News & Media Opinion Pieces Black Carbon and Tropospheric Ozone in Global Warming and Human Health

Black Carbon and Tropospheric Ozone in Global Warming and Human Health


The report from UNEP and World Meteorological “Association Integrated Assessment of Black Carbon and Tropospheric ozone” is a comprehensive review of a complicated topic.

Black Carbon (BC) and ozone (O3) have significant direct health impacts and are important factors in global warming. They can be countered by simple measures. This is an ABC of what we should know about them


Black carbon; the facts

  • Black carbon (BC) exists as atmospheric particles and is a component of ‘soot’ It is black because its particles absorb visible light.

  • This absorption leads to a disturbance of the planetary radiation balance and eventually to warming. The contribution to warming of 1 g of BC seen over a period of 100 years has been estimated to be anything from 100 to 2000 times higher than that of 1 g of CO2. However its lifetime in the atmosphere is only days to weeks and so emission reductions have an immediate benefit for climate and health.

  • 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.

  • Black carbon arises from the incomplete combustion of fossil fuels, wood and other biomass. The sources are cars and trucks, residential stoves, forest fires and some industrial facilities.

  • During the process of combustion, carbon monoxide (CO), volatile organic compounds (VOCs) and organic carbon (OC) particles are formed.

  • In contrast to the  warming action of BC particles, OC particles have a cooling effect and so combustion emits both a warming and a cooling agent; the proportions of the two is different  for each fuel but there is always more warming than cooling.


Tropospheric ozone- the facts

  • Ozone (O3) is a reactive gas that exists in two layers of the atmosphere: the troposphere (ground level to ~10–15 km) and the stratosphere (above this)

  • Tropospheric O3 is formed by the action of sunlight on Ozone precursors that have natural and anthropogenic sources. They are methane, nitrogen oxides (NOX), volatile organic compounds (VOCs) and CO.

  • In the stratosphere O3 is beneficial as it protects life on Earth from the sun’s harmful ultraviolet radiation.

  • In the troposphere, at ground level, 03 is a pollutant harmful to human health and ecosystems, and it is a major component of urban smog and a significant greenhouse gas. .

  • Reductions in both methane and CO emissions (but not NOX and VOCs) have the potential to substantially reduce ozone concentrations and reduce global warming.

  • Ozone concentration in the northern hemisphere has increased threefold during the past 100 years. It now the third most important contributor to the human enhancement of the global greenhouse effect, after carbon dioxide and methane.

  • Like black carbon, ozone has adverse impacts on human health leading to premature deaths worldwide.  In addition ozone is also the most important air pollutant responsible for reducing crop yields, and thus affects food security

Short-lived climate forcers (SLCFs)

BC, tropospheric O3 and methane (CH4) are often referred to as short-lived climate forcers  (SLCFs) as they have a short lifetime in the atmosphere (days to about a decade) relative to carbon dioxide.


Measures to reduce BC and Ozone

If we instituted all possible measures to reduce these, warming during the 2030s relative to the present day would be only half as much as if no measures were undertaken.

Over the same period an aggressive reduction of CO2 would do little to mitigate warming over the same period

There is some uncertainty about the impact of BC because it can influence clouds that have multiple effects on climate that are not fully understood.





Coal Mining. Improve the recovery of methane from ventilation air. This is a significant source world wide

Oil and natural gas production. There is widespread pollution which negates much of the advantage of gas. In the Four Corners program was seen leaking around the drill holes and continued for many months with no action from the company. This is a universal phenomenon with gas mining

Long-distance transmission pipelines. Reduce the leakage

Biodegradable municipal waste. Separate and treat through recycling, composting and anaerobic digestion as well as by landfill gas collection with combustion/utilisation

Wastewater treatment. Upgrade primary to secondary/tertiary treatment with gas recovery and overflow control

Emissions from livestock. Reduce through farm-scale anaerobic digestion of manure from cattle and pigs.

Rice paddies which are continuously flooded.Reduce emission by intermittent aeration




Diesel engines.Use efficient filters for particles for road and off-road vehicles

Eliminate of high-emitting vehicles in road and off-road transport

Replace coal by coal briquettes in cooking and heating stoves

Replace current wood-burning technologies in the residential sector in industrialized countries with fuel made from recycled wood waste or sawdust

Introduce clean-burning biomass stovesfor cooking and heating in developing countries

Substitute clean-burning cookstoves using modern fuels for traditional biomass cookstoves in developing countries

Ban open field burning of agricultural waste.  Ban forest floor burning.



However measures concerning cookstoves and open burning of biomass have some uncertainty because the proportion of OC is raised whereas there is much higher confidence for measures that mitigate diesel emissions because the proportion of co-emitted cooling OC particles is much lower


In terms of human health and particularly respiratory health, both BC, methane and therefore ozone are often present together. For example due to the coincidence of diesel pumps, methane and sunlight around the CSG well head.


The short term benefits of BC and ozone reductioncan be summarised as


Regional climate benefit,tropical rainfall benefits and Asian Monsoon

BC and ozone disturb tropical rainfall and regional circulation patterns such as the Asian monsoon, affecting the livelihoods of millions of people


Decreased warming in polar and glaciated regions


Benefits for human health

 “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”


Benefits for crop yields



These emissions in Australia could be tackled now probably for the cost of a new coal fired power station- with significant impacts. There is little action because the politicians do not understand it. Recently the clunkers program was abolished with the excuse it was an expensive way to reduce emissions. Clunkers impact on BC and ozone is more important and has an immediate health impact. Waste, forest floor burning, biomass burning, action on vehicular emissions and mining gases could all be fixed quickly. It requires understanding and leadership.


David Shearman