Greenhouse gas nitrous oxide is on the rise

During the last two decades agricultural practices and nitrogen-rich fertilisers have significantly increased the amount of nitrous oxide emissions in the atmosphere, according to a new study.

Nitrous oxide (N2O) is one of the main stratospheric ozone depleting substances. It is also a long-lived greenhouse gas – along with carbon dioxide and methane – which contributes to global temperature rise.

An international team of scientists, including Dr Chris Wilson and Professor Martyn Chipperfield both from the School of Earth and Environment, has found that N2O emissions have increased considerably during 1998-2016, but especially from 2009 onwards.

Their findings, published in Nature Climate Changesuggests that the steady rise of N2O emissions in the atmosphere is strongly linked to an increase in nitrogen substrates released to the environment from agricultural practices.

Since the mid-20th century, the production of nitrogen fertilisers, widespread cultivation of nitrogen-fixing crops – such as clover, soybeans, alfalfa, lupins, and peanuts – and the combustion of fossil and biofuels have enormously increased the availability of nitrogen substrates in the environment.

Dr Chris Wilson from the School of Earth and Environment and National Centre for Earth Observation (NCEO) at Leeds said: “We already knew that atmospheric N2O levels were increasing, but our study was able to assign this increase to specific regions and emission sources, which is important for informing strategies to limit the growth of this greenhouse gas.”

Study lead author Dr Rona Thompson, from the Norwegian Institute for Air Research, said: “Our results suggest that reducing nitrogen fertiliser use in regions where there is already a large nitrogen surplus, will result in larger than proportional reductions in N2O emissions. This is particularly relevant in regions such as East Asia, where nitrogen fertiliser could be used more efficiently, without reducing crop yields.”

The study determined N2O emissions from three global atmospheric inversion frameworks during 1998-2016, based on atmospheric N2O observations from global networks. The findings suggest that the emission of N2O has increased by a faster rate over the last decade than that estimated in the current Intergovernmental Panel on Climate Change (IPCC) guidelines.

The study found that between 2000-2005 and 2010-2015, N2O emissions increased globally by roughly 1.6 teragrams of nitrogen per year, accounting for approximately 10% of the global total.

This is about twice the amount reported to the United Nations Framework Convention on Climate Change based on the amount of nitrogen fertiliser and manure used and the default emission factor specified by the IPCC.

The authors suggest this discrepancy is due to an increase in the emission factor associated with a growing nitrogen surplus and the IPCC method, which assumes a constant emission factor, may underestimate emissions when the rate of nitrogen input and the nitrogen surplus are high.

Co-author Professor Martyn Chipperfield, also from the School of Earth and Environment at Leeds and NCEO, said: “The study highlights the need for more detailed algorithms and region-specific emission factors to estimate N2O.”

Further information:

The paper, "Acceleration of global N2O emissions seen from two decades of atmospheric inversion", is published in Nature Climate Change on 18 November 2019.