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Anthropogenic ozone radiative forcing is traditionally separately
attributed to tropospheric and stratospheric changes assuming that these
have distinct causes. Using the interactive composition-climate model
GISS-E2-R we find that this assumption is not justified. Our simulations
show that changes in emissions of tropospheric ozone precursors have
substantial effects on ozone in both regions, as do anthropogenic
halocarbon emissions. On the basis of our results, further simulations
with the NCAR-CAM3.5 model, and published studies, we estimate
industrial era (1850-2005) whole-atmosphere ozone forcing of
~0.5Wm-2 due to anthropogenic tropospheric precursors and
about -0.2Wm-2 due to halocarbons. The net troposphere plus
stratosphere forcing is similar to the net halocarbon plus precursor
ozone forcing, but the latter provides a more useful perspective. The
halocarbon-induced ozone forcing is roughly two-thirds the magnitude of
the halocarbon direct forcing but opposite in sign, yielding a net
forcing of only ~0.1Wm-2. Thus, the net effect of halocarbons
has been smaller, and the effect of tropospheric ozone precursors has
been greater, than generally recognized.
[Show abstract][Hide abstract] ABSTRACT:  Ozone changes and associated climate impacts in the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations are analyzed over the historical (1960–2005) and future (2006–2100) period under four Representative Concentration Pathways (RCP). In contrast to CMIP3, where half of the models prescribed constant stratospheric ozone, CMIP5 models all consider past ozone depletion and future ozone recovery. Multimodel mean climatologies and long-term changes in total and tropospheric column ozone calculated from CMIP5 models with either interactive or prescribed ozone are in reasonable agreement with observations. However, some large deviations from observations exist for individual models with interactive chemistry, and these models are excluded in the projections. Stratospheric ozone projections forced with a single halogen, but four greenhouse gas (GHG) scenarios show largest differences in the northern midlatitudes and in the Arctic in spring (~20 and 40 Dobson units (DU) by 2100, respectively). By 2050, these differences are much smaller and negligible over Antarctica in austral spring. Differences in future tropospheric column ozone are mainly caused by differences in methane concentrations and stratospheric input, leading to ~10 DU increases compared to 2000 in RCP 8.5. Large variations in stratospheric ozone particularly in CMIP5 models with interactive chemistry drive correspondingly large variations in lower stratospheric temperature trends. The results also illustrate that future Southern Hemisphere summertime circulation changes are controlled by both the ozone recovery rate and the rate of GHG increases, emphasizing the importance of simulating and taking into account ozone forcings when examining future climate projections.
[Show abstract][Hide abstract] ABSTRACT: We use simultaneous observations of tropospheric ozone and outgoing longwave radiation (OLR) sensitivity to tropospheric ozone from the Tropospheric Emission Spectrometer (TES) to evaluate model tropospheric ozone and its effect on OLR simulated by a suite of chemistry-climate models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). The ensemble mean of ACCMIP models show a persistent but modest tropospheric ozone low bias (5-20 ppb) in the Southern Hemisphere (SH) and modest high bias (5-10 ppb) in the Northern Hemisphere (NH) relative to TES ozone for 2005-2010. These ozone biases have a significant impact on the OLR. Using TES instantaneous radiative kernels (IRK), we show that the ACCMIP ensemble mean tropospheric ozone low bias leads up to 120 mW m(-2) OLR high bias locally but zonally compensating errors reduce the global OLR high bias to 39 +/- 41 mW m(-2) relative to TES data. We show that there is a correlation (R-2 = 0.59) between the magnitude of the ACCMIP OLR bias and the deviation of the ACCMIP preindustrial to present day (1750-2010) ozone radiative forcing (RF) from the ensemble ozone RF mean. However, this correlation is driven primarily by models whose absolute OLR bias from tropospheric ozone exceeds 100 mW m(-2). Removing these models leads to a mean ozone radiative forcing of 394 +/- 42 mW m(-2). The mean is about the same and the standard deviation is about 30% lower than an ensemble ozone RF of 384 +/- 60 mW m(-2) derived from 14 of the 16 ACCMIP models reported in a companion ACCMIP study. These results point towards a profitable direction of combining satellite observations and chemistry-climate model simulations to reduce uncertainty in ozone radiative forcing.
[Show abstract][Hide abstract] ABSTRACT: As part of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), we evaluate the historical black carbon (BC) aerosols simulated by 8 ACCMIP models against observations including 12 ice core records, long-term surface mass concentrations, and recent Arctic BC snowpack measurements. We also estimate BC albedo forcing by performing additional simulations using offline models with prescribed meteorology from 1996–2000. We evaluate the vertical profile of BC snow concentrations from these offline simulations using the recent BC snowpack measurements.
[Show abstract][Hide abstract] ABSTRACT: Results from simulations performed for the Atmospheric Chemistry and Climate Modeling Intercomparison Project (ACCMIP) are analysed to examine how OH and methane lifetime may change from present day to the future, under different climate and emissions scenarios. Present day (2000) mean tropospheric chemical lifetime derived from the ACCMIP multi-model mean is 9.8 ± 1.6 yr (9.3 ± 0.9 yr when only including selected models), lower than a recent observationally-based estimate, but with a similar range to previous multi-model estimates. Future model projections are based on the four Representative Concentration Pathways (RCPs), and the results also exhibit a large range. Decreases in global methane lifetime of 4.5 ± 9.1% are simulated for the scenario with lowest radiative forcing by 2100 (RCP 2.6), while increases of 8.5 ± 10.4% are simulated for the scenario with highest radiative forcing (RCP 8.5). In this scenario, the key driver of the evolution of OH and methane lifetime is methane itself, since its concentration more than doubles by 2100 and it consumes much of the OH that exists in the troposphere. Stratospheric ozone recovery, which drives tropospheric OH decreases through photolysis modifications, also plays a partial role. In the other scenarios, where methane changes are less drastic, the interplay between various competing drivers leads to smaller and more diverse OH and methane lifetime responses, which are difficult to attribute. For all scenarios, regional OH changes are even more variable, with the most robust feature being the large decreases over the remote oceans in RCP8.5. Through a regression analysis, we suggest that differences in emissions of non-methane volatile organic compounds and in the simulation of photolysis rates may be the main factors causing the differences in simulated present day OH and methane lifetime. Diversity in predicted changes between present day and future OH was found to be associated more strongly with differences in modelled temperature and stratospheric ozone changes. Finally, through perturbation experiments we calculated an OH feedback factor (F) of 1.24 from present day conditions (1.50 from 2100 RCP8.5 conditions) and a climate feedback on methane lifetime of 0.33 ± 0.13 yr K-1, on average. Models that did not include interactive stratospheric ozone effects on photolysis showed a stronger sensitivity to climate, as they did not account for negative effects of climate-driven stratospheric ozone recovery on tropospheric OH, which would have partly offset the overall OH/methane lifetime response to climate change.
[Show abstract][Hide abstract] ABSTRACT: We present multi-model global datasets of nitrogen and sulfate
deposition covering time periods from 1850 to 2100, calculated within
the Atmospheric Chemistry and Climate Model Intercomparison Project
(ACCMIP). The computed deposition fluxes are compared to surface wet
deposition and ice-core measurements. We use a new dataset of wet
deposition for 2000-2002 based on critical assessment of the quality of
existing regional network data. We show that for present-day (year 2000
ACCMIP time-slice), the ACCMIP results perform similarly to previously
published multi-model assessments. For this time slice, we find a
multi-model mean deposition of 50 Tg(N) yr-1 from nitrogen
oxide emissions, 60 Tg(N) yr-1 from ammonia emissions, and 83
Tg(S) yr-1 from sulfur emissions. The analysis of changes
between 1980 and 2000 indicates significant differences between model
and measurements over the United States but less so over Europe. This
difference points towards misrepresentation of 1980 NH3
emissions over North America. Based on ice-core records, the 1850
deposition fluxes agree well with Greenland ice cores but the change
between 1850 and 2000 seems to be overestimated in the Northern
Hemisphere for both nitrogen and sulfur species. Using the
Representative Concentration Pathways to define the projected climate
and atmospheric chemistry related emissions and concentrations, we find
large regional nitrogen deposition increases in 2100 in Latin America,
Africa and parts of Asia under some of the scenarios considered.
Increases in South Asia are especially large, and are seen in all
scenarios, with 2100 values more than double 2000 in some scenarios and
reaching > 1300 mg(N) m-2 yr-1 averaged over
regional to continental scale regions in RCP 2.6 and 8.5, ~30-50 %
larger than the values in any region currently (2000). The new ACCMIP
deposition dataset provides novel, consistent and evaluated global
gridded deposition fields for use in a wide range of climate and
[Show abstract][Hide abstract] ABSTRACT: Present day tropospheric ozone and its changes between 1850 and 2100 are considered, analysing 15 global models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). The ensemble mean compares well against present day observations. The seasonal cycle correlates well, except for some locations in the tropical upper troposphere. Most (75 %) of the models are encompassed with a range of global mean tropospheric ozone column estimates from satellite data, but there is a suggestion of a high bias in the Northern Hemisphere and a low bias in the Southern Hemisphere, which could indicate deficiencies with the ozone precursor emissions. Compared to the present day ensemble mean tropospheric ozone burden of 337 ± 23 Tg, the ensemble mean burden for 1850 time slice is ~30% lower. Future changes were modelled using emissions and climate projections from four Representative Concentration Pathways (RCPs). Compared to 2000, the relative changes in the ensemble mean tropospheric ozone burden in 2030 (2100) for the different RCPs are: −4% (−16%) for RCP2.6, 2% (−7%) for RCP4.5, 1% (−9%) for RCP6.0, and 7% (18%) for RCP8.5. Model agreement on the magnitude of the change is greatest for larger changes. Reductions in most precursor emissions are common across the RCPs and drive ozone decreases in all but RCP8.5, where doubled methane and a 40–150% greater stratospheric influx (estimated from a subset of models) increase ozone. While models with a high ozone burden for the present day also have high ozone burdens for the other time slices, no model consistently predicts large or small ozone changes; i.e. the magnitudes of the burdens and burden changes do not appear to be related simply, and the models are sensitive to emissions and climate changes in different ways. Spatial patterns of ozone changes are well correlated across most models, but are notably different for models without time evolving stratospheric ozone concentrations. A unified approach to ozone budget specifications and a rigorous investigation of the factors that drive tropospheric ozone is recommended to help future studies attribute ozone changes and inter-model differences more clearly.
[Show abstract][Hide abstract] ABSTRACT: The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) consists of a series of time slice experiments targeting the long-term changes in atmospheric composition between 1850 and 2100, with the goal of documenting composition changes and the associated radiative forcing. In this overview paper, we introduce the ACCMIP activity, the various simulations performed (with a requested set of 14) and the associated model output. The 16 ACCMIP models have a wide range of horizontal and vertical resolutions, vertical extent, chemistry schemes and interaction with radiation and clouds. While anthropogenic and biomass burning emissions were specified for all time slices in the ACCMIP protocol, it is found that the natural emissions are responsible for a significant range across models, mostly in the case of ozone precursors. The analysis of selected present-day climate diagnostics (precipitation, temperature, specific humidity and zonal wind) reveals biases consistent with state-of-the-art climate models. The model-to-model comparison of changes in temperature, specific humidity and zonal wind between 1850 and 2000 and between 2000 and 2100 indicates mostly consistent results. However, models that are clear outliers are different enough from the other models to significantly affect their simulation of atmospheric chemistry.
Geoscientific Model Development Discussions 02/2013; 6(1):179-206. DOI:10.5194/gmd-6-179-2013
[Show abstract][Hide abstract] ABSTRACT: We have analysed time-slice simulations from 17 global models, participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), to explore changes in present-day (2000) hydroxyl radical (OH) concentration and methane (CH4) lifetime relative to preindustrial times (1850) and to 1980. A comparison of modeled and observation-derived methane and methyl chloroform lifetimes suggests that the present-day global multi-model mean OH concentration is overestimated by 5 to 10% but is within the range of uncertainties. The models consistently simulate higher OH concentrations in the Northern Hemisphere (NH) compared with the Southern Hemisphere (SH) for the present-day (2000; inter-hemispheric ratios of 1.13 to 1.42), in contrast to observation-based approaches which generally indicate higher OH in the SH although uncertainties are large. Evaluation of simulated carbon monoxide (CO) concentrations, the primary sink for OH, against ground-based and satellite observations suggests low biases in the NH that may contribute to the high north-south OH asymmetry in the models. The models vary widely in their regional distribution of present-day OH concentrations (up to 34 %). Despite large regional changes, the multi-model global mean (mass-weighted) OH concentration changes little over the past 150 yr, due to concurrent increases in factors that enhance OH (humidity, tropospheric ozone, nitrogen oxide (NOx) emissions, and UV radiation due to decreases in stratospheric ozone), compensated by increases in OH sinks (methane abundance, carbon monoxide and non-methane volatile organic carbon (NMVOC) emissions). The large inter-model diversity in the sign and magnitude of preindustrial to present-day OH changes (ranging from a decrease of 12.7% to an increase of 14.6 %) indicate that uncertainty remains in our understanding of the long-term trends in OH and methane lifetime. We show that this diversity is largely explained by the different ratio of the change in global mean tropospheric CO and NOx burdens (Delta CO/Delta NOx, approximately represents changes in OH sinks versus changes in OH sources) in the models, pointing to a need for better constraints on natural precursor emissions and on the chemical mechanisms in the current generation of chemistry-climate models. For the 1980 to 2000 period, we find that climate warming and a slight increase in mean OH (3.5 +/- 2.2 %) leads to a 4.3 +/- 1.9% decrease in the methane lifetime. Analysing sensitivity simulations performed by 10 models, we find that preindustrial to present-day climate change decreased the methane lifetime by about four months, representing a negative feedback on the climate system. Further, we analysed attribution experiments performed by a subset of models relative to 2000 conditions with only one precursor at a time set to 1860 levels. We find that global mean OH increased by 46.4 +/- 12.2% in response to preindustrial to present-day anthropogenic NOx emission increases, and decreased by 17.3 +/- 2.3 %, 7.6 +/- 1.5 %, and 3.1 +/- 3.0% due to methane burden, and anthropogenic CO, and NMVOC emissions increases, respectively.