Some of the gases formed in seawater are not broken down in the troposphere, the lower layer of our atmosphere. This allows them to reach the upper layer of our atmosphere, which we call the stratosphere. The stratosphere is very important to our climate. It contains the ozone layer which protects us from harmful ultra-violet rays from the Sun. It also contains a layer of sulphate aerosols which prevent some of the energy from the Sun reaching the Earth. Even though the stratosphere starts over 11 km above the surface of the Earth, gases from seawater can affect it's chemistry.
Nitrous Oxide (N2O)
Another important biologically produced gas is nitrous oxide. Because it isn't broken down in the troposphere, it has a very long lifetime in the atmosphere, lasting around 120 years.
Where does nitrous oxide come from?
The largest source of nitrous oxide is soils, particularly in the tropics and emissions have risen over time probably due to increased fertiliser use. The oceans are also a very important source, particularly estuaries and coastal waters. Here nitrous oxide is produced by bacteria converting nitrogen compounds into nutrients.
Why is nitrous oxide important?
In the troposphere, nitrous oxide is 275 times more powerful a greenhouse gas than carbon dioxide so it contributes greatly to global warming. Because it has such a long atmospheric lifetime, it can get all the way to the stratosphere and affect climate there as well. In the stratosphere, nitrous oxide is destroyed by ultra-violet radiation to produce nitrogen oxide (NO) radicals. These are involved in the destruction of stratospheric ozone. Although a problem in the troposphere, we need stratospheric ozone to protect us from ultra-violet radiation.
Emissions of nitrous oxide from coastal waters are likely to increase as our climate gets warmer and sea levels rise. Further eutrophication may also lead to enhanced production of nitrous oxide. There is already evidence for increasing nitrous oxide emissions along the Indian coast as a result of human activity.
Carbonyl Sulphide (COS)
Carbonyl sulphide is the dominant sulphur gas in the atmosphere. It is formed mainly in the oceans but also has a small industrial source.
Where does marine carbonyl sulphide come from?
Marine derived carbonyl sulphide is formed by the action of sunlight on sulphur-containing organic matter in the upper layers of the ocean, particularly in coastal waters. Because it doesn't dissolve easily in water, it enters the atmosphere easily.
Why is carbonyl sulphide important?
Carbonyl sulphide isn't broken down in the troposphere so, like nitrous oxide, it can get all the way to the stratosphere. Here it is converted to sulphate aerosols. Sulphate aerosols reflect incoming sunlight back into space and so help cool our planet.
About this page:
author: Dr. Lucinda Spokes - Environmental Sciences, University of East Anglia, Norwich - U.K.
educational reviewers: Francis Mudge - School of Education and Professional Development, University of East Anglia, Norwich - U.K. and Trevor Leggett - Chemistry Teacher, Norwich - U.K.
last updated: 2003-10-01