Imperial College London

DrMatthewKasoar

Faculty of Natural SciencesDepartment of Physics

Research Associate
 
 
 
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Contact

 

m.kasoar12

 
 
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Location

 

714Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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26 results found

Hodnebrog Ø, Myhre G, Kramer RJ, Shine KP, Andrews T, Faluvegi G, Kasoar M, Kirkevåg A, Lamarque J-F, Mülmenstädt J, Olivié D, Samset BH, Shindell D, Smith CJ, Takemura T, Voulgarakis Aet al., 2020, The effect of rapid adjustments to halocarbons and N2O on radiative forcing, npj Climate and Atmospheric Science, Vol: 3, Pages: 1-7, ISSN: 2397-3722

Rapid adjustments occur after initial perturbation of an external climate driver (e.g., CO2) and involve changes in, e.g. atmospheric temperature, water vapour and clouds, independent of sea surface temperature changes. Knowledge of such adjustments is necessary to estimate effective radiative forcing (ERF), a useful indicator of surface temperature change, and to understand global precipitation changes due to different drivers. Yet, rapid adjustments have not previously been analysed in any detail for certain compounds, including halocarbons and N2O. Here we use several global climate models combined with radiative kernel calculations to show that individual rapid adjustment terms due to CFC-11, CFC-12 and N2O are substantial, but that the resulting flux changes approximately cancel at the top-of-atmosphere due to compensating effects. Our results further indicate that radiative forcing (which includes stratospheric temperature adjustment) is a reasonable approximation for ERF. These CFCs lead to a larger increase in precipitation per kelvin surface temperature change (2.2 ± 0.3% K−1) compared to other well-mixed greenhouse gases (1.4 ± 0.3% K−1 for CO2). This is largely due to rapid upper tropospheric warming and cloud adjustments, which lead to enhanced atmospheric radiative cooling (and hence a precipitation increase) and partly compensate increased atmospheric radiative heating (i.e. which is associated with a precipitation decrease) from the instantaneous perturbation.

Journal article

Mansfield L, Nowack P, Kasoar M, Everitt R, Collins WJ, Voulgarakis Aet al., 2020, Predicting global patterns of long-term climate change from short-term simulations using machine learning, npj Climate and Atmospheric Science, Vol: 3, ISSN: 2397-3722

Understanding and estimating regional climate change under different anthropogenic emission scenarios is pivotal for informing societal adaptation and mitigation measures. However, the high computational complexity of state-of-the-art climate models remains a central bottleneck in this endeavour. Here we introduce a machine learning approach, which utilises a unique dataset of existing climate model simulations to learn relationships between short-te¬rm and long-term temperature responses to different climate forcing scenarios. This approach not only has the potential to accelerate climate change projections by reducing the costs of scenario computations, but also helps uncover early indicators of modelled long-term climate responses, which is of relevance to climate change detection, predictability and attribution. Our results highlight challenges and opportunities for data-driven climate modelling, especially concerning the incorporation of even larger model datasets in the future. We therefore encourage extensive data sharing among research institutes to build ever more powerful climate response emulators, and thus to enable faster climate change projections.

Journal article

Richardson TB, Forster PM, Smith CJ, Maycock AC, Wood T, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Hodnebrog O, Kasoar M, Kirkevag A, Lamarque J-F, Muelmenstaedt J, Myhre G, Olivie D, Portmann RW, Samset BH, Shawki D, Shindell D, Stier P, Takemura T, Voulgarakis A, Watson-Parris Det al., 2019, Efficacy of climate forcings in PDRMIP models, Journal of Geophysical Research: Atmospheres, Vol: 124, Pages: 12824-12844, ISSN: 2169-897X

Quantifying the efficacy of different climate forcings is important for understanding the real‐world climate sensitivity. This study presents a systematic multimodel analysis of different climate driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near‐surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multimodel mean efficacies based on ERF are close to one for global perturbations of methane, sulfate, black carbon, and insolation, but there is notable intermodel spread. We do not find robust evidence that the geographic location of sulfate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of simulations. Despite these differences, we find that there is no evidence for an efficacy effect on historical GSAT trend estimates based on simulations with an impulse response model, nor on the resulting estimates of climate sensitivity derived from the historical period. However, the considerable intermodel spread in the computed efficacies means that we cannot rule out an efficacy‐induced bias of ±0.4 K in equilibrium climate sensitivity to CO2 doubling when estimated using the historical GSAT trend.

Journal article

Scannell C, Booth BBB, Dunstone NJ, Rowell DP, Bernie DJ, Kasoar M, Voulgarakis A, Wilcox LJ, Acosta Navarro JC, Seland O, Paynter DJet al., 2019, The influence of remote aerosol forcing from industrialized economies on the future evolution of East and West African rainfall, Journal of Climate, Vol: 32, Pages: 8335-8354, ISSN: 0894-8755

Past changes in global industrial aerosol emissions have played a significant role in historical shifts in African rainfall, and yet assessment of the impact on African rainfall of near-term (10–40 yr) potential aerosol emission pathways remains largely unexplored. While existing literature links future aerosol declines to a northward shift of Sahel rainfall, existing climate projections rely on RCP scenarios that do not explore the range of air quality drivers. Here we present projections from two emission scenarios that better envelop the range of potential aerosol emissions. More aggressive emission cuts result in northward shifts of the tropical rainbands whose signal can emerge from expected internal variability on short, 10–20-yr time horizons. We also show for the first time that this northward shift also impacts East Africa, with evidence of delays to both onset and withdrawal of the short rains. However, comparisons of rainfall impacts across models suggest that only certain aspects of both the West and East African model responses may be robust, given model uncertainties. This work motivates the need for wider exploration of air quality scenarios in the climate science community to assess the robustness of these projected changes and to provide evidence to underpin climate adaptation in Africa. In particular, revised estimates of emission impacts of legislated measures every 5–10 years would have a value in providing near-term climate adaptation information for African stakeholders.

Journal article

Hodnebrog O, Myhre G, Samset BH, Alterskjaer K, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Forster PM, Kasoar M, Kirkevag A, Lamarque J-F, Olivie D, Richardson TB, Shawki D, Shindell D, Shine KP, Stier P, Takemura T, Voulgarakis A, Watson-Parris Det al., 2019, Water vapour adjustments and responses differ between climate drivers, Atmospheric Chemistry and Physics, Vol: 19, Pages: 12887-12899, ISSN: 1680-7316

Water vapour in the atmosphere is the source of a major climate feedback mechanism and potential increases in the availability of water vapour could have important consequences for mean and extreme precipitation. Future precipitation changes further depend on how the hydrological cycle responds to different drivers of climate change, such as greenhouse gases and aerosols. Currently, neither the total anthropogenic influence on the hydrological cycle nor that from individual drivers is constrained sufficiently to make solid projections. We investigate how integrated water vapour (IWV) responds to different drivers of climate change. Results from 11 global climate models have been used, based on simulations where CO2, methane, solar irradiance, black carbon (BC), and sulfate have been perturbed separately. While the global-mean IWV is usually assumed to increase by ∼7 % per kelvin of surface temperature change, we find that the feedback response of IWV differs somewhat between drivers. Fast responses, which include the initial radiative effect and rapid adjustments to an external forcing, amplify these differences. The resulting net changes in IWV range from 6.4±0.9 % K−1 for sulfate to 9.8±2 % K−1 for BC. We further calculate the relationship between global changes in IWV and precipitation, which can be characterized by quantifying changes in atmospheric water vapour lifetime. Global climate models simulate a substantial increase in the lifetime, from 8.2±0.5 to 9.9±0.7 d between 1986–2005 and 2081–2100 under a high-emission scenario, and we discuss to what extent the water vapour lifetime provides additional information compared to analysis of IWV and precipitation separately. We conclude that water vapour lifetime changes are an important indicator of changes in precipitation patterns and that BC is particularly efficient in prolonging the mean time, and therefore like

Journal article

Sillmann J, Stjern CW, Myhre G, Samset BH, Hodnebrog O, Andrews T, Boucher O, Faluvegi G, Forster P, Kasoar MR, Kharin VV, Kirkevag A, Lamarque J-F, Olivie DJL, Richardson TB, Shindell D, Takemura T, Voulgarakis A, Zwiers FWet al., 2019, Extreme wet and dry conditions affected differently by greenhouse gases and aerosols, npj Climate and Atmospheric Science, Vol: 2, Pages: 1-7, ISSN: 2397-3722

Global warming due to greenhouse gases and atmospheric aerosols alter precipitation rates, but the influence on extreme precipitation by aerosols relative to greenhouse gases is still not well known. Here we use the simulations from the Precipitation Driver and Response Model Intercomparison Project that enable us to compare changes in mean and extreme precipitation due to greenhouse gases with those due to black carbon and sulfate aerosols, using indicators for dry extremes as well as for moderate and very extreme precipitation. Generally, we find that the more extreme a precipitation event is, the more pronounced is its response relative to global mean surface temperature change, both for aerosol and greenhouse gas changes. Black carbon (BC) stands out with distinct behavior and large differences between individual models. Dry days become more frequent with BC-induced warming compared to greenhouse gases, but so does the intensity and frequency of extreme precipitation. An increase in sulfate aerosols cools the surface and thereby the atmosphere, and thus induces a reduction in precipitation with a stronger effect on extreme than on mean precipitation. A better understanding and representation of these processes in models will provide knowledge for developing strategies for both climate change and air pollution mitigation.

Journal article

Stjern CW, Lund MT, Samset BH, Myhre G, Forster PM, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Iversen T, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Sand M, Shawki D, Shindell D, Smith CJ, Takemura T, Voulgarakis Aet al., 2019, Arctic amplification response to individual climate drivers, Journal of Geophysical Research: Atmospheres, Vol: 124, Pages: 6698-6717, ISSN: 2169-897X

The Arctic is experiencing rapid climate change in response to changes in greenhouse gases, aerosols, and other climate drivers. Emission changes in general, as well as geographical shifts in emissions and transport pathways of short‐lived climate forcers, make it necessary to understand the influence of each climate driver on the Arctic. In the Precipitation Driver Response Model Intercomparison Project, 10 global climate models perturbed five different climate drivers separately (CO2, CH4, the solar constant, black carbon, and SO4). We show that the annual mean Arctic amplification (defined as the ratio between Arctic and the global mean temperature change) at the surface is similar between climate drivers, ranging from 1.9 (± an intermodel standard deviation of 0.4) for the solar to 2.3 (±0.6) for the SO4 perturbations, with minimum amplification in the summer for all drivers. The vertical and seasonal temperature response patterns indicate that the Arctic is warmed through similar mechanisms for all climate drivers except black carbon. For all drivers, the precipitation change per degree global temperature change is positive in the Arctic, with a seasonality following that of the Arctic amplification. We find indications that SO4 perturbations produce a slightly stronger precipitation response than the other drivers, particularly compared to CO2.

Journal article

Tang T, Shindell D, Faluvegi G, Myhre G, Olivié D, Voulgarakis A, Kasoar M, Andrews T, Boucher O, Forster PM, Hodnebrog, Iversen T, Kirkevåg A, Lamarque JF, Richardson T, Samset BH, Stjern CW, Takemura T, Smith Cet al., 2019, Comparison of effective radiative forcing calculations using multiple methods, drivers, and models, Journal of Geophysical Research: Atmospheres, Vol: 124, Pages: 4382-4394, ISSN: 2169-897X

American Geophysical Union. All Rights Reserved. We compare six methods of estimating effective radiative forcing (ERF) using a set of atmosphere-ocean general circulation models. This is the first multiforcing agent, multimodel evaluation of ERF values calculated using different methods. We demonstrate that previously reported apparent consistency between the ERF values derived from fixed sea surface temperature simulations and linear regression holds for most climate forcings, excluding black carbon (BC). When land adjustment is accounted for, however, the fixed sea surface temperature ERF values are generally 10–30% larger than ERFs derived using linear regression across all forcing agents, with a much larger (~70–100%) discrepancy for BC. Except for BC, this difference can be largely reduced by either using radiative kernel techniques or by exponential regression. Responses of clouds and their effects on shortwave radiation show the strongest variability in all experiments, limiting the application of regression-based ERF in small forcing simulations.

Journal article

Richardson TB, Forster PM, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Hodnebrog O, Kasoar M, Kirkevag A, Lamarque J-F, Myhre G, Olivie D, Samset BH, Shawki D, Shindell D, Takemura T, Voulgarakis Aet al., 2018, Drivers of Precipitation Change: An Energetic Understanding, Journal of Climate, Vol: 31, Pages: 9641-9657, ISSN: 0894-8755

The response of the hydrological cycle to climate forcings can be understood within the atmospheric energy budget framework. In this study precipitation and energy budget responses to five forcing agents are analyzed using 10 climate models from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Precipitation changes are split into a forcing-dependent fast response and a temperature-driven hydrological sensitivity. Globally, when normalized by top-of-atmosphere (TOA) forcing, fast precipitation changes are most sensitive to strongly absorbing drivers (CO2, black carbon). However, over land fast precipitation changes are most sensitive to weakly absorbing drivers (sulfate, solar) and are linked to rapid circulation changes. Despite this, land-mean fast responses to CO2 and black carbon exhibit more intermodel spread. Globally, the hydrological sensitivity is consistent across forcings, mainly associated with increased longwave cooling, which is highly correlated with intermodel spread. The land-mean hydrological sensitivity is weaker, consistent with limited moisture availability. The PDRMIP results are used to construct a simple model for land-mean and sea-mean precipitation change based on sea surface temperature change and TOA forcing. The model matches well with CMIP5 ensemble mean historical and future projections, and is used to understand the contributions of different drivers. During the twentieth century, temperature-driven intensification of land-mean precipitation has been masked by fast precipitation responses to anthropogenic sulfate and volcanic forcing, consistent with the small observed trend. However, as projected sulfate forcing decreases, and warming continues, land-mean precipitation is expected to increase more rapidly, and may become clearly observable by the mid-twenty-first century.

Journal article

Smith CJ, Kramer RJ, Myhre G, Forster PM, Soden BJ, Andrews T, Boucher O, Faluvegi G, Fläschner D, Hodnebrog, Kasoar M, Kharin V, Kirkevåg A, Lamarque JF, Mülmenstädt J, Olivié D, Richardson T, Samset BH, Shindell D, Stier P, Takemura T, Voulgarakis A, Watson-Parris Det al., 2018, Understanding rapid adjustments to diverse forcing agents, Geophysical Research Letters, Vol: 45, Pages: 12023-12031, ISSN: 0094-8276

Rapid adjustments are responses to forcing agents that cause a perturbation to the top of atmosphere energy budget but are uncoupled to changes in surface warming. Different mechanisms are responsible for these adjustments for a variety of climate drivers. These remain to be quantified in detail. It is shown that rapid adjustments reduce the effective radiative forcing (ERF) of black carbon by half of the instantaneous forcing, but for CO2 forcing, rapid adjustments increase ERF. Competing tropospheric adjustments for CO2 forcing are individually significant but sum to zero, such that the ERF equals the stratospherically adjusted radiative forcing, but this is not true for other forcing agents. Additional experiments of increase in the solar constant and increase in CH4 are used to show that a key factor of the rapid adjustment for an individual climate driver is changes in temperature in the upper troposphere and lower stratosphere.

Journal article

Myhre G, Kramer RJ, Smith CJ, Hodnebrog, Forster P, Soden BJ, Samset BH, Stjern CW, Andrews T, Boucher O, Faluvegi G, Fläschner D, Kasoar M, Kirkevåg A, Lamarque JF, Olivié D, Richardson T, Shindell D, Stier P, Takemura T, Voulgarakis A, Watson-Parris Det al., 2018, Quantifying the importance of rapid adjustments for global precipitation changes, Geophysical Research Letters, Vol: 20, Pages: 11399-11405, ISSN: 0094-8276

Different climate drivers influence precipitation in different ways. Here we use radiative kernels to understand the influence of rapid adjustment processes on precipitation in climate models. Rapid adjustments are generally triggered by the initial heating or cooling of the atmosphere from an external climate driver. For precipitation changes, rapid adjustments due to changes in temperature, water vapor, and clouds are most important. In this study we have investigated five climate drivers (CO2, CH4, solar irradiance, black carbon, and sulfate aerosols). The fast precipitation responses to a doubling of CO2 and a 10-fold increase in black carbon are found to be similar, despite very different instantaneous changes in the radiative cooling, individual rapid adjustments, and sensible heating. The model diversity in rapid adjustments is smaller for the experiment involving an increase in the solar irradiance compared to the other climate driver perturbations, and this is also seen in the precipitation changes.

Journal article

Shawki D, Voulgarakis A, Chakraborty A, Kasoar MR, Srinivasan JSet al., 2018, The South Asian monsoon response to remote aerosols: global and regional mechanisms, Journal of Geophysical Research, Vol: 123, Pages: 11585-11601, ISSN: 0148-0227

The South Asian summer monsoon has been suggested to be influenced by atmospheric aerosols, and this influence can be the result of either local or remote emissions. We have used the Hadley Centre Global Environment Model Version 3 (HadGEM3) coupled atmosphere‐ocean climate model to investigate for the first time the centennial‐scale South Asian precipitation response to emissions of sulfur dioxide (SO2), the dominant anthropogenic precursor of sulfate aerosol, from different midlatitude regions. Despite the localized nature of the regional heating that results from removing SO2 emissions, all experiments featured a similar large‐scale precipitation response over South Asia, driven by ocean‐modulated changes in the net cross‐equatorial heat transport and an opposing cross‐equatorial northward moisture transport. The effects are linearly additive, with the sum of the responses from the experiments where SO2 is removed from the United States, Europe, and East Asia resembling the response seen in the experiment where emissions are removed from the northern midlatitudes as a whole, but with East Asia being the largest contributor, even per unit of emission or top‐of‐atmosphere radiative forcing. This stems from the fact that East Asian emissions can more easily influence regional land‐sea thermal contrasts and sea level pressure differences that drive the monsoon circulation, compared to emissions from more remote regions. Our results suggest that radiative effects of remote pollution should not be neglected when examining changes in South Asian climate and that and it is important to examine such effects in coupled ocean‐atmosphere modeling frameworks.

Journal article

Kasoar MR, Shawki D, Voulgarakis A, 2018, Similar spatial patterns of global climate response to aerosols from different regions, npj Climate and Atmospheric Science, Vol: 12, ISSN: 2397-3722

Anthropogenic aerosol forcing is spatially heterogeneous, mostly localised around industrialised regions like North America, Europe, East and South Asia. Emission reductions in each of these regions will force the climate in different locations, which could have diverse impacts on regional and global climate. Here, we show that removing sulphur dioxide (SO2) emissions from any of these northern-hemisphere regions in a global composition-climate model results in significant warming across the hemisphere, regardless of the emission region. Although the temperature response to these regionally localised forcings varies considerably in magnitude depending on the emission region, it shows a preferred spatial pattern independent of the location of the forcing. Using empirical orthogonal function analysis, we show that the structure of the response is tied to existing modes of internal climate variability in the model. This has implications for assessing impacts of emission reduction policies, and our understanding of how climate responds to heterogeneous forcings.

Journal article

Tang T, Shindell D, Samset BH, Boucher O, Forster PM, Hodnebrog O, Myhre G, Sillmann J, Voulgarakis A, Andrews T, Faluvegi G, Flaschner D, Iversen T, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Stjern CW, Takemura Tet al., 2018, Dynamical response of Mediterranean precipitation to greenhouse gases and aerosols, ATMOSPHERIC CHEMISTRY AND PHYSICS, Vol: 18, Pages: 8439-8452, ISSN: 1680-7316

Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and, thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the beginning of the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare the modeled dynamical response of Mediterranean precipitation to individual forcing agents in a set of global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive sea level pressure (SLP) pattern similar to the North Atlantic Oscillation–Arctic Oscillation, characterized by higher SLP at midlatitudes and lower SLP at high latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31±17%) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs, whereas global scattering sulfate aerosols would have negligible impacts. Aerosol–cloud interactions appear to have minimal impacts on Mediterranean precipitation in these models, at least in part because many simulations did not fully include such processes; these merit further study. The findings from this study suggest that future BC and WMGHG emissions may significantly affect regional water resources, agricultural practices, ecosystems and

Journal article

Liu L, Shawki D, Voulgarakis A, Kasoar M, Samset B, Myhre G, Forster P, Hodnebrog Ø, Sillmann J, Aalbergsjø S, Boucher O, Faluvegi G, Iversen T, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Shindell D, Takemura Tet al., 2018, A PDRMIP multi-model study on the impacts of regional aerosol forcings on global and regional precipitation, Journal of Climate, Vol: 31, Pages: 4429-4447, ISSN: 0894-8755

Atmospheric aerosols such as sulfate and black carbon (BC) generate inhomogeneous radiative forcing and can affect precipitation in distinct ways compared to greenhouse gases (GHGs). Their regional effects on the atmospheric energy budget and circulation can be important for understanding and predicting global and regional precipitation changes, which act on top of the background GHG-induced hydrological changes. Under the framework of the Precipitation Driver Response Model Inter-comparison Project (PDRMIP), multiple models were used for the first time to simulate the influence of regional (Asian and European) sulfate and BC forcing on global and regional precipitation. The results show that, as in the case of global aerosol forcing, the global fast precipitation response to regional aerosol forcing scales with global atmospheric absorption, and the slow precipitation response scales with global surface temperature response. Asian sulphate aerosols appear to be a stronger driver of global temperature and precipitation change compared to European aerosols, but when the responses are normalised by unit radiative forcing or by aerosol burden change, the picture reverses, with European aerosols being more efficient in driving global change. The global apparent hydrological sensitivities of these regional forcing experiments are again consistent with those for corresponding global aerosol forcings found in the literature. However, the regional responses and regional apparent hydrological sensitivities do not align with the corresponding global values. Through a holistic approach involving analysis of the energy budget combined with exploring changes in atmospheric dynamics, we provide a framework for explaining the global and regional precipitation responses to regional aerosol forcing.

Journal article

Myhre G, Samset BH, Hodnebrog Ø, Andrews T, Boucher O, Faluvegi G, Fläschner D, Forster PM, Kasoar M, Kharin V, Kirkevåg A, Lamarque J-F, Olivié D, Richardson TB, Shawki D, Shindell D, Shine KP, Stjern CW, Takemura T, Voulgarakis Aet al., 2018, Sensible heat has significantly affected the global hydrological cycle over the historical period, Nature Communications, Vol: 9, ISSN: 2041-1723

Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability.

Journal article

Richardson TB, Forster PM, Andrews T, Boucher O, Faluvegi G, Fläschner D, Kasoar M, Kirkevåg A, Lamarque JF, Myhre G, Olivié D, Samset BH, Shawki D, Shindell D, Takemura T, Voulgarakis Aet al., 2018, Carbon Dioxide Physiological Forcing Dominates Projected Eastern Amazonian Drying, Geophysical Research Letters, Vol: 45, Pages: 2815-2825, ISSN: 0094-8276

Future projections of east Amazonian precipitation indicate drying, but they are uncertain and poorly understood. In this study we analyze the Amazonian precipitation response to individual atmospheric forcings using a number of global climate models. Black carbon is found to drive reduced precipitation over the Amazon due to temperature-driven circulation changes, but the magnitude is uncertain. CO 2 drives reductions in precipitation concentrated in the east, mainly due to a robustly negative, but highly variable in magnitude, fast response. We find that the physiological effect of CO 2 on plant stomata is the dominant driver of the fast response due to reduced latent heating and also contributes to the large model spread. Using a simple model, we show that CO 2 physiological effects dominate future multimodel mean precipitation projections over the Amazon. However, in individual models temperature-driven changes can be large, but due to little agreement, they largely cancel out in the model mean.

Journal article

Samset BH, Myhre G, Forster PM, Hodnebrog O, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson TB, Shindell D, Takemura T, Voulgarakis Aet al., 2018, Weak hydrological sensitivity to temperature change over land, independent of climate forcing, npj Climate and Atmospheric Science, Vol: 1, ISSN: 2397-3722

We present the global and regional hydrological sensitivity (HS) to surface temperature changes, for perturbations to CO2, CH4, sulfate and black carbon concentrations, and solar irradiance. Based on results from ten climate models, we show how modeled global mean precipitation increases by 2–3% per kelvin of global mean surface warming, independent of driver, when the effects of rapid adjustments are removed. Previously reported differences in response between drivers are therefore mainly ascribable to rapid atmospheric adjustment processes. All models show a sharp contrast in behavior over land and over ocean, with a strong surface temperature-driven (slow) ocean HS of 3–5%/K, while the slow land HS is only 0–2%/K. Separating the response into convective and large-scale cloud processes, we find larger inter-model differences, in particular over land regions. Large-scale precipitation changes are most relevant at high latitudes, while the equatorial HS is dominated by convective precipitation changes. Black carbon stands out as the driver with the largest inter-model slow HS variability, and also the strongest contrast between a weak land and strong sea response. We identify a particular need for model investigations and observational constraints on convective precipitation in the Arctic, and large-scale precipitation around the Equator.

Journal article

Chen P, Wang T, Dong M, Kasoar M, Han Y, Xie M, Li S, Zhuang B, Li M, Huang Tet al., 2017, Characterization of major natural and anthropogenic source profiles for size-fractionated PM in Yangtze River Delta, SCIENCE OF THE TOTAL ENVIRONMENT, Vol: 598, Pages: 135-145, ISSN: 0048-9697

Journal article

Stjern CW, Samset BH, Myhre G, Forster PM, Hodnebrog Ø, Andrews T, Boucher O, Faluvegi G, Iversen T, Kasoar M, Kharin V, Kirkevåg A, Lamarque J-F, Olivié D, Richardson T, Shawki D, Shindell D, Smith CJ, Takemura T, Voulgarakis Aet al., 2017, Rapid Adjustments Cause Weak Surface Temperature Response to Increased Black Carbon Concentrations, Journal of Geophysical Research: Atmospheres, Vol: 122, Pages: 462-481, ISSN: 2169-897X

We investigate the climate response to increased concentrations of black carbon (BC), as part of the Precipitation Driver Response Model Intercomparison Project (PDRMIP). A tenfold increase in BC is simulated by nine global coupled-climate models, producing a model median effective radiative forcing of 0.82 (ranging from 0.41 to 2.91) W m−2, and a warming of 0.67 (0.16 to 1.66) K globally and 1.24 (0.26 to 4.31) K in the Arctic. A strong positive instantaneous radiative forcing (median of 2.10 W m−2 based on five of the models) is countered by negative rapid adjustments (−0.64 W m−2 for the same five models), which dampen the total surface temperature signal. Unlike other drivers of climate change, the response of temperature and cloud profiles to the BC forcing is dominated by rapid adjustments. Low-level cloud amounts increase for all models, while higher-level clouds are diminished. The rapid temperature response is particularly strong above 400 hPa, where increased atmospheric stabilization and reduced cloud cover contrast the response pattern of the other drivers. In conclusion, we find that this substantial increase in BC concentrations does have considerable impacts on important aspects of the climate system. However, some of these effects tend to offset one another, leaving a relatively small median global warming of 0.47 K per W m−2—about 20% lower than the response to a doubling of CO2. Translating the tenfold increase in BC to the present-day impact of anthropogenic BC (given the emissions used in this work) would leave a warming of merely 0.07 K.

Journal article

Myhre G, Forster PM, Samset BH, Hodnebrog O, Sillmann J, Aalbergsjo SG, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Iversen T, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson TB, Shindell D, Shine KP, Stjern CW, Takemura T, Voulgarakis A, Zwiers Fet al., 2017, PDRMIP A Precipitation Driver and Response Model Intercomparison Project-Protocol and Preliminary Results, Bulletin of the American Meteorological Society, Vol: 98, Pages: 1185-1198, ISSN: 0003-0007

As the global temperature increases with changing climate, precipitation rates and patterns are affected through a wide range of physical mechanisms. The globally averaged intensity of extreme precipitation also changes more rapidly than the globally averaged precipitation rate. While some aspects of the regional variation in precipitation predicted by climate models appear robust, there is still a large degree of intermodel differences unaccounted for. Individual drivers of climate change initially alter the energy budget of the atmosphere, leading to distinct rapid adjustments involving changes in precipitation. Differences in how these rapid adjustment processes manifest themselves within models are likely to explain a large fraction of the present model spread and better quantifications are needed to improve precipitation predictions. Here, the authors introduce the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where a set of idealized experiments designed to understand the role of different climate forcing mechanisms were performed by a large set of climate models. PDRMIP focuses on understanding how precipitation changes relating to rapid adjustments and slower responses to climate forcings are represented across models. Initial results show that rapid adjustments account for large regional differences in hydrological sensitivity across multiple drivers. The PDRMIP results are expected to dramatically improve understanding of the causes of the present diversity in future climate projections.

Journal article

Chen P, Wang T, Lu X, Yu Y, Kasoar M, Xie M, Zhuang Bet al., 2017, Source apportionment of size-fractionated particles during the 2013 Asian Youth Games and the 2014 Youth Olympic Games in Nanjing, China, SCIENCE OF THE TOTAL ENVIRONMENT, Vol: 579, Pages: 860-870, ISSN: 0048-9697

Journal article

Kasoar M, Voulgarakis A, Lamarque J-F, Shindell DT, Bellouin N, Collins WJ, Faluvegi G, Tsigaridis Ket al., 2016, Regional and global temperature response to anthropogenic SO2 emissions from China in three climate models, Atmospheric Chemistry and Physics, Vol: 16, Pages: 9785-9804, ISSN: 1680-7324

We use the HadGEM3-GA4, CESM1, and GISS ModelE2 climate models to investigate the global and regional aerosol burden, radiative flux, and surface temperature responses to removing anthropogenic sulfur dioxide (SO2) emissions from China. We find that the models differ by up to a factor of 6 in the simulated change in aerosol optical depth (AOD) and shortwave radiative flux over China that results from reduced sulfate aerosol, leading to a large range of magnitudes in the regional and global temperature responses. Two of the three models simulate a near-ubiquitous hemispheric warming due to the regional SO2 removal, with similarities in the local and remote pattern of response, but overall with a substantially different magnitude. The third model simulates almost no significant temperature response. We attribute the discrepancies in the response to a combination of substantial differences in the chemical conversion of SO2 to sulfate, translation of sulfate mass into AOD, cloud radiative interactions, and differences in the radiative forcing efficiency of sulfate aerosol in the models. The model with the strongest response (HadGEM3-GA4) compares best with observations of AOD regionally, however the other two models compare similarly (albeit poorly) and still disagree substantially in their simulated climate response, indicating that total AOD observations are far from sufficient to determine which model response is more plausible. Our results highlight that there remains a large uncertainty in the representation of both aerosol chemistry as well as direct and indirect aerosol radiative effects in current climate models, and reinforces that caution must be applied when interpreting the results of modelling studies of aerosol influences on climate. Model studies that implicate aerosols in climate responses should ideally explore a range of radiative forcing strengths representative of this uncertainty, in addition to thoroughly evaluating the models used against observat

Journal article

Stiller-Reeve MA, Heuze C, Ball WT, White RH, Messori G, van der Wiel K, Medhaug I, Eckes AH, O'Callaghan A, Newland MJ, Williams SR, Kasoar M, Wittmeier HE, Kumer Vet al., 2016, Improving together: better science writing through peer learning, Hydrology and Earth System Sciences, Vol: 20, Pages: 2965-2973, ISSN: 1027-5606

Science, in our case the climate and geosciences, is increasingly interdisciplinary. Scientists must therefore communicate across disciplinary boundaries. For this communication to be successful, scientists must write clearly and concisely, yet the historically poor standard of scientific writing does not seem to be improving. Scientific writing must improve, and the key to long-term improvement lies with the early-career scientist (ECS). Many interventions exist for an ECS to improve their writing, like style guides and courses. However, momentum is often difficult to maintain after these interventions are completed. Continuity is key to improving writing.This paper introduces the ClimateSnack project, which aims to motivate ECSs to develop and continue to improve their writing and communication skills. The project adopts a peer-learning framework where ECSs voluntarily form writing groups at different institutes around the world. The group members learn, discuss, and improve their writing skills together. Several ClimateSnack writing groups have formed. This paper examines why some of the groups have flourished and others have dissolved. We identify the challenges involved in making a writing group successful and effective, notably the leadership of self-organized groups, and both individual and institutional time management. Within some of the groups, peer learning clearly offers a powerful tool to improve writing as well as bringing other benefits, including improved general communication skills and increased confidence.

Journal article

Samset BH, Myhre G, Forster PM, Hodnebrog O, Andrews T, Faluvegi G, Flaeschner D, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Shindell D, Shine KP, Takemura T, Voulgarakis Aet al., 2016, Fast and slow precipitation responses to individual climate forcers: a PDRMIP multimodel study, Geophysical Research Letters, Vol: 43, Pages: 2782-2791, ISSN: 1944-8007

Precipitation is expected to respond differently to various drivers of anthropogenic climate change. We present the first results from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where nine global climate models have perturbed CO2, CH4, black carbon, sulfate, and solar insolation. We divide the resulting changes to global mean and regional precipitation into fast responses that scale with changes in atmospheric absorption and slow responses scaling with surface temperature change. While the overall features are broadly similar between models, we find significant regional intermodel variability, especially over land. Black carbon stands out as a component that may cause significant model diversity in predicted precipitation change. Processes linked to atmospheric absorption are less consistently modeled than those linked to top-of-atmosphere radiative forcing. We identify a number of land regions where the model ensemble consistently predicts that fast precipitation responses to climate perturbations dominate over the slow, temperature-driven responses.

Journal article

Tang T, Shindell D, Samset BH, Boucher O, Forster PM, Hodnebrog Ø, Myhre G, Sillmann J, Voulgarakis A, Andrews T, Faluvegi G, Fläschner D, Iversen T, Kasoar M, Kharin V, Kirkevåg A, Lamarque J-F, Olivié D, Richardson T, Stjern CW, Takemura Tet al., Mediterranean Precipitation Response to Greenhouse Gases andAerosols

<jats:p>Abstract. Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare Mediterranean precipitation responses to individual forcing agents in a set of state-of-the-art global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean, and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive North Atlantic Oscillation (NAO)/Arctic Oscillation (AO)-like sea level pressure (SLP) pattern, characterized by higher SLP at mid-latitudes and lower SLP at high-latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in Northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31 ± 17 %) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs whereas global scattering sulfate aerosols have negligible impacts. The results from this study suggest that future BC emissions may significantly affect regional water resources, agricultural practices, ecosystems, and the economy in the Mediterranean region. </jats:p>

Journal article

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