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.
Pamela Gray (07:44:49) :
Is it from Gauss that we get our term de-gauss, a step in computer screen maintenance to right a skewed monitor view? I had a problem with that once when I placed a tower speaker next to the screen and the view appeared to be magnetically attracted to the speaker.
It is magnetic fields that distort screens.
When working with a CERN experiment that had a field in the center of 15000 Gauss all the screen displays were skewed by the firnge fields and we had to turn our heads around to read them. I felt my head ringing too :).
Pamela Gray (08:21:58) :
His conclusions seem to place more emphasis one anthropogenic sources than the study merits.
I really don’t have any opinion on this [don’t know enough] but what caught my interest were the actual measurements of the solar radiance over ~ a century and how much it varied [from whatever source]. When we lived in Belgium we regularly has windblown dust from Sahara, so aerosols and dust travel far.
Pamela Gray (08:21:58) :
Anthropogenic means created by man. Are you sure that
. We don’t grow nearly the acres of wheat we used to grow since crop land was put into conservation programs. But this dry shrubby desert soil then gets kicked up by the wind during high wind seasons because there is little cover crop land to anchor it. In the old days, before high plains soil was planted, dust storms were as bad as ever.
the reason there is little cover is not due to man set fires at some point or other?
Even for the Sahara I have read studies that well digging and goats are advancing the desert in leaps and bounds because herders increase their goat herd once there is a well and the goats eat everything till the area dries up.
We get a lot of red rain, mud from the Sahara, in Greece.
In Oregon, we are not growing the tonnage we used to. We are getting more dust in the air as a result. The natural condition of the high desert plains is one of dusty dry palouse soil as well as fine sand. This, combined with high winds, becomes a natural aerosol ladened atmosphere. Close by, much of the soil now used for vineyards (and high-placed mansions) in around Pullman, WA was blown there long before tilled and irrigated agriculture became wide spread. So too the high desert plains of Oregon. However, efforts were made to reduce this dust in the air by planting and irrigating with that purpose in mind. But then it was decided that natural landscapes and soil left to itself is better. When land is placed in conservation programs, the emphasis is low or no tilling, natural cover crops, or none at all, and no irrigation. And so the dust is beginning to rise again. The point is that dust in the atmosphere is likely a natural decadel phenomena combined with agricultural practice that reduces it here and there, and/or now and then. As a pollution source, you will need to tease out natural phenomena from human sources.
I meant to say human sources from natural phenomena. This means that some satellite will have to be able to determine polluted dirt in the air from natural dirt in the air. I would hazard a guess that most aerosols are natural occurring phenomena (here again, the null hypothesis). It will be a neat trick to determine otherwise.
Anna, it is just the opposite. Man-made fire suppression has greatly endangered the forests and cause erosion prone soil. Without the regular vacuum of natural fires that are allowed to burn where they will, floor fuels build up everywhere and cause extremely hot wide-spread blanket fires that burn from below the ground to the tops of the trees. Natural fires follow a snake like path through the forest and only burn what little fuel there is at ground level. They don’t even sanitize the soil. Catastrophic fires of the kind only seen with large ground level fuel loads, sanitize the soil of seeds that could have otherwise sprouted after natural fires.
IMHO, to get back to healthy forests, all US national forests should be returned to the states and put under local control with the following conditions: fires should be allowed to burn and grazing should once again be the standard practice. Those that build houses in the forest are on their own and should carry rather large expensive insurance on their holdings. If this were to happen, forest management would not produce any added atmospheric or erosion problems.
Pamela Gray (08:21:58)
The issue of natural aerosols has interested me for some time. In fact, ever since the first satellite photos of Earth.
The fact is that even if there were no humans at all the continental areas would still generate huge volumes of atmospheric particulates and smoke from large scale fires caused by lightning strikes.
Putting the blame for much of the particulates in the air on humans is just silly. We reduce natural particulates and increase them in pretty much equal measure by our activities.
I was astonished to read once that a visible brown line seen at the horizon from polar regions was apparently caused by human pollution. In fact it was more likely a natural phenomenon from particulates raised over continental areas quite naturally.
That’s not to say we make no contribution at all. From time to time and on a limited scale I’m sure we have some effect but in my view it is grossly overstated.
There is a tendency for many to regard ANY newly observed phenomenon as a malign consequence of human activity.
It’s a modern extension of the witch doctors and soothsayers of the past.
At base we are far more primitive than is generally accepted.
All I was saying was that probably the pristine state of the land, before man started burning to create grazing land would be covered with grass and bushes, even in desert surroundings, so what is man’s intervention and what is not is very complicated.
Greece was 80% woodland in the 19th century. It is barely 30% now, and it is even worse for western Europe where one travels through thousands of miles of what would have been wooded hills and are now cultivated.
I would agree that it is very complicated to separate man induced from natural.
Yes, lightnings start fires, but usually they are accompanied by strong rains which limit the extent of fires. I live across a pine wood recovered mountain. In my fifty years in this area I have seen only once lightning strike a pine . The rain put the fire out. The probability is low.
It is the fires started by shepherds so that new grass will grow in the winter that have denuded most islands and mountains in Greece, and this has been happening over the last hundred of years or more .
anna v
There are large areas of the continents where lightning is frequently not accompanied by rain reaching the ground.
Lightning strikes without rain have been historically the main cause of large scale conflagrations.
Many forested area in the drier parts of the world have always been subject to regular burns and the local wildlife has adapted accordingly.
If anything human intervention has reduced the scale and frequency of such events so as to protect property.
Greece is mostly a dry area and, before the shepherds, natural fires were just as effective at clearing old undergrowth as anything the shepherds do now. Furthermore the encouragement of winter grass growth tends to fix the soil and reduce the transfer of particulates to the air.
In any event I would have thought the main influence on global dimming and brightening would be cloud cover variations from natural causes rather than anything humans could do.
Re: tallbloke (07:54:31)
Also, once you are proficient with the 2 different ways to use the offset function, it’s a piece of cake to get Excel to consider all possible lags (& hence run cross-correlation analyses). When it comes time for generating summaries, you may find the “match” and “index” functions useful.
I advise modular construction. If you try to create one large glorious workbook that “does everything” in Excel, you’ll end up spending most of your time rebooting your computer; however, if you keep workbooks under 100MB and submit just sheets of values from one book to another – never having more than one module open at a time – it’s clear sailing. It’s sort of like driving a well-engineered standard (manual transmission automobile) if you get the module sizes right …but if you get greedy with your algorithms: it’s crash, crash, crash.
Btw: Doing wavelet analysis in Excel is a breeze once you master the offset function.
My main complaint about Excel [hopefully someone influential at Microsoft is reading]: Color-contour plots do not (!) have a flexible aspect ratio.
You can make nicer color-contour plots in SPlus or R (since the color palette is more flexible), but the programming code is hideous and algorithm-debugging is inefficient since the code is not “live”. [Pouring over hundreds of lines of unaesthetic code looking for a missing comma? – & relying on a dissonant algebraic framework that forces endless transposes? – no thanks if there is an alternative …and no wonder people resort to using other peoples’ wavelet software without understanding it.]
Final Sales-Pitch:
Don’t underestimate what you can do in Excel once you master the offset function.
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Pamela, you bring up some interesting points about anthro-land-use influences on decadal-timescale variations.
We need to figure out the conditioning (switch-flipping – masking or enhancement) that drives the various intermittent “wiggle-matches” we see. It’s not necessarily about intermittent causation – it can be a third (lurking) variable influencing 2 study variables, for example.
There’s obsessive linear focus on amplitudes and a vast majority of researchers appear timid about studying phase. Study of phase relations is complex, so established researchers don’t seem keen about risking their reputations on it (“taking one for the team”, so to speak), but we’re just delaying vital progress by submitting to paralyzing cultural norms.
Expertise secures higher regard when “evaporating correlations” are explained rather than dismissed.
– –
Leif, have decisive conclusions been drawn about links (or lack thereof) between Earth Orientation Parameters (EOP) and geomagnetic indices – particularly aa index?
Noteworthy:
Nobel laureate Dr. Andrew Weaver appears to have briefly explored DTR.
http://climate.uvic.ca/people/weaver/
On his “Publications” page, search:
1) “diurnal temperature range” (without the quotes) and
2) “daily maximum and minimum temperature”.
Note the date on the relevant publications (2002 & 2003). Around that time, Dr. Weaver’s home province, British Columbia, Canada – which has a carbon tax – was publishing Tmax & Tmin trends based on LINEAR assumptions – which, if extrapolated into the future, produced Tmin GREATER THAN Tmax (!) – dragging along the average, of course, which is defined as (Tmax+Tmin)/2.
It is interesting to note that BC’s more recent trend-summaries have become less-easily-shot-down. I interpret the substantial changes as acknowledgment of a very serious weakness in earlier model-derived trend-summaries.
Many other jurisdictions have made the same serious mistake of using simple linear regression to summarize (& extrapolate) trends.
Historians: It might be worth archiving some of the (increasingly oldschool) linear trend-summary webpages before more administrators get panicky and issue directives to have them torn down (in haste). It might be informative to note which jurisdictions are the last to abandon the overly simplistic (& formerly conventional) linear-assumption-based summaries.
Paul Vaughan (15:04:03) :
Leif, have decisive conclusions been drawn about links (or lack thereof) between Earth Orientation Parameters (EOP) and geomagnetic indices – particularly aa index?
Nothing is ever ‘decisive’, but my take is that the energy in geomagnetic and solar wind coupling isn’t here to have any effect. Many people have looked at this and none have demonstrated anything significant, but I’m sure you can find links to anything to desire. A fertile ground is looking for semiannual correlations because aa has one [having three or four different causes, the dominant being the size of the magnetosphere controlled by the angle between the solar wind and the Earth’s magnetic axis], but you of course know what the best fertilizer is…
Stephen Wilde (13:39:53) :
I am not disputing that lightnings do set fires, or that there may not be dry lightning. Pine forests reproduce by fire ( the cones burst and bury the seeds in ash fertile ground)
I am saying that in my area, Greece, woodland was 80% in the 1800 hundreds and is 30% or less now. Destructive fires might have happened once in forty years naturally , now they go on every summer because from a lent observing mostly vegetarian diet we have turned to a meat eating society appreciating goat and lamb every week instead of just for Easter and feasts.
I think that people do have great influence in local weather from changes in land use and even a more global one if extensive irrigation or denudation takes place. These change cloud cover ultimately. It is fortunate that 3/4 of the earth is ocean.
Certainly CO2 should be the last on the list.
Re: Leif Svalgaard (18:30:48)
Thank you for this comment.
Strongly recommended reading:
Jan Vondrak (1999). Earth rotation parameters 1899.7-1992.0 after reanalysis within the hipparcos frame. Surveys in Geophysics 20, 169-195.
http://www.yspu.yar.ru/astronomy/lib/Rotation.pdf
[See particularly section 3.2.]
J. Vondrak & C. Ron (2005). The great Chandler wobble change in 1923-1940 re-visited. In: H.-P. Plag, B. Chao, R. Gross, & T. Van Dam (eds.), Forcing of polar motion in the Chandler frequency band: A contribution to understanding interannual climate variations, Cahiers du Centre Europeen de Geodynamique et de Seismologie 24, 39-47.
I am sorry but I must dispute that.
The ancient Greeks used timber for building to such an extent that they ran out of it, and had to turn to stone. To see how elegant their timber buildings must have been visit New England and its classical architecture in the wood: to see how clumsy it’s stone replacement was and is only look about you and see those mighty stone columns.
So abundant was timber that the Roman Empire did not run out of wood in either Hispania or Gaul, see one Julius and the seige of Massila, or indeed young Cicero
The disaster came with the Ottoman empire which introduced goats as a source of milk, flesh and wool. But goats eat everything including young saplings so the forests could not regrow.
To be fair to them, not that I care to, the Spanish government has recognised that pine trees are not natural in the South and that Cervantes had a point when he said that a squirrel could travel from Madrid to Malaga without touching the ground.
And so they are planting new decidious forests, but the project will take a hundred years so I for one shall not see the result. But I wish them well.
The point being that humans can do affect small areas of the world with their urbanisation and agriculture: but what is that given the tiny fraction of the globe they occupy?
Do you really imagine our puny efforts can affect the great natural forces that drive our global climate?
Sorry to be so pedantic.
Kindest Regards
Paul Vaughan (22:04:18) :
Re: Leif Svalgaard (18:30:48)
Thank you for this comment.
Perhaps you would be so kind as to tell me what ‘her central thesis’ was?
Paul Vaughan (22:28:05) :
Jan Vondrak (1999). Earth rotation parameters 1899.7-1992.0 after reanalysis within the hipparcos frame. Surveys in Geophysics 20, 169-195.
When reading papers about this, don’t be confused between tidal components and geomagnetic activity effects. Another frequent point of confusion is the difference between geomagnetic activity [caused by the solar wind] and expansion of the thermosphere [caused by solar irradiance variation in the UV]. The latter clearly having an effect on the LOD because of corresponding changes of the moment of inertia.
a jones (23:18:19) :
I am sorry but I must dispute that.
The ancient Greeks used timber for building to such an extent that they ran out of it, and had to turn to stone.
That Greece came out of the Ottoman rule with 80% forest coverage, and currently has about 30% is not disputable. I cannot discuss woodlands and ancient Greece, except noting that “running out of wood” would need many qualifications as: which part of Greece considering the many city states.n Considering also that the Aegis of Athena was made of goat skin, I doubt that it was the Ottoman who introduced goats to Greece.
Re: Leif Svalgaard (02:56:32)
If I understand what you are saying here:
You are cautioning me not to confuse (small?) radiation tides (~11a period) with geomagnetic effects (since there is statistical confounding of these variables).
–
I addressed your other concern (in a single sentence) some time ago.
I recently learned that the Earth’s distance from the sun varies more than i’d thought. The part I knew about was the eccentricity of the Earth’s orbit (+/-1.7% variation in distance). the other part is the size of the sun’s orbit around the solar system’s center of mass. The Sun’s orbital radius is +/-3% of the earth’s, chiefly because of Saturn’s and Jupiter’s motion. So, the variation in the Earth’s distance from the Sun can be as big as +/-4.7%. This amounts to 18.8% variation in how much light we receive.
Note that this 19% variation is about what the graph shows. Note also that the graph’s curve has a principal wavelength of about 20 years, which closely matches the time it takes for Earth, Jupiter, & Saturn to repeat their arrangements around the Sun. This 20-year period is called the synodic period for Jupiter & Saturn.
Paul Vaughan (19:01:34) :
If I understand what you are saying here:
You are cautioning me not to confuse (small?) radiation tides (~11a period) with geomagnetic effects (since there is statistical confounding of these variables).
That is one reason.
I addressed your other concern (in a single sentence) some time ago.Not to my satisfaction, would I otherwise continue to ask? I maintain that I have no idea what her central thesis is and that her papers must be judged on their merit, not on someone’s perceived thesis. So, one more time: please tell me what you think her central thesis is and how I have failed to take that into account and why it makes any difference.
Re: Leif Svalgaard (19:20:27) “That is one reason.”
And the other part? Annual & semi-annual tides vs. annual & semi-annual variation in aa?
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Resolving the misunderstanding appears to remain a fruitless endeavor.
Trying a more productive topic:
What caused the 1930s drought? (and would you answer this differently for the general public than for scientists?)
Paul Vaughan (20:09:36) :
And the other part? Annual & semi-annual tides vs. annual & semi-annual variation in aa?
For example, yes.
Resolving the misunderstanding appears to remain a fruitless endeavor.
Especially since you make no effort at all to make it fruitful. My question still stands.
What caused the 1930s drought? (and would you answer this differently for the general public than for scientists?)
I have no idea, but, no the answer [if I had one] would be the same, the exposition might be different, but if one really understands something, one can explain it to a five-year old.