Emissions of nitrous oxide — a greenhouse gas 300 times more potent than carbon dioxide — are going up faster than we thought.
It's countries in east Asia and South America that are making the biggest contribution to the increasing emissions, according to a study published today in the journal Nature Climate Change.
Natural sources of nitrous oxide include our oceans and rainforests, but it's the human sources that are of most cause for concern — specifically agriculture, including nitrogen fertiliser use and livestock manure.
We've known for decades that nitrous oxide emissions are increasing, but since 2009 there has been a "substantial increase" in these emissions, said Pep Canadell, executive director of the Global Carbon project and an author of the new research.
Most countries report their nitrous oxide emissions using the methodology of the Intergovernmental Panel on Climate Change. This approach assumes a linear relationship between the amount of nitrogen fertiliser used and the nitrous oxide emissions that come out at the other end, said Dr Canadell, who is also a chief research scientist at the CSIRO.
"Using atmospheric data for the first time, we show that the [linear] relationship is not true when there are regions around the world that over-fertilise," he said.
"So there's this threshold [of fertiliser use], and past that amount — which is well above and beyond what plants need — we find an exponential growth in nitrous oxide emissions."
Using atmospheric data and nitrous oxide concentrations from over 50 stations around the world, the scientists could model what emissions were expected from different regions.
What is nitrous oxide?
Nitrous oxide is a greenhouse gas 300 times more potent than carbon dioxide.
This means each molecule of nitrous oxide in the atmosphere can capture 300 times more heat than a molecule of carbon dioxide.
However, not only do we emit considerably more carbon dioxide than we do nitrous oxide, carbon dioxide also stays in the atmosphere for thousands and thousands of years — compared to just over 100 years for nitrous oxide.
So getting carbon dioxide emissions under control is still key to tackling climate change, but nitrous oxide and methane emissions are part of the picture too.
Carbon dioxide contributes to 65 per cent of global warming, while methane and nitrous oxide contribute 16 and 20 per cent, respectively.
Nitrous oxide is emitted predominantly from agriculture using nitrogen fertiliser, and to a lesser extent burning fossil fuels and biofuels, said Dr Canadell.
"The single, most important source of nitrous oxide is agriculture," he said.
In agriculture, nitrogen is added to the system in the form of fertiliser, but if you use more fertiliser than the crop needs, the excess nitrogen leaks from the system as ammonia or nitrous oxide, or as soluble nitrates leaching into the soil and then groundwater.
New on a global scale
The fact that nitrous oxide emissions are increasing faster than we thought is not surprising to scientist Peter Grace from the Queensland University of Technology, who previously coordinated the nitrous oxide program in Australia and was not involved in this study.
"A lot of the information we've collected over the years here and in other countries would show an exponential increase in nitrous oxide emissions as you apply more fertiliser," Professor Grace said.
He said that the atmospheric methods used in this study could take into account more variability than localised measuring is able to.
"We go out with chambers and take measurements of the atmosphere, but nitrous oxide is highly variable across the landscape so those numbers are a ballpark.
"But we also find hotspots — for example, right after you apply fertiliser, you get a big spike in nitrous oxide," he said.
"This paper is coming from a global perspective, so it's probably more accurate in terms of what the nitrous oxide emissions might be."
It's been known for a while that there isn't a linear relationship between nitrogen fertiliser input and nitrous oxide emissions, said Richard Eckard, director of the Primary Industries Climate Challenges Centre at the University of Melbourne.
"When you exceed the [plant] system's capacity to use that nitrogen fertiliser, the efficiency goes out the window, and the nitrogen can leak out of the cycle," Professor Eckard said.
"That plays out in some industries where the recommended amount of fertiliser is exceeded, and you get exponential loss of nitrogen."
Professor Eckard said he was pleased that this global analysis had picked up what was already known on a smaller scale.
"It's never been studied at this [global] level — recognising that it isn't a linear relationship," he said.
Different attitudes to fertiliser use
The big challenge with agriculture is that fertiliser is relatively cheap to use — so people put on a bit more to cover their bases, which is then lost to the environment (and atmosphere), Professor Eckard explained.
"While many developed countries are using nitrogen fertiliser up to recommendation, we've started seeing places like China, India and Brazil pushing to use more fertiliser in their cropping systems," he said.
"That's a concerning driver of the exponential increase nitrous oxide emissions, that they’ve picked up in this study as well."
Because of global food insecurity, more countries than ever before are using nitrogen fertiliser.
Professor Eckard said that if we encourage people to reduce livestock farming and associate methane emissions, the alternative is then for people to farm produce relying on synthetic nitrogen.
"It's not that they shouldn't be using nitrogen fertiliser, but if we all used the right amount we'd have significantly less nitrous oxide going into the atmosphere."
Nitrous oxide emissions in Europe and the United States have been flat over past 20 years despite increased agricultural output, which Dr Canadell said showed more accurate application of fertiliser was critical.
"You can use less nitrogen and maintain your productivity," he said.