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Keywords:

  • global dimming/brightening

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Papers in This Special Section
  5. 3. Concluding Remarks
  6. Acknowledgments
  7. References

[1] Global dimming and brightening is a popular expression which refers to the recent observational evidence for substantial decadal variations in solar radiation reaching the Earth's surface. For the first time, a special section is dedicated to this rapidly growing field of research. A comprehensive collection of more than 20 papers sheds new light on the phenomenon of global dimming and brightening. Here I provide a brief introduction and guideline to the special section and place the individual papers into context. To facilitate orientation in this special section, the papers are grouped according to the specific aspects of global dimming and brightening that they address.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Papers in This Special Section
  5. 3. Concluding Remarks
  6. Acknowledgments
  7. References

[2] The idea for this special section originated from a workshop held in February 2008 at the Kibbutz “Ein Gedi” in the Judean desert in Israel [Ohring et al., 2008]. In this most memorable, secluded oasis, 400 m below sea level on the shore of the Dead Sea, an international group of scientists gathered, united by a common curiosity in a phenomenon popularly known as “global dimming and brightening.” This phenomenon relates to the increasing observational evidence that the sunlight we receive at the Earth's surface is not stable over time, but undergoes substantial decadal variations. This evidence is based on long-term records of broadband surface solar radiation measurements, which started to become available in the 1950s on a widespread basis. Early analyses of these records pointed to a widespread decline of surface solar radiation from the 1950s up to the 1980s in various parts of the globe [e.g., Ohmura and Lang, 1989; Russak, 1990; Dutton et al., 1991; Stanhill and Moreshet, 1992, Liepert et al., 1994; Abakumova et al., 1996; Gilgen et al., 1998; Stanhill and Cohen, 2001; Liepert, 2002]. This phenomenon, largely attributed to increasing air pollution, was coined “global dimming” by Stanhill and Cohen [2001]. More recent analyses with data records updated to near present suggested that surface solar radiation shows no sign of decrease anymore since the 1980s or even started to recover at many locations [Wild et al., 2005; Wild, 2009a, and references therein]. Wild et al. [2005] thereby coined the term brightening to emphasize that global dimming has largely vanished after the mid-1980s. Air pollution control and the economic breakdown of the former communist countries were put forward as major influential factors for this transition [Wild et al., 2005; Streets et al., 2006]. Further the influence of the recovery from the dimming caused by Mt. Pinatubo volcanic eruption in 1991 and internal climate variability with associated cloud variations were suggested to contribute to the brightening in the 1990s [Romanou et al., 2007].

[3] The present special section builds upon these earlier works and states a comprehensive collection of novel studies which shed light on the phenomenon from various viewpoints. Out of a total of 32 manuscripts that have been submitted to this special section, 23 manuscripts could finally be accepted and published here. In the following, the various studies are briefly introduced and grouped according to the particular aspects of the phenomenon that they address.

2. Papers in This Special Section

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Papers in This Special Section
  5. 3. Concluding Remarks
  6. Acknowledgments
  7. References

2.1. Papers Presenting Evidence for Global Dimming/Brightening

[4] The special section provides new additional evidence for global dimming and brightening around the world. Two papers analyze a comprehensive data set of worldwide distributed surface solar radiation sites from the Global Energy Balance Archive (GEBA) [Ohmura, 2009; Gilgen et al., 2009]. They confirm earlier findings of a widespread dimming from the 1950s to the 1980s and a partial recovery thereafter, based on more extensive and updated data and differing analysis methods. Several other papers in this special section document dimming and brightening in various parts of the globe. In the continental United States, Long et al. [2009] note a widespread brightening from 1995 to 2007 under both all-sky and clear-sky conditions. Riihimaki et al. [2009] analyze data from three sites in Oregon from 1980 to 2007 and find a brightening in both total and direct normal irradiance. Chiacchio et al. [2010] investigate decadal variations in surface solar radiation in Alaska. In Asia, Norris and Wild [2009] note a statistically significant dimming in China and a nonsignificant dimming in Japan between 1971 and 1989, whereas between 1990 and 2002, a significant brightening is found at Japanese sites and a nonsignificant brightening at the Chinese sites. In Europe, Chiacchio and Wild [2010] see a predominant dimming and brightening in the periods 1970–1985 and 1985–2000, respectively. On a seasonal basis they find that this pattern is most pronounced in spring and summer, while autumn and winter rather show a continuous dimming and brightening, respectively, throughout both periods. In Estonia, Russak [2009] analyzes the long-term solar radiation record at Toravere and finds also a dimming between 1955 and the beginning of the 1990s and a brightening during more recent years. Ohvril et al. [2009] analyze multidecadal changes in atmospheric column transparency based on measurements of direct solar radiation at various sites in Eastern Europe. They detect an overall decreasing transparency until the mid-1980s and a recovery thereafter, modulated by strong volcanic eruptions, which fits well to the above-mentioned studies. The special section documents also dimming and brightening on the Southern Hemisphere. Liley [2009] finds dimming followed by brightening at a number of long-term monitoring sites in New Zealand. A summary of the evidence for dimming and brightening from direct observations is given by Wild [2009a].

[5] The sparse spatial and temporal distribution of direct observations of surface solar radiation makes the use of additional proxy information attractive. Makowski et al. [2009] demonstrate that the much more widely measured diurnal temperature range might be a useful proxy for surface solar radiation. Sanchez-Lorenzo et al. [2009] point out that sunshine duration records may also provide valuable proxy information for surface solar radiation, and find the characteristic dimming followed by brightening in extended sunshine duration records of the Iberian Peninsula and western Europe.

[6] Another promising approach is to infer surface solar radiation from satellite measurements and additional information on the atmospheric structure and composition. N. Hatzianastassiou et al. (Two-decadal trends of aerosol optical thickness and direct radiative effect on surface solar radiation and their role in global dimming and brightening, submitted to Journal of Geophysical Research, 2009) follow this approach and estimate an overall increase of surface solar radiation both over land and oceans over the period 1984–2001. Hinkelman et al. [2009] analyze a satellite-derived data set of surface solar radiation covering the period 1983–2004 and find a dimming between 1983 and 1991, followed by a brightening from 1991 to 1999 and a renewed dimming over 1999–2004, but no significant overall trend over the entire period.

[7] A review of studies on surface solar radiation changes inferred from surface proxies and satellite observations is given by Wild [2009a].

2.2. Papers Addressing the Causes of Global Dimming/Brightening

[8] As principal causes for global dimming/brightening, changes in cloud and aerosol characteristics as well as their interactions have been proposed and are discussed in several studies in this special section. A number of papers in this special section stress the importance of aerosol in the context of dimming and brightening. Streets et al. [2009] estimate trends of aerosol optical depth and the human contribution therein for eight world regions from 1980 to 2006, based on a global aerosol transport model. The estimated trends, largely caused by anthropogenic emissions, agree well with observed surface solar radiation trends in the majority of the investigated regions, indicative of anthropogenic aerosols as major contributors to global dimming and brightening. Norris and Wild [2009] show that after eliminating influences from cloud cover changes on surface solar radiation trends, the residual radiative fluxes still show a significant dimming in China between 1971 and 1989, and point out that only about half of the brightening in Japan and one third of the brightening in China between 1990 and 2002 can be attributed to changes in cloud amounts. This suggests that aerosol direct or indirect effects may be of particular importance in these areas. Using long-term sunshine duration data of the Iberian Peninsula, Sanchez-Lorenzo et al. [2009] find the characteristic dimming and subsequent brightening not only in the total records, but similarly also in records that only contained sunshine duration data of cloud-free days, pointing again to changes in aerosol radiative effects. In the United States, Riihimaki et al. [2009] cannot explain their clear-sky brightening at the Oregon sites with changes in volcanic aerosols and water vapor alone, and therefore imply that their increasing trend could be largely due to a reduction of anthropogenic aerosols. On the other hand, there are also papers in this special section that emphasize the importance of clouds in the dimming/brightening context. Long et al. [2009] show that changes in dry aerosols and/or direct aerosol effects alone cannot explain the observed brightening in the United States, but that likely also changes in cloudiness play a role. Further, Liley [2009] states that dimming and brightening on the Southern Hemisphere in New Zealand is caused by changes in clouds and not by the sparse aerosols in this area. Chiacchio et al. [2010] argue that the decadal changes in surface solar radiation in Alaska were predominantly caused by changes in cloud amount associated with a shift in the Pacific Decadal Oscillation and changes in the Pacific North American circulation pattern. Chiacchio and Wild [2010] suggest that over Europe, longer-term changes may be caused by aerosol changes, but seasonal and interannual changes are dominated by changes in cloud characteristics. Russak [2009] relates the strong dimming and subsequent brightening at the Estonian site to both changes in cloudiness and atmospheric clear-sky transmission due to aerosol changes. The latter is in line with the atmospheric transmission analyses by Ohvril et al. [2009] and with Ohmura [2009] who infers from selected sites around the globe that the aerosol direct and indirect effects played about an equal weight in changing surface solar radiation.

[9] A conceptual framework on the relative role of clouds, aerosols and their interactions in the context of dimming and brightening is introduced by Wild [2009a].

2.3. Papers Dealing With Global Dimming/Brightening in Climate Models

[10] Not many studies have investigated the representation of dimming and brightening in climate models so far. Three of them are found in this special section. Ruckstuhl and Norris [2009], focusing on Europe, assess the ability of IPCC-AR4 climate models to reproduce the observed trends in surface solar radiation under cloud-free conditions. They note large discrepancies in sign and magnitude between modeled and observed dimming and brightening trends in Europe, and attribute the differences in the model simulations primarily to differences in the aerosol burdens and emission histories used in the simulations. Wild [2009b] infers from the inability of the IPCC-AR4 models to reproduce decadal-scale variations in the twentieth century daily maximum and minimum temperatures, that dimming and brightening may not be reproduced adequately by the models, in line with Romanou et al. [2007]. Comparisons of model-calculated surface solar radiation with direct observations support this conclusion. Makowski et al. [2009] point to the inability of current regional climate models to simulate dimming and brightening adequately over Europe.

[11] A review of the studies focusing on the representation of global dimming and brightening in global climate models is given by Wild [2009a].

2.4. Papers Addressing the Impact of Global Dimming/Brightening on Climate Change

[12] Wild [2009a] gives an overview over recent studies investigating the potential impact of global dimming and brightening on various aspects of climate change. These include the impact on global warming, the intensity and components of the hydrological cycle, snow cover and glaciers retreat, and changes in terrestrial carbon uptake. Makowski et al. [2009] highlight the impact of global dimming and brightening on the evolution of the diurnal temperature range over Europe.

2.5. Papers Dealing With the Larger-Scale Significance of Global Dimming/Brightening

[13] The controversy on the larger-scale significance of dimming and brightening [e.g., Alpert et al., 2005] is debated in this special section (see Wild [2009a] for an overview). Kishcha et al. [2009] do not find significant changes in aerosol optical depth over midlatitude Ocean areas in satellite data after 2000 and infer from this that dimming/brightening is rather a local phenomenon. This stable aerosol optical depth over midlatitude oceans after 2000 is nevertheless not in contradiction with recent surface solar radiation and aerosol optical depth measurements over land, which overall show also no clear tendencies after 2000 [Wild et al., 2009] (see also section 2.6). Stanhill and Cohen [2009] point out that in Israel the maximum urban effects on surface solar radiation were insufficient to explain the large variability measured during the last 50 years and that population density as used by Alpert et al. [2005] is not a robust proxy for solar radiation change. Karnieli et al. [2009] find decreasing trends of sulfate aerosols over recent decades at a receptor site in southern Israel thousands of kilometers downwind of the original sources, and conclude that aerosol effects can have a long-range influence and are not just of local importance.

2.6. Papers Focusing on the Most Recent Developments in Global Dimming/Brightening

[14] Several papers in this special section deal with developments in global dimming/brightening after the year 2000. An update based on the worldwide surface observations is given by Wild et al. [2009]. They point out that brightening continues in several parts of the globe after 2000 (Europe, United States), but levels off (Japan) or even turns into a renewed dimming in other areas (China). The evidence for no substantial overall tendencies in surface solar radiation after 2000 is in broad agreement with Kishcha et al. [2009], who find stable aerosol optical depths over midlatitude oceans after 2000. The planetary albedo is a major factor governing global dimming/brightening. The evolution of the planetary albedo in the early 2000s has long been controversial, with evidence for decreasing planetary albedo from satellite measurements on one hand and increasing albedo from Earthshine measurements on the other hand [Wielicki et al., 2005]. This apparent discrepancy is reconciled in the study of Palle et al. [2009] by a reanalysis of the Earthshine data on one side, and a recalibration of the satellite data on the other side, resulting in a more constant planetary albedo in both approaches for the post 2000 period, in line with the evidence from the studies by Wild et al. [2009] and Kishcha et al. [2009].

[15] The potential future evolution of global dimming/brightening over the coming decades is discussed by Wild [2009a].

3. Concluding Remarks

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Papers in This Special Section
  5. 3. Concluding Remarks
  6. Acknowledgments
  7. References

[16] For the first time, a special section is exclusively dedicated to the rapidly growing research field of global dimming and brightening. This issue provides an overview over the phenomenon of global dimming and brightening from various viewpoints. Magnitude, origins, significance, model representation and possible impacts are discussed by the various studies included in this issue. A large number of open questions remain with respect to this phenomenon and its role in climate change. It is the hope of the editors that this special section further stimulates the discussion and will boost more research in this direction in the future.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Papers in This Special Section
  5. 3. Concluding Remarks
  6. Acknowledgments
  7. References

[17] I would like to thank all authors for their efforts to contribute with excellent and timely papers to this special section. My particular thanks go to Shep Cohen and Yinon Rudich, who put enormous efforts into the organization of the memorable international workshop on “global dimming and brightening” at Ein Gedi, Israel, in February 2008, where the idea for this special section was born. The Israel Science Foundation generously supported this workshop. This special section profited also from contributions presented at the session “Surface Radiation, Radiative Forcing and Climate Change” which we hold regularly at the European Geophysical Union (EGU) General Assembly in Vienna. Yinon Rudich, Editor of Journal of Geophysical Research, is further acknowledged for making the special section possible and for his great support during the review process of a total of 32 submitted papers. Global dimming and brightening research at ETH Zurich is supported by the National Centre for Competence in Climate Research (NCCR Climate) of the Swiss National Science Foundation.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Papers in This Special Section
  5. 3. Concluding Remarks
  6. Acknowledgments
  7. References