surface air pressure, Precipitation rate, surface wind, OLR and SST are not clear through the study period. For CFC 11 variability, like as CFC 12 no connection appears with weather elements variably over the globe through the period of study. CFC 11 returned to its normal values from the year 2006. For 15-MINOR, and sur- face air temperature and 500 hPa level of temperature anomaly, it is clear that, 15-MINOR became more than its normal values after 1993. All of these three parameters have a sharp increase with time through the period (2001-2011). With the continued increase of 15-MINOR, the geopotential height increase contradicted to sur- face air pressure almost time of the study period. Precipitation rate increase to became more its normal values on year 1995 up to the year 2011 over the globe likely the variability of 15-MINOR. Meanwhile the surface wind varies like a wave and not related to 15-MINOR variation through the period of study (1979-2011) as shown in Figure 8(c). Generally, OLR and SST vary and increase with time, typically with the 15-MINOR variability for all the period (1979-2011). It is noticed that the variability of total GRF and AGGI values is the same with the variability of the global annual weather elements through the period of study except with surface air pressure and wind. The total GRF and AGGI values has become above normal since 1995 with the same manner of the meteorological elements in general. Third one, the variability of GRF and climatic indices NAO, SOI, El- Nino 3.4 and SST has been studied. It is revealed that there is a positive trend of the NAO, El Nino 3.4 and total GRF, AGGI and for each of greenhouse gases in general. Meanwhile the connection between CFC 12 and CFC 11 and climatic indices variability are not clear. Last one, the correlation coefficient between the GRF and weather and climatic elements of the Earth through the period (1979-2011) has been studied. Through this section, the annual mean values of weather meteorological elements (surface air temperature, 500 hPa air temperature , sea level pressure, geopotential height at 500 hPa level, surface wind, precipitation rate, OLR, SST, NAO, SOI and El Nino 3.4 over the globe has been correlated with the GHGs and global radiative forcing has been analyzed through the period of (1979-2011). Anomaly and linear correlation coefficient methods had been used. It is no- ticed that:
34 Read more
With regard to climate change, ozone affects the radiative budget of the atmosphere through its interaction with both shortwave (SW) and longwave (LW) radiation and its chem- ical influence on other radiatively active trace gases such as methane and HCFCs. Radiative forcing is commonly defined as the imbalance in radiative flux at the tropopause resulting from a perturbation in the atmosphere, in order to determine the relative importance of different greenhouse gases and aerosols to climate. Connected with the absorption of solar SW radiation and the absorption and emission of LW radia- tion, reductions in lower stratospheric ozone imply a positive SW and a negative LW radiative forcing, while tropospheric ozone increases lead to a positive radiative forcing in both the SW and LW spectral regions. Several publications have established the strong dependence of radiative forcing on the altitude (Wang and Sze, 1980; Lacis et al., 1990; Forster and Shine, 1997; Hansen et al., 1997) and the horizontal distri- bution (Berntsen et al., 1997) of the ozone change. There are inadequate observations of the changes in tropospheric ozone on a global scale, even for recent decades, and this necessitates the use of models to estimate the changes. As ozone exhibits a highly spatially inhomogeneous distribu- tion, 3-D atmospheric models have been applied to estimate its global radiative forcing. In the past, several calcula- tions of radiative forcing due to tropospheric ozone increase since the preindustrial era have been published (e.g. Kiehl et al., 1999; Berntsen et al., 2000; Hauglustaine and Brasseur, 2001; Mickley et al., 2001). Chapter 6 of the IPCC Third
25 Read more
More work is needed to understand the factors that lead to this linearity, but they are likely to be linked to the global averaging of regions where the radiative response is in- creasingly buffered and more sensitive clean/pristine re- gions that are progressively affected by new emissions. If we accept that the global radiative forcing could respond line- arly to aerosol emissions, then this affects any limit implied from a simple energy balance constraint. Repeating the simple model framework outlined in S15 with a linear global aerosol forcing leads to a larger negative aerosol forcing that can be considered consistent with positive net 1950 forcing (2005 values up to 2 1.6 W m 22 ; Fig. 1b). This revised energy balance constraint (which rules out aerosol forcings of 2 1.6 W m 22 and larger) is interesting, but it still does not explain what we see in the CMIP5 ensemble. GFDL-CM3 simulates a net 21.6 W m 22 aerosol forcing but simulates a forced global temperature rise where the energy balance constraint suggests that there should be negligible warming at best.
More work is needed to understand the factors that lead to this linearity, but they are likely to be linked to the global averaging of regions where the radiative response is in- creasingly buffered and more sensitive clean/pristine re- gions that are progressively affected by new emissions. If we accept that the global radiative forcing could respond line- arly to aerosol emissions, then this affects any limit implied from a simple energy balance constraint. Repeating the simple model framework outlined in S15 with a linear global aerosol forcing leads to a larger negative aerosol forcing that can be considered consistent with positive net 1950 forcing (2005 values up to 2 1.6 W m 22 ; Fig. 1b). This revised energy balance constraint (which rules out aerosol forcings of 2 1.6 W m 22 and larger) is interesting, but it still does not explain what we see in the CMIP5 ensemble. GFDL-CM3 simulates a net 2 1.6 W m 22 aerosol forcing but simulates a forced global temperature rise where the energy balance constraint suggests that there should be negligible warming at best.
umented (e.g., Robock, 2000; Baxter, 2000; Delmelle et al., 2002; Schmidt et al., 2011). Major explosive volcanic erup- tions perturb stratospheric aerosol properties and the result- ing chemical, microphysical and radiative effects have been the subject of intensive investigation for several decades (a comprehensive review is provided by Robock, 2000). Re- cent advances include the use of global aerosol microphysics models due to a growing awareness that the evolution of the particle size distribution is critical to determining the mag- nitude of simulated climate forcings (e.g., Timmreck et al., 2009, 2010). In contrast, the atmospheric and climatic effects of volcanic aerosol released into the troposphere by contin- uously degassing and sporadically erupting volcanoes (here- after “volcanic degassing”) have only gradually become of greater interest to the geosciences community (Chin and Ja- cob, 1996; Graf et al., 1997, 1998; Stevenson et al., 2003a; Mather et al., 2003b; Textor et al., 2004; Gass´o, 2008; Yuan et al., 2011; Oppenheimer et al., 2011). In their recent re- view of sulphur degassing from volcanoes, Oppenheimer et al. (2011) concluded that “changes in time and space in this “background” emission could represent an important forcing factor that has yet to be characterized.”
20 Read more
Also the ice phase of clouds has improved in E63H23 compared to previous model versions. The low bias in IWP is reduced in E63H23 and the global mean vertical IWC is within the observational range (Fig. 7). This is because the Seifert and Beheng (2006) sticking efficiency used in E63H23 leads to a less efficient removal of ice crystals by snow. A subsequent reduction in the tuning parameter for stratiform snow formation by aggregation further increases IWP in E63H23. Only a few laboratory studies for stick- ing efficiency have been conducted, and even fewer theories for sticking efficiency have been developed (Phillips et al., 2015). We find that the simple formulation of Seifert and Beheng (2006) for sticking efficiency for the accretion of ice crystals by snow improves the simulation of cloud ice in E63H23. Furthermore, the altitude of the global mean maximum IWC agrees well with observations in E63H23, whereas in E61H22 and E55H20 it is at higher altitudes than observed. This can be explained by the changes in ICNC de- scribed in Sect. 2.1.5, such as the use of a consistent ice crys- tal shape (hexagonal plates), removal of an ICNC bug, or the changed treatment of detrained ice crystals. The subse- quent changes in precipitation formation and ice crystal sed- imentation can then lead to a different vertical distribution of cloud ice. In E61H22 the global ICNC burden is consider- ably higher than in the other two model versions because of an inconsistency between cloud droplet activation, conden- sation, vertical transport of CDNC, and homogeneous freez- ing of cloud droplets in cirrus clouds, which led to homoge- neous freezing of aerosol particles even when the water va- por pressure was too low for homogeneous nucleation. The higher ICNCs are also responsible for the LW component of ERF ari+aci in E61H22 being more than twice as large as
31 Read more
From the model simulations, we calculate global annual mean values for 1R and 1T , and obtain pairs of 1R i and 1T i for each year, i. Figure 3 shows the points of (1T i , 1R i ) for the early Eocene dark desert world simulation. In the initial equilibrium climate, savanna covers all continents but in the first year of the dark desert simulation, we replace savanna by dark bare soil. During the simulation, the sim- ulated climate progressively approaches a new equilibrium. The straight line fitted to the points of (1T i , 1R i ) reveals the parameters in Eq. (1) (Gregory et al., 2004). At the in- tersection of the regression line with the 1R axis, 1R is the radiative forcing 1Q. The slope of the regression line is the feedback parameter λ. Here, the regression line has a negative slope, λ, indicating that feedbacks counteract the perturbation in the TOA radiation balance. In other words, feedbacks stabilise climate.
13 Read more
The study of the last millennium climate has considerably increased during the last decades because it replaces the evo- lution of the global temperature for the last fifty years in a multi-centennial context, providing the necessary hindsight for a more comprehensive assessment of the recent climatic change. Studying this period gives the possibility to explore a relatively well-documented climate and weakly affected by anthropogenic greenhouse gases (GHGs). Before the indus- trial era, natural forcings such as volcanic aerosols and solar variability have likely dominated the forced variability of the Earth climate (Rind, 2002; Bauer et al., 2003; Bertrand et al., 2002; Mann et al., 1998; Robock, 2000; Hegerl et al., 2007; Crowley, 2000; Gerber et al., 2003; Jones and Mann, 2004). Several temperature reconstructions covering differ- ent areas of the globe are now available, ranging from multi- decadal to seasonal time scale. They were obtained from dif- ferent proxy data such as tree rings (Briffa et al., 2004; Cook et al., 2002; Esper et al., 2002; D’Arrigo et al., 2006; Os- born et al., 2006), fossil pollen (Bjune et al., 2004; Larocque and Hall, 2004), ice cores (Jouzel et al., 2001; Vinther et al.,
16 Read more
The stratosphere is being increasingly acknowledged as one of the keys to adding more skill to numerical models in a wide range of timescales and applications, from weather forecasts to climate studies, as well as a potential source of seasonal meteorological predictability (Solomon et al., 2010; Maycock et al., 2011; Scaife et al., 2012). To capture the variability of the stratosphere, radiative processes in this re- gion need to be realistically modelled. Stratospheric radiative heating rates strongly depend on the distribution of concen- trations of radiatively active gases in this region. Therefore, numerical models that consider the stratosphere but do not fully treat its stratospheric chemistry need realistic strato- spheric descriptions of, at least, the main greenhouse gases (GHGs), i.e. O 3 , H 2 O, CH 4 and chlorofluorocarbons (CFCs).
21 Read more
Atmospheric aerosols particles divided to primary and secondary aerosols; where primary aerosols are emitted directly at the source, and secondary aerosols are generally formed from gaseous precursors by various gas and aqueous phase oxidation pathways. Primary aerosols include, for example, fly ash activities, sea-salt particles emitted at the ocean surface (Zakey, et al., 2008), or mineral dust aerosol that is emitted by the effects of wind erosion on arid land (Zakey et al, 2006), the reflection of radiation to space may counteract the greenhouse warming by cooling the earth system (Charlson et al,1992) and the distribution of radiation is expected to change the temperature profile (Alpert et al.1998), the atmospheric stability and possibly cloud formation (Ackerman et al. 2000),so besides altering air quality (Wilson and Spengler, 1996; Prospero, 1999) aerosols affect atmospheric radiation transfer directly by scattering and absorbing light, and indirectly by influencing cloud formation. Because of the variety of aerosol sources, their short atmospheric residence time, and the dynamic processes that may alter them after generation, the physical and chemical characteristics of airborne particles are highly inhomogeneous in space and time, One of reasons for this variability is that dust optical properties at origin, as issued by the sources, are influenced by the potential changes during the path of the air masses as well as by the local aerosol properties at the reception site ( Bauer et al., 2011 ). Close to the source regions, mostly pure dust is found, but after long- range transport the aging of dust and mixing with other aerosol types modify the optical properties of Desert Dust “DD” (e.g. Bauer et al., 2011). Though, in situ ground-level measurements traditionally considered as the most reliable for aerosol characterization must undoubtedly be performed, more global and continuous observations providing a better spatial and temporal coverage are needed (Dubovik et al.,2002a). Aerosols
15 Read more
Radiation model simulations indicate that retained rBC and dust enhance radiative forcing in the eastern Sierra Nevada’s spring snowpack (Fig. 2b and c), with dust con- tributing a greater forcing than rBC based on modeling re- sults of each impurity. This feedback between rBC concen- tration, radiative forcing, and melt has been observed on Hi- malayan Glaciers (Xu et al., 2009) and may be an impor- tant process affecting the albedo of snowpacks and glaciers on a global scale (Hadley and Kirchstetter, 2012). A caveat to this study is that dry deposition of rBC and dust to the snowpack during the ablation season was not measured inde- pendently, and thus, some of the ablation season increase in surface concentrations may be explained by increasing dry deposition in late spring. However, long range transport of Asian dust and pollutants across the Pacific generally oc- curs earlier in the year (Perry et al., 1999), late spring wild- fires in the Mammoth region are uncommon, and local black carbon emissions from domestic burning and vehicle opera- tions (snowcats and snowmobiles) at the Mammoth Ski area are no higher in spring than in winter. While these measure- ments and snowpack radiation modeling suggest a significant role for rBC and continental dust in radiative forcing, en- ergy budgets, and melt generation, the results emphasize that further snowpack studies are needed, particularly to isolate the very local sources that are unimportant at the watershed scale from more regional deposition. At this site the com- bined forcing of rBC and dust ranges from 20 to 40 W m −2 during much of the ablation period, with typical grain sizes during melt (Fig. 3). During the period shown in Fig. 3, net daily solar radiation ranges from 100 to 200 W m −2 (Dozier et al., 2009), so the contribution of the absorbing impurities accounts for ∼ 20 % of the absorbed solar component of the energy balance.
10 Read more
The Intergovernmental Panel on Climate Change (IPCC) has reported that in the southeastern US and eastern China, the greenhouse warming due to anthropogenic gaseous emissions is dominated by the cooling effect of anthropogenic aerosols. In a recent study, we verified this model prediction by analyzing the trend in daily, maximum, minimum temperatures and diurnal temperature range at 52 stations in the southeastern US during 1949-94. In this study, we present an analysis of regional patterns of climate change at 72 stations in eastern China during 1951-94 (44 years) to detect the signal of aerosol radiative forcing. The results support the cooling effect of sulfate aerosols which was evident in the years following the eruption of Mt. Pinatubo. The enhancement in stratospheric aerosol optical depth was obtained by NASA’s (National Aeronautics and Spaces Administration) SAGE II (Stratospheric Aerosol and Gas Experiment) satellite measurements. A decreasing trend in the summer mean maximum temperature during the past 44 years and a two-year cooling trend following the Mt. Pinatubo eruption were found. This effect is similar to our finding in the southeastern US. However, a slightly overall warming trend in eastern China is evident; winters have become milder. This finding is explained by hypothesizing that increasing energy usage during the past 44 years has resulted in more fossil fuel consumption and biomass burning thus increasing the emission of absorbing soot and organic aerosols. Such emissions in addition to well-known Asian dust and greenhouse warming may be responsible for the winter warming trend that we have reported here.
264 Read more
tropopause as heat and mass exchange between the strato- sphere and troposphere can lead to changes in clouds and stratospheric water vapor. Although most cited studies in- clude the fast adjustment processes, and radiative forcing and response, there is a lack of clear and systematic understand- ing of the dependence of radiative forcing and climate re- sponse on the altitude of sulfate aerosols in the stratosphere. In this study, we use idealized climate model experiments to systematically study the sensitivity of the effective radia- tive forcing and the simulated surface climate to the height at which aerosols are prescribed in the stratosphere. The mod- els used in several previous studies include the effects of sev- eral processes that affect the aerosol microphysics, transport, and removal processes, and hence the altitude sensitivity es- timated in these studies is the net effect of all these processes. The idealized prescribed aerosol model used in this study has the advantage of isolating and analyzing the individual ef- fects, which will be challenging in complex models. In all our stratospheric aerosol experiments, we use the same total amount of aerosols but alter their altitude. Thus, our idealized simulations are intended to highlight the radiative influences of aerosol layer height and isolate these effects from effects associated with aerosol particle evolution and transport.
16 Read more
Better estimates of effective radiative forcing will refine understanding of how the Earth system responds to forc- ing, but the potentially knotty relationships between radia- tive forcing and response suggest value in subjecting models to ERFs that are as similar as possible. In signal processing it is common, when looking for a signal amidst a noisy back- ground, to reduce the noise as close to the source as possi- ble. In the context of ERF the largest source of variability is the treatment of atmospheric aerosol. The Radiative Forcing Model Intercomparison Project (RFMIP) therefore includes coupled atmosphere–ocean simulations in which aerosol ef- fective radiative forcing over the historical period is pre- scribed as much as possible, by analogy to protocols in which greenhouse gas concentrations over time are similarly spec- ified. This is not to diminish the true uncertainty in histori- cal concentration of anthropogenic aerosols but to ascertain what model responses robustly arise from a plausible histor- ical aerosol radiative forcing.
14 Read more
The 10 year period 1998–2008 can also be considered as a distinct period of anthropogenic emissions. In 1998, approximately 108 Tg of SO 2 was emitted globally [Lamarque et al., 2010] hence between 1998 and 2008 global SO 2 emissions declined more gradually than in previous decades [Granier et al., 2011]. The multidecadal trend in declining SO 2 emissions eased in Europe, yet became stronger in North America, during the 1998–2008 period. Asian emissions increased more rapidly than in the 1978–2008 period. The 1998–2008 period is also of interest because of the hiatus in global surface temperature rise which has been noted in the observational record since the late 1990s [Brohan et al., 2006]. Identifying the sign and magnitude of CAE forcing, along with the associated variance, will shed light on the role of CAE forcing during the hiatus period.
10 Read more
Abstract The usefulness of previous Coupled Model Intercomparison Project (CMIP) exercises has been hampered by a lack of radiative forcing information. This has made it dif ﬁ cult to understand reasons for differences between model responses. Effective radiative forcing (ERF) is easier to diagnose than traditional radiative forcing in global climate models (GCMs) and is more representative of the eventual temperature response. Here we examine the different methods of computing ERF in two GCMs. We ﬁ nd that ERF computed from a ﬁ xed sea surface temperature (SST) method (ERF_fSST) has much more certainty than regression based methods. Thirty year integrations are suf ﬁ cient to reduce the 5 – 95% con ﬁ dence interval in global ERF_fSST to 0.1 W m 2 . For 2xCO2 ERF, 30 year integrations are needed to ensure that the signal is larger than the local con ﬁ dence interval over more than 90% of the globe. Within the ERF_fSST method there are various options for prescribing SSTs and sea ice. We explore these and ﬁ nd that ERF is only weakly dependent on the methodological choices. Prescribing the monthly averaged seasonally varying model ’ s preindustrial climatology is recommended for its smaller random error and easier implementation. As part of CMIP6, the Radiative Forcing Model Intercomparison Project (RFMIP) asks models to conduct 30 year ERF_fSST experiments using the model ’ s own preindustrial climatology of SST and sea ice. The Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) will also mainly use this approach. We propose this as a standard method for diagnosing ERF and recommend that it be used across the climate modeling community to aid future comparisons.
16 Read more
for six projections of LULCC as a metric for quantifying climate impacts. The LULCC RF is attributed to three categories of LULCC activities: direct modifications to land cover, agriculture, and wildfire response, and sources of the forcing are ascribed to individual grid points for each sector. Results for the year 2010 show substantial positive forcings from the direct modifications and agriculture sectors, particularly from south and southeast Asia, and a smaller magnitude negative forcing response from wildfires. The spatial distribution of future sources of LULCC RF is highly scenario-dependent, but we show that future forest area change can be used as a predictor of the future RF from direct modification activities, especially in the tropics, suggesting that deforestation-prevention policies that value land based on its C-content may be particularly effective at mitigat- ing climate forcing originating in the tropics from this sector. However, the response of wildfire RF to tropical land cover changes is not as easily scalable and yet imposes a non-trivial feedback onto the total LULCC RF.
20 Read more
the continuum by using a χ factor to reduce the opacity of the Voight line shape out to 1000 cm −1 from the line centre to match the background absorption. We add to this CIA absorption which has been updated with recent results of Wordsworth et al. (2010). We believe that our radiative transfer runs are as accurate as possible given the poor understanding of continuum absorption. It is worthwhile comparing our calculated radiative forc- ings with previous results. In most studies, the greenhouse warming from a perturbation in greenhouse gas abundance is quantified as a change of the GAM surface tempera- ture. We convert our radiative forcings to surface temper- atures for comparison. This is achieved using climate sen- sitivity. Assuming the climate sensitivity to be in the range 1.5–4.5 W m −2 (medium confidence range, IPCC, 2013) for a doubling of atmospheric CO 2 and the radiative forcing for
23 Read more
tainties in the CTM calculations that were discussed in section 2 need to be resolved and emission scenarios have to be refined in order to get more accurate predictions for changes in ozone. Also, climate chemistry feedback mech- anisms identified in various studies [e.g., Granier and Shine, 1999; Stevenson et al., 2000; Johnson et al., 1999, 2001; Grewe et al., 2001b] could not be included in the CTM simulations and this lack adds to the uncertainty in predicted ozone changes. Major identified feedback mech- anisms include the change of chemical reaction rates due to tropospheric temperature increase and the enhanced photo- chemical destruction of tropospheric ozone related to the expected increases in water vapor. For example, Stevenson et al.  find a radiative forcing due to tropospheric ozone increase between 1990 and 2100 amounting to 0.43 W m 2 , ignoring climate change. This value falls to 0.27 W m 2 when climate feedback on chemistry is included. These results indicate the potential importance of climate feedbacks on chemistry.
21 Read more
the 1991 Mount Pinatubo eruption [Guo et al., 2004], and would require more than 1000 KC-135 tanker aircraft ﬂ ights per day during peak injection periods [Robock et al., 2009]. It remains uncertain whether the stratospheric aerosol concentration would increase linearly with injection rate [Heckendorn et al., 2009] and whether an ef ﬁ cient distribution of aerosol particle size could be sustained [Niemeier et al., 2011]. Accelerated climate change on termination of SRM, also demonstrated by Jones et al. , shows climate to be vulnerable to unplanned disruption of SRM injections [Baum et al., 2013]. We found statistically signi ﬁ cant differences in regional climate persisted into the 2090s even when global mean climate had returned close to the nongeoengineered state.