Janne Kotiaho and colleagues propose using a pre-degradation 'natural state' as a reference baseline for assessing the impact of humans on biodiversity and ecosystem function (Nature 532, 37; 2016). However, it is not possible for scientists to define a single such baseline objectively. This is because global ecosystems changed drastically during pre-human time periods, under otherwise 'natural' conditions.
As long as scientists set baselines to single time points in Earth's history, this problem will remain. The established solution is to compare the full ranges of variability in biodiversity and ecosystem functioning within and between pre-human and post-human worlds (see K. J. Willis and H. J. Birks Science 314, 1261–1265; 2006).
This approach avoids the need to set any arbitrary baselines for a 'natural state'. It also allows scientists to determine what effects human activities (such as nitrogen pollution or greenhouse-gas emissions) have on the planet, after expected ranges of natural changes have already been accounted for (see K. K. McLauchlan et al. Nature 495, 352–355; 2013).