We have performed high-precision measurements of the <sup>18</sup>O and position dependent <sup>15</sup>N isotopic composition of N<sub>2</sub>O from Antarctic firn air samples. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N<sub>2</sub>O isotope signatures since pre-industrial times. The heavy isotope content of atmospheric N<sub>2</sub>O is presently decreasing for all signatures at rates of about -0.038 %o yr <sup>-1</sup> for <sup>1</sup><font face="Symbol">d</font><sup>15</sup>N, -0.046 %o yr <sup>-1</sup> for <sup>2</sup><font face="Symbol">d</font> <sup>15</sup>N and -0.025 %o yr <sup>-1</sup> for <font face="Symbol">d</font><sup>18</sup>O. The total decrease since pre-industrial times is estimated to be about -1.8%o for <sup>1</sup><font face="Symbol">d</font><sup>15</sup>N at both positions and -2.2%o for <sup>2</sup><font face="Symbol">d</font><sup>15</sup>N. Isotope budget calculations using these trends and recent stratospheric measurements allow to isotopically characterize the present and the pre-industrial global average N<sub>2</sub>O source, as well as the additional N<sub>2</sub>O emissions that have caused the global N<sub>2</sub>O increase since pre-industrial times. The increased fluxes from the depleted surface sources alone are insufficient to explain the inferred temporal isotope changes. In addition, the global average N<sub>2</sub>O source signature is calculated to be significantly depleted today relative to the pre-industrial value, in agreement with recent indications from soil emission measurements.