New paper – Global dimming and brightening: A review

Global dimming and brightening: A review

Martin Wild

Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

There is increasing evidence that the amount of solar radiation incident at the Earth’s surface is not stable over the years but undergoes significant decadal variations. Here I review the evidence for these changes, their magnitude, their possible causes, their representation in climate models, and their potential implications for climate change. The various studies analyzing long-term records of surface radiation measurements suggest a widespread decrease in surface solar radiation between the 1950s and 1980s (“global dimming”), with a partial recovery more recently at many locations (“brightening”). There are also some indications for an “early brightening” in the first part of the 20th century. These variations are in line with independent long-term observations of sunshine duration, diurnal temperature range, pan evaporation, and, more recently, satellite-derived estimates, which add credibility to the existence of these changes and their larger-scale significance.

Current climate models, in general, tend to simulate these decadal variations to a much lesser degree. The origins of these variations are internal to the Earth’s atmosphere and not externally forced by the Sun. Variations are not only found under cloudy but also under cloud-free atmospheres, indicative of an anthropogenic contribution through changes in aerosol emissions governed by economic developments and air pollution regulations. The relative importance of aerosols, clouds, and aerosol-cloud interactions may differ depending on region and pollution level. Highlighted are further potential implications of dimming and brightening for climate change, which may affect global warming, the components and intensity of the hydrological cycle, the carbon cycle, and the cryosphere among other climate elements.

Received 14 November 2008; accepted 10 March 2009; published 27 June 2009.

Citation: Wild, M. (2009), Global dimming and brightening: A review,

J. Geophys. Res., 114, D00D16, doi:10.1029/2008JD011470.

I found this passage that parallels a lot of what I’ve been saying about data quality:

The assessment of the magnitude of these SSR (surface solar radiation) variations faces a number of challenges. One is related to data quality. Surface radiation networks with well-calibrated instrumentation and quality standards as those defined in BSRN [Ohmura et al., 1998] need to be maintained on a long-term basis and if possible expanded into underrepresented regions (see Figure 1b).

However in this figure, citing CRU surface temperature, he likely doesn’t understand what data quality issue might have contributed to the trend from 1960-2000

One of the effects of urbanization is the compression of the diurnal temperature variation. I recently was able to demonstrate this between two stations in Honolulu. One is in the middle of the Airport and had a sensor problem, the other was in a more “rural” setting about 4 miles away. Note how the ASOS station at the airport has an elevated temperature overall, but that the biggest difference occurs in the overnight lows, even when the ASOS sensor giving new record highs was “fixed”:

Urbanization affects Tmin more than Tmax. For example, here’s the nighttime UHI signature of Reno, NV that I drove as a measurement transect using a temperature datalogger:

Click for larger image

Even several hours after sunset, at 11:15PM, the UHI signature remained. The net result of  urbanization is that it increases Tmin more than Tmax, and thus minimizes the diurnal range, which we see in Wild’s diurnal range graph.

Even the IPCC misses it:

Wild probably has no idea of this type of issue in the CRU data, but again it speaks to data quality which he seems to be keen on. He’s looking for a global solar signature in temperature data, something Basil Copeland and I have done, to the tune of much criticism. The signature is there, but small. But, when diurnal temperature variation is looked at, any solar signature is likely swamped by the urbanization signal. I’m not saying there is no solar component to what Wild is looking at, but it seems fairly clear that UHI/urbanization/land use change plays a significant role also.

Even rural stations can be affected by our modern society, as Dr. John Christy demonstrated in California’s central valley:

A two-year study of San Joaquin Valley nights found that summer nighttime low temperatures in six counties of California’s Central Valley climbed about 5.5 degrees Fahrenheit (approximately 3.0 C) between 1910 and 2003. The study’s results will be published in the “Journal of Climate.”

The study area included six California counties: Kings, Tulare, Fresno, Madera, Merced and Mariposa.

While nighttime temperatures have risen, there has been no change in summer nighttime temperatures in the adjacent Sierra Nevada mountains. Summer daytime temperatures in the six county area have actually cooled slightly since 1910. Those discrepancies, says Christy, might best be explained by looking at the effects of widespread irrigation.

Wild’s study is a very interesting  and informative paper, I highly recommend reading the entire paper here (PDF 1.4 mb)

h/t and sincere thanks to Leif Svalgaard for bringing this paper to my attention.