Note, it is not about black carbon deposited on snow, but on its absorption properties when in the atmosphere.
If correct, I suppose the question is what past levels of black carbon have been and whether they could have been part of the cause of recent warming. If they have increased, then it means the effects of CO2 are less than expected. If they've been mostly constant, then the effects of CO2 are as predicted, but we could get a one-off cooling benefit if we were to significantly cut smoke production.
Steve_M, Does the soot in question reduce humidity by it's presence ie act as a particle to trigger condensation. If so that would increase water vapour if soot is reduced? So reduction might not be good news.
Does the soot in question reduce humidity by it's presence ie act as a particle to trigger condensation.
Yes, it does fill the category of CCN (cloud condensation nuclei) and this is why it's 'rained out' so easily, but it can still be found in the stratosphere. 'Relative humidity' is a 'constant' in the atmosphere and doesn't change. However, a cooler atmosphere saturates with less WV (water vapour) per volume of atmosphere, so without the 'soot' to warm the atmosphere the total WV per volume of atmosphere is 'lower'.
quote:
If so that would increase water vapour if soot is reduced? So reduction might not be good news.
As in the above, soot reduction would be good news because a cooler atmosphere will hold less WV.
It would seem that the mods of the Science forum don't think that "Science magazine" is a reputable website!
Here's the citation:
Science 28 March 2008: Vol. 319. no. 5871, p. 1745 DOI: 10.1126/science.319.5871.1745
News of the Week CLIMATE CHANGE: Study Fingers Soot as a Major Player in Global Warming Robert F. Service
They think the forcing could be up to 0.9W/m^2 which is 2/5 of the CO2 forcing, and more than the IPCC reported (0.2-0.4).
But the uncertainties are rather large.
Their estimate is higher because they have taken into account that black carbon is usually mixed with sulphates which reflect a lot of light. They say that black carbon absorbs a lot of this reflected light, but this absorption was not accounted for before.
This is only a news item Steve, but it comes from one of Asia's polluters so perhaps they try to 'down play' it. http://www.hindu.com/thehindu/holnus/008200803261550.htm There is also detail in the 'Oversight' link that RP made (the one with the video).
One other thing, I'm sure I read somewhere that BC (black carbon) also becomes a 'platform' for compounds of nitrogen, as well as sulphur. This strongly suggests that it can also be a generator of ozone.
Earth's atmospheric stress against WV survival apparently
OK Water vapour, thanks, but does anyone here know the scientific argument as to why it is a reasonable assumption that average relative humidity stays constant in the atmosphere as temperature and content may range?
Their estimate is higher because they have taken into account that black carbon is usually mixed with sulphates which reflect a lot of light. They say that black carbon absorbs a lot of this reflected light, but this absorption was not accounted for before.
So how does this now fit in with your explanation that the cooling period of 1940-1970 was caused by sulphate aerosols? The new soot theory seems to contradict it.
OK Water vapour, thanks, but does anyone here know the scientific argument as to why it is a reasonable assumption that average relative humidity stays constant in the atmosphere as temperature and content may range?
1. It's not an assumption. It's a model output. Models are not tuned to fix relative humidity.
2. As measurements get better, it is looking like the models are right in this respect.
So how does this now fit in with your explanation that the cooling period of 1940-1970 was caused by sulphate aerosols? The new soot theory seems to contradict it.
M Batchelor, putting aside the fact we had cooling from 1940-1950 and then steady temperatures or slight warming till 1970's, in my first post I noted that what would be interesting is to have an assessment of how black carbon levels have changed in time as this gives you an assessment of how much cooling or warming the change would cause.
If we start with the assumption that black carbon levels are approximately in proportion with other aerosols, then it suggests the other aerosols cause more cooling than we thought, and that the reduction in "dirty" emissions in the last few years may have contributed some of the warming that previously was attributed to CO2. But this assumption may not be correct, or the paper might be wrong. Note they do say the error bars are quite large, and the error bars on aerosols are also quite large.
On reading your post of 29-03-08 12:28 here, the Ramanathan & Carmichael paper shows China to be a major emitter of black carbon. Bearing in mind that black carbon is a 'particulate' platform for other regional aerosols (sulphates, nitrates and volatile organic compounds), we need to know the true ratios between these aerosols in order to get any idea of how the balance of any insolation forcings will affect the region. Also, the full range of aerosols are not always present in black carbon.
Once these ratios are understood the absorption (or other interaction) can be approximated for each aerosol within its ratio-metric limit. It is well to note though that during this recent period of low UV insolation, there is unlikely to be much interaction between UV and nitrates or UV and volatile organic compounds. Thus, little ozone generation with its added thermal forcing with both UV and IR.
I've found a paper that investigates the divergence between observations and model prediction with regard to China. It examines, among other issues, long-wave down-welling which, I believe, may be describing the actions of 'brown cloud' (black carbon with aerosols). Could this be the case? http://pubs.giss.nasa.gov/abstracts/2006/Rangwala_etal.html A full download is linked to this page.
It's a bit dated, but it follows on well and on remembering 'urban heat islands' I realised something that isn't mentioned in either article. From page 3. quote:
When searching for ship tracks, Coakley's team looked at the near-infrared light (light which has a wavelength longer than that of red light in the visible color spectrum) coming off the clouds. At this wavelength many ship tracks appear as bright lines that can be distinguished from the surrounding, uncontaminated clouds.
Surely, if the ship tracks are 'bright' in the near-infrared spectrum they must be 'exothermic' and hotter than the surrounding atmosphere. If this is the case then in both articles the 'culprit' looks to be 'black carbon' or 'soot', yes? And we all know what an abundant variety of pollutants black carbon can carry, as well as generating heat and ozone in the atmosphere!
I don't know why we're posting these here though, this is the antithesis of this thread. Sorry Batch!
Best regards, suricat.
I think this belongs in your thread for any proper comment.
It looks like you not only get an urban heat island effect but also an urban rain effect. So I guess as we build more cities they'll wring more water vapour out of the atmosphere, cause catastrophic cooling as well as increased flooding. Also it references our friends the aerosols again. More adjustments to the models required?
From my 'Ship Tracks' post, I'm interested in understanding better the 'forcings' involved and the polluting chemical 'aerosols' that cause these 'forcings' (and, of course, their 'eventual' overall effects upon climate).
Perhaps I'm 'old fashioned' but, although many ships now incorporate gas turbines, the majority of marine vessels continue to be driven by compression ignition engines (CI, or 'diesel'). The exhaust from this type of engine not only contains a high level of carbon particulates, but also sulphurous, nitrogenous and organic compounds (organic compounds from fuel additives etc).
BTW, biodiesel is 'organic' and whether or not this becomes a 'volatile organic compound' (VOC) depends upon the 'final' combustion products observed under 'all engine speed/load conditions' for each engine mark, or type. I'm quite sure that all these observations (tests) haven't been implemented before biodiesel use is sanctioned (the differing standards organisations don't seem to fully interconnect).
But I digress. The 'ship tracks' from my post state categorically that the 'tracks' can't be observed in the visual spectrum. This is because the 'expected' 'cloud' from WV (water vapour) due to fossil fuel combustion doesn't materialise. It seems it doesn't 'materialise' because the volume of atmosphere affected by the exhaust gasses has incurred an elevated temperature which permits the water product of this exhaust to continue as WV (re; RH) and not form a 'cloud droplet' consistency. This is verified by the manifestation of the 'plume' (track) when observed within the infrared spectrum. The 'plume' is thus a positive temperature forcing on the surrounding atmosphere until external events cause plume 'cloud formation' and, eventual, precipitation of the exhaust component that causes the forcing.
Is this component carbon, of a more 'nitrogenous' nature, or both (sulphates would only counter this effect)?
The reason I asked SoM to transfer his post is that the 'heat island' that causes a 'precipitation' scenario seems to be so closely aligned to the 'ship tracks' scenario that I can't distinguish the one from the other.
