Human demand is pushing ecosystems beyond safe limits – while weak policy, unrealistic emissions targets and the silencing of dissenting voices make the crisis harder to confront.

Humans take too much energy from the land

It is common knowledge that humanity’s footprint can be found almost everywhere over the world’s land surface. But how do you measure the heaviness of the footprint? How does the heaviness vary from place to place? How has it changed over time? And how does the heaviness affect the integrity and functioning of the ecosystems that form the biosphere in each place?

Almost all the energy within the biosphere is derived from photosynthesis by plants. Within any defined area, the energy produced by all the plants can be measured and is called the Net Primary Production (NPP). In many places, humans commandeer some of the NPP for their own uses (for instance by removing biomass for food, clothing and building materials and by land use change). The amount that humans have appropriated (HA) at any time can be expressed as a proportion of the NPP in pre-industrial times (the Holocene) and used as a measure of our impact on that patch of the environment. This proportion is abbreviated to the rather intimidating HANPPHol.

The energy contained in the NPP powers important (for humans and other species) ecosystem functions such as pollination, sediment retention, the health of soil microbiota, water retention and flow, and pest regulation (although the last is a very anthropocentric orientation). The more of that energy that humans remove, the less there is available to keep the ecosystem functioning and resilient.

So it’s not surprising that as the amount of NPP that humans appropriate in each location increases, the greater is the decrease in species richness and biodiversity in the area, although the precise effects are highly context specific.

Taking an HANPPHol of 10 per cent as the threshold for moving from a safe (sustainable) human footprint to one that puts the ecosystem at risk and 20 per cent as high risk, globally the 10 per cent boundary was crossed around 1900 and the 20 per cent boundary around 1990 – which seems logical considering the population growth and the industrial and agricultural development that occurred in both magnitude and coverage during the twentieth century. In 1900, just over a third of global land had passed the 10 per cent mark and 14 per cent was already in........

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