Global Dimming and Short Lived Climate Forcers (SLCF)

The carbon dioxide problem is hard to fix, however, because it comes mainly from the burning of fossil fuels, which is so essential to modern life and commerce. It will take decades and trillions of dollars to convert all the world’s fossil-fuel-based energy systems to cleaner systems like nuclear, solar and wind power. In the meantime, a fast-action plan is needed. But carbon dioxide is not the only kind of pollution that contributes to global warming. Other potent warming agents include three short-lived gases — methane (23 to 27x CO2 warming potential), some hydrofluorocarbons (1000x CO2 warming potential) and lower atmospheric ozone — and dark soot particles. The warming effect of these pollutants, which stay in the atmosphere for several days to about a decade, is already about 80 percent of the amount that carbon dioxide causes. The world could easily and quickly reduce these pollutants; the technology and regulatory systems needed to do so are already in place.

- Veerabhadran Ramanathan, Distinguished Professor of Atmospheric and Climate Sciences, Scripps Institution of Oceanography, University of California, San Diego, CA

As the complexities and time delays associated with fixing the CO2 emission problems becomes all too apparent, there is hope offered by solving a simpler concurrent problem. Short Lived Climate Forcers (SLCF) are anthropogenic substances such as methane, some hydrofluorocarbons,  lower atmospheric ozone and dark soot particles emitted into the atmosphere from a number of sources including:

  • methane – coal mines, landfills, rice paddies and livestock
  • hydrofluorocarbons – refrigerants in air-conditioners and other cooling systems
  • soot – agricultural burning, cooking burning, fossil fuel burning
  • ozone – formed in lower atmosphere from anthropogenic sources such as methane, carbon monoxide

and which stay in the atmosphere for only a few weeks instead of decades or centuries like CO2.

The 9/11 Episode – Confirmation of Impact of Aerosols?

A striking example that confirmed this low resident time in the atmosphere occurred on Sept 11, 2001, when terrorists attacked and destroyed the World Trade Center in New York city.  For security reasons, the US government took the unusual step of grounding the entire US airplane fleet worldwide for up to 3 days. On September 12th, 2001, the aftermath of the tragedy, as America mourned, the weather all over the country was unusually clear and sunny. Eight hundred miles west of New York, in Madison, Wisconsin, climate scientist David Travis of Wisconsin State University was on his way to work when he looked up in the sky and noticed these unusual conditions too. Travis took keen interest in the unusually bright and clear skies because he had been studying the effects of jet aircraft vapor trails on the climate for the past 15 years. Travis realized the  once-in-a-lifetime opportunity that presented itself.

Travis was hoping to detect some small change in temperature due to the grounding of the fleet and subsequent drop in atmospheric aerosols associated with them. He was surprised at his findings.  Travis says “We found that the change in the daily temperature range (DTR) during those three days was just over one degree centigrade. And you have to realize that from a layman’s perspective that doesn’t sound like much, but from a climate perspective that is huge.”

The daily temperature range (DTR) is the difference between the highest and the lowest temperatures in a 24-hour period. Usually, it is very stable, even if the weather changes, but not this time. Travis felt he had dramatic proof of the impact of aerosols and their global dimming potential on global temperatures, confirming the importance of research by Ramanathan and a host of other scientists over the past 40 years.

Travis analysed maximum and minimum temperature data from about 4,000 weather stations throughout the conterminous United States (the 48 states not including Alaska and Hawaii) for the period 1971–2000, and compared these to the conditions that prevailed during the three-day aircraft grounding period. All sites were inspected for data quality and adjusted for the time of observation.

Travis derived his results from 1971–2000 climatology data for the indicated three-day periods in September 2001. These periods included the three days before the terrorist attacks of 11 September; the three days immediately afterwards, when aircraft were grounded and there were therefore no contrails; and the subsequent three days

Departure of average diurnal temperature ranges (DTRs)
from the normal values(Source: Nature Brief Communications: Contrails reduce daily temperature range)

Since this 2002 paper, there has been some debate regarding whether contrail were really responsible for the anomally. Hong et al. wrote a paper in 2008 Do contrails significantly reduce daily temperature range?which disputed Travis’s findings and explains it by natural variations in cloud cover, rather than the absence of planes. Hong and his colleagues examined patterns of cloud cover and temperature in early September at US weather stations from 1971 to 2001. They found that thicker, low clouds are the dominant influence on temperature extremes while high clouds such as contrails have a minor effect at most and concluded that the 2001 temperature swings, including those during the 9/11 attacks seem to be within the range of natural variability over those decades.

Hong concludes:

We conclude that the increase of the diurnal temperature range over the United States during the three-day grounding period of 11 – 14 September 2001 cannot be attributed to the absence of contrails. While missing contrails may have affected the DTR, their impact is probably too small to detect with a statistical significance. The variationsin high cloud cover, including contrails and contrail-induced cirrus clouds, contribute weakly to the changes in the diurnal temperature range, which is governed primarily by lower altitude clouds, winds, and humidity.

Travis never claimed that contrails were solely responsible but that they interacted with natural weather conditions to create the anomally observed in the days immediately following 9/11. Travis maintains that Hong’s study does not come to any conclusions about the impact of contrials as it only accounts for natural cloud formations. Travis believes that Hong’s study, in fact, provides evidence showing just how rare a 1.8 Deg C DTR is.

For the fascinating modern history of global dimming research, go here.

Introduction to Importance of Black Carbon & Global Dimming Research to Global Warming

Leading researcher Professor Veerabhadran Ramanathan of the Scripps Oceanography Institute gives a clear introduction of the problem of Global Dimming and Short Lived Climate Forcers (SLCF) and its relevance to global warming  in a 2007  paper entitled:

Global Dimming by Air Pollution and Global Warming by Greenhouse Gases:
Global and Regional Perspectives

The global build up of greenhouse gases (GHGs), is the most vexing global environmental issue facing the planet. GHGs warm the surface and the atmosphere with significant implications for, rainfall, retreat of glaciers and sea ice, sea level, among other factors. What is less recognized, however, is a comparably major global problem dealing with air pollution. Until about ten years ago, air pollution was thought to be just an urban or a local problem. But new data have revealed that, due to fast long range transport, air pollution is transported across continents and ocean basins, resulting in trans_oceanic and trans-continental plumes of atmospheric brown clouds (ABCs) containing sub micron size particles, i.e, aerosols. ABCs intercept sunlight by absorbing as well as reflecting it, both of which, lead to a large surface dimming. The dimming effect is enhanced further because aerosols nucleate more cloud drops which makes the clouds reflect more solar radiation. The surface cooling from this dimming effect has masked the warming due to GHGs.

ABCs are concentrated in regional and mega-city hot spots. Long range transport from these hot spots gives rise to wide spread plumes over the adjacent oceans. Such a pattern of regionally concentrated surface dimming and atmospheric solar heating, accompanied by wide spread dimming over the oceans, gives rise to large regional effects. Only during the last decade, we have begun to comprehend the surprisingly large regional impacts. The large north-south gradient in the ABC dimming has altered the north-south gradients in sea surface temperatures, which in turn has been shown by models to decrease rainfall over the continents. In addition to their climate effects, ABCs lead to acidification of rain and also result in over few million fatalities worldwide.

The surface cooling effect of ABCs may have masked as much 50% of the global warming. This presents a dilemma since efforts to curb air pollution may unmask the ABC cooling effect and enhance the surface warming. Thus efforts to curb GHGs and air pollution should be done under one framework.. The uncertainties in our understanding of the ABC effects are large, but we are discovering new ways in which human activities are changing the climate and the environment.

Aerosols are considered the biggest uncertainty in global warming models, hence ongoing research is vital to developing models that can predict future warming trends with accuracy. The complexity of the relationship uncovered by professor Ramanathan and the INDOEX project show two competing forces:

  1. long-lived GHG’s which warm the atmosphere
  2. short-lived aerosols which can both warm and cool the atmosphere

Figure 1: Global Mean anthropogenic Forcing due to aerosols and GHGs (Source: Ramanathan)

In the above figure:

  • the two panels on the right is Forcing due to aerosols
  • the panel on the far left is Forcing due to greenhouse gas
  • The blue boxes show the atmospheric Forcing
  • the orange box at the bottom shows the surface forcing
  • the sum of the two (blue plus orange box) is the forcing at top-of-atmosphere (TOA).
  • The aerosol forcing is for 2001-2003 and is from the present study
  • the GHGs at TOA is from IPCC (2007)

Also see Crutzen and Ramanathan (2003) on the parasol effect of aerosols

The global mean estimates shown in the diagram show the relative contributions of aerosols and GHGs:

  • at the surface
  • the atmosphere

The resultant Forcing at the Top of the Atmosphere (TOA) is a summation of both aerosol and GHG components:

  • While at the surface, the aerosol dimming (negative forcing of -4.4 Wm-2) is much larger than the GHGs forcing of 1.6,
  • the positive atmospheric forcing of 3 Wm-2 within the atmosphere by aerosols (Atmospheric Brown Clouds) enhances the GHGs forcing of +1.4 Wm-2, such that the sum of the
  • surface (-4.4 Wm-2) and the atmospheric forcing (3 Wm-2) i.e, forcing at TOA, is -1.4 Wm-2 for ABCs and +3 Wm-2 ( =1.6 Wm-2 + 1.4 Wm-2) for GHGs.

Thus the net anthropogenic forcing by anthropogenic modification of the radiative forcing is positive. Globally, the ABC forcing (-1.4 Wm-2) has masked about 50% of the GHGs forcing (+3 Wm-2 ).

(Source: Global Dimming by Air Pollution and Global Warming by Greenhouse Gases: Global and Regional Perspectives, V. Ramanathan, May 2007)

Ramanathan’s pioneering research in the field of black carbon and global dimming has shed new light on the significance these processes play in global warming. This research clearly shows the interconnectedness between global warming and global dimming, between SLCF and GHG gases. We cannot eliminate airborne particulate problem without close coordination with GHG reduction stragegy, lest we risk worsening the global warming problem.

In Europe and North America, catalytic converters and scrubbers have already led to a large decrease in air pollution. While this reduces the pollution load of these SLCF’s, it can also increase global warming.  Global warming has increased global temperatures by 0.6 and 0.8 degree celcius in the last century but now, in light of global dimming, we must be very careful about the strategy we take for reducing global warming.

Scientist Peter Cox believes that due to the masking effect of aerosols, we can be creating the worst possible combination for global temperature if we reduce particle pollutants but keep pumping GHG into the atmosphere. If we continue reducing air pollutants while increasing GHG,  we are eliminating the cooling pollutant AND increasing the warming pollutant at the same time. This effectively doubles the global warming rate. If cooling is stronger than we thought, then warming is also stronger than we think.

Life in a 10 Deg C world

Scientist Peter Coxs’ computer models that incorporate global dimming and aerosols show that in the worse case, when aerosols are decreasing and GHG gases are increasing, then by 2100, this could escalate global temperatures by up to 10 Deg C.

  • many plant species could not survive such rapid change
  • Trees would die all over the planet
  • best agri land struck by draught and soil erosion, famine not far behind
  • in north, GHG from permafrost and ocean methane hydrates – 10,000 gt

Scientists found this happened last happened 50 million years, average temp was 13 and local temp was 25 deg hotter.

The Role of Black Carbon Aerosols

Black carbon is the primary absorbing aerosol in the atmosphere and plays a complex role in our climate system,  influencing global warming, cloud properties, and snow and ice albedo. It has a relatively short atmospheric life span of approximately one to two weeks and is part of a group of pollution sources known as Short-Lived Climate Forcers (SLCFs), which also include methane gas and ozone. It is formed by incomplete combustion and once it finds its way into the atmosphere become coated with airborne chemicals which can act like lenses capable of increasing the ability of the particles to absorb sunlight and heat the atmosphere. Black carbon particles are constantly changing,  collecting inorganic and organic materials, growing, changing shapes, and composition. All of these changes impact the the absorption and warming potential of the black carbon. That has raised a critical question: Can targeting black carbon emissions result in a rapid and significant decrease in global warming impact either regionally or globally?

In the 2013 landmark study Bounding the role of black carbon in the climate system, a team of multi-disciplinary scientists (T.C. Bond et al.)  concluded that black carbon (BC) has “twice the climate impact reported in previous assessments” and ranks black carbon as the “second most important human emission …; only carbon dioxide is estimated to have a greater forcing …”.

Black carbon is emitted in a variety of combustion processes and is found throughout the Earth system. Black carbon has a unique and important role in the Earth’s climate system because it absorbs solar radiation, influences cloud processes, and alters the melting of snow and ice cover. A large fraction of atmospheric black carbon concentrations is due to anthropogenic activities. Concentrations respond quickly to reductions in emissions because black carbon is rapidly removed from the atmosphere by deposition. Thus, black carbon emission reductions represent a potential mitigation strategy that could reduce global climate forcing from anthropogenic activities in the short term and slow the associated rate of climate change.

Black carbon absorbs much more light than it reflects, it warms the atmosphere through its interaction with sunlight. This warming effect contrasts with the cooling effect of other particles that are primarily scattering and, thus, reduce the amount of energy kept in the Earth system.

Radiative forcing (RF) by atmospheric BC stops within weeks after emissions cease because its atmospheric lifetime is short unlike the long timescale associated with the removal of CO2 from the atmosphere. Thus, sustained reductions in emissions of BC and other short-lived climate warming agents, especially methane and tropospheric ozone (O3), could quickly decrease positive climate forcing and hence climate warming.

(Source:  Bounding the role of black carbon in the climate system, T.C. Bond et al)

The short lived atmospheric lifespan of black carbon offers policy makers an important tool to fight global warming.  The 2011 UNEP report Integrated Assessment of Black Carbon and Tropospheric Ozone recommends rapid reductions in emissions of short-lived climate forcers, including black carbon, a component of fine particle pollution, and methane as the most effective strategy to slow warming and melting in the Arctic over the near term

The reports key findings are:

  1. Black carbon and ozone in the lower atmosphere are harmful air pollutants that have substantial regional and global climate impacts. They disturb tropical rainfall and regional circulation patterns such as the Asian monsoon, affecting the livelihoods of millions of people.
  2. Black carbon’s darkening of snow and ice surfaces increases their absorption of sunlight, which, along with atmospheric heating, exacerbates melting of snow and ice around the world, including in the Arctic, the Himalayas and other glaciated and snow-covered regions. This affects the water cycle and increases risks of flooding.
  3. Black carbon, a component of particulate matter, and ozone both lead to adverse impacts on human health leading to premature deaths worldwide.Ozone is also the most important air pollutant responsible for reducing crop yields, and thus affects food security.

Benefits from black carbon reduction

Major benefits reported by the 2011 UNEP report Integrated Assessment of Black Carbon and Tropospheric Ozone include:

  1. Full implementation of the identified measures would reduce future global warming by 0.5˚C (within a range of 0.2–0.7˚C). If the measures were to be implemented by 2030, they could halve the potential increase in global temperature projected for 2050 compared to the Assessment’s reference scenario based on current policies and energy and fuel projections. The rate of regional temperature increase would also be reduced.
  2. Both near-term and long-term strategies are essential to protect climate. Reductions in near-term warming can be achieved by control of the short-lived climate forcers whereas carbon dioxide emission reductions, beginning now, are required to limit long-term climate change. Implementing both reduction strategies is needed to improve the chances of keeping the Earth’s global mean temperature increase to within the UNFCCC 2˚C target.
  3. Full implementation of the identified measures would have substantial benefits in the Arctic, the Himalayas and other glaciated and snow-covered regions. This could reduce warming in the Arctic in the next 30 years by about two-thirds compared to the projections of the Assessment’s reference scenario. This substantially decreases the risk of changes in weather patterns and amplification of global warming resulting from changes in the Arctic. Regional benefits of the black carbon measures, such as their effects on snow- and ice-covered regions or regional rainfall patterns, are largely independent of their impact on global mean warming.
  4. Full implementation of the identified measures could avoid 2.4 million premature deaths (within a range of 0.7–4.6 million) and the loss of 52 million tonnes (within a range of 30–140 million tonnes), 1–4 per cent, of the global production of maize, rice, soybean and wheat each year .The most substantial benefits will be felt immediately in or close to the regions where action is taken to reduce emissions, with the greatest health and crop benefits expected in Asia.

Figure 1:  Global benefits from full implementation of the identified measures in 2030 compared to the reference
scenario. The climate change benefit is estimated for a given year (2050) and human health and crop benefits are
for 2030 and beyond. (Source: UNEP Integrated Assessment of Black Carbon and Tropospheric Ozone)

From a policy perspective, the major black carbon sources can be grouped into a small number of categories that we can tackle far more simply and quickly than carbon emissions:

  • Residential solid fuels (i.e., coal and biomass) contribute 60 to 80% of Asian and African emissions,
  • On-road and off-road diesel engines contribute about 70% of emissions in Europe, North America and Latin America.
  • Residential coal is a significant source in China, the former USSR and a few Eastern European countries.

These categories represent about 90% of black-carbon mass emissions. Emissions from aviation, shipping, and flaring make up another 9% (T.C. Bond et al).

A variety of measures could be put in place relatively easily that would dramatically reduce these short-lived emissions:

  • fitting diesel vehicles with exhaust-pipe filters,
  • using clean-burning stoves in place of open wood fires,
  • capturing methane from coal mines and landfill sites,
  • banning the burning of agricultural waste in fields
Figure 2: Effects of diesel filters on soot particles (Source UNEP Integrated Assessment of Black Carbon and Tropospheric Ozone)

Sources and Global Impact of Black Carbon

Figure 3: Sources of black carbon (BC) (Source:  Bounding the role of black carbon in the climate system )

Figure 4: Likely effects of black carbon (BC) globally (Source:  Bounding the role of black carbon in the climate system )

Figure 5: Black Carbon (BC) by latitude and source type (Source:  Bounding the role of black carbon in the climate system )

Figure 6: Black Carbon (BC) by region and source type (Source:  Bounding the role of black carbon in the climate system )

From looking at this graph, we can see major areas of black carbon focus area:

  • Agricultural, forests and grassland burning is a major concern in Africa and Latin America
  • Forest burning is a major concern in Southeast Asia
  • Biofuel cooking is a major concern in Africa and throughout Asia
  • Coal is a major focus concern is East Asia

Figure 7: Black Carbon (BC) co-emitted species – Primary Organic Aerosol  (POA) and SO2 – by region and source type (Source:  Bounding the role of black carbon in the climate system )

 Measures to Control Black Carbon

Figure 8: Dr. Drew Shindell of NASA GISS

Black carbon, also known as soot, is a type of dark particulate matter produced by the incomplete combustion of fossil fuels, wood, dung and other biofuels. It’s nasty stuff and recent research points to a surprisingly large global warming impact . The bit of good news is that it is actually very easy to control and can have a significant impact on global warming immediately.

Figure 9: Emissions of CO2, methane and black carbon (Source: NASA GISS)

Figure 10: Electron micrograph of black carbon attached to sulfate particles. A) The spherical structures are sulfates; the arrows point to smaller chains of black carbon. B) black carbon is shown in detail C)  fly ash, a product of coal-combustion, that’s often found in association with black carbon. While black carbon absorbs radiation and contributes to warming, sulfates reflect it and tend to cool Earth. Credit: Peter Buseck, Arizona State University

Eliminating soot can get rid of a whole host of problems.

  1. Soot is linked to a number of respiratory illnesses like lung cancer and asthma and contributes to 1.6 million premature deaths a year, mostly in developing countries
  2. It’s global warming potential comes from the fact the short lived particles absorb sunlight while it is in the atmosphere
  3. In addition,  in the 2 to 3 week aerosol lifetime that it has, black carbon emitted from North America, Europe or Asia is carried by prevailing winds north to the Arctic, whereit is deposited onto glaciers and ice pack. It’s darkness dramatically affects the albedo and draws in heat, further accelerating melting.

The good news is that, unlike CO2, black carbon is easy and cost effective to control. A team of 70 scientists, led by New York City-based Goddard Institute for Space Studies climatologist Drew Shindell, has concluded that just a handful of measures could yield major benefits in the next fifty years.

As shown in the diagram above, keeping black carbon and methane under control will make a huge impact on total temperature rise in the next few decades.  In fact, Shindell’s team has established that controlling black carbon successfully can mitigate up to 2/3 of the warming happening in the Arctic. This has important ramifications for the Arctic, the region which is heating up twice as fast as any other region on the planet.

Scientists named 2012 the “Goliath melt year”, after observing melting on over 90% of the mammoth Greenland Ice Sheet’s surface and sea ice retreating to half the size it was when measurements began in 1979.  Eliminating black carbon does not mean, however, that we can relax efforts with CO2 because CO2 stays trapped in the atmosphere for centuries. What it does do is relieve some of the radiative forcing on the Arctic which may otherwise cause it to reach tipping points sooner.

Shindell’s team also estimated that adoption of 16 control measures would prevent  between 700,000 and 4.7 million premature deaths each year and increase global crop yields by up to 135 million tons per season.

Because soot is an aeresol, it is particulate matter which is more easily removed than CO2. For this reason, if soot-causing pollution is reduced, a positive impact would occur quickly. Among the  measures which could be easily adopted are:

  •  installing filters for diesel engines
  • using more efficient cookstoves
  • curbing the open burning of agricultural waste
  • modernizing brick kilns
  • capturing methane released by landfills, coal mines, oil and gas wells, leaky pipelines, wastewater treatment plants and rice paddies
Figure 11: Seasonal plot of biomass burning  (Source:  Bounding the role of black carbon in the climate system )

Black Carbon in the Arctic and the CLRTAP Convention

In May 2012, all eight Arctic state members of the Convention on Long-range Transboundary Air Pollution (CLRTAP) became parties to the first multilateral agreement to address black carbon. The CLRTAP’s Gothenburg Protocol was amended to establish emissions standards for fine particulate matter. The new protocol urge Parties to “… seek reductions from those source categories known to emit high amounts of black carbon, to the extent it considers appropriate.” The group recognized that reducing black carbon emissions will lead to a whole host of improvements including:

  1. air quality
  2. significant public health benefits
  3. regional climate benefits by protecting the Arctic and glaciated mountainous regions, in particular from accelerated rates of melting of ice, snow and permafrost
Rapid reductions in emissions of short-lived climate forcers, including black carbon, a component of fine particle pollution, and methane have been identified as the most effective strategy to slow warming and melting in the Arctic over the near term.

- Earthjustice

UNEP assessment showed that emissions reductions before 2030 will have the greatest impact but emissions ceilings of the new agreement don’t apply until 2020 and black carbon reductions goals are voluntary. Subsequently, there is an urgent need for immediate action. The Arctic Council nations are better positioned than anyone else to create a regional agreement on black carbon. They have studied science-based mitigation opportunities in two working groups for more than four years and the Task Force on SLCF and the UNEP Integrated Assessment have shown that that mitigation using available technologies and known practices can have a significant temperature impact in the region.

Such a regional agreement could consist of:

  • agreement to submit black carbon emissions inventories, based on CLRTAP guidelines soon to be finalized;
  • tracking regional trends and identifying mitigation opportunities;
  • establishing a mechanism for reporting and joint consultation on national mitigation action through the Arctic Council
  • adoption of a common, circumpolar vision for black carbon emissions reductions
  • development of national mitigation action plans for black carbon
  • creation of a mechanism for technology transfer and financing to facilitate enhanced mitigation action