Of my three areas of interest, global heating, biodiversity and agriculture, it would be useful to identify some measure that tells us how bad things are. We are looking for something that relates well to the quantity of pollutant or substance being extracted, and this might be a concentration of pollutant, or an observable, measurable, state of the system . This is the control variable. We could also use some measure of the impact on the system; what things are changing. Once we know the impact on the system we can identify boundaries on the control variable that relate to the impact on humans. We know this for global heating. The control variable is atmospheric CO2 and we have a long list of things that go wrong as CO2 levels rise. Johan Rockström and his colleagues, of the Stockholm Resilience Centre, have done just this by identifying observed issues arising from human actions. The result is a proposal for a breakdown of 9 planetary boundaries¹. To summarise, we have:

• climate change
• ocean acidification
• Stratospheric ozone depletion
• atmospheric aerosol loading
• biogeochemical flows (phosphorous and nitrogen cycles)
• global fresh water use
• land-system change
• biodiversity loss
• chemical pollution

The items in bold have, as of 2015, already broken through the safe boundary². Rockström and his colleagues are very clear that these are systems that we are dealing with. The first boundary acts as a warning that something needs to be done, and can still be done, while the second boundary, a threshold, indicates the risk of a non-linear transition that may be impossible to reverse. The 2015 update indicates that the Nitrogen cycle and genetic diversity (part of the biodiversity area) have crossed their thresholds into the high risk areas.

So, I have left out some things, and I have bundled together other things. Many of these systems interact anyway, so, at one level, the breakdown we have here is not going to alter public or political perception. On the other hand, the point of this analysis is to identify management actions, and to warn us to be wary of side effects. Even with a focus on global heating we need to be sure that chemical pollution is not forgotten, and, we particularly need to be aware that any technical solutions to global heating do not get us into trouble over the likes of atmosphere aerosol loading.