Johan Rockström: 9 Planetary Boundaries

Scientists hope identifying and quantifying planetary boundaries "that must not be transgressed" could keep humans from wrecking the climate any further.

A new study published in Science argues:

To avoid catastrophic environmental change, humanity must stay within defined 'planetary boundaries' for a range of essential, and interlinked, Earth-system processes, argues a Feature article in this week's Nature. The authors warn that the maximum acceptable limit has been passed for climate change, species loss and the nitrogen cycle, and is nearing for three more processes.

What are the boundaries? These are the nine:

1. climate change rate 
2. biodiversity loss (terrestrial and marine)
3. interference with the nitrogen and phosphorus cycles
4. stratospheric ozone depletion 
5. ocean acidification
6. global freshwater use
7. change in land use
8. chemical pollution
9. atmospheric aerosol loading

 The authors write, "The boundaries in three systems (rate of biodiversity loss, climate change and human interference with the nitrogen cycle), have already been exceeded."  I've highlighted these in red.

Why is this concept useful?  "There are at least three reasons for our proposed climate boundary."

1. First, current climate models may significantly underestimate the severity of long-term climate change for a given concentration of greenhouse gases¹². Most models¹¹ suggest that a doubling in atmospheric CO₂ concentration will lead to a global temperature rise of about 3 °C (with a probable uncertainty range of 2–4.5 °C) once the climate has regained equilibrium. But these models do not include long-term reinforcing feedback processes that further warm the climate, such as decreases in the surface area of ice cover or changes in the distribution of vegetation. If these slow feedbacks are included, doubling CO₂ levels gives an eventual temperature increase of 6 °C (with a probable uncertainty range of 4–8 °C). This would threaten the ecological life-support systems that have developed in the late Quaternary environment, and would severely challenge the viability of contemporary human societies.
2. The second consideration is the stability of the large polar ice sheets. Palaeoclimate data from the past 100 million years show that CO₂ concentrations were a major factor in the long-term cooling of the past 50 million years. Moreover, the planet was largely ice-free until CO₂ concentrations fell below 450 p.p.m.v. (100 p.p.m.v.), suggesting that there is a critical threshold between 350 and 550 p.p.m.v. (ref. 12). Our boundary of 350 p.p.m.v. aims to ensure the continued existence of the large polar ice sheets.
3. Third, we are beginning to see evidence that some of Earth's subsystems are already moving outside their stable Holocene state. This includes the rapid retreat of the summer sea ice in the Arctic ocean¹³, the retreat of mountain glaciers around the world¹¹, the loss of mass from the Greenland and West Antarctic ice sheets¹⁴ and the accelerating rates of sea-level rise during the past 10–15 years¹⁵.