Strong present-day aerosol cooling implies a hot future

Atmospheric aerosols counteract the warming effects of anthropogenic greenhouse gases by an uncertain, but potentially large, amount. This in turn leads to large uncertainties in the sensitivity of climate to human perturbations, and therefore also in carbon cycle feedbacks and projections of climate change. In the future, aerosol cooling is expected to decline relative to greenhouse gas forcing, because of the aerosols' much shorter lifetime and the pursuit of a cleaner atmosphere. Strong aerosol cooling in the past and present would then imply that future global warming may proceed at or even above the upper extreme of the range projected by the Intergovernmental Panel on Climate Change.     

The impact of humidity above stratiform clouds on indirect aerosol climate forcing

Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei. The resulting cooling tendency, termed the indirect aerosol forcing, is thought to be comparable in magnitude to the forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect climate forcing.     

A climatologically significant aerosol longwave indirect effect in the Arctic

The warming of Arctic climate and decreases in sea ice thickness and extent observed over recent decades are believed to result from increased direct greenhouse gas forcing, changes in atmospheric dynamics having anthropogenic origin, and important positive reinforcements including ice-albedo and cloud-radiation feedbacks. The importance of cloud-radiation interactions is being investigated through advanced instrumentation deployed in the high Arctic since 1997 (refs 7, 8). These studies have established that clouds, via the dominance of longwave radiation, exert a net warming on the Arctic climate system throughout most of the year, except briefly during the summer. The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes. Here we use multisensor radiometric data to show that enhanced aerosol concentrations alter the microphysical properties of Arctic clouds, in a process known as the 'first indirect' effect. Under frequently occurring cloud types we find that this leads to an increase of an average 3.4 watts per square metre in the surface longwave fluxes. This is comparable to a warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant.     

Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface

The effect of radiative forcing by anthropogenic aerosols is one of the largest sources of uncertainty in climate predictions. Direct observations of the forcing are therefore needed, particularly for the poorly understood tropical aerosols. Here we present an observational method for quantifying aerosol forcing to within +/-5 per cent. We use calibrated satellite radiation measurements and five independent surface radiometers to quantify the aerosol forcing simultaneously at the Earth's surface and the top of the atmosphere over the tropical northern Indian Ocean. In winter, this region is covered by anthropogenic aerosols of sulphate, nitrate, organics, soot and fly ash from the south Asian continent. Accordingly, mean clear-sky solar radiative heating for the winters of 1998 and 1999 decreased at the ocean surface by 12 to 30 Wm(-2), but only by 4 to 10 Wm(-2) at the top of the atmosphere. This threefold difference (due largely to solar absorption by soot) and the large magnitude of the observed surface forcing both imply that tropical aerosols might slow down the hydrological cycle.     

Forced response of atmospheric oscillations during the last millennium simulated by a climate system model

Variations in global atmospheric oscillations during the last millennium are simulated using the climate system model FGOALS_gl. The model was driven by reconstructions of both natural forcing (solar variability and volcanic aerosol) and anthropogenic forcing (greenhouse gases and sulfate aerosol). The model results are compared against proxy reconstruction data. The reconstructed North Atlantic Oscillation (NAO) was out of phase with the Pacific Decadal Oscillation (PDO) in the last millennium. During the ...     

Global estimate of aerosol direct radiative forcing from satellite measurements

Atmospheric aerosols cause scattering and absorption of incoming solar radiation. Additional anthropogenic aerosols released into the atmosphere thus exert a direct radiative forcing on the climate system. The degree of present-day aerosol forcing is estimated from global models that incorporate a representation of the aerosol cycles. Although the models are compared and validated against observations, these estimates remain uncertain. Previous satellite measurements of the direct effect of aerosols contained limited information about aerosol type, and were confined to oceans only. Here we use state-of-the-art satellite-based measurements of aerosols and surface wind speed to estimate the clear-sky direct radiative forcing for 2002, incorporating measurements over land and ocean. We use a Monte Carlo approach to account for uncertainties in aerosol measurements and in the algorithm used. Probability density functions obtained for the direct radiative forcing at the top of the atmosphere give a clear-sky, global, annual average of -1.9 W m(-2) with standard deviation, +/- 0.3 W m(-2). These results suggest that present-day direct radiative forcing is stronger than present model estimates, implying future atmospheric warming greater than is presently predicted, as aerosol emissions continue to decline.     

Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone

Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed. This expansion is important because it is associated with shifts in large-scale atmospheric circulation and major climate zones. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion, the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents--including black carbon aerosols and tropospheric ozone--are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion. Mechanistically, atmospheric heating from black carbon and tropospheric ozone has occurred at the mid-latitudes, generating a poleward shift of the tropospheric jet, thereby relocating the main division between tropical and temperate air masses. Although we still underestimate tropical expansion, the true aerosol forcing is poorly known and could also be underestimated. Thus, although the insensitivity of models needs further investigation, black carbon and tropospheric ozone, both of which are strongly influenced by human activities, are the most likely causes of observed Northern Hemisphere tropical expansion.     

Possible methane-induced polar warming in the early Eocene

Reconstructions of early Eocene climate depict a world in which the polar environments support mammals and reptiles, deciduous forests, warm oceans and rare frost conditions. At the same time, tropical sea surface temperatures are interpreted to have been the same as or slightly cooler than present values. The question of how to warm polar regions of Earth without noticeably warming the tropics remains unresolved; increased amounts of greenhouse gases would be expected to warm all latitudes equally. Oceanic heat transport has been postulated as a mechanism for heating high latitudes, but it is difficult to explain the dynamics that would achieve this. Here we consider estimates of Eocene wetland areas and suggest that the flux of methane, an important greenhouse gas, may have been substantially greater during the Eocene than at present. Elevated methane concentrations would have enhanced early Eocene global warming, and also might specifically have prevented severe winter cooling of polar regions because of the potential of atmospheric methane to promote the formation of optically thick, polar stratospheric ice clouds.     

Dust direct radiative effects on the earth-atmosphere system over east Asia: Early spring cooling and late spring warming

Focusing on three dust storms occurring in spring 2001,we developed a detailed aerosol parameterization scheme and integrated it in a radiative transfer model to characterize possible impacts of solar altitude angle on dust direct radiative effects over China desert regions and the North Pacific,using actual daily solar altitude angles.Increasing solar altitude angle from early spring (or winter) to late spring (or summer) leads to increase of positive clear sky radiative forcing,and decrease of negative radiative forcing due to dust aerosols at the top of the atmosphere.Because solar altitude angle increases from early to late spring,dust-clear sky radiative forcing may change from negative to positive at the top of atmosphere,showing a change from cooling to heating of the earth-atmosphere system over high-albedo deserts and nearby regions.Over low-albedo ocean negative clear sky radiative forcing by dust may decrease,suggesting a change from strong to weak cooling on the earth-atmosphere system.The impacts of solar altitude angle on cloudy sky radiative forcing due to dust are similar to those of clear sky.Impacts of low cloud on dust radiative forcing are the same as increasing surface albedo.This causes the transition of dust cooling effects into heating effects over deserts to occur earlier,and causes decrease of negative radiative forcing over the ocean and even cause a change from weak negative radiative forcing to weak positive forcing over local areas.Even in the same East Asian desert regions and nearby areas,the strength and sign of the radiative forcings depend on storm dates and thus solar altitude angle.The nearer to early spring (or winter) a dust storm occurs,the easier it leads to negative radiative forcing at the top of atmosphere,which indicates cooling effects on the earth-atmosphere system.In contrast,the nearer to late spring (or summer) a dust storm occurs,the easier it leads to positive radiative forcing at the top of atmosphere,showing heating effects.Over East Asian deserts and nearby regions,dust layers may be regarded as cooling sources in early spring (winter) and warming sources in late spring (summer).     

Climate sensitivity constrained by temperature reconstructions over the past seven centuries

The magnitude and impact of future global warming depends on the sensitivity of the climate system to changes in greenhouse gas concentrations. The commonly accepted range for the equilibrium global mean temperature change in response to a doubling of the atmospheric carbon dioxide concentration, termed climate sensitivity, is 1.5-4.5 K (ref. 2). A number of observational studies, however, find a substantial probability of significantly higher sensitivities, yielding upper limits on climate sensitivity of 7.7 K to above 9 K (refs 3-8). Here we demonstrate that such observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. We use large-ensemble energy balance modelling and simulate the temperature response to past solar, volcanic and greenhouse gas forcing to determine which climate sensitivities yield simulations that are in agreement with proxy reconstructions. After accounting for the uncertainty in reconstructions and estimates of past external forcing, we find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5-95 per cent range shrinks to 1.5-6.2 K, thus substantially reducing the probability of very high climate sensitivity.     

The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing

Air traffic condensation trails, or contrails, are believed to have a net atmospheric warming effect, although one that is currently small compared to that induced by other sources of human emissions. However, the comparably large growth rate of air traffic requires an improved understanding of the resulting impact of aircraft radiative forcing on climate. Contrails have an effect on the Earth's energy balance similar to that of high thin ice clouds. Their trapping of outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) is partly compensated by their reflection of incoming solar radiation (negative radiative forcing). On average, the longwave effect dominates and the net contrail radiative forcing is believed to be positive. Over daily and annual timescales, varying levels of air traffic, meteorological conditions, and solar insolation influence the net forcing effect of contrails. Here we determine the factors most important for contrail climate forcing using a sophisticated radiative transfer model for a site in southeast England, located in the entrance to the North Atlantic flight corridor. We find that night-time flights during winter (December to February) are responsible for most of the contrail radiative forcing. Night flights account for only 25 per cent of daily air traffic, but contribute 60 to 80 per cent of the contrail forcing. Further, winter flights account for only 22 per cent of annual air traffic, but contribute half of the annual mean forcing. These results suggest that flight rescheduling could help to minimize the climate impact of aviation.     

复杂干扰下配电线路无人机巡视目标检测方法研究

配电线路所处环境复杂,传统配电线路无人机巡视目标检测方法无法有效去除外界环境干扰,导致检测结果不可靠。为此,提出一种新的基于视觉感知的配电线路无人机巡视目标检测方法。通过可见光相机与紫外相机对无人机巡线图像进行采集,分析了紫外相机采集原理。通过直方图均衡化处理对采集图像进行增强处理,选用高斯滤波器对采集的配电线路无人机巡视图像进行滤波处理。对配电线路密集度区域和偏心度区域进行提取,获取配电线路无人机巡视目标特征向量。依据得到的特征向量确定无人机巡视目标,求出目标质心坐标,从而实现配电线路无人机巡视目标检测。实验结果表明,所提方法能够在干扰环境下有效实现配电线路无人机巡视目标的检测,检测结果可靠性高。     

Increased Arctic cloud longwave emissivity associated with pollution from mid-latitudes

There is consensus among climate models that Arctic climate is particularly sensitive to anthropogenic greenhouse gases and that, over the next century, Arctic surface temperatures are projected to rise at a rate about twice the global mean. The response of Arctic surface temperatures to greenhouse gas thermal emission is modified by Northern Hemisphere synoptic meteorology and local radiative processes. Aerosols may play a contributing factor through changes to cloud radiative properties. Here we evaluate a previously suggested contribution of anthropogenic aerosols to cloud emission and surface temperatures in the Arctic. Using four years of ground-based aerosol and radiation measurements obtained near Barrow, Alaska, we show that, where thin water clouds and pollution are coincident, there is an increase in cloud longwave emissivity resulting from elevated haze levels. This results in an estimated surface warming under cloudy skies of between 3.3 and 5.2 W m(-2) or 1 and 1.6 degrees C. Arctic climate is closely tied to cloud longwave emission, but feedback mechanisms in the system are complex and the actual climate response to the described sensitivity remains to be evaluated.     

基于海洋大气波导的无人机高速数据链信道模型研究

由于海水的蒸发以及风的湍流扩散作用,使得自海面向上一定高度内,大气湿度随高度递减形成大气波导,无人机在海航时其高速数据链会受到这一异常大气结构的严重影响。针对于这一问题分析了大气波导的形成机理,并运用APM模型计算了其传输损耗,该计算结果表明大气波导内能量衰减较小,致使其可以产生较强的多经分量和弥散作用。从实际雷达测量数据出发,结合大气波导的范围分布与密度分布2个方面的特点提出了弥散离散混合信道模型,给出了该模型的理论分析过程与具体实现方法。确定了延时功率分布特性为该信道的决定因素,为进一步实测信道统计特性提供了重要的理论基础。     

Forced response of atmospheric oscillations during the last millennium simulated by a climate system model

Variations in global atmospheric oscillations during the last millennium are simulated using the climate system model FGOALS_gl. The model was driven by reconstructions of both natural forcing (solar variability and volcanic aerosol) and anthropogenic forcing (greenhouse gases and sulfate aerosol). The model results are compared against proxy reconstruction data. The reconstructed North Atlantic Oscillation (NAO) was out of phase with the Pacific Decadal Oscillation (PDO) in the last millennium. During the Medieval Warm Period (MWP), the NAO was strong while the PDO was weak. During the Little Ice Age (LIA), the NAO was weak while the PDO was strong. A La Niña-like state prevailed in the MWP, while an El Niño-like state dominated in the LIA. This phenomenon is particularly obvious in the 15th, 17th and 19th centuries. Analysis of the model output indicates that the NAO was generally positive during 1000?C1400 AD and negative during 1650?C1900 AD. There is a discrepancy between the simulation and reconstruction during 1400?C1650 AD. The simulated PDO generally varies in parallel with the reconstruction, which has a negative phase during the MWP and a positive phase during the LIA. The correlation coefficient between the reconstruction and simulation is 0.61, and the correlation is statistically significant at the 1% level. Neither the La Niña-like state of the MWP nor the El Niño-like state of the LIA is reproduced in the model. Both the reconstructed and the simulated Antarctic Oscillations had a negative phase in the early period of the last millennium and a positive phase in the late period of the last millennium. The Asian-Pacific Oscillation was generally strong during the WMP and weak during the LIA, and the correlation coefficient between the simulation and reconstruction is 0.50 for the period 1000?C1985 AD. The analysis suggests that the specified external forcings significantly affected the evolution of atmospheric oscillation during the last millennium.     

Direct detection of variable tropospheric clouds near Titan's south pole

Atmospheric conditions on Saturn's largest satellite, Titan, allow the possibility that it could possess a methane condensation and precipitation cycle with many similarities to Earth's hydrological cycle. Detailed imaging studies of Titan have hitherto shown no direct evidence for tropospheric condensation clouds, although there has been indirect spectroscopic evidence for transient clouds. Here we report images and spectra of Titan that show clearly transient clouds, concentrated near the south pole, which is currently near the point of maximum solar heating. The discovery of these clouds demonstrates the existence of condensation and localized moist convection in Titan's atmosphere. Their location suggests that methane cloud formation is controlled seasonally by small variations in surface temperature, and that the clouds will move from the south to the north pole on a 15-year timescale.     

黑碳气溶胶研究新进展

黑碳气溶胶是气溶胶的重要组成部分,在大气物理、大气化学、大气光学、大气光化学等过程中具有重要作用。近年来研究表明,黑碳气溶胶对于全球变暖、区域气候变化有重要贡献,黑碳气溶胶可能是影响全球变暖的第二大重要因子,其作用仅次于CO2。因此,应控制黑碳的排放。考虑到黑碳气溶胶在全球变暖、区域气候、环境与健康等方面的作用,研究和评价黑碳气溶胶的作用已十分必要和迫切。     

对大气气溶胶的辐射效应的数值试验

采用两流近似辐射模式,应用半球常数法近似,采用累加法和倍加法,计算了不同月份大气气溶胶的辐射加热率和减温率的日变化。结果表明,从气候角度而言,气溶胶浓度的差别使得即使是在6月份,气溶胶层既可以是冷源也可以是热源;从天气尺度而言,气溶胶的垂直分布和浓度对层结稳定度有着重要影响,并由此从物理上指出了沙尘暴天气过程中辐射效应所起的重要作用。     

Increased soil emissions of potent greenhouse gases under increased atmospheric CO2

Increasing concentrations of atmospheric carbon dioxide (CO(2)) can affect biotic and abiotic conditions in soil, such as microbial activity and water content. In turn, these changes might be expected to alter the production and consumption of the important greenhouse gases nitrous oxide (N(2)O) and methane (CH(4)) (refs 2, 3). However, studies on fluxes of N(2)O and CH(4) from soil under increased atmospheric CO(2) have not been quantitatively synthesized. Here we show, using meta-analysis, that increased CO(2) (ranging from 463 to 780 parts per million by volume) stimulates both N(2)O emissions from upland soils and CH(4) emissions from rice paddies and natural wetlands. Because enhanced greenhouse-gas emissions add to the radiative forcing of terrestrial ecosystems, these emissions are expected to negate at least 16.6 per cent of the climate change mitigation potential previously predicted from an increase in the terrestrial carbon sink under increased atmospheric CO(2) concentrations. Our results therefore suggest that the capacity of land ecosystems to slow climate warming has been overestimated.     

Non-CO2 greenhouse gases and climate change

Earth's climate is warming as a result of anthropogenic emissions of greenhouse gases, particularly carbon dioxide (CO(2)) from fossil fuel combustion. Anthropogenic emissions of non-CO(2) greenhouse gases, such as methane, nitrous oxide and ozone-depleting substances (largely from sources other than fossil fuels), also contribute significantly to warming. Some non-CO(2) greenhouse gases have much shorter lifetimes than CO(2), so reducing their emissions offers an additional opportunity to lessen future climate change. Although it is clear that sustainably reducing the warming influence of greenhouse gases will be possible only with substantial cuts in emissions of CO(2), reducing non-CO(2) greenhouse gas emissions would be a relatively quick way of contributing to this goal.