Temperature stabilization, ocean heat uptake and radiative forcing overshoot profiles

Political leaders in numerous nations argue for an upper limit of the global average surface temperature of 2 K above the pre-industrial level, in order to attempt to avoid the most serious impacts of climate change. This paper analyzes what this limit implies in terms of radiative forcing, emissions pathways and abatement costs, for a range of assumptions on rate of ocean heat uptake and climate sensitivity. The primary aim is to analyze the importance of ocean heat uptake for radiative forcing pathways that temporarily overshoot the long-run stabilization forcing, yet keep the temperature increase at or below the 2 K limit. In order to generate such pathways, an integrated climate-economy model, MiMiC, is used, in which the emissions pathways generated represent the least-cost solution of stabilizing the global average surface temperature at 2 K above the pre-industrial level. We find that the level of overshoot can be substantial. For example, the level of overshoot in radiative forcing in 2100 ranges from about 0.2 to 1 W/m2, where the value depends strongly and positively on the effective diffusivity of heat in the oceans. Measured in relative terms, the level of radiative forcing overshoot above its longrun equilibrium level in 2100 is 20% to 60% for high values of climate sensitivity (i.e., about 4.5 K) and 8% to 30% for low values of climate sensitivity (i.e., about 2 K). In addition, for cases in which the radiative forcing level can be directly stabilized at the equilibrium level associated with a specific climate sensitivity and the 2 K limit, the net present value abatement cost is roughly cut by half if overshoot pathways are considered instead of stabilization of radiative forcing at the equilibrium level without an overshoot.     

Importance of instantaneous radiative forcing for rapid tropospheric adjustment

To better understand CFMIP/CMIP inter-model differences in rapid low cloud responses to CO₂ increases and their associated effective radiative forcings, we examined the tropospheric adjustment of the lower tropospheric stability (LTS) in three general circulation models (GCMs): HadGEM2-A, MIROC3.2 medres, and MIROC5. MIROC3.2 medres showed a reduction in LTS over the sub-tropical ocean, in contrast to the other two models. This reduction was consistent with a temperature decrease in the mid-troposphere. The temperature decrease was mainly driven by instantaneous radiative forcing (RF) caused by an increase in CO₂. Reductions in radiative and latent heating, due to clouds, and in adiabatic and advective heating, also contribute to the temperature decrease. The instantaneous RF in the mid-troposphere in MIROC3.2 medres is inconsistent with the results of line-by-line (LBL) calculations, and thus it is considered questionable. These results illustrate the importance of evaluating the vertical profile of instantaneous RF with LBL calculations; improved future model performance in this regard should help to increase our confidence in the tropospheric adjustment in GCMs.     

Stratosphere adjusted radiative forcing calculationsin a comprehensive climate model

Summary  A new technical procedure is introduced to determine the stratosphere adjusted radiative forcing at the tropopause in the framework of the 3-D climate model ECHAM4. However, the procedure appears to be appropriate for other GCMs as well. It allows to study in a straightforward way the problem of the general usefulness of radiative forcing as a reliable predictor of climate change. Some examples are given for illustration. It is, once again, confirmed that the climate sensitivity is practically equal for experiments with increased solar insolation and increased CO₂ concentration. However, a higher climate sensitivity is obtained for ozone perturbations with a horizontally or vertically inhomogeneous distribution. The latter finding is in qualitative agreement with respective results reported in other studies, whereas the value of the climate sensitivity is exceptionally high in our model. The physical reasons for the unique model behaviour in the ozone experiments are currently not understood. Received August 28, 2000 Revised January 2, 2001     

Future change of Asian-Australian monsoon under RCP 4.5 anthropogenic warming scenario

We investigate the future changes of Asian-Australian monsoon (AAM) system projected by 20 climate models that participated in the phase five of the Coupled Model Intercomparison Project (CMIP5). A metrics for evaluation of the model’s performance on AAM precipitation climatology and variability is used to select a subset of seven best models. The CMIP5 models are more skillful than the CMIP3 models in terms of the AAM metrics. The future projections made by the selected multi-model mean suggest the following changes by the end of the 21st century. (1) The total AAM precipitation (as well as the land and oceanic components) will increase significantly (by 4.5 %/°C) mainly due to the increases in Indian summer monsoon (5.0 %/°C) and East Asian summer monsoon (6.4 %/°C) rainfall; the Australian summer monsoon rainfall will increase moderately by 2.6 %/°C. The “warm land-cool ocean” favors the entire AAM precipitation increase by generation of an east-west asymmetry in the sea level pressure field. On the other hand, the warm Northern Hemisphere-cool Southern Hemisphere induced hemispheric SLP difference favors the ASM but reduces the Australian summer monsoon rainfall. The combined effects explain the differences between the Asian and Australian monsoon changes. (2) The low-level tropical AAM circulation will weaken significantly (by 2.3 %/°C) due to atmospheric stabilization that overrides the effect of increasing moisture convergence. Different from the CMIP3 analysis, the EA subtropical summer monsoon circulation will increase by 4.4 %/°C. (3) The Asian monsoon domain over the land area will expand by about 10 %. (4) The spatial structures of the leading mode of interannual variation of AAM precipitation will not change appreciably but the ENSO-AAM relationship will be significantly enhanced.     

基于高分辨率模拟数据RCP4.5情景下的华中区域气候变化预估

基于RegCM4.4高分辨率区域气候模式数据和华中区域1986—2005年逐日气象观测资料,在对模式模拟性能检验的基础上,对中国华中区域未来不同时期、1.5℃和2℃温升阈值下气候变化进行预估.结果表明:模拟结果能较准确反映出区域气温、降水年内变化特征及空间分布特征;与观测值相比,气温模拟值偏低、降水模拟值偏大;与198...     

On aspects of the concept of radiative forcing

 The concept of radiative forcing has been extensively used as an indicator of the potential importance of climate change mechanisms. It allows a first order estimate of the global-mean surface temperature change; and it is possible to compare forcings from different mechanisms, on the assumption that similar global-mean forcings produce similar global-mean surface temperature changes. This study illustrates two circumstances where simple models show that the conventional definition of radiative forcing needs refining. These problems arise mainly with the calculation of forcing due to stratospheric ozone depletion. The first part uses simple arguments to produce an alternative definition of radiative forcing, using a time-dependent stratospheric adjustment method, which can give different forcings from those calculated using the standard definition. A seasonally varying ozone depletion can produce a quite different seasonal evolution of forcing than fixed dynamical heating arguments would suggest. This is especially true of an idealised and extreme “Antarctic ozone hole” type scenario where a sudden loss of ozone is followed by a sudden recovery. However, for observed ozone changes the annually averaged forcing is usually within 5% of the forcing calculated using the fixed dynamical heating approximation. Another problem with the accepted view of radiative forcing arises from the definition of the tropopause considered in the second part of this study. For a correct radiative forcing estimate the “tropopause” needs to separate the atmosphere into regions with a purely radiative response and those with a radiative-convective response. From radiative-convective model results it is found that radiative equilibrium conditions persist for several kilometres below the tropopause (the tropopause being defined as where the lapse rate reaches 2 K km^-1). This region needs to be included in stratospheric adjustment calculations for an accurate calculation of forcing, as it is only the region between the surface and the top of the convection that can be considered as a single, forced, system. Including temperature changes in this region has a very large effect on stratospheric ozone forcing estimates, and can reduce the magnitude of the forcing by more than a factor of two. Although these experiments are performed using simple climate models, the results are of equal importance for the analysis of forcing-response relationships using general circulation models. Received: 25 October 1996/Accepted: 14 April 1997     

Why radiative forcing might fail as a predictor of climate change

Radiative forcing has been widely used as a metric of climate change, i.e. as a measure by which various contributors to a net surface temperature change can be quantitatively compared. The extent to which this concept is valid for spatially inhomogeneous perturbations to the climate system is tested. A series of climate model simulations involving ozone changes of different spatial structure reveals that the climate sensitivity parameter is highly variable: for an ozone increase in the northern hemisphere lower stratosphere, it is more than twice as large as for a homogeneous CO₂ perturbation. A global ozone perturbation in the upper troposphere, however, causes a significantly smaller surface temperature response than CO₂. The variability of the climate sensitivity parameter is shown to be mostly due to the varying strength of the stratospheric water vapour feedback. The variability of the sea-ice albedo feedback modifies climate sensitivity of perturbations with the same vertical structure but a different horizontal structure. This feedback is also the origin of the comparatively larger climate sensitivity to perturbations restricted to the northern hemisphere extratropics. As cloud feedback does not operate independently from the other feedbacks, quantifying its effect is rather difficult. However, its effect on the variability of for horizontally and vertically inhomogeneous perturbations within one model framework seems to be comparatively small.This revised version was published online March 2005 with corrections to table 5.     

Adjusted radiative forcing and global radiative feedbacks in CNRM-CM5, a closure of the partial decomposition

This study provides a comprehensive global analysis of the climate radiative feedbacks and the adjusted radiative forcing for a CO₂ increase perturbation in the CNRM-CM5 climate model using the partial radiative perturbations (PRP) method. Some methodological key points of the PRP are investigated, with a particular focus on the consideration of the effect of fast adjustments. First, the standard PRP method is applied by neglecting certain fast adjustments. The effect of the field decorrelation is highlighted by performing a PRP across two different periods of a control experiment and by analyzing second-order terms. Sensitivity tests to the field substitution frequency, the sampling period and the perturbed experiment used are performed. The impact of the definition of the top of the climate system (top-of-the-atmosphere or tropopause) in the feedback estimate is also discussed. Secondly, the fast adjustment processes are taken into account by combining the PRP framework with the method of linear regression of the partial net radiative flux change against the mean surface air temperature change using a step forcing experiment. This method allows us to quantify the contribution of the different constituents to the forcing adjustment and to improve the estimation of the radiative feedbacks. It is shown that such decomposition allows the retrieval of the adjusted radiative forcing, the radiative feedbacks and the climate sensitivity as estimated with the linear regression method with a high level of accuracy, validating the partial decomposition.     

Aerosol and ozone changes as forcing for climate evolution between 1850 and 2100

Global aerosol and ozone distributions and their associated radiative forcings were simulated between 1850 and 2100 following a recent historical emission dataset and under the representative concentration pathways (RCP) for the future. These simulations were used in an Earth System Model to account for the changes in both radiatively and chemically active compounds, when simulating the climate evolution. The past negative stratospheric ozone trends result in a negative climate forcing culminating at ?0.15 W m^?2 in the 1990s. In the meantime, the tropospheric ozone burden increase generates a positive climate forcing peaking at 0.41 W m^?2. The future evolution of ozone strongly depends on the RCP scenario considered. In RCP4.5 and RCP6.0, the evolution of both stratospheric and tropospheric ozone generate relatively weak radiative forcing changes until 2060–2070 followed by a relative 30 % decrease in radiative forcing by 2100. In contrast, RCP8.5 and RCP2.6 model projections exhibit strongly different ozone radiative forcing trajectories. In the RCP2.6 scenario, both effects (stratospheric ozone, a negative forcing, and tropospheric ozone, a positive forcing) decline towards 1950s values while they both get stronger in the RCP8.5 scenario. Over the twentieth century, the evolution of the total aerosol burden is characterized by a strong increase after World War II until the middle of the 1980s followed by a stabilization during the last decade due to the strong decrease in sulfates in OECD countries since the 1970s. The cooling effects reach their maximal values in 1980, with ?0.34 and ?0.28 W m^?2 respectively for direct and indirect total radiative forcings. According to the RCP scenarios, the aerosol content, after peaking around 2010, is projected to decline strongly and monotonically during the twenty-first century for the RCP8.5, 4.5 and 2.6 scenarios. While for RCP6.0 the decline occurs later, after peaking around 2050. As a consequence the relative importance of the total cooling effect of aerosols becomes weaker throughout the twenty-first century compared with the positive forcing of greenhouse gases. Nevertheless, both surface ozone and aerosol content show very different regional features depending on the future scenario considered. Hence, in 2050, surface ozone changes vary between ?12 and +12 ppbv over Asia depending on the RCP projection, whereas the regional direct aerosol radiative forcing can locally exceed ?3 W m^?2.     

Time of emergence of climate signals over China under the RCP4.5 scenario

The signal of climate change is emerging against a background of natural internal variability. The time of emergence (ToE) is an indicator of the magnitude of the climate change signal relative to this background variability and may be useful for climate impact assessments. In this work, we examined the ToE of surface air temperature and precipitation over China under a medium mitigation scenario Representative Concentration Pathway 4.5 based on 30 satisfactory global climate models that are chosen from the Coupled Model Intercomparison Project Phase 5. Major conclusions are: the earliest ToE of annual and seasonal temperature occurs in the eastern Qinghai-Tibetan Plateau between 2006 and 2012 for S/N?>?1.0 and between 2020 and 2030 for S/N?>?2.0, which is 10–20 years sooner than in Northeast China where the latest ToE appears in the country. Consistent with previous studies at the global scale, the median ToE for most of China occurs sooner in summer (2008–2020 for S/N?>?1.0 and 2020–2045 for S/N?>?2.0), while for Northeast and North China the median ToE occurs sooner in autumn (2015–2025 for S/N?>?1.0 and 2040–2050 for S/N?>?2.0). For the ToE of temperature, the inter-model uncertainty is at least 24 years in all five regions of concern and more than 85 years in some seasons, and the inter-model uncertainty in one season for which the earliest median ToE occurs is the smallest among the seasons. For precipitation, the early ToE occurs in the northeastern Qinghai-Tibetan Plateau for the annual mean, and seasonally it occurs first in winter in northern Northeast China and southwestern Northwest China and in winter and spring in the northeastern Qinghai-Tibetan Plateau. For southern China, the median ToE will not occur until 2090.     

Relative effects of solar and infrared radiative forcing in a mesoscale model

A new efficient parameterization scheme for solar short-wave radiative heating, as a component of the net radiative effects in the atmosphere, is tested in a three-dimensional mesoscale model. This model is designed with moist convective processes in mind, so that the radiative parameterization (solar plus thermal infrared) are interactive with the cloud field. Previous work by the authors with only an infrared scheme has demonstrated that cloud-radiation interactions are characterized by strong cloud-top cooling, leading to upper cloud-layer destabilization. The effects of including solar heating are to modulate the strength of the strong infrared cooling, thereby leading to weaker interactions between clouds, radiation, and mesoscale fields. The present study shows that even on the mesoscale and for relatively short time-spans, radiative processes in the presence of clouds are not negligible.As a further step, a simple fractional cloud cover parameterization is introduced and the model response is compared with results omitting this parameterization.     

RCP4.5情景下中国季风区及降水变化预估

本文使用国际耦合模式比较计划第五阶段(CMIP5)中共46个全球气候模式的数值试验结果,通过对中国区域的年、夏季和冬季降水气候态的模拟能力评估,择优选取了18个气候模式用来预估RCP4.5情景下21世纪中国季风区范围、季风降水及其强度变化。结果表明,相对于1986~2004年参考时段,RCP4.5情景下多数模式和所有模式集合平均在不同时段内均模拟出中国季风区面积、季风降水及其强度的增加趋势,最明显的时段出现在2081~2099年。其中,季风区面积扩张是导致季风降水增加的主要因素。在机制上,热力与动力条件变化均有利于季风降水强度的增加以及更多的水汽进入中国东部,从而引起季风区范围的扩大。     

Radiative energy partition and cloud radiative forcing at a Po valley site

Values of downward and upward flux densities of solar and terrestrial radiation were continuously recorded between 1 December 2001 and 30 November 2002 using a four-components radiometer at S. Pietro Capofiume (SPC) in northern Italy (44°39′N, 11°37′E, alt. 11 m a.m.s.l.), which is characterized by a weakly-reflective surface. The aim of the study was to investigate the effects of clouds on surface radiation balance (SRB); the cloud fraction (N) has been retrieved through the inverted form of the parameterization proposed by Kasten and Czeplak [Solar Energy 24 (1980) 177] and cloud types estimated following the methodology of Duchon and O'Malley [J. Appl. Meteorol. 38 (1999) 132]. The cloud radiative forcing (CRF) was evaluated through the Bintanja and Van den Broeke [Int. J. Climatol. 16 (1996) 1281] formula and then associated with cloud type. Experimental results showed that during the measuring period the net shortwave (Sw) balance always decreased with increasing N, whereas the net longwave (Lw) balance increased in all seasons. The net radiation available at the surface decreased with increasing N in all seasons except in winter, where no significant dependency was detected.The analysis of the cloud radiative forcing indicates that all seasons were characterized by a net cooling of the surface except winter, where clouds seem to have no effects on the surface warming or cooling. Taking into account the dependence on solar radiation cycle, an intercomparison between the retrieved cloud types seems to indicate that the effect of stratus was a slight cooling whereas that of cumulus clouds was a stronger cooling of the surface. On the contrary, cirrus clouds seem to have slight warming effect on the surface.The annual trends of mean monthly values of shortwave and longwave radiation balances confirmed that the measurement site is characterized by a temperate climate. Moreover, in spite of the cooling effect of clouds, a monthly radiative energy surplus is available all year long for surface–atmosphere energy exchanges. The analysis is also instrumental for the detection of SRB variations.     

飞机尾迹云识别及其辐射强迫的研究进展

喷气式飞机在对流层上层的航空活动形成的尾迹云能够影响区域气候,对全球变暖有正的贡献。飞机尾迹云的辐射强迫与飞机尾迹云的区域覆盖率、物理特性以及光学特性密切相关。本文回顾了近几十年来线状尾迹云的相关研究进展,并分析总结了线状尾迹云的不同识别方法。首先讨论了欧美主流的尾迹云检测算法(Contrail Detection Algorithm,CDA)及其延伸算法,并总结了线状尾迹云在西欧、北美等地区覆盖率的季节和昼夜变化特点;接着讨论了以往多种线状尾迹云光学厚度的计算方案及其计算不确定性;最后分析了线状尾迹云的辐射强迫与覆盖率、光学厚度的关系,并指出目前飞机尾迹云相关研究存在的问题以及未来发展方向。     

On summing the components of radiative forcing of climate change

 Radiative forcing is a useful concept in determining the potential influence of a particular mechanism of climate change. However, due to the increased number of forcing agents identified over the past decade, the total radiative forcing is difficult to assess. By assigning a range of probability distribution functions to the individual radiative forcings and using a Monte-Carlo approach, we estimate the total radiative forcing since pre-industrial times including all quantitative radiative forcing estimates to date. The resulting total radiative forcing has a 75–97% probability of being positive (or similarly a 3–25% probability of being negative), with mean radiative forcing ranging from +0.68 to +1.34 W m⁻², and median radiative forcing ranging from +0.94 to +1.39 W m⁻². Received: 14 March 2001 / Accepted: 1 June 2001     

RCP4.5情景下中国未来干湿变化预估

本文采用国际耦合模式比较计划第五阶段(CMIP5)中21个气候模式的试验数据, 利用土壤湿度以及由其他8个地表气象要素计算所得的干旱指数, 预估了RCP4.5(Representative Concentration Pathway 4.5)情景下21世纪中国干湿变化。结果表明:全球气候模式对1986～2005年中国现代干湿分布具备模拟能力, 尽管在西部地区模式与观测间存在一定的差异。在RCP4.5情景下, 21世纪中国区域平均的标准化降水蒸散发指数和土壤湿度均有减小趋势, 与之对应的是短期和长期干旱发生次数增加以及湿润区面积减小。从2016到2100年, 约1.5%～3.5%的陆地面积将从湿润区变成半干旱或半湿润区。空间分布上, 干旱化趋势明显的区域主要位于西北和东南地区, 同时短期和长期干旱发生次数在这两个地区的增加幅度也最大, 未来干旱化的发生时间也较其他地区要早;只在东北和西南地区未来或有变湿倾向, 但幅度较小。在季节尺度上, 北方地区变干主要发生在暖季, 南方则主要以冷季变干为主。造成中国干旱化的原因主要是由降水与蒸散发所表征的地表可用水量减少。     

A three-dimensional study of mesoscale model response to radiative forcing

An infrared radiation parameterization has been applied to a detailed three-dimensional mesoscale model in order to determine whether radiative forcing significantly affects mesoscale atmospheric processes. By taking into account water vapor, liquid water, and carbon dioxide absorption, the scheme differentiates between cloud and clear air regions. The parametric model is presented, along with an overview of the associated mesoscale model.Comparisons between a control run in which only a uniform cooling rate of l K day^–1 is specified, and runs with the infrared scheme are made for 12-hr simulations. The major feature of the radiative forcing is seen to be strong cloud-top cooling. This leads to enhanced destabilization of the upper cloud layer, which in turn results in faster growth of clouds (and which extend to higher levels) than in the control experiment. The deeper clouds force a more vigorous secondary circulation, in which thermodynamic feedbacks between clouds and their environment are substantially stronger than in the case with only a constant cooling rate. This confirms findings made in previous studies undertaken in small-scale numerical models. The discussion also focuses upon a simulation in which the cloud-top infrared cooling has been smoothed out over neighboring vertical levels, in order to represent a cloud-top height distribution crudely. The results indicate that although the absolute values of cloud-top cooling are reduced with respect to the unfiltered case, the fact that cooling extends even higher than previously predicted leads to the formation of thicker, more vigorous clouds. These clouds interact more intensely with their environment than in the unfiltered situation, thereby considerably modifying the mesoscale atmosphere.