Greenhouse Gas Warming Research Articles

Multidecadal Sea Level Rise in the Southeast Indian Ocean: The Role of Ocean Salinity Change

AbstractRegional sea level rise in the southeast Indian Ocean (SEIO) exerts growing threats to the surrounding Australian and Indonesian coasts, but the mechanisms of sea level rise have not been firmly established. By analyzing observational datasets and model results, this study investigates multidecadal steric sea level (SSL) rise of the SEIO since the mid-twentieth century, underscoring a significant role of ocean salinity change. The average SSL rising rate from 1960 through 2018 was 7.4 ± 2.4 mm decade−1, and contributions of the halosteric and thermosteric components were ∼42% and ∼58%, respectively. The notable salinity effect arises primarily from a persistent subsurface freshening trend at 400–1000 m. Further insights are gained through the decomposition of temperature and salinity changes into the heaving (vertical displacements of isopycnal surfaces) and spicing (density-compensated temperature and salinity change) modes. The subsurface freshening trend since 1960 is mainly attributed to the spicing mode, reflecting property modifications of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the southern Indian Ocean. Also noteworthy is a dramatic acceleration of SSL rise (20.3 ± 7.0 mm decade−1) since ∼1990, which was predominantly induced by the thermosteric component (16.3 ± 5.5 mm decade−1) associated with the heaving mode. Enhanced Ekman downwelling by surface winds and radiation forcing linked to global greenhouse gas warming mutually caused the depression of isopycnal surfaces, leading to the accelerated SSL rise through thermosteric effect. This study highlights the complexity of regional sea level rise in a rapidly changing climate, in which the role of ocean salinity is vital and time-varying.

Vatnajökull Mass Loss Under Solar Geoengineering Due to the North Atlantic Meridional Overturning Circulation

AbstractThe objective of solar geoengineering by stratospheric aerosol injection (SAI) is to lower global temperatures, but it may also have adverse side effects. Iceland is situated close to the overturning regions of the Atlantic Meridional Overturning Circulation (AMOC) that warms the North Atlantic area. Hence, this may be one region where reduced irradiance by SAI may not be successful in reducing impacts from greenhouse gas warming. We examine this proposition by estimating how the Icelandic Vatnajökull ice cap (VIC) surface mass balance (SMB) and surface runoff changes in response to greenhouse gas and solar geoengineering scenarios over the period 1982–2089. We use the surface energy and mass balance model SEMIC driven by Earth System Model output under the GeoMIP G4, and CMIP RCP4.5 and RCP8.5 greenhouse gas scenarios. Geoengineering significantly reduces VIC near‐surface air temperature by 0.4°C, downward longwave radiation by 2.4 Wm−2 and increases snowfall by 4.9 mm yr−1 relative to RCP4.5. During the SAI period 2020–2069, modeled annual mean SMB under G4, RCP4.5 and RCP8.5 are −0.34 ± 0.18 m yr−1, −0.56 ± 0.06 m yr−1 and −0.66 ± 0.04 m yr−1, respectively; surface runoff reduction under G4 is 6 ± 6% and 7 ± 6% (95% confidence interval uncertainties) compared with that under RCP4.5 and RCP8.5, which is far smaller than the 20 ± 2% and 32 ± 2% reductions for the Greenland ice sheet. The differences may be attributed to the reinvigoration of AMOC under G4 relative to RCP4.5 which brings more heat to Iceland than Greenland, leading to around half the cooling and longwave radiation reductions than for Greenland.

Study proposal of the impact of gas flaring on health of communities in Delta state Nigeria

<p class="abstract">Gas flaring is the continuous discharge of gaseous fuel into the atmosphere during oil and gas operations. Over the past years, there has been an increased concern of the impact of gas flaring on the environment and recently on human health. The impact of gas flaring in Niger Delta, Nigeria is of local and global environmental concern. The uncontrolled and wasteful flaring of gas has caused negative impacts on the flora, fauna and human health and livelihood in the region. Reports indicate that gas flaring in Nigeria contributed more greenhouse gases (GHGs) such as carbon dioxide, methane, nitrous oxide, chlorofluorocarbons to the atmosphere than the combined contribution of gas flaring on GHGs in the Sub-Saharan African countries. The GHGs emitted during gas flaring contribute significantly to global warming which may result in sea level rise and hasten the effects of climate change. Gas flaring, commonly carried out by oil exploration companies in Nigeria poses a hazard to the health of populations and environment by pollution, warming and release of GHGs. This study seeked to identify the risks associated with gas flaring in relation to human health in Delta region of Nigeria.</p>

Green Computing Approaches - A Survey

The upsurge in global warming and release of greenhouse gases is a major issue that intensified over the past years due to the increasing usage of technological resources in our daily routines. That is why a call for going green in the technological field is highly recommended. This paper reviews various approaches of green computing in five main models - software engineering model, cloud computing, mobile computing, data centers, and the educational sector.

Global Near-Surface Wind Speed Changes over the Last Decades Revealed by Reanalyses and CMIP6 Model Simulations

AbstractNear-surface (10 m) wind speed (NWS) plays a crucial role in many areas, including hydrological cycles, wind energy production, and air pollution, but what drives its multidecadal changes is still unclear. Using reanalysis datasets and model simulations from phase 6 of the Coupled Model Intercomparison Projection (CMIP6), this study investigates recent trends in the annual mean NWS. The results show that the Northern Hemisphere (NH) terrestrial NWS experienced significant (p < 0.1) decreasing trends during 1980–2010, when the Southern Hemisphere (SH) ocean NWS was characterized by significant (p < 0.1) upward trends. However, during 2010–19, global NWS trends shifted in their sign: NWS trends over the NH land became positive, and trends over the SH tended to be negative. We propose that the strengthening of SH NWS during 1980–2010 was associated with an intensified Hadley cell over the SH, while the declining of NH land NWS could have been caused by changes in atmospheric circulation, alteration of vegetation and/or land use, and the accelerating Arctic warming. The CMIP6 model simulations further demonstrate that the greenhouse gas (GHG) warming plays an important role in triggering the NWS trends over the two hemispheres during 1980–2010 through modulating meridional atmospheric circulation. This study also points at the importance of anthropogenic GHG forcing and the natural Pacific decadal oscillation to the long-term trends and multidecadal variability in global NWS, respectively.

Short-term CO[formula omitted] emissions forecasting based on decomposition approaches and its impact on electricity market scheduling

The world is facing major challenges related to global warming and emissions of greenhouse gases is a major causing factor. In 2017, energy industries accounted for 46% of all CO2 emissions globally, which shows a large potential for reduction. This paper proposes a novel short-term CO2 emissions forecast to enable intelligent scheduling of flexible electricity consumption to minimize the resulting CO2 emissions. Two proposed time series decomposition methods are developed for short-term forecasting of the CO2 emissions of electricity. These are in turn bench-marked against a set of state-of-the-art models. The result is a new forecasting method with a 48-hour horizon targeted the day-ahead electricity market. Forecasting benchmarks for France show that the new method has a mean absolute percentage error that is 25% lower than the best performing state-of-the-art model. Further, application of the forecast for scheduling flexible electricity consumption is studied for five European countries. Scheduling a flexible block of 4 h of electricity consumption in a 24 h interval can on average reduce the resulting CO2 emissions by 25% in France, 17% in Germany, 69% in Norway, 20% in Denmark, and just 3% in Poland when compared to consuming at random intervals during the day.

Attribution of historical near-surface permafrost degradation to anthropogenic greenhouse gas warming

Given the current confirmed permafrost degradation and its considerable impacts on ecosystems, water resources, infrastructure and climate, there is great interest in understanding the causes of permafrost degradation. Using the surface frost index (SFI) model and multimodel data from the fifth phase of the Coupled Model Intercomparison Project (CMIP5), this study, for the first time, investigates external anthropogenic and natural forcing impacts on historical (1921–2005) near-surface permafrost change in the Northern Hemisphere. The results show that anthropogenic greenhouse gas (GHG) forcing produces a significant decrease in the area of near-surface permafrost distribution at a rate of 0.46 × 106 km2 decade−1, similar to observations and the historical simulation (ALL). Anthropogenic aerosol (AA) forcing yields an increase in near-surface permafrost distribution area at a rate of 0.25 × 106 km2 decade−1. Under natural (NAT) forcing, there is a weak trend and distinct decadal variability in near-surface permafrost area. The effects of ALL and GHG forcings are detectable in the observed change in historical near-surface permafrost area, but the effects of NAT and AA forcings are not detected using the optimal fingerprint methods. This indicates that the observed near-surface permafrost degradation can be largely attributed to GHG-induced warming, which has decreased the near-surface permafrost area in the Northern Hemisphere by approximately 0. 21 × 106 km2 decade−1 on average over the study period, according to the attribution analysis.

Climate engineering to mitigate the projected 21st-century terrestrial drying of the Americas: a direct comparison of carbon capture and sulfur injection

Abstract. To mitigate the projected global warming in the 21st century, it is well-recognized that society needs to cut CO2 emissions and other short-lived warming agents aggressively. However, to stabilize the climate at a warming level closer to the present day, such as the “well below 2 ∘C” aspiration in the Paris Agreement, a net-zero carbon emission by 2050 is still insufficient. The recent IPCC special report calls for a massive scheme to extract CO2 directly from the atmosphere, in addition to decarbonization, to reach negative net emissions at the mid-century mark. Another ambitious proposal is solar-radiation-based geoengineering schemes, including injecting sulfur gas into the stratosphere. Despite being in public debate for years, these two leading geoengineering schemes have not been directly compared under a consistent analytical framework using global climate models. Here we present the first explicit analysis of the hydroclimate impacts of these two geoengineering approaches using two recently available large-ensemble (>10 members) model experiments conducted by a family of state-of-the-art Earth system models. The CO2-based mitigation simulation is designed to include both emission cuts and carbon capture. The solar-radiation-based mitigation simulation is designed to inject sulfur gas strategically at specified altitudes and latitudes and run a feedback control algorithm to avoid common problems previously identified such as the overcooling of the tropics and large-scale precipitation shifts. Our analysis focuses on the projected aridity conditions over the Americas in the 21st century in detailed terms of the potential mitigation benefits, the temporal evolution, the spatial distribution (within North and South America), the relative efficiency, and the physical mechanisms. We show that sulfur injection, in contrast to previous notions of leading to excessive terrestrial drying (in terms of precipitation reduction) while offsetting the global mean greenhouse gas (GHG) warming, will instead mitigate the projected drying tendency under RCP8.5. The surface energy balance change induced by sulfur injection, in addition to the well-known response in temperature and precipitation, plays a crucial role in determining the overall terrestrial hydroclimate response. However, when normalized by the same amount of avoided global warming in these simulations, sulfur injection is less effective in curbing the worsening trend of regional land aridity in the Americas under RCP8.5 when compared with carbon capture. Temporally, the climate benefit of sulfur injection will emerge more quickly, even when both schemes are hypothetically started in the same year of 2020. Spatially, both schemes are effective in curbing the drying trend over North America. However, for South America, the sulfur injection scheme is particularly more effective for the sub-Amazon region (southern Brazil), while the carbon capture scheme is more effective for the Amazon region. We conclude that despite the apparent limitations (such as an inability to address ocean acidification) and potential side effects (such as changes to the ozone layer), innovative means of sulfur injection should continue to be explored as a potential low-cost option in the climate solution toolbox, complementing other mitigation approaches such as emission cuts and carbon capture (Cao et al., 2017). Our results demonstrate the urgent need for multi-model comparison studies and detailed regional assessments in other parts of the world.

Large‐Scale Industrial Cloud Perturbations Confirm Bidirectional Cloud Water Responses to Anthropogenic Aerosols

AbstractAerosols offset a poorly quantified fraction of anthropogenic greenhouse gas warming, and the aerosol impact on clouds is the most uncertain mechanism of anthropogenic climate forcing. Here observations of a relatively weak average response of liquid cloud water to aerosol perturbations are extended to much larger areas than in previous studies, with the polluted cloud areas covering hundreds by hundreds of kilometers. Polluted clouds detected in satellite images at the global anthropogenic air pollution hot spot of Norilsk, Russia, and at other various major aerosol source regions show close compensation between aerosol‐induced cloud water increases and decreases. In the sampled Norilsk cloud perturbations the decrease in liquid water path offsets 3% of the radiative forcing through the Twomey effect on average. Weak cloud water response on average in large polluted areas is in very good agreement with previous results based on observations of small‐scale ship‐track‐like industrial cloud perturbations, helping to reduce the uncertainty associated with the anthropogenic aerosol impacts on clouds.

The Impact of Financial Development and Economic Growth on Environmental Quality of Kenya

Of recent times many countries are suffering from environmental problems such as global warming and emission of greenhouse gases. Emissions of carbon dioxide as been recognized as the major contributor to global warming and climate change. This paper examines the long run relationship among the variables environmental quality, financial development and economic growth in Kenya using time series for the period 1970-2019. Autoregressive distributed lag bounds test is used to investigate long run relationship and Granger causality method is used to test for causality among the variables. Empirical results indicate that there is long-run relationship among the variables. Long run results suggest that increases in financial development, lagged CO 2 , energy consumption, population growth, and trade openness significantly worsens environmental quality in Kenya. Natural resources significantly improve environmental quality in Kenya. According to the results the relationship between CO 2 and financial development in Kenya is non-linear suggesting presence of EKC between CO 2 and financial development. The empirical results confirm that the Environmental Kuznets curve does not exist between CO 2 and economic growth in Kenya in the long run. Short-run results also show that financial development, lagged CO 2 , FDI, population growth, and trade openness increase CO 2 emissions while natural resources reduce it. Causality results show unidirectional causality running from financial development to environmental quality and from CO 2 to GDP. According to the findings, there is evidence of neutrality hypothesis between financial development in Kenya and economic growth. Existence of long run relationship suggests that the government of Kenya needs to implement appropriate environmental policies that reduce pollution during economic growth. The government should set policies and guidelines to the financial sector so that the sector offers credit to firms that reduce air pollution. Keywords : Financial development, economic growth, CO 2 , Autoregressive distributed lag model, Kenya JEL Classification: E44, C32, Q43, Q56 DOI: 10.7176/JESD/11-12-03 Publication date: June 30th 2020

To what extent can cirrus cloud seeding counteract global warming?

The idea of modifying cirrus clouds to directly counteract greenhouse gas warming has gained momentum in recent years, despite disputes over its physical feasibility. Previous studies that analyzed modifications of cirrus clouds by seeding of ice nucleating particles showed large uncertainties in both cloud and surface climate responses, ranging from no effect or even a small warming to a globally averaged cooling of about 2.5 °C. We use two general circulation models that showed very different responses in previous studies, ECHAM6-HAM and CESM-CAM5, to determine which radiative and climatic responses to cirrus cloud seeding in a 1.5 × CO2 world are common and which are not. Seeding reduces the net cirrus radiative effect for −1.8 W m−2 in CESM compared with only −0.8 W m−2 in ECHAM. Accordingly, the surface temperature decrease is larger in CESM, counteracting about 70% of the global mean temperature increase due to CO2 and only 30% in ECHAM. While seeding impacts on mean precipitation were addressed in past studies, we are the first to analyze extreme precipitation responses to cirrus seeding. Seeding decreases the frequency of the most extreme precipitation globally. However, the extreme precipitation events occur more frequently in the Sahel and Central America, following the mean precipitation increase due to a northward shift of the Intertropical Convergence Zone. In addition, we use a quadratic climate damage metric to evaluate the amount of CO2-induced damage cirrus seeding can counteract. Seeding decreases the damage by about 50% in ECHAM, and by 85% in CESM over the 21 selected land regions. Climate damage due to CO2 increase is significantly reduced as a result of seeding in all of the considered land regions.

Opposite spatial variability of climate change‐induced surface temperature trends due to soil and atmospheric moisture in tropical/subtropical dry and wet land regions

AbstractRegional responses to surface warming are strongly influenced by the availability of surface or atmospheric moisture. For example, differences in evapotranspiration, atmospheric temperature lapse rate and heat capacity due to the availability of water over oceans and its lack over land contribute to an observed land/ocean warming ratio that is greater than one. This study shows that mechanisms related to the distributions of soil or atmospheric moisture could result in variations in regional climate change over land in the tropics and subtropics. Using atmospheric reanalyses and observations for the satellite era (1979–2016) within ±30° latitude, the analysis shows that dry regions such as northern and southern Africa and south‐central Australia have a significant negative regional spatial correlation between time‐averaged soil moisture/evapotranspiration and surface temperature trends. In contrast, wet tropical regions such as Amazonia and the Congo Basin display a significant positive spatial correlation between these quantities. Spatial correlations suggest that a regional amplification of surface temperature trends in wet regions could be associated with a robust positive longwave radiative effect of high clouds which co‐occur with wet soils. This positive relationship is strongest during austral summer, dominated by the summer seasons in the Amazon and Congo basins. These differences in regional sensitivity to greenhouse gas warming can be consequential for regional climate change assessment, for both tropical forests, which could already be operating at their high temperature limit, and also for dry desert regions, which are already marginal for supporting life.

Climate Sensitivity and the Direct Effect of Carbon Dioxide in a Limited-Area Cloud-Resolving Model

AbstractEven in a small domain, it can be prohibitively expensive to run cloud-resolving greenhouse gas warming experiments due to the long equilibration time. Here, a technique is introduced that reduces the computational cost of these experiments by an order of magnitude: instead of fixing the carbon dioxide concentration and equilibrating the sea surface temperature (SST), this technique fixes the SST and equilibrates the carbon dioxide concentration. Using this approach in a cloud-resolving model of radiative–convective equilibrium (RCE), the equilibrated SST is obtained as a continuous function of carbon dioxide concentrations spanning 1 ppmv to nearly 10 000 ppmv, revealing a dramatic increase in equilibrium climate sensitivity (ECS) at higher temperatures. This increase in ECS is due to both an increase in forcing and a decrease in the feedback parameter. In addition, the technique is used to obtain the direct effects of carbon dioxide (i.e., the rapid adjustments) over a wide range of SSTs. Overall, the direct effect of carbon dioxide offsets a quarter of the increase in precipitation from warming, reduces the shallow cloud fraction by a small amount, and has no impact on convective available potential energy (CAPE).

Observational constraints on the effective climate sensitivity from the historical period

The observed warming in the atmosphere and ocean can be used to estimate the climate sensitivity linked to present-day feedbacks, which is referred to as the effective climate sensitivity (Shist). However, such an estimate is affected by uncertainty in the radiative forcing, particularly aerosols, over the historical period. Here, we make use of detection and attribution techniques to derive the surface air temperature and ocean warming that can be attributed directly to greenhouse gas increases. These serve as inputs to a simple energy budget to infer the likelihood of Shist in response to observed greenhouse gases increases over two time periods (1862–2012 and 1955–2012). The benefit of using greenhouse gas attributable quantities is that they are not subject to uncertainties in the aerosol forcing (other than uncertainty in the attribution to greenhouse gas versus aerosol forcing not captured by the multi-model aerosol response pattern). The resulting effective climate sensitivity estimate, Shist, ranges from 1.3 °C to 3.1 °C (5%–95% range) over the full instrumental period (1862–2012) for our best estimate, and gets slightly wider when considering further uncertainties. This estimate increases to 1.7 °C–4.6 °C if using the shorter period (1955–2012). We also evaluate the climate model simulated surface air temperature and ocean heat content increase in response to greenhouse gas forcing over the same periods, and compare them with the observationally-constrained values. We find that that the ocean warming simulated in greenhouse gas only simulations in models considered here is consistent with that attributed to greenhouse gas increases from observations, while one model simulates more greenhouse gas-induced surface air warming than observed. However, other models with sensitivity outside our range show greenhouse gas warming that is consistent with that attributed in observations, emphasising that feedbacks during the historical period may differ from the feedbacks at CO2 doubling and from those at true equilibrium.

A Business Model for Producing Clean Energy in Developing Countries

Similar to other business projects, clean energy projects also has orientation for profit maximization in developing countries. Environmental problems caused by industrial wastes have been becoming serious issues in developing countries. Hence, recycling industrial wastes, in order to create more renewable and clean energy, has been recognized as one of ways to reduce adverse impact of global warming and negative effect of greenhouse gases. According to statistics, Viet Nam discharges about 400,000 tons of waste tires annually and this number in the US is estimated about 4,200,000 tons per year (source: vnu.edu.vn). This creates many environmental issues. Hence, this paper aims to propose a business model to solve problems mentioned below in the paper. Building a tire shredding plant in California, USA (for example) to cut the whole tires into small shredded tires then export to developing countries like Vietnam is one method to convert wastes into clean energy and protecting our environment. This is one main objective of this research paper. Another purpose of this study is to find out a financial model to evaluate socio-economic values of renewable energy projects that help to protect our environment, as well as a modern viewpoint of not including or adding (+) new debt issuances to increase net cash flow when estimating FCFE cash flow. Using pyrolysis technology to crack carbon linkage into smaller linkages, and then convert waste tires into renewable energy (FO-R oil, carbon black and steel). This is an application of chemical engineering. Through the economic and technical analysis of this model, we can see the practical benefits of the energy project in terms of economic efficiency, profitability, which bring surplus value for investors, effective solutions for customers and a quality energy product for the society. And it also suggest the relevant government of developing countries to consider proper policies to encourage environment protection and businesses in the field of converting industrial wastes such as tires into clean energy.

Density-compensated overturning in the Labrador Sea

The Atlantic Meridional Overturning Circulation, a key constituent of the climate system, is projected to slow down in the twenty-first century due to a weakening of the Labrador Sea convection, itself a response to greenhouse gas warming and/or enhanced freshwater flux from the Arctic. However, the first observations from the Overturning in the Subpolar North Atlantic Program reveal a minimal response of the Meridional Overturning Circulation to the strong Labrador Sea convection during the winters of 2015–2016. From an analysis of the observational and reanalysis data, we show here that this weak response can be explained by a strong density compensation in the Labrador Sea. Although convection induces important changes of temperature and salinity in the basin interior, the export of the thermal and haline anomalies to the boundary current largely takes place along density surfaces. As a result, the transformation across density surfaces, that is, the imprint on the overturning circulation, is relatively small. This finding highlights the critical relationship between temperature and salinity in determining the overturning strength in the Labrador Sea and underlines the necessity of accurate freshwater flux estimates for improved Meridional Overturning Circulation predictions. Compensation of the effects of salt and temperature changes on water density in the Labrador Sea can explain the minimal response of the Atlantic overturning circulation to convection, according to analyses of observations and reanalysis data.

Indian Ocean Warming Trend Reduces Pacific Warming Response to Anthropogenic Greenhouse Gases: An Interbasin Thermostat Mechanism

AbstractA greater warming trend of sea surface temperature in the tropical Indian Ocean than in the tropical Pacific is a robust feature found in various observational data sets. Yet this interbasin warming contrast is not present in climate models. Here we investigate the impact of tropical Indian Ocean warming on the tropical Pacific response to anthropogenic greenhouse gas warming by analyzing results from coupled model pacemaker experiments. We find that warming in the Indian Ocean induces local negative sea level pressure anomalies, which extend to the western tropical Pacific, strengthening the zonal sea level pressure gradient and easterly trades in the tropical Pacific. The enhanced trade winds reduce sea surface temperature in the eastern tropical Pacific by increasing equatorial upwelling and evaporative cooling, which offset the greenhouse gas warming. This result suggests an interbasin thermostat mechanism, through which the Indian Ocean exerts its influence on the Pacific response to anthropogenic greenhouse gas warming.

Wind energy generation from Nigeria continental shelf: A review

The menace of global warming and increase of green-house gases have motivated many countries to embrace renewable energy sources as an alternative to fossil fuels. Some of the means by which sustainable energy is tapped are solar, biofuel, and wind. Wind energy have been around for quite some time and the power in the wind have been harnessed on land. However, due to some constraints related to land use, visual impacts and turbulence induced by buildings and trees, offshore wind farms have been engaged to provide quality alternative to onshore wind energy. This study reviews various works that have been done in the offshore wind development and tries to make a case for the generation of electricity from this type of renewable energy source to boost the nation’s electricity demand. Work done in offshore energy assessment, wake models, offshore turbine layout, future innovation in offshore wind turbine and Nigeria onshore energy scenario were reviewed. To speed up development of offshore wind energy in Nigeria, recommendation was made for government to invest in wind energy research, industries that operate in the offshore region should be willing to share meteorological data with researchers and tertiary institutions should encourage student to work on “issue-based” projects like proffering solution to the energy deficit crisis in the country.

Weak average liquid-cloud-water response to anthropogenic aerosols

The cooling of the Earth's climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. An increase in the amount of water inside liquid-phase clouds induced by aerosols, through the suppression of rain formation, has been postulated to lead to substantial cooling, which would imply that the Earth's surface temperature is highly sensitive to anthropogenic forcing. Here we provide direct observational evidence that, instead of a strong increase, aerosols cause a relatively weak average decrease in the amount of water in liquid-phase clouds compared with unpolluted clouds. Measurements of polluted clouds downwind of various anthropogenic sources-such as oil refineries, smelters, coal-fired power plants, cities, wildfires and ships-reveal that aerosol-induced cloud-water increases, caused by suppressed rain formation, and decreases, caused by enhanced evaporation of cloud water, partially cancel each other out. We estimate that the observed decrease in cloud water offsets 23% of the global climate-cooling effect caused by aerosol-induced increases in the concentration of cloud droplets. These findings invalidate the hypothesis that increases in cloud water cause a substantial climate cooling effect and translate into reduced uncertainty in projections of future climate.

Aerosol versus Greenhouse Gas Effects on Tropical Cyclone Potential Intensity and the Hydrologic Cycle

AbstractAerosol cooling reduces tropical cyclone (TC) potential intensity (PI) more strongly, by about a factor of 2 per degree of sea surface temperature change, than greenhouse gas warming increases it. This study analyzes single-forcing and historical experiments from phase 5 of the Coupled Model Intercomparison Project, aiming to understand the physical mechanisms behind this difference. Calculations are done for the tropical oceans of each hemisphere during the relevant TC seasons, emphasizing multimodel means. PI theory is used to interpret the difference in the PI response to aerosol and greenhouse gas forcings in terms of three factors. The net surface turbulent heat flux (sum of the latent and sensible heat fluxes) explains half of the difference, thermodynamic efficiency explains at most a small fraction, and surface wind speed does not explain the remainder, perhaps because of the use of monthly mean data. Changes in turbulent surface heat fluxes are interpreted as responses to surface radiative flux changes in the context of the energy balance of the ocean mixed layer. Radiative kernels are used to estimate what fractions of the surface radiative flux changes are feedbacks due to temperature and water vapor changes. The greater effect of aerosol forcing occurs because shortwave forcing has a greater direct, temperature-independent component at the surface than does longwave forcing, for a forcing amplitude that provokes the same SST change. This conclusion recalls prior work on the response of precipitation to radiative forcing, and the similarities and differences between precipitation and potential intensity in this regard are discussed.