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Black Carbon May be Second-Most Significant Global Warming Pollutant After Carbon Dioxide; Alters Picture of Diesel Engine Benefits

Primary contribution to observed global warming since 1750. Click to enlarge. Source: Testimony of Dr. Jacobson

Black carbon—contained in soot from the combustion of biomass and fossil fuels—may be responsible for around 16% of the gross warming the planet is currently experiencing and may be the second-most significant global warming pollutant after carbon dioxide and ahead of methane, according to testimony provided by five scientists before the US House Committee on Oversight and Government Reform in October.

Because of their increased fuel efficiency relative to gasoline-engined vehicles, diesels are seen as an improvement overs gasoline vehicles with respect to global warming issues. However, once soot warming is factored in, the difference between the two platforms is greatly reduced, as diesel emits more soot than gasoline. 

Testifying before the committee were:

  • Dr. Mark Z. Jacobson, Prof. of Civil and Environmental Engineering, Atmosphere/Energy Program, Stanford University

  • Dr. Tami C. Bond, Asst. Prof. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign

  • Dr. V. Ramanathan, Prof. of Climate and Atmospheric Sciences, Scripps Institute of Oceanography, University of San Diego

  • Dr. Charles Zender, Assoc. Prof. of Earth System Science, University of California at Irvine.

  • Dr. Joel Schwartz, Professor of Environmental Epidemiology, Harvard University

The black carbon in soot performs its warming by absorbing sunlight, converting it into infrared (heat) radiation, and emitting that heat radiation to the air around it.  Soot on the surface of snow and sea ice contribute to both the melting of those surfaces as well as the warming of the air.

Because of the relatively short lifetime of soot in the atmosphere compared to greenhouse gases, control of soot may be the fastest method of slowing warming for a specific period, according to Dr. Jacobson. 

Black carbon, noted Dr. Bond, adds 2-3 order of magnitude more energy to the climate system than an equivalent mass of CO2 because black carbon is an extremely good absorber of visible light. While carbon dioxide stays in the atmosphere for decades, it absorbs just a small amount of infrared radiation.

Particles from burning biomass (which differ from biofuel particles) are less oily and contain a much lower black carbon fraction than fossil fuel soot particles, according to Dr. Jacobson. Biomass-burning particles thus tend to cool climate on a global scale (although the biomass-burning gas warming exceeds its global cooling due to permanent deforestation. The panel thus focused on soot particles resulting from the combustion of fossil fuels and biofuels.

...fossil fuel plus biofuel soot may contribute to about 16% of gross global warming (warming due to all greenhouse gases plus soot plus the heat island effect), but its control in isolation could reduce 40% of net global warming.

—Dr. Jacobson

Methods proposed to control fuel soot include improving engines; switching fuels; adding particle traps; and changing vehicle technologies.

In sum, there is not an advantage and a potential disadvantage of diesel versus gasoline in terms of climate and air pollution impact. However, neither type of vehicle is satisfactory or useful for solving climate and health problems as the emissions from both are very high. Even modest improvements in mileage standards for all vehicles are beneficial, but will only delay the eventual increase in emissions due to a larger population.

A more certain method of reducing global arming caused by both fossil-fuel soot and carbon dioxide is to convert vehicles from fossil fuels to electric, plug-in hybrid or hydrogen fuel cell vehicles, where the electricity or hydrogen is produced by a renewable energy sources [sic], such as wind, solar geothermal, hydroelectric, wave, or tidal power.

—Dr. Jacobson

(A hat-tip to Green Car Congress reader MR!)




A couple of questions:

Does the amount of soot (black carbon) put out the exhaust of diesel engines vary with technology, or in other words do the new low emmission diesels put out more black carbon than gasolene engines, or is this comparing the old black smoke diesels with fuel injected gasolene engines.

And two, where does home heating oil fit into the picture, because you can find "gray" snow near those flues.


Wow, this is great news for carbon-negative bioenergy systems.

If biomass particles cool the planet, and we know that biomass can be used for the production of carbon-negative electricity, then we may have the best system to tackle climate change.

Unlike ordinary renewables (wind, solar, geothermal, etc...), biomass can generate negative emissions and take historic emissions out of the atmosphere.

The future is bio-electric!

"although the biomass-burning gas warming exceeds its global cooling due to permanent deforestation"


So let me get this straight...we've heard elsewhere that the sun may be responsible for 16% - 36% of observed global warming, and now we're hearing that soot may be another 16%, which means that anywhere from 1/3 to 1/2 of global warming is potentially explained by things other than carbon dioxide.

But let me guess...we're still supposed to have complete faith in the climate modelers, right?


My electric vehicle, charged with solar power did not produce any black carbon driving to work today.

Carl the new low emmission diesels put out more black carbon than gasolene engines...

According to this link, PM emissions from diesel engines with particulate filters (DPF) are lower than those from equivalent gasoline engines, something with which even CARB (certainly not a diesel proponent) concurs: link. DPFs are most effective in removing the black carbon fraction of diesel particulate matter.

Actually, there are several studies which show diesel engine exhaust contains fewer aerosols than ambient levels, i.e., DPF-equipped diesel engines are actually serving as air filters if these studies are accurate.


Question: What happens to a diesel engine's emission numbers when it reaches high mileage? And, will a high mileage diesel engine smoke as badly as the current ones under load? Seems to me we are again being sold a bill of goods only to discover diesels were a terrible idea later when we have to clean up the mess. In other words I don't trust the PR. There is nothing more obnoxious that to be caught next to a diesel truck pucking out sickening exhaust fumes. And did you ever notice how the smallest people drive the largest diesels? Some kind of a social message here?

Rafael Seidl

If your diesel has a wall-flow DPF - as virtually all late-vintage European models do and 100% of upcoming T2B5 US models will - over 98% of what little soot comes out of the engine is burned off.

If diesel soot is a global warming problem, the sources are legacy diesels, off-road equipment, locomotive and ships. Not to mention heating oil combustion, industrial furnaces and the big ones - wood-burning stoves, slash-and-burn clearing of forests as well as wildfires. Emerging economies tend to produce a disproportionate amount of soot because they are stuck with antiquated technology.

In other words, soot emissions from *modern* diesel-powered cars featuring wall-flow DPFs are small compared to other sources.


It should be noted that GDI combustion systems also produce soot. The latest European gasoline standards also have a soot limit now to reflect this.

This is not just a diesel problem, although they are clearly larger producers of soot, the latest limits are the same for gasoline and diesel which means that the diesel engines require the use of a DPF.

Megadittoes Matt! Don't let those environazis rule the roost!

Harvey D


Tks for reminding us that soot (BC and OC) come from many other sources including wood stoves, coal fired power plants etc etc.

Beside absorbing solar energy and warming the troposhere like other GHG do, BC & OC can be extremely damaging to humans and animals. The fine carbon particles will clog and damage your lungs much the same as tobacco smoking do.

In Europe, children attending schools within one Km from a highway have a much higher incident of respiratory diseases



Biomass plantations - grass, eucalyptus - have nothing to do with deforestation. They have to do with reforestation and afforestation.


@Matthew: Something to read in your spare time.
There's also this...

Does the amount of soot (black carbon) put out the exhaust of diesel engines vary with technology

Yes, it most certainly does. My 2001 Jetta TDI does not spew black smoke. Diesel isn't perfect, but it's gone from bad (in 1980) to pretty good (MY 2008+). Also, the EPA Tier 2 Bin 5 regulations (which took effect around 2007ish) hold light duty diesels to the same standard as gasoline engines.

Another plus is the ability to run biodiesel, which, in my humble opinion, is the least pie-in-the-sky of the first generation biofuels. Plus, I've found diesel passenger cars can be much more pleasant to drive than their gasoline equivalent, due to their torque and hill-climbing ability.

Yes, I'm a dieselhead, so you may take my comments with a grain of salt and, no, diesel isn't right for everybody or all applicatins -- but you really should take a ride in a modern (with an ECU that controls the fuel metering and timing) diesel passenger car to see if your opinions still reflects the current state of technology.


A properly calibrated maintained diesel does not spew black smoke, and more than a gasoline engine spews blue or white smoke. The old dump-trucks that you see spewing black smoke are not representative of the diesels on the road, though they are very visible. I hate following them in my Jetta TDI (also a diesel vehicle) because I don't want to have to wash my shiny little zippy car. If my car were as bad as one of those, or as bad as a Mercedes 300d from the early 1980s, then I would have to wash my back bumper every few hundred miles. That is not the case in my car.


I am interested in making my non-smoking diesel even cleaner. (In my humble opinion, having changed the oil in both in my garage, the exhaust on my 2001 Jetta TDI with 116k miles is roughly about as nasty as the exhaust from my 1998 Ford Ranger with 165k miles.)

I read in this blog (or was it autobloggreen?) that some Germans are fitting their diesel cars with aftermarket diesel particulate filters, to augment the catalytic converter. Does anyone know where one might get one of those for a Jetta TDI in the USA?



maybe your EV produced no black carbon, but some tiny amount of carbon black was left by the tires ;)

Cameron Dell

I drive an old 85 VW turbo diesel with absolutely no sight of a black bumper. I have my fuel enrichment screw backed right off. I have sacrificed and little bit of power but its pretty hard to get any black smoke to come out of this baby. The bad thing about mechanical injected diesel is that their's no computer to change mixture for you but if you stay on the lean side and don't floor it the black smoke is minimal. I installed a high ratio tranny to slow the rpm's down and now I get 63mpg Imp. The TDI get 68 with a lot of extra power and torque. But hey I only paid 500 for this thing with a leaky injection pump that I fix myself. I think if we just kepted driving cars like this instead of building less efficient vehicle we could save on GHG. It would be niece if they would sell an after market catalysis muffler.

Rafael Seidl

@ Harvey D -

respiratory population health and concerns about possible carcinogens in PM for diesel engines were the reason for introducing DPFs to begin with - something customers insisted on. Euro 5 in 2009 will make the mandatory, they weren't considered feasible when the Euro 4 regs (effective since Oct 2005) were drafted.

That means the respiratory health issues associated with PM from diesel engines should lessen as the European fleet churns. Some cities in Germany have announced they will ban older diesels on days with low air quality starting in 2008, to comply with an EU directive. Most other countries are still dragging their feet.

Btw, poor respiratory health and living (or going to school) near high-traffic roads are correlated. That's not the same thing as a causal relationship. The people living there also tend to have lower income and education levels, are more likely to be smokers, have a poor diet, exercise less etc. There are a lot of confounding factors.

All this is basically a moot point for the introduction of T2B5 diesel LDVs in the US, because those are 1-2 orders of magnitude cleaner than in terms of both PM and NOx than the average diesel on Europe's roads today.

And this is my point: just because some scientists claim that soot particles are implicated in global warming actually means squat diddley for upcoming diesel engine options for the US LDV market - in spite of the article's controversial headline. That interpretation is plain fearmongering.

Rafael Seidl

... closing the silly HTML italics.

Cameron Dell

How much GHG is created to produce all these new diesel that really aren't the best solution for sustainability. A lot of these cars are unreliable and expensive the service. Their's only a few mechanics around that can work on them so bring extra cash for any work you get done. It's really a dealer driven product that only they can work on and they love it that way. Mass Flow Air Meter were blowing out of TDI's like popcorn a while back at 750 dollars a pop. Someone was selling ones for a Mercedes for 130 on Ebay hacked to work it a TDI. Who knows how well they worked these owner didn't care after they have forked out thousands for the OEM MFA.

Brian P

Cameron, the MAF problems that certain VW models had is not specific to the diesel engine; it is specific to that component (and it has been solved). My 2006 Jetta TDI has not had any problems. Our family has had a few VW diesels, and we've found long-term maintenance costs to be no different from other vehicles. (By the way, I sold my previous VW diesel still running at 462,000 km.)

To the anonymous poster with the 2001 Jetta TDI, you can purchase the particulate-trap from VW in Europe; it is a bolt-in replacement for your existing catalytic converter. The part number for your car is 1J0 254 800 X, BUT ... it costs $1100, and it is a passive-regeneration filter, and your car doesn't have a warning system to tell you to change your driving patterns to initiate regeneration. If you do a decent amount of highway driving it should be OK. If it's all stop and go and/or short trips, it's gonna clog. You must use ULSD fuel, but that's a non-issue nowadays.


Meanwhile, millions continue to go hungry.


Carbon black, yet another rationale for converting centrally fueled MDVs and HDVs to CNG and LNG.


...just because some scientists claim that soot particles are implicated in global warming actually means squat diddley for upcoming diesel engine options for the US LDV market - in spite of the article's controversial headline. That interpretation is plain fearmongering.

I completely agree with Rafael here.

I don't dispute the potential global warming impacts of BC. However, I glanced through some of the testimonies provided in the link above and Jacobson makes an unsubstantiated statement (unreferenced) that "even with a particle trap, diesel vehicles still emit more particles than gasoline vehicles."

That runs completely contrary to the references I posted above along with several other references I've run across. Jacobson has published several papers that specifically bash diesel vehicles (on virtually all air quality fronts) and I am becoming suspicious that he (in particular) has some sort of hidden agenda going.

At any rate, if diesel vehicles in fact have lower direct CO2, PM, CH4, and N20 greenhouse gas emissions than equivalent gasoline vehicles, along with the lower energy requirements in the refining of diesel fuel than gasoline (and thus lower indirect CO2 emissions there), it seems that it is a real stretch to claim that diesel vehicles have a greater global warming potential than equivalent gasoline vehicles.

Diesel? Lower PM than gasoline? Put down the crack pipe.

Harvey D


Could a small light weight on-board diesel engine be designed and calibrated to run cleaner and more efficiently, at a more or less fix (ideal) speed, to charge batteries in a series PHEV?

If so, initial cost, maintenance and operation cost could be reduced, making it a worthwhile choice.

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