Reactive nitrogen impacts climate through its impact on the
concentrations of ozone, methane and nitrous oxide; through its impact
on the carbon cycle and through its impact on aerosols, particularly
ammonium nitrate. The anthropogenic component of reactive nitrogen is
added to the environment primarily through combustion (as NOX) or
through its addition to soils as fertilizer or manure, where the latter
process dominates. Whereas future emissions of NOX are expected to
decrease due to the increased adaptation of emission controls and
changes in the energy sector, the use of fertilizer is expected to
increase in response to population pressures and increased global food
equality. In the future the role of ammonium nitrate aerosols is
expected to be of increased importance. Despite its central role in the
nitrogen cycle the emissions of ammonia from agriculture are crudely
represented in Earth System Models and do not respond changes in
climate. Here I present an interactive parameterization of ammonia
emissions in an Earth System Model based on model physics, discuss the
consequences of this as the agricultural sector expanded over the 20th
century, and discuss future consequences and uncertainties.