Moses Amweelo
There are several ways to reduce the amount of greenhouse gases in the atmosphere and mitigate climate change. To ensure these are effective, however, it’s important to understand the full consequences of the action taken.
Research provides the vital information needed to inform these choices and enable careful planning for mitigation strategies. Here, we look at the impact of the shipping and aviation industries, as well as black carbon.
These areas raise the complex issues of trying to compare the impacts of pollutants that have very different effects and timescales in the atmosphere.
These issues will become increasingly important as greenhouse gas emissions are reduced from the bigger polluting sectors, in particular carbon dioxide (CO2) from the energy sector. Civil aviation currently accounts for about 2.6% of global fossil fuel emissions of CO2, but the relative importance of aviation emissions is expected to increase significantly in future due to rapid aviation industry growth and reductions in greenhouse gas
emissions from other sectors. While significant, this contribution to CO2 emissions is not the only impact aviation has on our climate. Water vapour in the hot, moist exhausts from high-flying aircraft can trigger condensation trails, known as contrails. They are normally short-lived, but depending on atmospheric conditions, they can last much longer. Observational evidence shows contrails can develop into high-level cirrus clouds. Contrails and aviation-induced cirrus have two competing effects on climate: they reflect sunlight into space to create a cooling effect, but also trap thermal radiation to create a warming effect. Studies show that, overall, the warming effect is stronger. Climate model simulations allow the distribution of contrails to be mapped, and their warming effect is now estimated to be smaller than initially thought.
However, when it comes to aviation-induced cloudiness, there is still considerable uncertainty about its geographic extent. This means there is also uncertainty in the amount of additional warming this process creates.
Areas of high aviation activity are those most likely to be affected, and research is ongoing to get a better understanding of the global impacts. Shipping accounts for around 3.4% of global fossil fuel emissions of CO2, and emissions are growing at around 3% per year.
This is a significant contribution to global greenhouse gas emissions, but shipping is currently not included in the Kyoto Protocol. Besides CO2 emissions, shipping has another impact on global climate due to the relatively high sulphur content of the fuel used.
When burnt, shipping fuel releases a high amount of sulphur dioxide, which then reacts to form sulphate aerosols in the atmosphere. These act as a global coolant by reflecting sunlight into space. Model simulations have shown that, after both the warming effect of CO2 emissions and the cooling effect of sulphate aerosols are taken into account, shipping has created a net cooling effect on the climate.
Shipping emissions are, therefore, helping to cool the planet in the short-term, although the extent of this cooling is uncertain. The sulphate aerosols are removed fairly quickly from the atmosphere in rainfall, whereas the CO2 continually builds up. After about 50 year,s the warming from the accumulated CO2 overcomes the cooling from the sulphate. Sulphate aerosol is also an air pollutant, harmful to both humans and plants (through acid rain).
New international rules aim to reduce its damaging effects by significantly reducing the sulphur content in shipping fuel. While this will tackle air pollution by removing the aerosols, it will have a warming effect on our climate. The Meteorological (MET) office researched the climate impacts of reducing the sulphur content of shipping fuel to get a clearer understanding of the overall impact of climate change. Black carbon, or soot, is a by-product of combustion. The main sources are energy- generation and transport, but many sources such as domestic cooking and slash-and-burn forest clearance also contribute. It enters the air as fine particles produced by incomplete burning, and has serious effects on air quality and human health. It has a relatively short lifespan in the atmosphere of about 15 days, compared to about 100 years for CO2.
Although it has a short lifespan, black carbon also has a dual warming effect on climate: it absorbs sunlight when in the atmosphere and darkens reflective surfaces, reducing the amount of sunlight they reflect into space. Black carbon is often released with other greenhouse gases and aerosols, which have contrasting effects on our climate, meaning mitigation measures must be assessed carefully.