solar dimming/brightening in the mediterranean

CEST2013 – Athens, Greece Ref no: XXX

SOLAR DIMMING/BRIGHTENING WITHIN THE MEDITERRANEAN

P.T. NASTOS 1 , H.D. KAMBEZIDIS2 and D. DEMETRIOU1

1Laboratory of Climatology and Atmospheric Environment, Departmentof Geology and Geoenvironment, University of Athens, University

Campus, GR-15784 Athens, Greece2Atmospheric Research Team, Institute for Environmental Research

and Sustainable Development, National Observatory of Athens, LofosNymphon, P.O. Box 20048, GR-11810 Athens, Greece.

Email: [email protected])

EXTENDED ABSTRACT

The energy source of life in our planet, which largely determinesthe climatic conditions, is the solar radiation that incidents onthe earth's surface. The amount of solar energy that reaches onsurface regulates processes which take place there, such asevaporation, melting ice, plant photosynthesis and the absorptionof carbon dioxide by terrestrial organisms. Therefore, any changein the intensity of solar radiation reaching the earth's surface,might have serious environmental, social and economic impacts.These effects can be created by natural causes or human’sinterference in the climate.

Early analyses of solar radiation records have pointed to awidespread decline of surface solar radiation from the 1950s up tothe 1980s in various parts of the world. This phenomenon wasattributed to increasing air pollution and has been named “globaldimming”. More recent analyses with data records updated to nearpresent suggested that surface solar radiation shows no sign ofdecrease anymore since the 1980s or even started to recover atmany locations. This recovery has been named “solar brightening”and is very likely due to air pollution control and the economicbreakdown of the former communist countries. Further the influenceof the recovery from the dimming caused by Mt. Pinatubo volcaniceruption in 1991 and internal climate variability with associatedcloud variations were suggested to contribute to the brighteningin the 1990s.

mailto:[email protected]


The objective of the present work is to examine the phenomenon ofglobal brightening and dimming in the Mediterranean during theperiod 1979 to 2004. The Mediterranean region was chosen becauseit is the crossroad of air masses and aerosols and acts as the airinterface (neutral zone) between Atlantic Ocean, North Africa,Middle East and Europe. To examine this phenomenon in theMediterranean region, satellite and re-analysis data werecollected from the Giovanni website(http://disc.sci.gsfc.nasa.gov/giovanni). The monthly datacollected refer to the MERRA 2D option and concern the short-waveradiation (SWR) at the surface for clear-and all-skies conditions.The findings of the analysis showed that a mixed pattern of solardimming/brightening is apparent in the Mediterranean within theexamined period and thus, it is not quite certain that solarbrightening (recovery of the solar dimming effect in the period1979-1999) is occurring.

KEYWORDS: Solar dimming and brightening, Mediterranean

1. INTRODUCTION

Changes in the electromagnetic radiation of short wavelength (upto 4mm) were instrumental to the impact of the climate system ofEarth and are responsible for the amount of solar energy reachingthe Earth’s surface (Wild, 2009). Much analysis took place overvarious worldwide locations from the early 20th century studyingsignificant variations in solar radiation intensity considered assolar dimming or brightening phenomenon (Stanhill and Moreshet,1992; Stanhill and Cohen, 2001; Wild et al., 2004, 2005, 2007;Romanou et al., 2007, Zerefos et al., 2009). Changes in global solarradiation could be attributed to changes in aerosolsconcentrations (Wild et al., 2005; Romanou et al., 2007, Zerefos et al.,2009) or cloud cover (Wild et al., 2005).

These studies have been concluded to a decrease in solar radiationduring the period 1960-1990 (solar dimming) and a recovery (solarbrightening) at some locations in the last two decades. Morespecifically, Stanhill and Moreshet (1992), from measurements withthermoelectric pyranometers at stations of the World RadiationNetwork observed that the amount of radiation had largelydecreased in midlatitude zone in the northern hemisphere. Between1958 and 1985 the reduction was measured to be 9 W m-2 or 5.3% onaverage, which was calculated by two methods as the mean weighted

http://disc.sci.gsfc.nasa.gov/giovanni


difference in solar radiation over the Earth's surface. Gilgen etal. (1998) using data from the Global Energy Balance Archive(GEBA) database found out that in large areas in Africa, Asia,Europe and North America, shortwave solar radiation decreasessignificantly with relative changes 22% of the mean in 10 years.Significant positive trends observed in only four small areas.Ohmura (2006) examined global radiation based on pyronometermeasurements at about 400 regions and observed that globalradiation generally increased in Europe from the 1920s to the1950s. After the late 1950s and early 1960s global radiation beganto decrease in most areas of the world at a mean rate of 0.7Wm−2a−1 until 1980s, thereafter 75% of the stations showed arecovery at a mean rate of 0.7 Wm−2a−1. All stations in the Polarregion, which are far from aerosol sources, also show this patternof change. At the remaining 25% of the stations the decrease hascontinued to present. These regions are a part of China, most ofIndia, and Central Africa. Collecting data from 65 widelydistributed stations over the 1890 to 2002 period from the JapanMeteorological Agency's database, Stanhill and Cohen (2008)studied the variation of global radiation in Japan and observedthat during the 20th century, sunshine duration increased 10%,half of which occurred between 1900 and 1940, followed by anirregular decrease until 1950. Guang-Yu Shi et al., (2007) bycollecting data from 122 meteorological stations in China from1957 to 2000 observed clear decreasing trends during 1961 to 1989,followed by an increasing trend starting in 1990.The averagedecrease of global irradiance is 2.54% (10 yr)-1, which becomesmore obvious during 1961–89, during which it reaches up to about4.61% (10 yr)-1, and the increase after 1990 is about 1.76% (10yr)-1. For the seasonal change in global irradiance, the decreaseis largest in winter with 4.82% (10 yr)-1 with the magnitudes ofthe decrease for autumn, summer, and spring being 2.63%, 2.15%,and 1.39% (10 yr)-1, respectively. In addition, the most obviousdecreases occur in southern China, the Sichuan basin, and theGuizhou area, as well as in the middle and lower reaches of theYangtze River areas. The decreases are smaller in the northeasternand western parts of China, with magnitudes of no more than 4% (10yr)-1.

Despite the interest in the solar dimming/brightening phenomenonthe Mediterranean area has not attracted the attention of thescientists in this respect so far. Therefore, the present worktries to fill this gap by providing spatio-temporal analysis ofthe incoming short-wave solar radiation (SWR) in the whole area ofthe Mediterranean Sea in the period 1979-2004 taken from


satellites. The Mediterranean region was chosen because it is thecrossroad of air masses and aerosols and acts as the air interface(neutral zone) between Atlantic Ocean, North Africa, Middle Eastand Europe.

To give better spatial information about the phenomenon theMediterranean region has been divided into three sub-regions: theWest Mediterranean, from Gibraltar to Corsica, the CentralMediterranean, from Corsica to the Ionian Sea, and the EastMediterranean, from the Ionian Sea to the shores of Syria. Theanalysis shows that the three sub-regions have not undergone thesame spatio-temporal pattern of the phenomenon probably due to thedifferent distribution of aerosols in the region.

2. DATA AND METHODS

To examine the solar dimming/brightening phenomenon in theMediterranean region satellite and re-analysis data were collectedfrom the Giovanni website(http://disc.sci.gsfc.nasa.gov/giovanni). The data collected referto the MERRA 2D option covering the period 1979 – 2004. The datacorrespond to the short-wave radiation (SWR) at the surface forclear- and all-skies conditions and are monthly values for eachpixel in the area of interest. The term "clear skies" refers tothe flows which are calculated with the aerosol load to zero.There are 2016 pixels covering the Mediterranean region, eachpixel with size of 0.5o x 0.667o. The four corners of the selectedarea have the following geographical coordinates [latitude,longitude]: [+30o, -6.667o], [+45o, -6.667o], [+30o, +36o], and[+45o, +36o]. The monthly values of SWR were averaged over the 26-year period for each pixel and maps of the selected Mediterraneanarea were produced showing the spatial distribution of theparameter for the whole period for the months of January, April,July and October (representative months of winter, spring, summerand autumn, respectively). Further, the slope of the parameterwithin the examined period was also derived. The formula used isslope (%): inclination of fitted curve x 26 years x 100 / average.Here the inclination refers to the slope of the straightregression line, which fits best to the mean value for eachspecific month within the 26 years.

3. RESULTS AND DISCUSSION


Figure 1 refers to all-skies and atmospheric conditionsencountered in the 26-year period. This means that the effect ofclouds and aerosols is included. During the winter (January) theMediterranean area, and more specifically its eastern part, hasreceived higher levels of solar radiation in comparison withsouthern Europe and north Africa. There is also observed agradation of SWR from southern Europe to the Mediterranean areaprobably due to more cloudy-sky conditions and heavier aerosolloading in the atmosphere of Europe.

Similar results are found for spring (April), summer (July) andautumn (October). Of course, the radiation levels at the surfaceare higher in the summer than the winter, as expected. The sameobservations are made for clear-skies conditions (not shown here),a fact that leads to the conclusion that it is most aerosols thathave driven the solar dimming/brightening phenomenon in the Mediterranean area. This canbe true since human agglomerations exist in the continental partof the selected area, which have contributed to the solar dimmingphenomenon. Anthropogenic aerosols have been and are beingproduced leading to a “contamination” of the atmosphere andresulting to lower solar radiation values due to greaterabsorption and scattering.


Figure 1. Spatial distribution of monthly (January, April, July,October) SWR values averaged along the period of 1979 – 2004 inthe Mediterranean region under all-skies conditions. The bar codeshows discrete values of SWR in Wm-2.


The intra annual variation of the monthly area average of SWR forall-skies (Figure 2, left graph) and clear-skies (Figure 2, rightgraph) is depicted in terms of Box & Whiskers, in order to theminimum, maximum, median, lower quartile, and upper quartile for aparticular monthly SWR. The caps at the end of each box indicatethe extreme values (minimum and maximum), the box is defined bythe lower and upper quartiles, and the line in the center of thebox is the median. The area average of SWR shows simple patternappearing however higher variability within the warm period of theyear (April-September) against the cold period (October-March).Besides, the monthly distribution of SWR presents greaterdispersion for all-skies than clear-skies, indicating the impactof atmospheric aerosols.

Figure 2. The intra annual variation of the monthly area averageof SWR for all-skies (left graph) and clear-skies (right graph),in terms of Box & Whiskers averaged along the period of 1979 –2004 in the Mediterranean region.

Figure 3 shows the slope (%) of SWR within the 26-year period.Positive values of slope imply increase in SWR and negative valuesa decreasing trend in SWR. Since the analysis has been done forthe whole period of study, it has not taken into account sub-periods in which solar dimming and brightening have been reportedto have occurred (solar dimming until mid ‘80, solar brighteningafterwards for Europe at least). Therefore, any trends shown inthe Figure refer to the whole period. It is seen that most of thebroader Mediterranean area shows signs of continuous increasingSWR levels (signs of solar dimming recovery), but there are somesmall areas (part of Turkey, part of Bulgaria, FYROM and part of


the Aegean Sea) where signs of solar dimming still exist in winter(January) and spring (April). The latter is probably due toproduction and transport of anthropogenic aerosols because ofprevailing weather conditions as this is also indicated by theslope of SWR for clear-skies conditions (not shown here).

In order to have an overall estimation of the SWR trends for thewhole Mediterranean region for the representative months wecalculated the area average SWR for January, April, July andOctober under all-skies and clear skies conditions (Figure 4).There is not observed a clear significant trend in all seasons,but it is worthy to remark that the seasonal area average SWRunder all-skies depict higher variability against the respectiveSWR under clear-skies. Furthermore, SWR under clear-skies standsfor higher area averages during the examined period than therespective ones under all-skies, indicating that, as it has beenmentioned before, the crucial involvement of aerosols in SWRmodification reaching the surface of the earth.

Improving our knowledge on how anthropogenic aerosols, affect theregional and global climate in the view of changing cloud cover,cloud optical depth and albedo, would be


Figure 3. Spatial distribution of monthly (January, April, July,October) SWR slope values averaged along the period of 1979 – 2004in the Mediterranean region under all-skies conditions. The barcode shows discrete values of slope in %.


useful for evaluating the climate models and reducing theuncertainties in the indirect aerosol effects, which remain high(IPCC, 2007).


Figure 4. Inter annual variability and trends of monthly areaaverage of SWR (Wm-2) for January, April, July and October underall-skies (left y-axis; blue cross) and clear-skies (right y-axis;red circle) conditions in the Mediterranean region

CONCLUSIONS

A simple analysis of the short-wave solar radiation reaching thesurface of the broader Mediterranean area shows that the solarbrightening phenomenon (recovery from the solar dimming one)exists in most of the area.

There are, though, some small parts of the area (parts of Turkey,parts of Bulgaria, FYROM and part of the Aegean Sea) where thesolar dimming phenomenon seems to exist still. Since thisphenomenon appears in the winter and spring, it implies that it isvery close of been diminished as when the SWR levels become higherduring summer and autumn than the other two seasons, thephenomenon turns to solar brightening. Further detailed analysisis needed in order to give deeper insight. On the other hand, theaccuracy of the data provided needs to be investigated.

REFERENCES

1. Gilgen H., Wild M. and Ohmura A. (1998) Means and trends of shortwaveirradiance at the surface estimated from GEBA, J. Climate, 11, 2042–2061.

2. IPCC (2007) Summary for policymakers, In: S. Solomon, D. Qin, M.Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller,Editors, Climate Change 2007: The Physical Science Basis.Contribution of Working Group I to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change, Cambridge UniversityPress, Cambridge, United Kingdom/New York, NY, USA.

3. Ohmura A. (2006) Observed long-term variations of solar irradiance atthe earth’s surface, Space Sci. Rev., 125, 111–128.

4. Romanou A., Liepert B., Schmidt G.A., Rossow W.B., Ruedy R.A., ZhangY. (2007) 20th century changes in surface solar irradiance insimulations and observations. Geophys. Res. Lett., 34 (5), L05713.

5. Shi G-Yu et all. (2007) Data Quality Assessment and the Long-TermTrend of Ground Solar Radiation in China, J. Appl. Meteorol Clim., 47,1006-1016.

6. Stanhill G. and Cohen S. (2001) Global dimming: A review of theevidence for a widespread and significant reduction in globalradiation, Agric. For. Meteorol., 107, 255–278.


7. Stanhill G. and Cohen S. (2008) Solar Radiation Changes in Japanduring the 20th Century Evidence from Sunshine Duration Measurements,J. Meteorol. Soc.JPN., 86 (1), 57-67.

8. Stanhill G. and Moreshet S. (1992) Global radiation climate changes:The world network, Clim. Change, 21, 57– 75.

9. Wild M., Ohmura A. and Makowski K. (2007) Impact of global dimmingand brightening on global warming, Geophys. Res. Lett., 34, L04702.

10. Wild M., Ohmura A., Gilgen H. and Rosenfeld D. (2004) On theconsistency of trends in radiation and temperature records andimplications for the global hydrological cycle, Geophys. Res. Lett., 31,L11201.

11. Wild M., et al. (2005) From dimming to brightening: Decadal changesin surface solar radiation, Science, 308, 847–850.

12. Zerefos C.S., Eleftheratos K., Meleti C., Kazadzis S., Romanou A.,Ichoku C., Tselioudis G. and Bais A. (2009) Solar dimming andbrightening over Thessaloniki, Greece, and Beijing, China, Tellus, B, 61(4), 657-665.

solar dimming/brightening in the mediterranean

Documents