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Soot and global warming

Posted on 2 April 2011 by Sarah

Black Carbon emissions regionWe have long known that there are plenty of reasons to reduce emissions of soot, also called black carbon (BC). Black carbon has serious and well documented health effects; the same crud that turns buildings and your laundry black also coats your lungs. Industrialized countries recognized the dangers of billowing clouds of soot over a century ago; emissions in North America have been decreasing since their peak around 1910.

We now have an additional and urgent reason to minimize soot emissions: black carbon gives a short-term, but powerful boost to heating the planet. Black carbon (BC), contributes to climate warming in two ways. First, black soot particles in the air absorb sunlight and directly heat the surrounding air. Second, soot falling on snow or ice changes those reflecting surfaces into absorbing ones, that is, soot decreases the albedo. Therefore, soot deposits increase the melting rate of snow and ice, including glaciers and the arctic ice.

Black carbon is a “short-term” climate forcer, acting for a few days in the atmosphere and a few months on snow and ice. Over these short times it is an important contributor to warming. Reducing soot will have immediate benefits; by gradually eliminating BC emissions over the next 40 years we could slow warming, perhaps by 0.1-0.2°C globally. Decreasing black carbon deposits in the arctic may also slow amplification of feedbacks from melting arctic snow and ice.

Black carbon does not accumulate in the atmosphere like carbon dioxide (CO2). BC stops warming the air when it is washed out in a few days or weeks; it stops warming the snow when it gets covered with more snow or carried away by melting. So, reductions in BC have immediate, but not long-term effects on global warming. Each CO2 molecule continues to block heat loss from the Earth for the 100+ years that it stays in the atmosphere. That is why CO2 is known as the “biggest control knob” for our climate. Climate change cannot be prevented without reducing carbon dioxide emissions. Reductions in long and short-term forcers, CO2 and BC and methane and ozone will be necessary to keep global temperatures from rising more than 2°C above preindustrial levels in the next 50 years.

“It is important to emphasize that BC reduction can only help delay and not prevent unprecedented climate changes due to CO2 emissions.” (Ramanathan and Carmichael. Global and regional climate changes due to black carbon. Nature Geoscience (2008) vol. 1 (4) pp. 221-227)

“Short-lived climate forcers – methane, black carbon and ozone – are fundamentally different from longer-lived greenhouse gases, remaining in the atmosphere for only a relatively short time. Deep and immediate carbon dioxide reductions are required to protect long-term climate, as this cannot be achieved by addressing short-lived climate forcers.” (Integrated Assessment of Black Carbon and Tropospheric Ozone; United Nations Environment Programme,, 2011)

Because of its short lifetime in the atmosphere, the effects of BC are most important regionally, especially in East Asia and South America. Other hotspots occur in Mexico, Brazil, Peru, and parts of Africa. In Asia, BC contributes to regional heating and disrupts rainfall patterns. BC is of great concern in the Himalayas where it accelerates melting of glaciers, which supply water to millions.

The largest sources of BC are incomplete burning of biomass and unfiltered diesel exhaust. Major reductions could be achieved by replacing traditional cook and heat stoves in developing countries with clean-burning biomass stoves or alternative fuel systems. Installation of filters on diesel vehicles reduces BC, as can be seen by comparing bus exhaust in different countries. Industrial coke ovens and brick kilns should also be updated to employ newer, cleaner technologies. Finally, open field burning of agricultural waste should be eliminated. These old technologies are primarily used in developing countries and many are already acting to replace them.

In the industrialized northern hemisphere, residential heating stoves are the primary source of BC. Emissions from North America and Europe are major controllable sources of BC to the Arctic, contributing to northern warming and loss of ice. Recent work (Doherty, et a) suggests that BC in the Arctic has been relatively constant or slowly declining since the 1980s.

The major sources of BC (biomass burning for cooking and heating, and diesel engines) are not the biggest global sources of CO2 (coal and fossil fuel burning). Therefore, both problems can and must be addressed independently and simultaneously. Immediate reductions in BC can buy a little time as we convert to low carbon energy sources.

Health effects alone should motivate the rapid limitation of soot emissions. Worldwide reductions in soot emissions would prevent an estimated 2.4 million premature deaths. Furthermore, emissions of BC are accompanied by carbon monoxide, and volatile organic compounds (VOCs), which have additional adverse health effects. The climate change effects of BC add urgency to the issue because global warming will only exacerbate the known health consequences of breathing dirty air.



Integrated Assessment of Black Carbon and Tropospheric Ozone: Summary for Decision Makers. United Nations Environment Programme and World Meteorological Organization (2011) pp. 1-36.

Doherty et al. Light-absorbing impurities in Arctic snow. Atmos. Chem. Phys. (2010) vol. 10 (23) pp. 11647-11680.

Lacis et al. Atmospheric CO2: Principal Control Knob Governing Earth's Temperature. Science (2010) vol. 330 (6002) pp. 356-359.

Ramanathan and Carmichael. Global and regional climate changes due to black carbon. Nature Geoscience (2008) vol. 1 (4) pp. 221-227.

Graphic by Riccardo Pravettoni, UNEP/GRID-Arendal, 'Black Carbon Emissions', UNEP/GRID-Arendal Maps and Graphics Library, 2009, <> [Accessed 30 March 2011].

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Comments 1 to 31:

  1. What a load of rubbish, such bad science does not deserve funding. It is a fact that particulates in the atmosphere DECREASE global temperatures and not increase. Soot in the atmosphere increases cloud cover, increases the planet's planet's albedo and thus reflects more radiation back into space, causing the body of the planet and the lower atmosphere to cool. This has been amply demonstrated on Earth and Mars. On Earth Major volcanic eruptions have been conclusively shown to lower global mean temperatures and this has been shown to also occur when major dust storms on Mars cause the surface temperature to decrease. This is science {snip}
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    Moderator Response: [muoncounter] Ease up on the accusations please; portions violating Comments Policy were snipped. If you have an opinion, please substantiate it more thoroughly than 'amply demonstrated'.
  2. LandyJim - this article refers to black carbon on the ground, not aerosols in the atmosphere. Please read more carefully next time.
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  3. LandyJim: Your statements are not factual. While overall, aerosol particles are likely to have a cooling effect due to increased scattering back into space and to increased cloud reflectivity, soot particles are different. They absorb light and heat the level of the atmosphere where they are found. Soot=black. When they combine with other particles to form a core surrounded by a nonabsorbing coating, their absorbing power can be almost doubled due to a lensing effect. There is a vast literature on these topics, almost all of which indicates that airborne soot has positive contribution to global warming. Adding to this is the effect soot has on snow by slightly darkening it, thus increasing the snowmelt rate and exposing the dark underlying surface to sunlight earlier in the spring. Again, lots of literature on these topics. Your bluster doesn't help promote your viewpoint. One of my fields of study; you don't want to get me going ;-)
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  4. RealClimate has an interesting discussion here of the downside of controlling soot in lieu of going after the big fish--CO2 regulation.
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  5. LJ: I agree with cbrock. Soot has a very positive greenhouse effect in the atmosphere. The imaginary part of the index of refraction plays a huge role in whether a particular aerosol will cool (scatter) or heat (absorb). The imaginary index of refraction for soot is huge. It heats, regardless of location. Soot-containing particles are implicated in closing some of the unknowns in radiative transfer in the atmosphere, where the measured scattering coefficients don't match what would be expected based on the inorganic chemical constituents (the imaginary index is way too high). However, small amounts of soot boost the imaginary index up, so that the scattering is less and the absorption is greater. Don't make those comments around aerosol chemists or radiative transfer specialists, they will be very forceful in telling you how wrong you are. If you do just a bit of googling on strings like "soot radiative impact atmosphere" you will clear up these misconceptions in very short order. -Bill
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  6. LandyJim I ask you this simple question: Since 1900 where have the billions (and billions and billions!) of pounds of rubber and asphalt dust gone? The short simple answer is anywhere and everywhere. In addition to these there is brake dust, the stuff that builds up on the sidewall of tires. Lots of reddish-brown rust falls off motor vehicles. Other sources of rust are ships, steel rails, wheels of railcars, brake drums and disk rotors. Modern synthetic rubber does not decompose upon exsposure to sunlight, air or microbes. Once in the environment, this stuff is there forever. Try this. Take a Post-It note and dab in the dusty top surface of car until it does not stick anymore. Then examine the stcky surface with viewer with 30-40x magnification. You will tiny black flat flecks. These are rubber particles. You sill also see bright highly reflective particles. These are sand particles from concrete. There are also particles you can't see at low magnification. These particles are few microns or less in size. And you breath in these. All rubber contains some natural latex because it improves sidewall flexibility. Natural latex contains proteins and fine rubber dust may contribute to the increasing incidence asthma in children.
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  7. "What a load of rubbish, such bad science does not deserve funding." I'm getting a little tired of "skeptics" trying to tell the rest of us what scientific work does and doesn't deserve funding, while making basic factual and logical errors that virtually no peer-reviewed scientist could get away with. The phrase "unskilled and unaware of it" springs to mind, for some reason.
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  8. cbrock is right. The atmospheric warming effect was directly observed by Ramanathan in his UAV study of the Asian brown cloud.
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  9. RE: Cryconite in Greenland You all should check the June 2010 issue of Nat Geo. There is a really good article on cryconite, the brown mineral dust that settles on the ice sheets in Greenland. In particular check pp 38-39 to see the black water produced by evil stuff.
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  10. Landy Jim #1. There are particulates and particulates. The dark ones absorb heat, the lighter ones from your dust storms reflect heat, as do sulfate aerosols.
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  11. Thanks for pointing out Doherty et al (2010). One key point at the end of the abstract "Nevertheless, the BC content of Arctic snow appears to be no higher now than in 1984, so it is doubtful that BC in Arctic snow has contributed to the rapid decline of Arctic sea ice in recent years." The impact on glacier melt can be real, but likewise would not have accelerated on the Arctic glaciers or Greenland due to BC. The impact on glaciers is limited to the zone where the material can accumulate at the surface and alter albedo. This is not in accumulation zone, nor in zones where there is heavy debris cover. The latter is not seen in Greenland. However, in the Himalaya many glaciers have heavy debris cover near the terminus, and the accumulation zones recieve frequent summer monsoon accumulation limiting the impact of BC to a small section of the glacier near and below the ELA, such as on Boshula Glacier or Gangotri Glacier
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  12. Cryoconite Holes Apparently cryconite holes have been around since 1870. They are part of the ecosystem. Effect of Life in Holes. Some of the melting is due to heat released from the organisms that grow in cryoconite. Cryoconites in the Antarctic Weblog of Belgians studying cryoconites. The cryoconites in the Taylor valley support an active, diverse assemblage of organisms despite the fact that they may remain sealed from the atmosphere for decades. Given the density of the cryoconites in the dry valleys (~ 4-6% of ablation zone surfaces), flushing of the cryoconites during warm years could provide a vital nutrient and organic carbon source to the surrounding polar desert. LINK Some cryoconites have been around a while. It seems more plausible that it isn't the carbon's heat absorbing properties, but rather it's food value that is important in the formation of cryoconite holes. Part of the food chain.
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  13. Doherty just measured BC. They haven't yet verified it's use in models to predict reductions in albedo. This winter I was especially observant of the snow melt next to my driveway. What Doherty says about the importance of crystal size to melt is apparently true. I observed that the snow was melting horizontally more or less in line with the sun's elevation. That wasn't the dirt on the snow that did that.
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  14. A few papers on BC and snow melt. DUST AND BLACK CARBON IN SEASONAL SNOW ACROSS NORTHERN CHINA IMPURITIES IN SNOW: EFFECTS ON ALBEDO Darkening of Soot-doped Natural Snow: Measurements and Model Sources of light-absorbing aerosol in arctic snow and their seasonal variation This last study suggests that biomas burning is responsible for a lot of the BC in snow. I wasn't able to find anyting about the affect of BC on heat absorbtion over the oceans.
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  15. Yes. And reduced summer ice cover in the Arctic is clearly caused by Chinese soot. However, in this case the phenomenon can't be used as a proof of high climate sensitivity to atmospheric CO2 concentrations.
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  16. BP, that graph doesn't make a 'clear' case for correlation... let alone causation. Since it is a short term atmospheric particulate effect we'd expect to see close correlation between peaks and valleys if it was impacting Arctic sea ice... similar to the way that volcanic eruptions and ENSO events can easily be identified in the temperature record. Instead, what we see is no correspondence at all. The unusually high (in comparison to the ongoing trend) ice extent of 1996 is matched with the highest soot output to that point... and the next year there was unprecedentedly huge soot output with no corresponding spike in ice decline. Ditto the high soot output in 2004 and 2005. Basically, the chart takes two factors which are changing and scales them such that the start and end points roughly overlap. You could do the same thing with Arctic sea ice and cell phone sales.
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  17. BP: You've given us sufficient inductive proof in the past that your understanding of correlation methods is woefully deficient. I tell you what. Grab a range of different data sets, run some multiple regression models showing how your graph at #15 is seemingly the best explanation of some causal relationship and I'll give you some kudos. Meanwhile I'm going to go out on a limb and conclude that there's a 95% chance or greater that your causal explanation is only a very very small part of the story. This conclusion is based on the multiple regression modeling I have done in the past.
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  18. #17 kdkd at 21:36 PM on 3 April, 2011 I'm going to go out on a limb and conclude that there's a 95% chance or greater that your causal explanation is only a very very small part of the story. While on that limb, please read the peer reviewed literature, then come down safely. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, D04204, 2005 doi:10.1029/2004JD005296 Distant origins of Arctic black carbon: A Goddard Institute for Space Studies ModelE experiment Dorothy Koch and James Hansen "The (former) Soviet Union (FSU) was implicated as a major source of Arctic haze in many studies. Novakov et al. [2003] found that black carbon emissions from the FSU in the late 1990s was less than 1/4 their peak levels of 1980. European emissions are also about 1/3 their levels in the 1970s. However China and India have doubled their BC emissions since the late 1970s. Thus BC emissions are more heavily weighted toward south Asia than they were in the 1970s and 1980s, when many of the Arctic haze studies took place."
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  19. kdkd#17: "Grab a range of different data sets ... " Here's one. What a correlation! By BP's logic, "reduced summer ice cover in the Arctic is clearly caused by ... " atmospheric CO2. Time series of annual arctic sea-ice extent and atmospheric concentrations of CO2 for 1900-2007. Sea-ice observations are from the Walsh and Chapman dataset 1900-78, merged with sea-ice concentration retrieved from satellite passive-microwave data (1979-2007) using the NORSEX algorithm, with ice extent updated to 2007. The CO2 scale is inverted. -- from The Nansen Group, Responding to Climate Change 2010
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  20. mspelto- I think the primary mechanism of depositing soot onto the Himalayan glaciers is snowfall, known as "wet deposition". This is in contrast to "dry deposition", which is particles impacting the surface and sticking. Wet deposition is a very efficient process, as ice crystals may form on soot particles, and falling snow can also capture soot on the way down. Thus the main "accumulation zones" are probably also where most of the soot is found. During a thaw cycle, the top layer of snow melts and the water runs down into the snowpack, leaving the soot behind. The accumulated result of many such cycles can be an increased concentration of soot at the top of the snowpack--it can get concentrated there. So I wouldn't expect the main effect of the soot to be limited to the terminus of the glacier. The important thing about soot in snow is that it doesn't directly absorb a lot of sunlight and melt the whole snow surface. Instead, it changes the local snow grain in which it is embedded, causing the grain to partially melt and form a larger, rounder grain when it refreezes. These larger grains absorb more energy than the fluffy snow, and they also allow sunlight to penetrate more deeply into the snowpack. It is these feedbacks of snow grain morphology that cause the bulk of the warming impact of the soot. Complicated stuff, and not very well understood. Soot is only one of many causes of snow metamorphosis. BP- Concentrations of soot in the Arctic atmosphere have declined quite a bit since the breakup of the USSR, although they seem to have leveled off and may be slightly increasing now due to Chinese emissions. Most soot in the Arctic snow seems to come from forest and agricultural burning, anyway. It is a big stretch to attribute the dramatic Arctic sea-ice decline in recent years to Chinese soot emissions--it is probably a minor player given the big changes in air and ocean temperature that have occurred in the last 40 years. It's also interesting to see you pointing to one of Hansen's GISS models to support one of your points. I presume then that you accept the general findings from that group's other climate simulations?
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  21. Berényi Péter at 04:14 AM on 4 April, 2011 You need to read a fair slice of the recent peer reviewed literature to gain a better picture. Your proposed correlation of Arctic Ice reduction and soot from China does not correspond to the actual data from the Arctic. Overall amounts of Black Carbon has reduced in the Arctic in the past few decades. We discussed this here in 2010 where I gave several references from 2009 and 2010. Obviously more work has been done since and here are some further references from 2011. Matsui 2011 shows Russian Biomass Burning is still the most important source of Black Carbon in the snow in the North American Arctic, in agreement with previous studies, whilst Skeie 2011 shows using a model cross checked with an assembly of recent measurements, with emphasis on the Arctic, that Black Carbon reached a measured peak in the 1960s, falling off since, and interestingly (as mentioned before) also reached relatively high levels in the 1920s.
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  22. cbrock the point you miss is that unlike all other alpine glaciers where summer snowfall is modest and dust and particles accumulate on the surface and enhance melt, summer monsoon dominated glaciers have there main accumulation in the summer with nearly daily snowfall burying the soot so it cannot accumulate at the surface. It is not that complicated, but nor can we extrapolate the same process from all glaciers. If we go further north into Tibet monsoon accumulation is not nearly as important and soot can have a role.
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  23. "....necessary to keep global temperatures from rising more than 2°C above preindustrial levels in the next 50 years" Why is the benchmark for T 'pre-industrial levels', when this time period (ie around 1700) was the amongst the coldest in the last 10,000 years?, ie the Little Ice Age? Shouldn't a T baseline be when c02 effects on T apparently became stronger?, such as from about 1950? Using 'pre industrial T' as a baseline might appeal to ideological conveniences (if one is anti-industry, anti-caplitalist, anti-fossil fuels, pro-socialist control, pro-alternative energy etc etc), but it is generally scientifitcally accepted that most of the warming from ~1700-1950 was caused by the sun, not by human industrial activities. Moreover, the T at 'pre industrial levels' could be argued to be not the most optimal for human and ecological health, (there is no real 'optimum' in any case); in terms of human health, optimum T is certainly a little warmer than Little Ice Age conditions, so neither such a temperature, nor climate period, should be used as a baseline for social objectives.
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  24. Does anyone know why West Europe is not included in the figure? I think I've tracked down 'Bond et al. 2000' (poor referencing there) to "A technology-based global inventory of black and organic carbon emissions from combustion. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109" but that does not break down the geographic units in the same way as the figure does (e.g. Europe is taken as a whole and North Americas is not split into the US & Canada as in the figure).
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  25. Thingadonta: "Why is the benchmark for T 'pre-industrial levels'..." We should determine the total change in temperatures caused by industrial carbon dioxide emissions from some point other than when those emissions started? Which, by the way, was around 1800... not 1700. On the whole, 'earlier temperature changes were caused by the Sun' bit... temperature forcings from solar fluctuations are obviously temporary. When the solar output changes the forcing 'goes away'. Thus, if solar forcing were currently equal to the 1800 level then none of the current cumulative temperature change would be due to the Sun. Since solar forcing is actually negative compared to the baseline that means the warming from greenhouse forcing is actually >greater< than the total warming observed thus far.
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    Moderator Response: [Dikran Marsupial] I suspect the 1700 startpoint is because of land use changes, which are as much industrial as agricultural (particularly iron smelting - for which coke was also used in the 18th century).
  26. "Since solar forcing is actually negative compared to the baseline that means the warming from greenhouse forcing is actually >greater< than the total warming observed thus far." No, solar activity gradually increased from around 1800-1950, meaning the solar forcing would continue until at least then. On top of this, you get at least a 20 year time lag from soal effects, which has been shown by various research papers. There are also could cover changes which are immedaite and sloightly delayed. Aside from this, the reason I mentioned 1700 rsther than 1800 as pre-industrial T is because of the very reason I outlined, 'baseline' should not be chosen irrespective of the relative natural climate at the time, which just happened to be very cold-Littel Ice Age (LIA). The LIA bottomed earlier than 1800. Also, if oen takes into account natural factors, baseline should be ~1950, because it was naturally colder than average over the last 10 years during pre industrial period. You dont chose as a baseline something well below a running mean unless one has an agenda, the very fact that the European Climate Union (or whatever they call themselves) chose this date is largely ideological, it has nothing to do with optimal temperatures, effects of industrualisation, or a running mean upon which to use as a baseline against human activities.
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  27. Sorry, that should read "20 year time lag from solar effects", and also "cloud cover changes which are both immediate and delayed", and "it was colder than the average over the last 10,000 years during the immediate pre-industrial period". (So don't chose it as a baseline!) I'll try to spell check next time.
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  28. thingadonta, you'll notice that we are not currently >in< the 1950s... or even 20 years thereafter. Any impact of that solar forcing or feedbacks set off by it are long gone. Atmospheric carbon dioxide was level at 270 +/- 10 ppm for thousands of years until ~1800. Then it started increasing. Thus, the optimal time to compare current temperatures against would be ~1800. That said, all three of the major temperature anomaly records start in 1850 or 1880... because we don't have significant thermometer readings before then. No "agenda" involved. Just using all the data we have available. Thus, the available anomaly records are technically not "pre-industrial"... they pick up after the first 10 - 15 ppm of CO2 increase. However, this is close enough that people are usually referring to the warming shown in those records when they say 'pre-industrial'. In essence, the current ~0.75 C total warming anomaly generally cited leaves out the small amount of warming from 1800 - 1880 caused by industrial greenhouse gas emissions. In any case, the accusation of "ideological" selection bias is pure nonsense... 1880 was long after the Little Ice Age and obviously 'chosen' based on the fact that it is the data we have.
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  29. Some perspective on the LIA : However, the timing of maximum glacial advances in these regions differs considerably, suggesting that they may represent largely independent regional climate changes, not a globally-synchronous increased glaciation (see Bradley, 1999). Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of "Little Ice Age" and "Medieval Warm Period" appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries. Mann et al. (1998) and Jones et al. (1998) support the idea that the 15th to 19th centuries were the coldest of the millennium over the Northern Hemisphere overall. However, viewed hemispherically, the "Little Ice Age" can only be considered as a modest cooling of the Northern Hemisphere during this period of less than 1°C relative to late 20th century levels (Bradley and Jones, 1993; Jones et al., 1998; Mann et al., 1998; 1999; Crowley and Lowery, 2000). Was there a "Little Ice Age" and a "Medieval Warm Period"?
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  30. Chris S at 18:51 PM on 4 April, 2011 The Lamarque 2010 paper linked in "here" (comment 21 above) describes a new global historical (back to 1850) gridded anthropogenic and biomass burning aerosol emissions dataset (Bond is co-author) and Bond 2011 also offers a more updated geographical breakdown, including the significant contribution from Europe (see section 4.5 on the Arctic). This paper also contains a wealth of other information and references.
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  31. Thanks Peter.
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