NASA has been using satellite imagery to collect data on the tracks ships leave when they cross bodies of water. Tracking this visible shipping pollution has demonstrated that a tremendous amount of nitrogen dioxide (NO2) emissions has resulted.
NASA collected the data using its Aura satellite from 2005 to 2012.
Scientists are trying to figure out just how much shipping contributes to global NOx emissions, but current data finds that an estimate of between 15% and 30% is a reasonable assessment. The rest of the emissions are the direct result of oil drilling, agricultural burning, and oddly, enough, lightening.
Carbon dioxide often gets the spotlight, but nitrogen dioxide can cause major health problems, not least of which are respiratory problems. It also creates fine particle pollution and ground-level ozone.
Maps tracking this NO2 pollution demonstrate that the issues lie in the most trafficked and busiest shipping routes. The route between Singapore and Sri Lanka in the Indian Ocean currently seems to be the most predominant. The Gulf of Aden, the Red Sea, and the Mediterranean Sea also have major pollution.
However, North America, Europe, and China are not without issue. Despite the fact the maps often do not demonstrate their pollution levels visibly, the pollution is still there. The Atlantic and Pacific oceans seem clear because they are so large the ship tracks get dispersed. Bad weather often makes data collection in the Arctic inaccurate, and in Europe, North American, and China, the ship tracks are often overshadowed by the existing pollution from coastal cities and offshore drilling.