Mark Sutton, BBC Green Room 9 Jul 08;
Farming and industry are producing too much of a substance we ought to be concerned about, says Mark Sutton; not carbon, but nitrogen. And he would like to hear your ideas on what society should do about it.
Over the last decade, you have surely heard many views as to why you should worry about carbon and climate change.
But the chances are you're not worrying about nitrogen.
In fact, there is a global nitrogen threat out there, yet the world seems not to notice!
It's an issue that has recently been highlighted by two reviews in the journal Science.
In many regions of the world, humans are producing too much nitrogen, creating a host of different environmental threats.
Most of this nitrogen is made for a reason - we need it to fertilise crops and feed ourselves. Without it, it has been estimated that around half of the world's population would not be alive.
Put these parts of the problem together and you get what we might call the "NitroNet" - a complex web of nitrogen interactions that are difficult to explain and even harder for governments to manage.
And here you can already see why people on the streets are not yet talking about nitrogen.
Complex web
There are many different nitrogen forms, from atmospheric ammonia, nitrogen oxides and particulate matter, to the greenhouse gas nitrous oxide and nitrates in aquatic systems.
Each has different effects: increased air pollution threatens human health and biodiversity, disturbance of the greenhouse gas balance, and loss of drinking and bathing water quality.
It is the kind of complexity that is not easy to chat about casually on a bus journey.
All this makes for a double challenge to the scientific community; to understand and deal with an extremely complex system, while distilling out the simple messages.
This is where the Science papers start to help.
One of the ideas they contained is that we can distinguish nitrogen into two main forms - unreactive and reactive.
There is plenty of unreactive nitrogen in the world; this is the N2 that makes up 78% of the earth's atmosphere. But it can't be used directly by most plants or animals.
By contrast, reactive nitrogen (Nr) is all the other nitrogen forms that can be used.
In natural conditions, reactive nitrogen is in extremely short supply. Biologically, it can only be made by special nitrogen fixing bacteria, typically associated with legumes like clover and beans.
A century ago, a serious shortage of reactive nitrogen in agriculture limited food production in Europe, and encouraged careful re-use of manures.
Since that time, two major new sources have appeared.
Firstly, high temperature combustion in vehicles and industry now oxidises more N2 to Nr. This contributes to acid rain, photochemical smog and particulate air pollution, the last of which may, for example, reduce average life expectancy across south-east England by 8-12 months.
Secondly, development of the Haber-Bosch process has allowed industrial-scale manufacture of reactive nitrogen fertilisers.
The benefits of this process for global food production have been immense, with synthetic fertilisers being the foundation of the Green Revolution. But along with the benefits have come hidden costs, as the extra reactive nitrogen pollutes air, land and water.
Perhaps the most dramatic changes are seen immediately downwind of large livestock farms, where atmospheric ammonia can completely wipe out certain wild flowers, bog mosses and lichens.
Seasonal problem
One message from the developing scientific assessment is that there are clear choices to be made.
How much nitrogen do we really need for food production? And how can we weigh up the environmental costs and benefits?
For example, Nobel Prizewinner Paul Crutzen has recently argued that emissions of nitrous oxide from fertilised biofuel crops can outweigh the carbon benefits of avoided fossil fuel use.
Others have highlighted a possible benefit of nitrogen in making forests grow faster, absorbing more carbon dioxide from the atmosphere.
But the decisions get even harder when dealing with multiple nitrogen threats.
For example, policies to reduce nitrates in water have banned wintertime spreading of farm manures across much of Europe's farmland. The resulting focus on springtime manure spreading has intensified peak ammonia emissions, giving a new threat to biodiversity and air quality.
The recent "odour episode" experienced in London is not unrelated, and likely to be repeated.
Food for thought
Future policies will, I hope, emphasise a smarter overall management of Nr in agriculture. The big challenge, however, will be for governments to weigh up trade-offs between the different nitrogen threats, and make better informed choices in international agreements.
Yet, there is also a simple challenge for each of us.
The cascade of reactive nitrogen from fertiliser bag to food on our plate is extremely inefficient, with losses to the environment occurring at every stage.
Eating meat and dairy products adds an extra step to the food chain, massively increasing the Nr losses. This observation can help us in our search for clear messages.
At its very simplest, the carbon story might be summarised in three short words: use less fuel.
At this level, the matching nitrogen challenge for developed countries becomes clear: eat less meat.
Of course, we all know that both stories are more complicated. But for nitrogen, this is a message that needs to be shouted much more loudly.
According to World Health Organization guidelines, many of us eat more animal products than is good for us.
As we begin to untangle the NitroNet, we could even find some health benefits too.
Mark Sutton is based at the Edinburgh Research Station of the Centre for Ecology & Hydrology
He is co-chair of the UN's Task Force on Reactive Nitrogen (TFRN), director of the European Centre of the International Nitrogen Initiative (INI) and co-ordinator of the NitroEurope Integrated Project
The Green Room is a series of opinion articles on environmental topics running weekly on the BBC News website
