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Global Dimming in the Hottest Decade

Posted on 25 October 2012 by Rob Painting

Key points

  • Global surface temperatures in the "noughties" decade (2000-2009) were the warmest in at least 120 years of record-keeping. However, the rate of surface warming, was slower than in the two decades that preceded it.
  • Even though global warming is not expected to result in incremental warming year-after-year, given that the greenhouse gas forcing increased steadily, surface temperatures through the "noughties" decade might have been expected to be warmer than they were.
  • Analysis of surface and satellite-based observations show that, on a global scale, sunlight reaching the Earth's surface decreased markedly during 2001-2006.  
  • This decline in surface solar radiation was caused by  an increase in cloud cover, with a much smaller contribution due to increased concentrations of light-scattering fine particles in the atmosphere called aerosols.
  • Based on these observations alone, a slower rate of warming of the upper ocean and global surface air temperatures between 2001-2006 should have occurred.

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).

A Background Primer 

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.  

The Hottest Decade Could Have Been Hotter

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.

Validation

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).

Dimming & Brightening: A longer view

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.

Summarizing Global Dimming in the 21st Century

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  

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Comments

Comments 1 to 31:

  1. If the effect of cloud dimming is now considered by the authors to be far greater than that of aerosols, does this mean that Cox's work is less important than we thought?

    How do Lindzen's theories fit into this paper?

    Since overall warming will result in more water vapor and dimming, will the .52 w/sqm negative feedback cited during the 2000's increase in the future, offsetting a substantial portion of forcing from emissions?
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  2. "a slower rate of warming of the surface ocean and global surface temperatures should have occurred in the noughties."

    That's what actually happened in the noughties, yes? But how do you know a slower rate of warming should have occurred, and not cooling?
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  3. Mike Roddy - the indirect effect of aerosols, i.e. their capacity to modify cloud characteristics, is far greater than their direct effect - the absorbing or scattering of sunlight, by the presence of aerosols themselves. This has been borne out by aircraft-based observations over the last few decades. The fact that aerosols did in fact increase over this period, suggests they played a part in this dimming trend. How much we don't yet know.

    Lindzen? - does his hypothesis predict dimming, brightening, dimming? Note also the solar trend during this period. Aside from the energetics problem, the solar trend make his idea untenable. Changes in human-made aerosol composition, and emission, at least match the dimming/brightening observations.

    I'll discuss more of this in upcoming posts, this post is focused on the results of the Hatzianastassiou paper.
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  4. mwsmith12 - Global surface temperatures in the noughties warmed, albeit slightly, that's why that sentence is constructed so.

    There is no expectation that it should have cooled given that the oceans, the repository of over 90% of global warming, have indeed warmed substantially through the noughties - save for a period 2004-2008.

    In other words; part of the trend in global surface temperatures is due to the burial of heat in the subsurface ocean during the La Nina-dominant period in the latter part of that decade, and the hiatus in ocean warming between 2004-2008. This will become apparent in later posts.
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  5. mwsmith12 @2,

    Model runs made without GHGs suggest that global temperatures should have decreased if only natural variability is taken into account (see image below). This is even more so for the naughts given the prolonged solar minimum and the increased aerosol loading. Yet, because of the positive radiative imbalance/forcing from anthropogenic greenhouses gases, temperatures and energy in the climate system have continued to increase.



    [Source]
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  6. If the southern hemisphere is experiencing increased dimming, does that mean that aerosols have increased there also? If so, what is the source for the increase of aerosols in the southern hemisphere?
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  7. Perhaps relevant is Norris 1999, On Trends and Possible Artifacts in Global Ocean Cloud Cover between 1952 and 1995:

    "Trends in zonal mean total and low cloud cover in 10°-latitude bands between 40°S and 60°N are all positive, and trends in the Southern Hemisphere and Tropics are generally as large or larger than trends in the midlatitude Northern Hemisphere. This argues against attribution of increased cloud cover to increased anthropogenic aerosol."

    These are ship-based observations - Norris notes that crosschecking with satellite based data will be part of validating these observations.
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  8. Is there an explanation for more dimming in the SH? The paper implies it's due to clouds. Wouldn't there be more ocean thermal inertia in the SH to delay the temperature response?
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  9. Doesn't anyone else find this at least a little scary?
    Looking forward to part 2
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  10. Martin - global dimming over the period examined was almost entirely due to aerosol and cloud increases in the Southern Hemisphere. See the text in the boxed areas of Figure 1. This paper does not look at what may have caused the dimming.
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  11. Eric (skeptic) - an emerging area of research is that the dimming effect can saturate in highly polluted regions - such as over much of China and India. In heavily polluted areas, once a certain threshold is exceeded, the high concentration of dark-coloured aerosols (such as black carbon) begins to dominate the reflective particles. This darkening absorbs more solar radiation, heats up and evaporates the cloud layers, and therefore reduces cloud lifetime.

    So one would observe a brightening, or little change, over such regions when concentrations of aerosols increase, and large changes in areas where the atmosphere is far less polluted - such as the Southern Hemisphere. It's a very new area of research, so we'll have to wait and see how it develops.

    What I'm suggesting is that the dimming of the pristine Southern Hemisphere atmosphere might seem counter-intuitive, but there may be a logical reason for this. Especially if the transfer of the main pollution source to China enables aerosols to reach the Southern Hemisphere via the large-scale atmospheric circulation. Remember the discussion in this SkS article for example?: Why Wasn't The Hottest Decade Hotter?"
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  12. A very well written post. I'm thinking that a good deal more time is needed before any trend can be declared, but its great to see this study.
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  13. If only the dimming totally compensated for the GHG forcing, then we would only have to deal with less glamorous problems, such as Ocean Acidification and the survival of polar bears. I can just imagine the denialisti crowing about our miraculous self-balancing atmosphere ...
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  14. Hello all, this is my first post here. Sorry for my poor english.
    If there is a decrease in downward solar radiation that must be due either to increase atmospheric absorption or refelction or both . If this is due to reflection there should be a corresponding albedo increase. Is there the case? I have the feeling that CRES results do net show a consistent amlbedo increase but I may be wrong. Anyone to confirm or not?

    Considering the methods to derive surface solar radiation fron satellite observations et albedo derivations from CERES, I Think the accuracy of CERES measurements is far much better. Without consistant trends in CERES derived albedoes isn't there a sreious doubdt as for the reality of the reported trends?
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  15. I have hard time understanding the numbers in Fig1, relative to GHG forcings.

    Firstly, I think the explanation of those numbers in the text is incorrect. For example, while the text sais: "Southern Hemisphere [experienced] a dimming of -2.88 W/m2 per year", the eyeballing of the black trendline on Figure 1b reveals that it cannot be so. It looks more like dSSR=-2.88 is shown for the entire period of the study (2001-2007). If it was -2.88, then it must have been -17.28 over 6 years, which is clearly not the case.

    Secondly, -2.88W/m2 (or -2.71W/m2 over both NH+SH) is a large forcing. It does overwhelm the net anthropogenic forcing since preindustrial 1750AD (estimated by IPCC in AR4 to be 1.6W/m2), assuming Earth was in rediative equilibrium in 1750AD. Hansen has recently asserted that the current radiative imbalance was +0.5W/m2, because we did warm since 1750AD enough to compensate for just about 1.1W/m2. Now, a new "forcing of the noughties" -2.71W/m2 discussed here, overwhelms the current +0.5W/m2 imbalance, resulting in a substantial negative forcing and cooling, as some commenters (e.g. @2) suggested. What am I missing here? Or maybe the number in question (-2.71W/m2) is not comparable to other climate forcings, and why so?
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  16. I take it that:
    It is reasonable to say, that the earth's albedo has increased slightly over the noughties, despite the polar amplifications . ?
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  17. I notice in figure 2 that the area with the highest rate of dimming globally is in Brazil, in the vicinity of Rio, Sao Paulo and Brasillia. I'm wondering if Part 2 of this series will explore why this region of the world is so particularly affected?
    To the northwest of the area is the region of the Amazon that is being the most deforested, but I'm disinclined to think that is the cause as the tradewinds typically blow to the northwest

    Also, off topic, I think we've got a new Monckton Myth on our hands
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  18. DMCarey @17 - I wouldn't call that a new Monckton Myth. We've debunked economics myths from Monckton many times, here and here for example. If he can get a paper on the subject published by a peer-reviewed journal, we'll have a look at it. The article claims "The paper will be published by the World Federation of Scientists". I hadn't previously heard of that organization, and as far as I know they don't have a peer-reviewed journal, but their climatology panel chariman is Christopher Essex, who is a climate denier, so I guess they would probably be open to publishing Monckton's economic ignorance, if they have a journal to publish it in.
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  19. #1 mike roddy, I think that the fact we warmed from 1980-2000 whilst SSR increased shows that either the long term cloud feedback is positive, and/or there are other effects such as aerosol direct and indirect effects which can overpower it.

    add in #6 martin and #8 Eric, there appear to be a number of reasons. Amazonia is a big regions of change, and that's ascribed to aerosol. I also suspect the weather has something to do with it. They say the Australian change is because over that short period it went from dry and clear to wet and cloudy.

    Over such a short time period the effects of El Nino etc are likely to be quite important as it strongly affects the regional cloudiness distribution. Combine that with aerosol changes and that could perhaps explain the funny patterns even in the presence of a positive cloud feedback.

    I'm aware of papers detecting positive cloud feedback to temperatures in the short term, but not any that explain these more recent global data.
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  20. Chriskoz - there was some missing text with the annually averaged rate of change - now fixed.

    "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)"

    Thanks.
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  21. Chriskoz - "Hansen has recently asserted that the current radiative imbalance was +0.5W/m2"

    Actually, closer to +0.6W/m2. Perhaps the simplest way to appreciate this is that the ocean warming demonstrated, most recently in Levitus & Nuccitelli (2012), is actually a proxy for the Earth's energy imbalance. 93% of global warming has gone into the oceans over the last 50-60 years because they are the planet's largest reservoir of heat.

    That the oceans continue to warm shows that an imbalance still exists - as we would expect it to given the long atmospheric lifetime of carbon dioxide, and the mechanism of greenhouse gas-induced warming of the oceans. Estimates of the Earth's energy imbalance cover the 2001-2006 global dimming period shown in Hatzianastassiou (2011), and despite the slowdown in ocean heating (between 2004-2008 in particular), the oceans continue to accumulate heat.

    So no, this study result does not mean the Earth's energy imbalance should have been negative over this time period. Indeed, a slowdown in ocean heating during the latter part of the 2001-2006 period is highly suggestive that this global dimming did in fact occur.

    I'm working on the follow up - the primary purpose of this post is to show that global dimming occurred during the "noughties" - a period where global surface temperatures barely increased, but ocean warming continued.

    BTW, I've also seen bloggers naively link to the data from satellite products which measure the transparency of the atmosphere, as if that somehow invalidates peer-reviewed research - a recent example being here. There are a number of reasons why this cannot be relied upon, much of which is detailed in previous work by these authors. That will be covered in a general post on aerosols in the near-future.
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  22. @Rob Painting

    Rob.

    Firstly sorry for my bad English
    Second..Great Post!!! , but are something that not match for me... the study afirms that the South Hemisphere is dimming and the North Hemisphere is Brightening Correct? So how is it possible that as GISS temperature data can show just the opposite of what is expected???







    If the study is correct and the S.H is dimming, and this brightening NH, we should not expect the temperature falling in the SH and the NH will increase in the period in question? If I did not misread the previous graphs see just the opposite in recent years, the SH is heated to the "same speed" as before, but the NH "apparently" stopped warming to the "same speed" even appear to have "stopped", this interpretation is correct?

    I would really appreciate an answer to my question...
    many Thanks
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  23. DOOMDAY, I am confused by your question. All the graphs you posted show greater warming in the northern hemisphere than in the southern. If the study is correct and there has been dimming in the south vs brightening in the north, we would expect the solar forcing to promote greater warming in the north and that is what your graphs show.

    The bigger question is the amount of forcing this dimming/brightening gives. Was the temperature of the globe in the decade 2001-2010 more influenced by aerosols, or GHGs? Did the dimming/brightening over-ride the GHG forcing? To my mind, the forcing from aerosols is not sufficient to completely negate the forcing from GHGs, so we should see both hemispheres warming. Your graphs show both hemispheres warming, so they support my expectations.

    Have I misunderstood your question?
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  24. True, before the 2000s decade warming in the NH is more pronounced in the SH qeu but I think that has more to do with the more continental NH vs SH, although it certainly could and should have also influenced the decline in anthropogenic aerosols emissiones in the 80s and 90s.
    But I did not mean for that period, but the period after the 2000s, (which is also part of the study period analyzed 2001-2006) where, for me, in the NH the heating rate is less than 2 decades whereas in the above SH is almost equal.

    Sorry I can not express myself better ... It is clearer now???
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  25. To understand that I'm "questioning" that indeed the aerosol forcing is negative in the SH and positive in the NH in the last decade of the 21st century
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  26. DOOMDAYS, I think we are missing each other's point. I am trying to see the change in global temperature: you seem to be looking at the change in land surface temperature, which is barely useful as a measure of global warming, because global warming includes oceans, cryosphere, land surface and atmosphere. Therefore, looking at recent dimming due to atmospheric pollution and expecting to see a correlated change in land surface temperature is missing the point. Dimming may change the rate of increase of global temperature, but it cannot magically eliminate all the other positive forcings and cannot eliminate all the energy used to heat the ocean and melt Arctic ice.

    As long as the relatively constant sun shines on a planet with increasing levels of CO2 in its atmosphere and as long as the ingoing radiation exceeds the amount reflected/radiated back into space, the planet will warm. The more aerosols in the air, the more gets reflected without heating the planet, but no-one is suggesting that the aerosols are currently reflecting enough to stop the globe from warming. Thus, trying to conflate global dimming with hemispheric land surface temperature changes is not going to get enough legs to run with, IMO.

    Trying to extrapolate warming/cooling trends for the short time-span 2001-2006, is always going to be a bad idea, because such a short time span will have the signal so swamped by the noise that no robust conclusions can be drawn one way or the other.
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  27. Doomsdays - I'm not sure that hemispheric surface temperatures are the best way to approach this. Those will be contaminated by the ENSO trend over this short interval because La Nina is when ocean surface temperatures drop due to greater-than-average heat being buried in the subsurface layers, and El Nino is when the greater-than-normal heat wells to the surface and is lost to the atmosphere -temporarily boosting surface temperatures.

    During 2001-2006 the latter part of that period was dominated by El Nino (see here), so there would be a tendency toward greater surface temperatures during the interval - a tendency which would counteract the Southern Hemisphere dimming, and therefore cooling, trend.

    A global brightening trend during the 1990's, and a dimming trend during the noughties would, however, help explain their trends relative to each other, i.e a strong rate of global warming in the earlier decade followed by very little warming. But again, the picture is contaminated by the ENSO trend, and what appears to be a change in ocean mixing processes in the noughties - i.e greater heat transport to the deep ocean.

    A less complex approach is to ask what would happen at the surface if less sunlight was reaching it? Think of heat absorption into the upper layers of the ocean, the sea level trend, and evaporative trends. These are all strongly influenced by surface solar radiation, and are discussed in part 2.
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  28. @Rob Painting

    Thenquiu Very Much for you answer ROB.

    Is true that in short timeframes (less than a decade or so) ENSO dominates the other forcings like GHGs or Aerosols, so if not misunderstood your words, it is misleading to take this decade as a real trend of what would happen in a dimming world ..Iam right ??? ...Questions? As we would expect that the aerosol forcing is seen reflected in the surface temperatures of the HS??? 15 to 20 Years ?
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  29. @Rob Painting

    Another Question : Why in 1975 when The CLean Air Act. is fully implemented and the aerosols stall and becoming to fall ,the resultant brightening of N.H is aparently and inmediately reflected in the temperature graph of N.H in aprox. 1975??? Why ENSO No mask the warming here for a short time...???

    sorry for so many questions and thanks
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  30. @Rob Painting

    Useful article thanks. Has the Part 2 follow-up appeared? I cannot find it....

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  31. Sorry Ian, still on my to do list. It does appear, however, that since about 2007/2008 we've seen a period of brightening (more sunlight reaching the Earth's surface) due to reduced sulfate pollution from China. See Figure 1 in Klimont (2013).

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