The oceans would be solid ice. Greenhouse gases are thus atmospheric gases that are transparent to visible light but which absorb IR radiation. The relative effect of greenhouse gases depends on their absorption characteristics and concentrations. The chief greenhouse gas is carbon dioxide, but there are many others that contribute to the greenhouse effect, including water vapor, methane, ozone, nitrous oxide, and CFCs. That means that more fixed N is being nitrified and denitrified.
Nitrous oxide is a greenhouse gas and an ozone destroyer.
Human fixation of N has other effects. Deforestation People have been deforesting the planet for a long time. The forests of Europe were mostly gone by the time Columbus sailed. European settlers were soon busy clearing eastern deciduous forests. Forests in Washington began to be seriously tackled by the late 1800s.