Key points
Figure 1 - Anomalies of monthly downward surface solar radiation (with seasonal signal removed) over the period 2001-2006. Average over land (green lines), ocean (blue) and land+ocean (black lines). Global Dimming & Brightening magnitudes are shown in boxed area for Northern Hemisphere (a) and Southern Hemisphere (b). SSR = surface solar radiation. From Hatzianastassiou (2011).
Global Dimming and Global Brightening describes the variation in solar radiation reaching the Earth's surface. This has nothing to do with changes in the intensity of sunlight emitted by the sun, but rather with changes in the transparency of the Earth's atmosphere. Unsurprisingly, the largest contributors to this variation in surface solar radiation are clouds and aerosols, which absorb or scatter incoming sunlight depending on their physical characteristics, and thereby contribute toward either warming or cooling the planet.
Aerosols? These are microscopic particles suspended in the air, and are derived from natural sources - such as dust, sea salt, and material from volcanic eruptions, and from human activity - especially air pollution. Aerosols are a key component of the climate system because they act as "seed particles" for water to condense, forming clouds. If there were no aerosols, there would be no clouds. This is a physically impossible scenario of course, but it does serve to illustrate how climatologically influential aerosols are.
Hatzianastassiou (2011) is the second peer-reviewed paper to examine the global surface solar radiation (SSR) trend in the 21st Century, and to find a dimming trend. A previous paper, Hinkelmann (2009), found a decline in surface solar radiation for the period 1999-2004 amounting to -0.52 W/m2 per year (watts per square metre) - enough to substantially impact Earth's climate.
This more recent study takes a look at the period 2000-2007, with an emphasis on 2001-2006, because of incomplete data for the start and end-point years. Utilizing satellite-based observations from a number of global datasets, and a radiative transfer model, the authors compute the surface solar radiation changes on a pixel-by-pixel basis - meaning that they divide the Earth into a series of small grids, and calculate the linear trend over the period for each grid (or pixel). The result of those calculations (with seasonal signals removed) are shown below:
Figure 2 - anomalies of deseasonalized model-computed monthly downward surface solar radiation (in Wm2 per year). The white regions are those where fluxes are corrupted by bright surfaces - i.e. ice-covered and desert regions, and therefore have yet to be sufficiently validated. From Hatzianastassiou (2011).
To the naked eye the change over this time is remarkably patchy, with opposing tendencies even within the same continent. Analysis demonstrates that the land-dominant Northern Hemisphere underwent a slight solar brightening over the period, whereas the ocean-dominated Southern Hemisphere experienced a strong dimming trend. This can be seen in Figure 1, where the Northern Hemisphere experienced a brightening of 0.17 W/m2 (0.028 W/m2 per year), and the Southern Hemisphere a dimming of -2.88 W/m2 (0.-48W/m2 per year).
Readers will notice that, despite this decline over the period, the global surface solar radiation trend exhibits huge year-to-year variability. Perhaps this large annual variability shouldn't come as any great surprise. Aerosols are generally short-lived in the atmosphere and their persistence is strongly controlled by the source of emission, be that natural or of human-made origin, and by local weather patterns - which may vary from year-to-year.
To determine if their observationally-derived calculations were supported by other measurements, the authors compared their results with two global surface-based datasets - the Global Energy Balance Archive (GEBA) and the Baseline Surface Radiation Network (BSRN). These datasets come from skyward-looking ground-based instruments which detect the amount of sunlight reaching the surface. Although these instruments have a great advantage over the satellites in that they directly measure sunlight hitting the detectors, they are handicapped by a poor global distribution, and being very limited in number. This makes the ground-based datasets less robust as a means to determine trends on a planetary scale, but very useful for validating the satellite data.
Discarding stations with incomplete data over the 2001-2006 period, gave the authors 91 GEBA, and 14 BSRN, stations with which to make comparision. Despite comparing one specific location (land-based) with a grid/pixel about 280x280 km in size (the satellite data), the trends in the two datasets show good agreement - as determined statistically by the authors. See Figure 3.
Figure 3 - Comparison of model-computed tendencies of surface solar radiation against GEBA (a) and BSRN (b) land-based station measurements over the 2001-2006 period. Similar tendencies are shown in blue colour and opposite in red, and the larger the circle, the stronger is the statistical correlation. From Hatzianastassiou (2011).
The global decline in surface solar radiation shown in Hatzianastassiou (2011), for half of the first decade of the 21st Century, is contrary to the trend of the previous decades. Since the late 1980's the Earth experienced a Global Brightening trend (Hatzianastassiou [2005], Pinker [2005]), a trend which reversed a dimming trend in the decades before that - the 1950's to 1980's (Wild [2007]). Therefore, since the mid 20th Century the Earth's surface has globally undergone dimming, then brightening, and now dimming again.
Figure 4 - Schematic representation of global dimming/brightening in the latter half of the 20th Century. GH=ground heat flux, SSR=surface solar radiation, SH=sensible heat flux, LH=latent heat flux (evaporation), LW↑=heat emitted from the surface, and LW↓=downwelling heat from greenhouse gases. Global Dimming & brightening have substantial impacts upon global temperature and the global water cycle. See Wild (2012) for further detail.
I won't delve any further into the topic here, the point is simply to show that over decadal time frames, and longer, the Earth has undergone substantial variations in the amount of sunlight it receives at its surface. The magnitude and direction of these changes are such that changes in the brightness of the sun can be immediately eliminated as a potential culprit because the solar flux has been far too small, and total solar output has in fact declined over the last 4-5 decades.
Analysis of satellite-based observations by Hatzianastassiou (2011) reveal that the Earth experienced a substantial decline in the amount of solar radiation received at its surface between 2001-2006. This decline was primarily the result of increased cloud cover, however a much smaller contribution came from increased aerosol concentrations.
The Northern Hemisphere underwent slight brightening during this time, but was overcompensated for by a strong dimming of surface solar radiation in the Southern Hemisphere - whose surface area is largely dominated by ocean.
Based solely on these observations, and taking into account the physical understanding of the Earth system, a slower rate of warming of the surface ocean and global surface temperatures should have occurred in the noughties. This short-term global dimming should have counteracted a larger fraction of the long-term warming effect of the greenhouse gas forcing during this interval.
Time will tell if the results of this paper are affirmed by further research, but there are other global observations which suggest that Global Dimming did indeed occur during the 21st Century.
Next: Part 2 - A Closer Look at 21st Century Global Dimming
Posted by Rob Painting on Thursday, 25 October, 2012
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