The anthropogenic component (this includes both industrial processes like Haber-Bosch and changes brought about by high-intensity farming) to most legs of the nitrogen cycle equals or exceeds that of the natural component. To argue that humanity is incapable of screwing with climate in any significant way when we are clearly overriding natural ecological cycles is pure inanity.
When talking about the "significance," we forget that we're talking about differences of 5K in an ambient temperature of 300K.
We're not "significantly" screwing with the climate. That's not the issue. The issue is just how sensitive our civilization is to such a small temperature change.
