CLIMATE CHANGE:

The planet is warming due to human activities. This will almost cetainly wreck havoc on the planet's ecosystems.

Consequently, it is important to focus climate-protection efforts towards the human activities that are causing the most warming. One important contributor is transport.

Different modes of transport release different gases and levels of gases, each with differing lifetimes, into the atmosphere. Thus, they each warm (or even cool) the planet by differing amounts.

One can imagine that predicting the future warming contribution of each primary mode of transport is difficult. One must compare (1) different pollutants (2) emitted at different locations, (3) impacting different regions of the atmosphere, and (4) instigating effects that span a time scale from hours to centuries.

Terje Berntsen and Jan Fuglestvedt at the Center for International Climate and Environmental Research (Norway) have risen to the challenge. They have predicted the future changing contributions of transport by road, rail, air, and sea, focusing on climate impacts of widely differing time scales.

The model.

The scientists' model enables a comparison of the global warming impact of road, rail, air, and sea transport over time. They include the major contributors of emissions related to combustion.

The gases considered include carbon dioxide, carbon and sulfate aerosols, ozone, and methane. These last two are impacted by nitrogen oxides, carbon monoxide, and volatile (easily evaporated) carbon-based compounds. For air transport, water vapor and various clouds are also included.

Contribution of each gas.

The scientists used this model to predict dramatically changing impacts with respect to time and mode of transport. They classify three impact groups:

• Short-lived species give a maximum response within one year, declining in impact to less than 10% within 13 years.

• Due to its degredation by other compounds, methane gives a maximum response after 8 years, declining in impact to less than 10% within 45 years.

• Carbon dioxide is the only gas that persists in large quantities for centuries.

Contribution of each mode of transport.

The previous results predict the global warming impact of each gas, with respect to time. The scientists then turned their focus on predicting the impact of each mode of transport.

Road and air transport initially warm the planet. Rail and shipping transport initially cool the planet; for shipping, this cooling effect lasts for decades.

How does shipping give a long-term cooling effect? The main reason is that engines on ships give very efficient combustion, releasing large quantities of nitrogen oxides, and comparatively small levels of carbon monoxide and volatile carbon-based compounds.

Although shipping has a long-term cooling effect, the other modes of transport collectively conspire to warm the planet after more than two decades. This is due to the dominating effects of carbon dioxide.

The scientists then used the available data and their model to predict the impact of each mode of transport within a 67% probability range. Primarily due to its much higher fuel consumption and carbon dioxide output, their prediction is that road transport will be largest contributor to global warming in the coming decades.

Over the next 20 and 100 years, warming caused by road transport will be roughly four times that caused by air transport (due to carbon dioxide emissions). This assumes constant emissions at year 2000 levels.

A focus on carbon dioxide emitted by road transport is needed.

Politicians and others need this kind of time-dependent global warming prediction. This is because we need to consider both short-term ("tipping point") and long-term global warming, and how to best mitigate one or both.

For example, the European Union has set as its goal the prevention of a 2ºC temperature increase above preindustrial levels, predicted to arrive after approximately 40 more years. Under these considerations, aviation will yield 15%, rail 4%, and shipping 2% of the warming caused by road transport.

Clearly, there needs to be a focus on reducing carbon dioxide emissions due to road transport. If emissions remain much the same as they are now into the future, such a focus will have the largest mitigating effect on global warming than will a focus on any other greenhouse gas or means of transport.

for more information:
Berntsen, T.; Fuglestvedt, J. Global temperature responses to current emissions from the transport sectors. Proc. Natl. Acad. Sci. USA 2008, 105, 19154-19159.