Earth beyond six of nine planetary boundaries

Abstract

This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.

Fig. 1.. Current status of control variables for all nine planetary boundaries.

Six of the nine boundaries are transgressed. In addition, ocean acidification is approaching its planetary boundary. The green zone is the safe operating space (below the boundary). Yellow to red represents the zone of increasing risk. Purple indicates the high-risk zone where interglacial Earth system conditions are transgressed with high confidence. Values for control variables are normalized so that the origin represents mean Holocene conditions and the planetary boundary (lower end of zone of increasing risk, dotted circle) lies at the same radius for all boundaries (except for the wedges representing green and blue water, see main text). Wedge lengths are scaled logarithmically. The upper edges of the wedges for the novel entities and the genetic diversity component of the biosphere integrity boundaries are blurred either because the upper end of the zone of increasing risk has not yet been quantitatively defined (novel entities) or because the current value is known only with great uncertainty (loss of genetic diversity). Both, however, are well outside of the safe operating space. Transgression of these boundaries reflects unprecedented human disruption of Earth system but is associated with large scientific uncertainties.

Fig. 2.. Impact of the combined effect of land system change and climate change boundary states on trajectories of terrestrial carbon stocks and global land temperature.

Results are based on idealized Earth system model experiments with varying planetary boundary status, ranging from maintaining the planetary boundary (85%/50%/85% boreal/temperate/tropical forest remaining, 350-ppm atmospheric CO₂, green), the upper end of the zone of increasing risk (60%/30%/60%, 450 ppm, orange), and beyond the zone of increasing risk (40%/20%/40%, 550 ppm, red). Open circles represent the short-term changes (1988–2100) of the system, while colored circles the long-term changes (2100–2770). Their colors denote the state of the land system change boundary, while the climate change boundary is shown on the y axis. The locations of the circles on the x axis represent the changes in the land carbon stocks, and the associated land temperature changes are given next to each circle, both compared to the year 1988. Transgressing the climate change boundary (y axis) is mostly connected to an increase in temperature, while the transgression of land system change leads to a loss of terrestrial carbon stocks (source) of 100 to 200 Gt of C.