Thresholds risk prolonged degradation

William Schlesinger in Nature Reports: For nitrogen deposition as for other pollution, waiting until we approach the limits of environmental degradation merely allows us to continue our bad habits until it's too late to change them.

Thresholds are comforting for decision-makers. There is no controversy when a high-jumper makes the bar, in contrast to a figure-skater who wins based on form and execution. When the skater doesn't make the grade, there is endless debate about whether the judges were too harsh and what revisions are needed in scoring procedures.

In personal health, as long as we are alive we can be pretty sure we haven't crossed a threshold of dire consequence. But in many cases, identifying and waiting for thresholds also allows misbehaviour that might be better nipped in the bud. Humans don't die of the first cigarette they inhale, but the slow cumulative effects of smoking can hasten the journey towards one's ultimate personal threshold.

Ecologists believe there are numerous thresholds in nature (Nature 413, 591–596; 2001). As we see anthropogenic changes in the Earth system, we need to decide whether we want to allow human activities to disrupt Earth's life-support processes, or whether to begin now to sustain something that is pleasant and potentially more healthful for humans and the other species that share this planet with us. Ongoing changes in global chemistry should alarm us about threats to the persistence of life on Earth, whether or not we cross a catastrophic threshold anytime soon.

Rockström et al. (Nature 461, 472–475; 2009) guess that an acceptable human impact on the global nitrogen cycle should not exceed 25 per cent of the current anthropogenic transfer of nitrogen from the atmosphere to the land surface. This threshold for nitrogen seems arbitrary and might just as easily have been set at 10 per cent or 50 per cent. Since nitrogen can also be denitrified by soil bacteria and ecosystem remediation is theoretically possible, greater human impacts might potentially be tolerated with proper management (Proc. Natl Acad. Sci. USA 106, 203–208; 2009)…..

The delta of the Atchafalaya River on the Gulf of Mexico. US Army Corps of Engineers