Supplementary material to “Global Dimming and Brightening”

George Ohring, NOAA National Environmental Satellite, Data, and Information Service, Camp Springs, Maryland

Shabtai Cohen, Institute of Soils, Water and Environmental Sciences, Volcani Center, Bet Dagan, Israel

Joel Norris, Scripps Institution of Oceanography, La Jolla, California

Alan Robock, Rutgers University, New Brunswick, New Jersey

Yinon Rudich, Weizmann Institute of Science, Rehovot, Israel

Martin Wild, Swiss Federal Institute of Technology, Zurich, Switzerland

Warren Wiscombe, NASA Goddard Space Flight Center, Greenbelt, Maryland

Citation:

Ohring, G. et al. (2008), Global dimming and brightening, Eos Trans. AGU, 89(23), 212. [Full Article (pdf)]

Global Dimming and Brightening (GDB)

International Workshop of the Israel Science Foundation
on Global Dimming and Brightening
Ein Gedi, Israel Feb. 10 - 14, 2008

Motivation

Observations indicate that the amount of solar radiation reaching the surface of the Earth decreased significantly (global dimming) over Northern Hemisphere mid-latitude land areas from the late 1950s to the late 1980s. Increases in solar radiation have been observed in many places in Europe and North America, but not in India and China, since the late 1980s (global brightening). Global dimming and brightening (GDB) have profound effects on the Earth's environment. For example, GDB counteracts or supplements greenhouse warming.

The International Workshop of the Israel Science Foundation on Global Dimming and Brightening (GDB) was organized to evaluate the observational evidence for GDB, the global extent of the phenomenon, the possible causes, and the impacts on the Earth's climate, hydrology, agriculture, and land use. The workshop brought together some 35 international experts in surface and satellite observations of surface solar radiation, scientists studying trends in clouds and aerosols, specialists in cloud and aerosol physics, climate scientists, and agricultural and land use experts.

Findings

Measurements of GDB

A widespread decrease of about 2-3 W/m²/decade occurred in surface solar radiation over mid-latitude land surfaces of the Northern Hemisphere between the late 1950s – when extensive measurements became available during the International Geophysical Year (IGY) – and the late 1980s. Since then, the trend has reversed at many of the observation sites and an increase of the order of 2 W/m²/decade has been observed in Europe and N. America. Despite this reversal, most measurements show that current surface solar radiation levels remain below IGY levels. GBD inferred from surface radiation measurements is in line with trends seen in atmospheric transmission and sunshine duration measurements (Europe, USA, and Japan). The temporal coverage and accuracy of surface radiation data varies greatly.

Satellite estimates of surface radiation inferred from reflected solar irradiance measurements show a global decline prior to 1991, followed by an increase through 2000. Comparisons of satellite estimates of surface radiation and direct surface measurements during 1983-2002 are consistent to 3 W/m² on average, but larger differences are observed in polar regions, where snow cover impacts the accuracy of satellite retrievals. The satellite-based trend for the average of all comparison sites is consistent with the corresponding surface measured trend to within 0.5-1 W/m² per decade. Satellite observations of aerosol amounts, available since the early 1980s, but only over the oceans, indicate a downward trend since about 1990, consistent with the observed brightening during this period.

A major uncertainty is the true global extent of GDB. Unfortunately, the surface measurement sites are skewed toward the developed nations in the Northern Hemisphere. Some analyses of the surface observations indicate that the rates of dimming and brightening are highest in major population centers and fall off significantly in areas with lower population densities. The satellite estimates of GDB and aerosol trends are only available for the last few decades. While they do indicate a global average brightening, they suffer from calibration uncertainties and other satellite issues.

Causes of GDB

The dominant contributors to GDB are very likely to be changes in anthropogenic aerosol and/or cloud properties, and potential interactions between them. Changes in water vapor can only have a small affect on surface solar radiation through absorption in the near IR part of the spectrum. Volcanic stratospheric aerosols can have a large global impact on surface solar radiation, but this only lasts for a few years.

Previous studies have attributed GDB to changes in clear-sky flux, changes in cloud cover, and/or changes in cloud optical thickness, depending on the specific region and time period. For example, trends in cloud cover have had a relatively small impact on trends in surface solar radiation in central Europe, whereas a decrease in cloud cover has partially counteracted dimming in cloud-free conditions due to increasing aerosol over eastern China. For many locations, dimming and brightening have predominantly occurred when skies are clear, but some sites report substantial trends in surface solar radiation when skies are overcast.

There is evidence that the long-term regional trends in surface solar radiation flux in cloud-free conditions are due to changes in anthropogenic aerosol emissions. Depending on chemical composition, aerosols deplete incoming solar radiation through scattering and/or absorption processes. Anthropogenic aerosols may also contribute to changes in cloud properties through various mechanisms and thereby to GDB. In pristine conditions, an increase in aerosol concentration may lead to greater cloud albedo and cloud amount through reduced precipitation loss, but in heavily polluted conditions over land, an increase in absorbing aerosol concentration may suppress cloudiness through atmospheric heating and surface cooling. Both of these effects have been observed locally, but the impact of anthropogenic aerosol on cloud albedo and cloud amount at large spatial scales and over long time periods remains uncertain. In some cases, trends in cloud amount can be definitely attributed to natural low-frequency variations in atmospheric circulation.

Implications of GDB

We have clear evidence that global dimming and brightening affect both temperature and the hydrological cycle. Several statements about the relationship between global warming and GDB are proposed as working hypotheses. Global greenhouse warming from the late 1800's to 1940 was intensified by the lack of volcanic eruptions during the latter half of this period and the associated “early” global brightening – observed in the few available surface radiation records of the time. From about 1940 to 1970, global surface temperature remained rather constant, in spite of the continuing increase in greenhouse gases. Global dimming, caused by rapidly increasing industrialization, counteracted the positive radiative forcing from increasing greenhouse gases. Although aerosol emissions have continued to increase in Asia and the Southern Hemisphere, this has been offset by the decreasing emissions in North America and Europe resulting from clean air legislation. This has lead to global brightening, which may have supplemented greenhouse radiative forcing and related global warming over the past few decades. It is likely that brightening will spread over the coming decades and contribute to an acceleration of greenhouse warming as China, India and other developing nations clean up their air.

Changes in solar radiation affect maximum daytime temperatures and, consequently, the diurnal temperature range (DTR). The observations indicate a decrease in DTR for many parts of the earth from the 1950s to about 1980 and an increase since then, reasonably consistent with GDB trends.

Global dimming and brightening affect the hydrological cycle in two different ways. By changing evaporative demand, GDB affects the surface latent heat flux and changes the surface water balance. But by differentially changing surface temperature, atmospheric circulation can change, particularly in the African and Asian monsoons, and may affect precipitation patterns. The practical measure of evaporative demand, pan evaporation responds to global dimming and brightening. In energy-limited sites, pan evaporation is a good surrogate for actual evapotranspiration. The longest available observations of soil moisture show an increasing trend of summer soil moisture in Ukraine and Russia – consistent with declining pan evaporation in spite of increasing temperature and no significant trend in precipitation. The reduction in evaporative demand has been attributed to solar dimming. In water-limited sites, like arid environments, the trend in actual evapotranspiration is mostly determined by changing precipitation.

Studies show that a shift to more diffuse radiation caused by global dimming has positive impacts on agriculture and tree growth, while dimming in arid zones may reduce plant water stress and increase productivity.

Recommendations

To determine its distribution over the globe and obtain reliable estimates of the global average value of GDB:

Improve the observational basis through: global extension of the surface radiation network; more reliable calibration/intercalibration of satellite sensors; more accurate satellite retrieval algorithms for surface radiation, clouds, and aerosols; and development of additional observational techniques, e.g., Earthshine, that have the potential for filling in the measurement gaps.

To determine the relative contributions of changes in aerosol and cloud properties, and natural and anthropogenic forcing, to GDB and the extent to which GDB been driven by changes in cloud properties in response to anthropogenic aerosols:

Conduct more comprehensive analyses of existing measurements to evaluate the relative contributions of aerosol and cloud trends, and aerosol//cloud interactions, to the observed record of GDB.
Add measurements at all surface radiation sites of spectra to infer aerosol and cloud properties, SW components (diffuse and/or direct as well as total), and basic meteorological parameters (T/RH and air pressure);
Improve the modeling of aerosol-cloud interactions in GCMs and regional models.

To evaluate the impacts of GDB on the Earth's environment:

Perform modeling studies on aerosol/clouds/radiation/circulation feedbacks to assess the roles and impacts, causes and effects, and feedbacks of different aerosol types (e.g., volcanic, anthropogenic, biomass burning, dust). The models should be constrained by the observations of aerosol quantities, surface radiation change, and aerosol/radiation trends.
Carry out controlled shading experiments to study the relative effects of solar radiation, temperature, and precipitation on agricultural and natural ecosystems.
Use remote sensing methods to constrain the effects of GDB on land use and agriculture.

Workshop participants are planning to prepare a review paper and research articles on GDB for publication in a special issue of JGR-Atmospheres. The workshop was supported by the Israel Science Foundation, NASA, NIST, Weizmann Institute of Science, Israel Ministry of Agriculture & Rural Development, and Tel Aviv University.

—George Ohring (NOAA/NESDIS, Camp Springs, MD, george.ohring@noaa.gov), Shabtai Cohen (Institute of Soil, Water and Environmental Sciences, Israel), Joel Norris (Scripps Institution of Oceanography), Alan Robock (Rutgers University), Yinon Rudich (Weizmann Institute, Israel), Martin Wild (Swiss Federal Institute of Technology), Warren Wiscombe (NASA, Greenbelt MD)