Differences In Radiative Forcing Research Articles

An investigation of the relationship between tropical monsoon precipitation changes and stratospheric sulfate aerosol optical depth

Abstract Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.

Simulation and evaluation study of atmospheric aerosol nonsphericity as a function of particle size

Aerosol nonsphericity causes great uncertainty in radiative forcing assessments and climate simulations. Although considerable studies have attempted to quantify this uncertainty, the relationship between aerosol nonsphericity and particle size is usually not considered, thus reducing the accuracy of the results. In this study, a coupled inversion algorithm combining an improved stochastic particle swarm optimization algorithm and angular light scattering is used for the nonparametric estimation of aerosol nonsphericity variation with particle size, and the optimal sample selection method is employed to screen the data. Based on the verification of inversion accuracy, the variation of aerosol aspect ratio with particle size based on the ellipsoidal model in global regions has been obtained from Aerosol Robotic Network (AERONET) data, and the effect of nonsphericity on radiative forcing and dry deposition has been studied. The results show that the aspect ratio increases with particle size in all regions, with the maximum ranging from 1.4 to 1.8 in the desert, reflecting the differences in aerosol composition at different particle sizes. In radiation calculations, considering aerosol nonsphericity makes the aerosol cooling effect weaker and surface radiative fluxes increase, but hardly changes the aerosol absorption, with maximum differences of 9.22% and 22.12% at the bottom and top of the atmosphere, respectively. Meanwhile, the differences in radiative forcing between aspect ratios as a function of particle size and not varying with particle size are not significant, averaging less than 2%. Besides, the aspect ratio not varying with particle size underestimates the deposition velocity of small particles and overestimates that of large particles compared to that as a function of particle size, with maximum differences of 7% and 4%, respectively.

Differences in Radiative Forcing, Not Sensitivity, Explain Differences in Summertime Land Temperature Variance Change Between CMIP5 and CMIP6

AbstractHow summertime temperature variability will change with warming has important implications for climate adaptation and mitigation. CMIP5 simulations indicate a compound risk of extreme hot temperatures in western Europe from both warming and increasing temperature variance. CMIP6 simulations, however, indicate only a moderate increase in temperature variance that does not covary with warming. To explore this intergenerational discrepancy in CMIP results, we decompose changes in monthly temperature variance into those arising from changes in sensitivity to forcing and changes in forcing variance. Across models, sensitivity increases with local warming in both CMIP5 and CMIP6 at an average rate of 5.7 ([3.7, 7.9]; 95% c.i.) × 10−3°C per W m−2 per °C warming. We use a simple model of moist surface energetics to explain increased sensitivity as a consequence of greater atmospheric demand (∼70%) and drier soil (∼40%) that is partially offset by the Planck feedback (∼−10%). Conversely, forcing variance is stable in CMIP5 but decreases with warming in CMIP6 at an average rate of −21 ([−28, −15]; 95% c.i.) W2 m−4 per °C warming. We examine scaling relationships with mean cloud fraction and find that mean forcing variance decreases with decreasing cloud fraction at twice the rate in CMIP6 than CMIP5. The stability of CMIP6 temperature variance is, thus, a consequence of offsetting changes in sensitivity and forcing variance. Further work to determine which models and generations of CMIP simulations better represent changes in cloud radiative forcing is important for assessing risks associated with increased temperature variance.

Cloud Feedbacks from CanESM2 to CanESM5.0 and their influence on climate sensitivity

Abstract. The newest iteration of the Canadian Earth System Model (CanESM5.0.3) has an effective climate sensitivity (EffCS) of 5.65 K, which is a 54 % increase relative to the model's previous version (CanESM2 – 3.67 K), and the highest sensitivity of all current models participating in the sixth phase of the coupled model inter-comparison project (CMIP6). Here, we explore the underlying causes behind CanESM5's increased EffCS via comparison of forcing and feedbacks between CanESM2 and CanESM5. We find only modest differences in radiative forcing as a response to CO2 between model versions. We find small increases in the surface albedo and longwave cloud feedback, as well as a substantial increase in the SW cloud feedback in CanESM5. Through the use of cloud area fraction output and cloud radiative kernels, we find that more positive low and non-low shortwave cloud feedbacks – particularly with regards to low clouds across the equatorial Pacific, as well as subtropical and extratropical free troposphere cloud optical depth – are the dominant contributors to CanESM5's increased climate sensitivity. Additional simulations with prescribed sea surface temperatures reveal that the spatial pattern of surface temperature change exerts controls on the magnitude and spatial distribution of low-cloud fraction response but does not fully explain the increased EffCS in CanESM5. The results from CanESM5 are consistent with increased EffCS in several other CMIP6 models, which has been primarily attributed to changes in shortwave cloud feedbacks.

Comparing Greenhouse Gas Impacts from Domestic Coal and Imported Natural Gas Electricity Generation in China

In 2019, coal power accounted for 65% of China’s electricity and coal accounted for 71% of its 10.2 GTpa CO2 emissions. To meet its pledge of peak carbon by 2030 and carbon neutrality by 2060, China plans to shift away from coal. Fuel switching to natural gas for electrical generation is an interim solution that is in the portfolio of measures for meeting these goals. However, because of variability in emissions from coal and natural gas supply chains, the potential for benefiting from fuel switching also varies. Here, we use a high-resolution basin-to-power plant analysis for estimating methane and CO2 emissions in natural gas and coal supply chains. We illustrate how the variations in supply chains at the level of the sourcing of natural gas and coal from individual production regions can lead to differences in the effectiveness of fuel switching in reducing greenhouse gas emissions. The analyses are applied to four switching scenarios from domestic coal to imported natural gas at representative power plants in China. Life cycle emission factors for the natural gas supply chains evaluated in this study range from 650 to 1000 kg carbon dioxide/MWh of electricity generated and from 2.3 to 13 kg methane/MWh, depending on the allocation choice, power generation technology, and production region. For the coal supply chain, emissions range from 850 to 1100 kg carbon dioxide/MWh and from 0.4 to 4.0 kg methane/MWh, depending on the power generation technology and production region. We show that for these scenarios, in the short term fuel switching can increase radiative forcing by a factor of 3 or decrease it by 70%, depending on the supply chains, generation technology, and, in the case of natural gas, co-product allocation choices in the supply chain. We also quantify consumptive losses of natural gas over long supply chains and show that fuel switching results in differences in radiative forcing that evolve over time due to the influence of methane emissions.

Improving a Biogeochemical Model to Simulate Surface Energy, Greenhouse Gas Fluxes, and Radiative Forcing for Different Land Use Types in Northeastern United States

AbstractLand use changes exert important impacts on climate, primarily through altering greenhouse gas (GHG) and surface energy fluxes. Biogeochemical models have incorporated a relatively complete suite of biogeochemical processes to simulate GHG fluxes. However, these models often lack detailed processes of surface energy exchange, limiting their ability to assess the impacts of land use change on climate. In this study, we incorporated processes of surface energy exchange into a widely used biogeochemistry model, DeNitrification‐DeComposition (DNDC), so that it can quantify both GHG and energy fluxes between the biosphere and the atmosphere. When tested against field observations for the three dominant land use types (forest, hayfield, and cornfield) in the northeastern United States, the improved DNDC successfully captured the observed fluxes of outgoing shortwave radiation, latent heat, sensible heat, net ecosystem exchange of CO2, and their differences among the three land use types. To evaluate the differences in radiative forcing among these land use types, we conducted 100‐year simulations and converted the modeled GHG fluxes to radiative forcing using an atmospheric impulse response model. Our results show that the 100‐year cumulative differences in net radiative forcing are 3.35 nW m−2 between the hayfield and forest (slight warming) and 43.2 nW m−2 between the cornfield and forest (warming) per hectare land use difference. The cooling effects of increased albedo after the conversion of forest to hayfield or cornfield (observed and modeled in recent years) are gradually offset by the warming effects of the increasing release of GHG as the forest becomes older.

Understanding the links between climate feedbacks, variability and change using a two-layer energy balance model

A simple, two-layer energy balance model (EBM) is used to investigate climate variability in Coupled Model Intercomparison Project Phase 5 (CMIP5) models and examine possible links between variability and climate sensitivity, and the roles of stochastic variability, radiative feedbacks and ocean mixing. The EBM represents global variability that, while somewhat stronger than the CMIP5 models, simulates reasonable ratios between shorter and longer timescales. Variability in the EBM to the range of parameters from the Global Climate Models is found to be particularly sensitive to stochastic variability, especially on interannual time-scales. Radiative feedbacks and ocean mixing parameters are also important, particularly for decadal timescales. A modest amount of the model-to-model spread in the variance of global temperature in the CMIP5 models is explained by the EBM. The EBM exhibits a stronger link between equilibrium climate sensitivity and the magnitude of the variability than it does for transient climate response, especially on decadal and longer timescales. The EBM results suggests that spread in stochastic forcing across the CMIP5 models is the single greatest factor degrading the correlation between variability and climate sensitivity, although model to model differences in radiative forcing and mixing into the deep ocean are also important. The findings suggest that from a theoretical point of view investigating constraints from variability may be a fruitful exercise. However, they also suggest that normalizing variability in general circulation models by stochastic forcing, uptake into the deep ocean and radiative forcing are all important first steps to reduce factors that will otherwise confound the correlations.

Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer

Abstract. Reduction of surface temperatures of the planet by injecting sulfate aerosols in the stratosphere has been suggested as an option to reduce the amount of human-induced climate warming. Several previous studies have shown that for a specified amount of injection, aerosols injected at a higher altitude in the stratosphere would produce more cooling because aerosol sedimentation would take longer. In this study, we isolate and assess the sensitivity of stratospheric aerosol radiative forcing and the resulting climate change to the altitude of the aerosol layer. We study this by prescribing a specified amount of sulfate aerosols, of a size typical of what is produced by volcanoes, distributed uniformly at different levels in the stratosphere. We find that stratospheric sulfate aerosols are more effective in cooling climate when they reside higher in the stratosphere. We explain this sensitivity in terms of effective radiative forcing: volcanic aerosols heat the stratospheric layers where they reside, altering stratospheric water vapor content, tropospheric stability, and clouds, and consequently the effective radiative forcing. We show that the magnitude of the effective radiative forcing is larger when aerosols are prescribed at higher altitudes and the differences in radiative forcing due to fast adjustment processes can account for a substantial part of the dependence of the amount of cooling on aerosol altitude. These altitude effects would be additional to dependences on aerosol microphysics, transport, and sedimentation, which are outside the scope of this study. The cooling effectiveness of stratospheric sulfate aerosols likely increases with the altitude of the aerosol layer both because aerosols higher in the stratosphere have larger effective radiative forcing and because they have higher stratospheric residence time; these two effects are likely to be of comparable importance.

Description and basic evaluation of simulated mean state, internal variability, and climate sensitivity in MIROC6

Abstract. The sixth version of the Model for Interdisciplinary Research on Climate (MIROC), called MIROC6, was cooperatively developed by a Japanese modeling community. In the present paper, simulated mean climate, internal climate variability, and climate sensitivity in MIROC6 are evaluated and briefly summarized in comparison with the previous version of our climate model (MIROC5) and observations. The results show that the overall reproducibility of mean climate and internal climate variability in MIROC6 is better than that in MIROC5. The tropical climate systems (e.g., summertime precipitation in the western Pacific and the eastward-propagating Madden–Julian oscillation) and the midlatitude atmospheric circulation (e.g., the westerlies, the polar night jet, and troposphere–stratosphere interactions) are significantly improved in MIROC6. These improvements can be attributed to the newly implemented parameterization for shallow convective processes and to the inclusion of the stratosphere. While there are significant differences in climates and variabilities between the two models, the effective climate sensitivity of 2.6 K remains the same because the differences in radiative forcing and climate feedback tend to offset each other. With an aim towards contributing to the sixth phase of the Coupled Model Intercomparison Project, designated simulations tackling a wide range of climate science issues, as well as seasonal to decadal climate predictions and future climate projections, are currently ongoing using MIROC6.

Why do the dark and light ogives of Forbes bands have similar surface mass balances?

ABSTRACTBand ogives are a striking and enigmatic feature of Mer de Glace glacier flow. The surface mass balances (SMBs) of these ogives have been thoroughly investigated over a period of 12 years. We find similar cumulative SMBs over this period, ranging between −64.1 and −66.2 m w.e., on the dark and light ogives even though the dark ogive albedo is ~40% lower than that of the light ogives. We, therefore, looked for another process that could compensate for the large difference of absorbed short-wave radiation between dark and light ogives. Based on in situ roughness measurements, our numerical modeling experiments demonstrate that a significant difference in turbulent flux over the dark and light ogives due to different surface roughnesses could compensate for the difference in radiative forcing. Our results discard theories for the genesis of band ogives that are based on the assumption of a strong ice ablation contrast between dark and light ogives. More generally, our study demonstrates that future roughness changes are as important to analyze as the radiative impacts of a potential increase of aerosols or debris at the surface of glaciers.

Stratospheric ozone chemistry feedbacks are not critical for the determination of climate sensitivity in CESM1(WACCM)

AbstractThe Community Earth System Model‐Whole Atmosphere Community Climate Model (CESM1‐WACCM) is used to assess the importance of including chemistry feedbacks in determining the equilibrium climate sensitivity (ECS). Two 4×CO2 model experiments were conducted: one with interactive chemistry and one with chemical constituents other than CO2 held fixed at their preindustrial values. The ECS determined from these two experiments agrees to within 0.01 K. Similarly, the net feedback parameter agrees to within 0.01 W m−2 K−1. This agreement occurs in spite of large changes in stratospheric ozone found in the simulation with interactive chemistry: a 30% decrease in the tropical lower stratosphere and a 40% increase in the upper stratosphere, broadly consistent with other published estimates. Off‐line radiative transfer calculations show that ozone changes alone account for the difference in radiative forcing. We conclude that at least for determining global climate sensitivity metrics, the exclusion of chemistry feedbacks is not a critical source of error in CESM.

Last glacial maximum radiative forcing from mineral dust aerosols in an Earth system model

AbstractThe mineral dust cycle in preindustrial (PI) and Last Glacial Maximum (LGM) simulations with the Coupled Model Intercomparison Project Phase 5 model Hadley Centre Global Environment Model 2‐Atmosphere (HadGEM2‐A) is evaluated. The modeled global dust cycle is enhanced at the LGM, with larger emissions in the Southern Hemisphere, consistent with some previous studies. Two different dust uplift schemes within HadGEM2 both show a similar LGM/PI increase in total emissions (60% and 80%) and global loading (100% and 75%), but there is a factor of 3 difference in the top of the atmosphere net LGM‐PI direct radiative forcing (−1.2 W m−2 and −0.4 W m−2, respectively). This forcing is dominated by the short‐wave effects in both schemes. Recent reconstructions of dust deposition fluxes suggest that the LGM increase is overestimated in the Southern Atlantic and underestimated over east Antarctica. The LGM dust deposition reconstructions do not strongly discern between these two dust schemes because deposition is dominated by larger (2–6 μm diameter) particles for which the two schemes show similar loading in both time periods. The model with larger radiative forcing shows a larger relative emissions increase of smaller particles. This is because of the size‐dependent friction velocity emission threshold and different size distribution of the soil source particles compared with the second scheme. Size dependence of the threshold velocity is consistent with the theory of saltation, implying that the model with larger radiative forcing is more realistic. However, the large difference in radiative forcing between the two schemes highlights the size distribution at emission as a major uncertainty in predicting the climatic effects of dust cycle changes.

Sensitivity of regional climate to global temperature and forcing

The sensitivity of regional climate to global average radiative forcing and temperature change is important for setting global climate policy targets and designing scenarios. Setting effective policy targets requires an understanding of the consequences exceeding them, even by small amounts, and the effective design of sets of scenarios requires the knowledge of how different emissions, concentrations, or forcing need to be in order to produce substantial differences in climate outcomes. Using an extensive database of climate model simulations, we quantify how differences in global average quantities relate to differences in both the spatial extent and magnitude of climate outcomes at regional (250–1250 km) scales. We show that differences of about 0.3 °C in global average temperature are required to generate statistically significant changes in regional annual average temperature over more than half of the Earth’s land surface. A global difference of 0.8 °C is necessary to produce regional warming over half the land surface that is not only significant but reaches at least 1 °C. As much as 2.5 to 3 °C is required for a statistically significant change in regional annual average precipitation that is equally pervasive. Global average temperature change provides a better metric than radiative forcing for indicating differences in regional climate outcomes due to the path dependency of the effects of radiative forcing. For example, a difference in radiative forcing of 0.5 W m−2 can produce statistically significant differences in regional temperature over an area that ranges between 30% and 85% of the land surface, depending on the forcing pathway.

Interactive ozone and methane chemistry in GISS-E2 historical and future climate simulations

Abstract. The new generation GISS climate model includes fully interactive chemistry related to ozone in historical and future simulations, and interactive methane in future simulations. Evaluation of ozone, its tropospheric precursors, and methane shows that the model captures much of the large-scale spatial structure seen in recent observations. While the model is much improved compared with the previous chemistry-climate model, especially for ozone seasonality in the stratosphere, there is still slightly too rapid stratospheric circulation, too little stratosphere-to-troposphere ozone flux in the Southern Hemisphere and an Antarctic ozone hole that is too large and persists too long. Quantitative metrics of spatial and temporal correlations with satellite datasets as well as spatial autocorrelation to examine transport and mixing are presented to document improvements in model skill and provide a benchmark for future evaluations. The difference in radiative forcing (RF) calculated using modeled tropospheric ozone versus tropospheric ozone observed by TES is only 0.016 W m−2. Historical 20th Century simulations show a steady increase in whole atmosphere ozone RF through 1970 after which there is a decrease through 2000 due to stratospheric ozone depletion. Ozone forcing increases throughout the 21st century under RCP8.5 owing to a projected recovery of stratospheric ozone depletion and increases in methane, but decreases under RCP4.5 and 2.6 due to reductions in emissions of other ozone precursors. RF from methane is 0.05 to 0.18 W m−2 higher in our model calculations than in the RCP RF estimates. The surface temperature response to ozone through 1970 follows the increase in forcing due to tropospheric ozone. After that time, surface temperatures decrease as ozone RF declines due to stratospheric depletion. The stratospheric ozone depletion also induces substantial changes in surface winds and the Southern Ocean circulation, which may play a role in a slightly stronger response per unit forcing during later decades. Tropical precipitation shifts south during boreal summer from 1850 to 1970, but then shifts northward from 1970 to 2000, following upper tropospheric temperature gradients more strongly than those at the surface.

Feedback between windblown dust and planetary boundary-layer characteristics: Sensitivity to boundary and surface layer parameterizations

Inclusion of direct radiative forcing by mineral dust is important for accurate simulation of meteorological conditions, and planetary boundary-layer (PBL) parameterization plays a critical role in proper representation of such forcing. The direct radiative forcing of mineral dust and its feedback effects on boundary-layer dynamics are investigated using the Weather Research and Forecasting with Chemistry (WRF/Chem) model. Furthermore, this study examines the sensitivity of dust radiative effects associated with two different PBL schemes: the Yonsei University (YSU) scheme with both Noah and RUC (Rapid Update Cycle) land-surface models (LSMs), and the Mellor–Yamada–Janjic (MYJ) scheme. By reflecting and absorbing solar radiation, dust aerosols are predicted to cool the atmosphere from the surface to near the boundary-layer top, while warm the boundary-layer top and lower free atmosphere by absorbing both solar and infrared radiation. The simulated surface cooling and heating at the boundary-layer top stabilizes the lower atmosphere, causing a reduction of boundary-layer depth. The stabilized atmosphere restricts vertical exchange of momentum, resulting in an overall decrease of wind speeds in the lower boundary layer and an increase within the upper boundary layer and lower free atmosphere. Use of the YSU-RUC scheme resulted in larger dust feedback effects on atmospheric characteristics, while the MYJ scheme produced lower radiative feedback effects because of lower dust concentrations and reduced vertical mixing of dust. The differences in radiative forcing by dust in model runs are found to be mainly due to differences in PBL scheme, rather than the LSM used in the model.

Abrupt CO2 experiments as tools for predicting and understanding CMIP5 representative concentration pathway projections

A fast simple climate modelling approach is developed for predicting and helping to understand general circulation model (GCM) simulations. We show that the simple model reproduces the GCM results accurately, for global mean surface air temperature change and global-mean heat uptake projections from 9 GCMs in the fifth coupled model inter-comparison project (CMIP5). This implies that understanding gained from idealised CO2 step experiments is applicable to policy-relevant scenario projections. Our approach is conceptually simple. It works by using the climate response to a CO2 step change taken directly from a GCM experiment. With radiative forcing from non-CO2 constituents obtained by adapting the Forster and Taylor method, we use our method to estimate results for CMIP5 representative concentration pathway (RCP) experiments for cases not run by the GCMs. We estimate differences between pairs of RCPs rather than RCP anomalies relative to the pre-industrial state. This gives better results because it makes greater use of available GCM projections. The GCMs exhibit differences in radiative forcing, which we incorporate in the simple model. We analyse the thus-completed ensemble of RCP projections. The ensemble mean changes between 1986–2005 and 2080–2099 for global temperature (heat uptake) are, for RCP8.5: 3.8 K (2.3 × 1024 J); for RCP6.0: 2.3 K (1.6 × 1024 J); for RCP4.5: 2.0 K (1.6 × 1024 J); for RCP2.6: 1.1 K (1.3 × 1024 J). The relative spread (standard deviation/ensemble mean) for these scenarios is around 0.2 and 0.15 for temperature and heat uptake respectively. We quantify the relative effect of mitigation action, through reduced emissions, via the time-dependent ratios (change in RCPx)/(change in RCP8.5), using changes with respect to pre-industrial conditions. We find that the effects of mitigation on global-mean temperature change and heat uptake are very similar across these different GCMs.

Characterizing the Morphology of Organic Aerosols at Ambient Temperature and Pressure

The aerosol direct effect, which characterizes the interaction of radiation with aerosol particles, remains poorly understood. By determining aerosol composition, shape, and internal structure, we can predict aerosol optical properties. In this study, we performed a feasibility study to determine if tapping-mode atomic force microscopy (TM-AFM) and Raman microscopy can be effectively used to obtain information on aerosol composition, shape, and structure. These techniques are advantageous because they operate under ambient pressure and temperature. We worked with model aerosol particles composed of organic components of varying solubility mixed with ammonium sulfate. In particular, we explored whether aerosols could be differentiated on the basis of the solubility of the organic component. We also characterized the aerosol internal structure and investigated how this structure changes as the solubility of the organic compound is varied. To obtain indirect chemical information from AFM, we imaged particles supported on both polar, SiO(x)/Si(100), and nonpolar, highly ordered pyrolytic graphite, surfaces. We have found that AFM can be used to differentiate the solubility of the organic component. In some cases, AFM can also be used to identify internal structure. With Raman microscopy, we can differentiate between core-shell structures and homogeneous structures. Surprisingly, we find that even for the most soluble compounds, core-shell structures are observed. To discuss consequences of our results for climate studies, we calculate the difference in radiative forcing caused by having a core-shell aerosol rather than a homogeneous particle. Overall, these techniques are promising for characterizing composition, shape, and internal structure of atmospheric particles.