From Dimming to Brightening

Recent studies [46,47] have found evidence for a reversal in the negative trends in solar radiation, which, for many sites changed to positive trends in the late 1980s and early 1990s. The data sets analysed were from the GEBA archive [46] and, for the first time, long term trends in satellite data from 1983 to 2001 [47]. However, there is an inconsistency between the two studies, since the satellite data show brightening over the oceans and no trend over the land surfaces while the surface GEBA and BSRN measurements are mostly land based and show clear brightening during this period. The reversal in the trend is thought to be related to the decreases in air pollution in Europe and other parts of the western world following legislation that limited air pollution. The positive trend has not led to a full recovery in Eg# and current levels of solar radiation in most places where dimming took place are still below the values measured during the 1950s. A selection of widespread trends reported for Eg# is given in Table 2.

A list of the publications on global dimming, brightening and related topics was compiled by M. Roderick at ANU and is kept more or less up to date. It can be found on the web at http://www.rsbs.anu.edu.au/ResearchGroups/EBG/ index.php. Several international meetings have been held to discuss these topics (Table 1).

TABLE 1 International meetings held on changing surface solar radiation and related changes in evaporation

Organizing Organization and event

Date Session title

Location

Reference

AGU/CGU joint assembly

17 18 May Magnitude and Causes 2004 of Decreasing Surface Solar Radiation

Montreal, Canada

[91]

Australian Academy of Science International workshop

22 23

November

2004

Pan evaporation: An example of the detection and attribution of trends in climate variables

Canberra, Australia

Australian Academy of Science International workshop

22 23

November

2004

Pan evaporation: An example of the detection and attribution of trends in climate variables

Canberra, Australia

EGU general assembly

15 20 April 2007

Surface Radiation Budget, Radiative Forcings and Climate Change

Vienna, Austria

AGU fall meeting

10 14

December 2007

Pan Evaporation Trends: Observations, Interpretations, and the Ecohydrological Implications

Francisco, CA, USA

Israel Science Foundation international workshop

10 14 February 2008

Global dimming and Ein Gedi, brightening Israel

EGU general assembly

13 18 April

2008

Surface Radiation Budget, Radiative Forcings and Climate Change

Vienna, Austria f \ TABLE 2 Selected estimates of widespread trends in surface solar radiation from surface measurements and satellite-based estimates. Based on Ref. [94]

Surface

Study

Time period

Energy trend per decade/ (Wm~2)

Comments

[23]

From mid 1950s to 1992

3

Trend analysis of about 30 sites of various lengths, and data from five years from 1957 to 1992 for >145 stations

[95]

1960 1990

2

Trend analysis of GEBA and US NREL data sets from 1960 to 1990

[44]

From mid 1950s to 1990

3

Statistics of the GEBA data set based on about 300 sites of various length

[51]

From mid 1950s to 1990

1.6 4.1

Analysis of GEBA data to constrain the "urbanization" effect. Separation of sparsely populated sites (<0.1 million inhabitants) and populated sites (>0.1 million inhabitants)

[96]

1977 1990

2

Trend analysis of five records of the GMD data set from remote sites from South Pole to Barrow, Alaska

[46]

1993 2004

4.7

Trend analysis of 18 BSRN records

[46]

1985 2005

2.2

Decadal change between (1985 1995) and (1995 2005) based on 320 GEBA sites

Satellite

[47]

1983 2001

0.5

Global. University of Maryland algorithm with ISCCP Clouds Global average Ocean surfaces Land surfaces

[97]

1984 2000

2.4

Global. ISCCP Clouds with own RT model

[58]

1984 2000

0.4 1 1

Global (ISCCP FD) Ocean (ISCCP FD) Land (ISCCP FD)

Notes: GMD Global monitoring division of NOAA, ISCCP FD International Satellite Cloud Climatology Project result data sets.

Notes: GMD Global monitoring division of NOAA, ISCCP FD International Satellite Cloud Climatology Project result data sets.

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