Changes in mid-latitude variability due to increasing greenhouse gases and sulphate aerosols

 Intra-seasonal variability in the Northern Hemisphere winter is investigated in ensembles of experiments using the Hadley Centre coupled ocean-atmosphere model. Synoptic techniques which identify low centres and blocking anticyclones have been used in preference to time-filtered variances of geopotential height as these may not uniquely identify true synoptic systems. Changes in mid-latitude variability are qualitatively similar in experiments that include changes in greenhouse gases only and those that also include the direct effects of sulphate aerosols. The presence of aerosols reduces the warming of the northern continents but the relatively larger warming of the land compared to the oceans remains in both experiments and at higher latitudes this leads to a reduced poleward transient flux of energy. There is an increase in the transient flux of energy at most latitudes, due to a greater transport of latent heat arising from the greater moisture availability. The total number of Northern Hemisphere storms decreases but there is a tendency towards deeper low centres. There are fewer storms in the North Pacific and North Atlantic source regions where the local baroclinicity is reduced. The climatological tracks from these regions are shorter with decreases at the northeastern ends of the tracks and increases in the regions with maximum counts towards the centre of the ocean basins. These changes are not generally statistically significant. The greatest changes in blocking anticyclones occur in the North Pacific where there is a downstream shift in the region of maximum activity. Changes in stationary waves show a strong increase in the PNA oscillation which may be influencing the changes in blocking as has been found by other authors. Possible mechanisms for forcing this response are speculated but it will need further experiments to unravel them. Received: 10 September 1997/Accepted: 2 December 1997     

Sensitivity of the hydrological cycle to increasing amounts of greenhouse gases and aerosols

The coupled atmosphere–ocean Climate Model of the Centre National de Recherches Météorologiques (CNRM) has been used to run a time-dependent climate change experiment to study the impact of increasing amounts of greenhouse gases and aerosols on the simulated water cycle. This simulation has been initialised with the oceanic temperature and salinity profiles and the atmospheric trace gas concentrations observed in the 1950s, and has been carried out for 150 years after a 20-year spin-up. The simulated climate change has been analysed as the difference between two 30-year time slices: 1970–2000 and 2070–2100 respectively. The model achieves a reasonable simulation of present-day climate and simulates a general increase in precipitation throughout the twenty first century. The main exceptions are the subtropics, where the enhanced Hadley circulation has a drying impact, and the mid-latitude continents, where the increased evaporation in spring and decreased moisture convergence in summer lead to a relative summer drying. Global and regional analyses suggest that the precipitation increase is generally limited by a decrease in the water vapour cycling rate and in the precipitation efficiency, which appear as key parameters of the simulated water cycle. In order to reduce the spread between climate scenarios, more efforts should be devoted to estimate these parameters from satellite observations and meteorological analyses, and their possible evolution over recent decades. In the present study, the impacts of global warming on the surface hydrology have been also investigated. The main findings are the amplification of the annual cycle of soil moisture in the mid-and-high latitudes, and the decrease in the Northern Hemisphere snow cover, at a rate that is consistent with recent satellite estimations and should increase during the twenty first century. The runoff simulated over the 1950–2100 period has been converted into river flow using a linear river routeing model. The trends simulated over recent decades are surprisingly consistent with the river flow measurements available from the Global Runoff Data Centre. These trends can differ from those estimated over the whole 150-year integration, thereby indicating that it is not safe to predict hydrological impacts just by extrapolating the trends found in the available observations. Our climate model seems likely to provide qualitative hydrological scenarios over large river basins, but it still shows serious biases in the simulation of present-day river flows. Regional hydrological projections remain a challenge for the global climate modelling community and downscaling techniques are still necessary for this purpose.     

Competition of NAO regime changes and increasing greenhouse gases and aerosols with respect to Arctic climate projections

Regional magnitudes and patterns of Arctic winter climate changes in consequence of regime changes of the North Atlantic Oscillation (NAO) are analyzed using a regional atmospheric climate model. The regional model has been driven with data of positive and negative NAO phases from a control simulation as well as from a time-dependent greenhouse gas and aerosol scenario simulation. Both global model simulations include a quite realistic interannual variability of the NAO with pronounced decadal regime changes and no or rather weak long-term NAO trends. The results indicate that the effects of NAO regime changes on Arctic winter temperatures and precipitation are regionally significant over most of northwestern Eurasia and parts of Greenland. In this regard, mean winter temperature variations of up to 6 K may occur over northern Europe. Precipitation and synoptic variability are also regionally modified by NAO regime changes, but not as significantly as temperatures. However, the climate changes associated with the NAO are in some regions clearly stronger than those attributed to enhanced greenhouse gases and aerosols, indicating that projected global changes of the atmospheric composition and internal circulation changes are competing with each other in their importance for the Arctic climate evolution in the near future. The knowledge of the future NAO trend on decadal and longer time scales appears to be vitally important in terms of a regional assessment of climate scenarios for the Arctic.     

The 1983 drought in the West Sahel: a case study

Some drought years over sub-Saharan west Africa (1972, 1977, 1984) have been previously related to a cross-equatorial Atlantic gradient pattern with anomalously warm sea surface temperatures (SSTs) south of 10°N and anomalously cold SSTs north of 10°N. This SST dipole-like pattern was not characteristic of 1983, the third driest summer of the twentieth century in the Sahel. This study presents evidence that the dry conditions that persisted over the west Sahel in 1983 were mainly forced by high Indian Ocean SSTs that were probably remanent from the strong 1982/1983 El Ni?o event. The synchronous Pacific impact of the 1982/1983 El Ni?o event on west African rainfall was however, quite weak. Prior studies have mainly suggested that the Indian Ocean SSTs impact the decadal-scale rainfall variability over the west Sahel. This study demonstrates that the Indian Ocean also significantly affects inter-annual rainfall variability over the west Sahel and that it was the main forcing for the drought over the west Sahel in 1983.     

The effects of anthropogenic greenhouse gases and aerosols on the inter-decadal change of the South China Sea summer monsoon in the late twentieth century

Climate Dynamics - Analysis of observational precipitation indicates that in last few decades, the precipitation in boreal summer (June–August) over the South China Sea (SCS) exhibited an...     

The dynamics of the Indian Ocean sea surface temperature forcing of Sahel drought

Given the pronounced warming in the Indian Ocean sea surface temperature (SST) during the second half of the twentieth century and the empirical relationship between the Indian Ocean SST and Sahel summer precipitation, we investigate the mechanisms underlying this relationship using the GFDL atmospheric model AM2.0 to simulate the equilibrium and transient response to the warming of the Indian Ocean. Equatorial wave dynamics, in particular the westward propagating equatorial Rossby waves, communicates the signal of tropospheric warming and stabilization from the Indian Ocean to the African continent. The stabilization associated with the Rossby wave front acts to suppress the convection. Feedbacks with local precipitation and depletion of moisture amplify the dynamically induced subsidence. While this stabilization mechanism is expected to operate in climate change response, the future prospects for the Sahelian climate under global warming are complicated by the intricate sensitivities to the SSTs from different ocean basins and to the direct radiative forcing of greenhouse gases.     

The role of synoptic systems in the interannual variability of Sahel rainfall

Summary Components of the June-September climate over the Sahel are investigated in simulations with the GCM of the NASA/Goddard Institute for Space Studies, forced by SST observed during 1987 and 1988. The study analyzes the role of the synoptic patterns in determining precipitation differences between the two seasons, with special attention given to African wave disturbances (AWD). Emphasis is placed on deducing the characteristics of individual systems which may be missed by spectral and/or composite analyses. Results are derived from time-longitude cross-sections and spatial distributions of daily and weekly averages of key climatological variables. Despite the overall rainier season, rainless AWD are more prevalent in the simulations corresponding to June–September 1988 forcing than for 1987. Daily precipitation is shown to be highly correlated with mid-tropospheric vorticity, near surface convergence and 200 mb divergence. August daily rainfall was some-what better correlated with implied large scale vertical motion for 1988 forcing, emphasizing the dominance of broad circulation influences during that summer. While significant rainfall variability is attributed to AWD, quasistationary mechanisms cannot be ignored. In these simulations, upper tropospheric divergence modulated by changes in the Tropical Easterly Jet serves to both intensify the rainfall triggered by AWD and to sustain broader rainfall patterns during events of massive uplift.With 16 Figures     

Economics- and physical-based metrics for comparing greenhouse gases

A range of alternatives to the Global Warming Potential (GWP) have been suggested in the scientific literature. One of the alternative metrics that has received attention is the cost-effective relative valuation of greenhouse gases, recently denoted Global Cost Potential (GCP). However, this metric is based on complex optimising integrated assessment models that are far from transparent to the general scientist or policymaker. Here we present a new analytic metric, the Cost-Effective Temperature Potential (CETP) which is based on an approximation of the GCP. This new metric is constructed in order to enhance general understanding of the GCP and elucidate the links between physical metrics and metrics that take economics into account. We show that this metric has got similarities with the purely physical metric, Global Temperature change Potential (GTP). However, in contrast with the GTP, the CETP takes the long-term temperature response into account.     

The modification of greenhouse gas warming by the direct effect of sulphate aerosols

 The Canadian Centre for Climate Modelling and Analysis (CCCma) second generation climate model (GCMII) consists of an atmospheric GCM coupled to mixed layer ocean. It is used to investigate the climate response to a doubling of the CO₂ concentration together with the direct effect of scattering by sulphate aerosols. As expected, the aerosols offset some of the greenhouse gas (GHG) warming; the global annual mean screen temperature change due to doubled CO₂ is 3.4 °C in this model and this is reduced to 2.7 °C when an estimate of the direct effect of anthropogenic sulphate aerosols is included. The pattern of climate response to the comparatively localized aerosol forcing is not itself localized, and it bears a striking resemblance to the response pattern that arises from the globally distributed change in GHG forcing. This “non-local” response to “localized” forcing indicates that the pattern of climate response is determined, to first order, by the overall magnitude of the change in forcing rather than its detailed nature or structure. Feedback processes operating in the system apparently determine this pattern by locally amplifying and suppressing the response to the magnitude of the change in forcing. The influence of the location of the change in forcing is relatively small. These “non-local” and “local” effects of aerosol forcing are characterized and displayed and some of their consequences discussed. Effects on the moisture budget and on the energetics of the global climate are also examined. Received: 10 June 1997 / Accepted: 8 January 1998     

Assessment of greenhouse gases emission from civil aviation in Russia

The greenhouse gases emission (CO₂, CH₄, and N₂O) from domestic and international aviation in the Russian Federation is assessed. In 2007, the total emission of CO₂, CH₄, and N₂O amounted to 18.4 million tons of CO₂-equivalent, which is 21% below the 1990 level. Carbon dioxide dominates in the component composition of the emissions, its part in 2007 accounted for 99.1% of the emission. Taking into account the tendency towards increasing fuel consumption due to intense aircraft traffic it can be expected that compared to the present level the greenhouse gases emissions in 2012 and 2020 will increase by 15 and 45%, respectively. Accounting for the increased aircraft emissions as well as plans of foreign countries to include the international aviation into the scheme of greenhouse gases emission allowance (trade credits) it is expedient to make more precise the greenhouse gases emissions from the Russian aviation based on the detailed flight data for all types of the aircraft.     

In-cloud scavenging of gases and aerosols at a mountain site in Central Switzerland

An in-cloud scavenging case study of the major ions (NH₄ ⁺, SO₄ ^2- and NO₃ ^-) determining the cloudwater composition at a mountain site (1620 m.a.s.l.) is presented. A comparison between in-cloud measurements of the cloudwater composition, liquid water content, gas concentrations and aerosol concentrations and pre-cloud gas and aerosol concentrations yields the following results. Cloudwater concentrations resulted from scavenging of about half of the available NH₃, aerosol NH₄ ⁺, aerosol NO₃ ^-, and aerosol SO₄ ^2-. Approximately a third of the SO₂ was scavenged by the cloudwater and oxidized to SO₄ ^2-. Cloud acidity during the first two hours of cloud interception (pH 3.24) was determined mostly by the scavenged gases (NH₃, SO₂, and HNO₃); aerosol contributions to the acidity were found to be small. Observations of gas and aerosol concentrations at three elevations prior to several winter precipitation events indicated that NH₃ concentrations are typically half (12–80 %) of the total (gas and aerosol) N (-III) concentrations. HNO₃ typically is present at much lower concentrations (1–55 %) than aerosol NO₃ ^-. Concentrations of SO₂ are a substantial component of total sulfur, with concentrations averaging 60 % (14–76 %) of the total S (IV and VI).     

Transient climatic response to an increase of greenhouse gases

The physical factors governing the transient climatic response to an increase of greenhouse gases are discussed, reasons for remaining uncertainties are identified, and recent climate modelling results are briefly summarized. The relevance of the transient response, and of uncertainties in the transient response, to questions such as the applicability of equilibrium climate model simulations to a gradual greenhouse gas increase, the verification of model projections, rates of climatic change, and the impacts of preventative strategies for dealing with the buildup of greenhouse gases is also discussed.     

Simulated ENSO-tropical rainfall teleconnections in present-day and under enhanced greenhouse gases conditions

El-Niño/Southern Oscillation (ENSO) variability and its relationship with precipitation in the tropics and subtropics are analysed using the ARPEGE-OPA ocean-atmosphere coupled model. Three 150-year simulations are considered, differing by greenhouse gases (GHG) and aerosols concentrations. The first one has constant (1950 level) concentrations, and the two others follow observed values till 1999, then the SRES B2 scenario until 2099. The model is able to reproduce most present-day features characteristic of ENSO in the Pacific. It also displays ENSO as the leading mode of sea-surface temperature (SST) variability, with spatial patterns and explained variance both quite similar to the observation. A detailed analysis of its teleconnections with rainfall variability is carried out on a seasonal basis. Patterns for the last part of the twentieth century compare favourably with the observation, with the notable exception of parts of the Atlantic sector. The overall strong rainfall response arises from the strong interannual variability of simulated ENSO, and also suggests an ability to simulate atmospheric dynamics in a realistic way. In the future climate, the model does not exhibit major changes in the ENSO/rainfall teleconnections. However, on a regional basis, there is some evidence of strengthening (e.g., in parts of Southern Africa) and weakening (e.g., East Africa) in the course of the twenty-first century. In most cases, decadal swings in the correlations suggest that these alterations may partly reflect natural changes in the teleconnections with ENSO, long-term correlation trends (possibly GHG-induced) being comparatively weaker.     

The African Sahel 25 years after the great drought: assessing progress and moving towards new agendas and approaches

This paper introduces a special issue of Global Environmental Change: Human dimensions on the Sahel of West Africa. It reviews the seminar to which the papers were presented, and brings together some conclusions. Despite the quarter century of research into the West African Sahel that followed the great droughts of the 1970s, there are still strong disagreements about how to achieve more prosperous, yet sustainable livelihood systems in the region. There are conflicts between those who believe in indigenous capacities to maintain rural livelihoods, those who believe that various forms of external support are necessary, and those wedded to a vision of a Sahel directed by regional urban growth. Under economic and cultural globalisation, the future of this region is, at best, unclear. The papers in this collection do agree that Sahelian environments are diverse, and that Sahelian people cultivate and exploit diversity and flexibility. They also suggest that there are no quick-fix development solutions, except to build upon this historical diversity with renewed purpose.     

大气污染物与温室气体减排协同效应评估方法及应用

基于二维四象限图构建了一个量化大气污染控制和温室气体减排协同效应的评估指标,建立了量化评估协同效应方法;针对《大气污染防治行动计划》评估中能源结构调整和产业结构调整措施进行了协同效应量化实施效果评估.结果显示:所有实施的减排污染物的措施均有正的CO2减排协同效应,应该积极鼓励和推荐.实现CO2和SO2减排最大协同效应的...     

Changes of interannual NAO variability in response to greenhouse gases forcing

Observations show that there was change in interannual North Atlantic Oscillation (NAO) variability in the mid-1970s. This change was characterized by an eastward shift of the NAO action centres, a poleward shift of zonal wind anomalies and a downstream extension of climate anomalies associated with the NAO. The NAO interannual variability for the period after the mid-1970s has an annular mode structure that penetrates deeply into the stratosphere, indicating a strengthened relationship between the NAO and the Arctic Oscillation (AO) and strengthened stratosphere-troposphere coupling. In this study we have investigated possible causes of these changes in the NAO by carrying out experiments with an atmospheric GCM. The model is forced either by doubling CO₂, or increasing sea surface temperatures (SST), or both. In the case of SST forcing the SST anomaly is derived from a coupled model simulation forced by increasing CO₂. Results indicate that SST and CO₂ change both force a poleward and eastward shift in the pattern of interannual NAO variability and the associated poleward shift of zonal wind anomalies, similar to the observations. The effect of SST change can be understood in terms of mean changes in the troposphere. The direct effect of CO₂ change, in contrast, can not be understood in terms of mean changes in the troposphere. However, there is a significant response in the stratosphere, characterized by a strengthened climatological polar vortex with strongly enhanced interannual variability. In this case, the NAO interannual variability has a strong link with the variability over the North Pacific, as in the annular AO pattern, and is also strongly related to the stratospheric vortex, indicating strengthened stratosphere-troposphere coupling. The similarity of changes in many characteristics of NAO interannual variability between the model response to doubling CO₂ and those in observations in the mid-1970s implies that the increase of greenhouse gas concentration in the atmosphere, and the resulting changes in the stratosphere, might have played an important role in the multidecadal change of interannual NAO variability and its associated climate anomalies during the late twentieth century. The weak change in mean westerlies in the troposphere in response to CO₂ change implies that enhanced and eastward extended mid-latitude westerlies in the troposphere might not be a necessary condition for the poleward and eastward shift of the NAO action centres in the mid-1970s.     

Anthropogenic aerosols and gases in the lower troposphere at Alert Canada in April 1986

During April 1986, as part of an international arctic air chemistry study (AGASP-2), ground level observations of aerosol trace elements, oxides of sulphur and nitrogen and particle number size distribution were made at Alert Canada (82.5N, 62.3W). Pollution haze was evident as indicated by daily aerosol number (size > 0.15 m diameter) and SO₄ ⁼ concentrations in the range 125 – 260 cm^–3 and 1.6 – 4.5 g m^–3, respectively. Haze and associated acidic gases tended to increase throughout the period. SO₂ and peroxyacetylnitrate (PAN) mixing ratios were in the range 140 – 480 and 370 – 590 ppt(v), respectively. About 88% of the total end-product nitrogen was in the form of PAN. In air dried to 2% relative humidity by warming to room temperature, the aerosol mass size distribution had a major mode at 0.3 m diameter and a minor one at 2.5 m. Aerosol mass below 1.5 m was well correlated with SO₄ ⁼, K⁺ and PAN. There was a steady increase in the oxidized fraction of total airborne sulphur and nitrogen oxide throughout April as the sun rose above the horizon and remained above. The mean oxidation rate of SO₂ between Eurasia and Alert was estimated as 0.25 – 0.5% h^–1. The molar ratio of total nitrogen oxide to total sulphur oxide in the arctic atmosphere (0.67±0.17) was comparable to that in European emissions. A remarkably strong inverse correlation of filterable Br and O₃ led to the conclusion that O₃ destruction and filterable Br production below the Arctic surface radiation inversion is associated with tropospheric photochemical reactions involving naturally occurring gaseous bromine compounds.     

Global and regional evolution of short-lived radiatively-active gases and aerosols in the Representative Concentration Pathways

In this paper, we discuss the results of 2000?C2100 simulations following the emissions associated with the Representative Concentration Pathways (RCPs) with a chemistry-climate model, focusing on the changes in 1) atmospheric composition (troposphere and stratosphere) and 2) associated environmental parameters (such as nitrogen deposition). In particular, we find that tropospheric ozone is projected to decrease (RCP2.6, RCP4.5 and RCP6) or increase (RCP8.5) between 2000 and 2100, with variations in methane a strong contributor to this spread. The associated tropospheric ozone global radiative forcing is shown to be in agreement with the estimate used in the RCPs, except for RCP8.5. Surface ozone in 2100 is projected to change little compared from its 2000 distribution, a much-reduced impact from previous projections based on the A2 high-emission scenario. In addition, globally-averaged stratospheric ozone is projected to recover at or beyond pre-1980 levels. Anthropogenic aerosols are projected to strongly decrease in the 21st century, a reflection of their projected decrease in emissions. Consequently, sulfate deposition is projected to strongly decrease. However, nitrogen deposition is projected to increase over certain regions because of the projected increase in NH₃ emissions.