Lucy Sandbach

Nitrogen - The Bad Guy of Global Warming

Carbon dioxide emissions, rising global temperatures, melting ice caps

and climate change make news on a daily basis. But is our love affair

with carbon dioxide blinding us to the threat posed by a more dangerous

agent? The global warming culprit in question is nitrogen, and ignoring

it could lead to immense costs to both human health and the

environment.  

Natural Nitrogen

Nitrogen gas is taken from the atmosphere and converted by bacteria into nitrogen compounds that plants and animals can use.©EPA

Nitrogen is an

essential part of life. Plants, animals and bacteria all use nitrogen

in fundamental building blocks called amino acids, and these are joined

together to make proteins. Proteins not only allow us to grow and

function properly, but they form the basis of almost every chemical

reaction in the human body.
Our main source of nitrogen is the

atmosphere, where it is present as nitrogen gas (formula N₂). However

in its gaseous form, nitrogen is very inert (unreactive) and only a

small number of organisms are able to utilise it. The natural process

of taking nitrogen gas and converting it into useful compounds is known

as nitrogen fixation, and is carried out by nitrogen-fixing bacteria

(and more occasionally, lightning). These ‘fix’ nitrogen into another

nitrogen-containing compound: ammonia (NH₃). 

Ammonia is more

biologically accessible than nitrogen gas and is used by nitrifying

bacteria to form nitrites (NO_2-) and then nitrates (NO_3-). These

nitrates are the form of nitrogen that plants can process, and thus the

form that introduces nitrogen into our food chain. But if all

atmospheric nitrogen eventually ended up in plants or animals, there

would soon be a shortage. Fortunately there are denitrifying bacteria

that complete the cycle and convert nitrates back into the inert and

unreachable N₂.

This cycle is naturally regulated by the speed at

which bacteria can change one compound into another, and by the amount

of bacteria available in the soil. In the past this led to a natural

upper limit of nitrogen available for use in the biosphere at any one

time. However, technological advances have dramatically increased

this natural limit, and the consequences have been far-reaching. So

what happened?

Causes of the nitrogen overdose

The dawning of

the Industrial Revolution heralded a major change that greatly affected

the nitrogen balance. Large-scale burning of fossil fuels such as coal

and oil released high levels of nitrogen oxides (including nitrous

oxide or N₂O) as fumes. The nitrogen problem escalated further by World

War I with the development of the Haber-Bosch Process, which allowed

inert N₂ gas to be made into ammonia without the use of slow

nitrogen-fixing bacteria. The ammonia produced became a valuable

resource and could be used to make cheap fertilisers for use on crops.

Other contributors to increased levels of nitrogen compounds were the

burning of trees and plants for agriculture, and the manufacture of

nylon. But seeing as successful industry and agriculture are crucial

across the globe, do we really want to stop artificially creating our

own useful nitrogen compounds? Why would we want to go back to the

natural limits of the nitrogen cycle?

Why should we worry?

There are two main things that these nitrogen compounds affect: the

environment and human health.  When nitrous oxide (N₂O) reaches

the stratosphere it helps destroy the ozone layer, resulting in higher

levels of UV radiation and increasing the risk of skin cancer and

cataracts. Ironically, when N₂O is nearer to the Earth’s surface it can

actually make ozone, which can become smog on a still and sunny day.

Smog has been linked to respiratory problems, lung damage, increased

risks of cancer and a weakening of the immune system. 

Smog over Santiago in Chile. This can be caused by nitrogen oxides in the atmosphere.

As

well as its tricks with ozone, nitrogen oxides dissolve in atmospheric

water to make acid rain, which corrodes stone and metal work and

damages buildings.  In 1967 a bridge over the Ohio River collapsed

due to acid rain corrosion, killing 46 people. But it’s not only

building damage that’s cause for concern; plants (including our food

crops) and even humans are at risk. Links between acid rain,

Alzheimer’s disease and brain damage have been suggested, as well as

with many respiratory problems. So, overall, not good news!

But the

problems extend further. The overuse of fertilisers on fields and of

nitrogen compounds in animal feed leads to nitrogen leaching into

streams and rivers. Algae, whose growth is usually limited by nitrogen

availability, use this flood of nitrogen to grow out of control,

leading to big algal blooms. These use up all the oxygen in the water

and block out the light, suffocating aquatic life and preventing

underwater plants from photosynthesising. Worryingly, nitrate levels in

the Norwegian lakes have doubled in the last ten years, and in northern

Europe we are depositing nitrogen compounds at over 100 times the

natural rate. The outlook for these lakes seems bleak.

Returning to

the land, higher nitrogen levels in the soil mean that a few plants are

able to out-compete the rest. These tend to be plants able to quickly

utilise the excess nitrogen for rapid growth, leaving fewer resources

and more shade for other species. This can lead to many species of

plant becoming extinct, and will in turn have knock-on effects on all

the animals, insects and birds that use them. Many species-rich

heathlands in the Netherlands have been taken over by species-poor

forests for precisely this reason. 

Finally, nitrogen oxides

contribute to global warming. Although the concentration of nitrous

oxide in the atmosphere is considerably lower than that of carbon

dioxide, the global warming potential of nitrous oxide is over 300

times greater. So although carbon dioxide causes climate change and its

associated problems, nitrogen compounds are arguably worse. They have a

greater global warming potential, could lead to more exaggerated

climate change problems, and cause havoc with health and the

environment to boot! So what can we do about it?

The remedies

Currently, 80% of the nitrogen compounds in the atmosphere are from

human sources. This problem is a by-product of our highly

technology-driven societies, but therein lies the solution. The same

technological innovation can be used to reduce emissions, and catalytic

converters can convert nitrogen oxides into harmless nitrogen gas.

Legislation can also play a role. In California, large farms with over

a thousand dairy cows must now apply to the Air Resources Board for a

 

license, controlling the levels of concentrated leaching from animals.

 Ultimately though, there is one solution guaranteed to deal with

this nitrogen problem: reduce the amount of nitrogen we use to fuel our

daily lives. This is all well and good, but as with all solutions to

big problems, it’s going to be very, very hard work.

- March 2007