According to the article “Transport emissions sizable, and rising” in Science News (January 19, 2008, subscription needed), the researchers report that the transporting of goods and services has caused about 15% of the carbon dioxide and about 31% of the ozone artificially produced by humankind and emitted into the atmosphere.
Their conclusion is based on data taken since the time just before the Industrial Revolution, which occurred between the late eighteenth century and the early nineteenth century.
The researchers stated that the percentage of carbon dioxide (CO2) and ozone (O3) produced by the transportation of goods and services was not exactly known before their research.
They state that railroads were the primary mode of transportation for goods and services beginning around the 1850s, ships were added around the 1870s, motorized vehicles in the 1900s, and aircraft around the1930s.
Using motorized vehicles running with internal combustion engines to transport goods and services on roadways caused more emission of carbon dioxide than railroads and ships beginning in the 1940s.
Aviation increased in its emissions of carbon dioxide emissions beginning in the 1960s.
Now, in the 2000s, aircraft emit over 600 million metric tons of carbon dioxide each year, while motorized vehicles produce about 4.2 billion metric tons annually.
Motorized vehicles, they conclude, have been the worst way to transport goods and services, based on the amount of greenhouse gases emitted into the atmosphere.
Their paper “Climate forcing from the transport sectors” was published in the January 15, 2008 issue of Proceedings of the National Academy of Sciences and in the online version on January 7.
The researchers for the article include Jan Fuglestvedt, Terje Berntsen, and Gunnar Myhre, from the Center for International Climate and Environmental Research (CICER, Oslo, Norway) and the Department of Geosciences at the University of Oslo; and Kristin Rypdal and Ragnhild Bieltvedt Skeie from CICER.
In their abstract, the researchers state: “Although the transport sector is responsible for a large and growing share of global emissions affecting climate, its overall contribution has not been quantified. We provide a comprehensive analysis of radiative forcing from the road transport, shipping, aviation, and rail subsectors, using both past- and forward-looking perspectives.”
“We find that, since preindustrial times, transport has contributed 15% and 31% of the total man-made CO2 and O3 forcing, respectively. A forward-looking perspective shows that the current emissions from transport are responsible for 16% of the integrated net forcing over 100 years from all current man-made emissions. The dominating contributor to positive forcing (warming) is CO2, followed by tropospheric O3.”
“By subsector, road transport is the largest contributor to warming. The transport sector also exerts cooling through reduced methane lifetime and atmospheric aerosol effects. Shipping causes net cooling, except on future time scales of several centuries. Much of the forcing from transport comes from emissions not covered by the Kyoto Protocol.”
However, some scientists point out problems with their study. Please read on.
Critics point out that their abstract statement that “Shipping causes net cooling…” is inaccurate because, in actuality, it produces greenhouse gases (like carbon dioxide) and fine particle matter (such as sulfates and soot).
The greenhouse gases warm the atmosphere but the fine particles cool the atmosphere.
However, as these scientists point out, the greenhouse gases remain in the atmosphere for a hundred years or so, while particulate matter falls back to Earth’s surface within a matter of days (from precipitation).
In other words, the cooling effect from particulate matter is short-term (matter of days) while the warming effect from carbon dioxide and other greenhouse gases is long term (a hundred years or so).
Thus, scientists--countering this statement from the Norwegian scientists that shipping does not have a warming effect on the Earth but rather a net cooling effect (more cooling than warming)--point out that the cooling effect from shipping is not really there to counter the warming effect.
Additional information on these critical remarks is found in the New Scientist article “Transport emissions study 'misleading' say experts.” (subscription needed)