Energy Use And Carbon Emissions Environmental Sciences
Nowadays, a constant growth of carbon emissions into the atmosphere has been observed, possibly leading to earth's climate change. Transportation contributes to the aforementioned phenomenon through emissions of carbon dioxide (CO2), carbon monoxide (CO), hydrocarbons (HCs) and other types of pollutants occurring from the combustion of fossil fuels. In this paper modes of sea and air transport will be compared in regards with their energy use and carbon emissions. Moreover, some measures will be suggested for reducing these emissions.2. Energy use and carbon emissions2.1 Energy use
Worldwide, sea and air transport demand has steadily increased the last decades mostly because of globalization and economic growth leading to an increase of fossil fuel used in these sectors . According to the Intergovernmental Panel on Climate Change (IPCC) the demand for aviation increased by 5.9% during 2005 alone and the respective percentage for marine transport was 5.1% the same year .
Basically all the fuels used for both marine and aircraft transportation are petroleum based. Fossil fuels have dominated in energy consumption due to their density and flexibility, despite any damage they cause to the environment . According to Energy Information administration (EIA), 100 EJ are used every year for transportation, 96% of which are oil products . More precisely, over 99% of all aviation fuel is kerosene type jet fuel whereas domestic marine vessels consume a mix of diesel oil and residual fuel oil and international marine vessels consume residual fuel oil .
Predominantly, marine energy use accounts for 13,6% of total energy consumption whereas the corresponding percentage for aviation is 12,8%. Specifically, according to the European Environmental energy (EEA), on 2008 54591 Mega tonnes oil equivalent (Mtoe) were consumed on bunker sea transport, 7781 Mtoe in international navigation (river, lake, canal shipping) and 58525 Mtoe on aviation . Obviously more fuel energy is required in the maritime sector however air transport's demand is expected to quickly rise the following years.2.2 Carbon emissions
Both marine and aviation modes emit high amounts of CO2, through fuel combustion, which is considered to be the most important of their gas pollutants contributing to climate change and the greenhouse effect.
In more details, according to the International Marine Organization (IMO), CO2 emissions from international shipping reached 843 million tonnes of CO2 (which represents 2,7% of world's total anthropogenic CO2 emissions). Moreover, if 176 million tonnes from domestic shipping and fishing vessels are added, maritime emissions reach 3,3% of the world total anthropogenic emissions .
As far as air transport is concerned, it is estimated that aviation accounts for 2% of global CO2 emissions (676 million tonnes of CO2) [8,9]. Despite the lower percentage, it is worth mentioning that because of the fact that aircraft emissions occur into the higher troposphere or lower stratosphere their impact on the greenhouse effect and climate change is considered more serious . In addition, there is high concern over the sharp incline of aviation's demand the last years. This incline is expected to continue, meaning that air modes' emissions are expected to rise even more unless immediate measures are taken.
Other types of carbon emissions from ships and airplanes are CO, Hydrocarbons and black carbon (only from ships). Their contribution to air pollution is however substantially lower [11,12].
Overally, despite shipping's higher amount of emissions, it is regarded to be one of the most energy efficient means of transport whereas aviation is the least efficient. In figure 1 the range of CO2 emissions of all means of transport is presented where it becomes obvious that aircrafts have low efficiency when it comes to emissions of CO2/ton km . Nevertheless, due to both means' increasing demand, more attention should be paid on the reduction of carbon emissions in the future.
Figure 1. Typical range of ship CO2 emissions compared to rail road and air freight (IMO 2009)3. Solutions
In order to avoid the worst impacts of climate change, transport should share the responsibility of minimizing carbon emissions into the atmosphere and even though aviation and marine are not the main carbon polluters, measures should be taken to decrease their contribution to air pollution as well.3.1 Technological Measures regarding sea transport
As far as vessel's technology is concerned, several measures can be taken to decrease carbon emissions. Larger ships, hull and propeller optimization, more efficient engines along with speed reductions can have positive results by reducing CO2 emissions by approximately 50% [14,15].
Alternative fuels are another means of reducing emissions. Substituting marine diesel oil or liquefied natural gas for heavy fuel oil seems to be a viable option . Furthermore, bio-oils have been suggested as fuel for small low-power combustion engines since long ago. For large diesel engines, hydrogen diesel can be an alternative, more environmental friendly, fuel .
Additionally, wind energy can prove beneficial either as a power producer through wind turbines or as a thruster producer through sails. Solar cells can be used as well, if a way is found to confront their high cost and big size of panels needed to provide power. A combination of the above measures can prove very efficient as well [17,18].3.2 Technological Measures regarding air transport
In the technological field, new aircraft designs with more light materials and reduced aerodynamic drag can lead to a reduction of fuel consumption and carbon emissions . In addition, improved operational practices like weight reductions or efficient navigation can contribute even more.
When it comes to the use of alternative fuels in aviation, one of the problems is that aircrafts are restricted in the types of fuel they use, meaning that fuel switching is rather difficult . Fischer-Tropsch fuels and hydro processed fuels seem to be the only option so far, however they come with the need for new engine designs in order to use them .3.3 Political measures
Finally, political measures should be taken so that the aforementioned solutions are implied. One major problem is that both international maritime and aviation do not fall under any legislation regarding their carbon emissions so far. The Kyoto protocol covers only domestic transport in those fields since it is difficult to determine boundaries between countries when it comes to international air pollution . Furthermore, calls for mandatory measures have met industries resistance as well .
Apparently, global policies should be created to ensure the reduction of emissions. Governments should find a common base in order to give incentives to large companies to implement new technologies on their vessels. This, along with better information, education and maybe some taxation measures regarding pollution could have positive results in minimizing carbon emissions from these sectors.4. Conclusion
Concluding, despite the fact that marine has proven to be more efficient than aviation in terms of energy and carbon emissions, more factors should be taken into account when deciding which means is overal better. More types of chemical compounds that impact the atmosphere should be counted and compared, such as NOx or SOx emissions. Noise and sea pollution should be taken into account as well in order to form an overall opinion.
Nonetheless, efforts to reduce carbon emissions from air and sea modes of transport are of paramount importance particularly due to the expected expansion of these sectors in the upcoming years.
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