Will Steffen

“Coral reefs are a good example of threshold and step-change behaviour. Reefs are subject to a wide variety of natural disturbances, from hurricanes to episodic outbreaks of crown-of-thorns starfish. Over the last several decades human stresses - nutrient and sediment loadings from adjacent coastal areas, fishing and tourism - have begun to interact with natural disturbances to put reefs under increasing stress. Global change is adding even more stresses of a quite different nature. Increasing atmospheric CO2 is changing the carbonate chemistry in the surface waters of the ocean, making it more difficult for reef organisms to form their hard shells. At the same time, warming of the upper ocean is leading to widespread bleaching events. These new, global-scale stresses operate everywhere, and are both persistent and inexorably increasing in severity. Given sufficient pressure from these interacting local to global stresses, coral reefs can cross a threshold with widespread death of the coral and a rapid change to colourless algal beds.”
― Will Steffen, Global Change and the Earth System: A Planet Under Pressure: Executive Summary

“The development of the ozone hole was an unforeseen and unintended consequence of widespread use of chlorofluorocarbons as aerosols in spray cans, solvents, refrigerants and as foaming agents. Had, inadvertently, bromofluorocarbons been used instead, the result could have been catastrophic. In terms of function as a refrigerant or insulator, bromofluorocarbons are as effective as chlorofluorocarbons. However, on an atom-for-atom basis, bromine is about 100 times more effective at destroying ozone than is chlorine. As Nobel Laureate Paul Crutzen has written “This brings up the nightmarish thought that if the chemical industry had developed organobromine compounds instead of the CFCs – or, alternatively, if chlorine chemistry would have run more like that of bromine – then without any preparedness, we would have been faced with a catastrophic ozone hole everywhere and at all seasons during the 1970s, probably before the atmospheric chemists had developed the necessary knowledge to identify the problem and the appropriate techniques for the necessary critical measurements. Noting that nobody had given any thought to the atmospheric consequences of the release of Cl or Br before 1974, I can only conclude that mankind has been extremely lucky.” (Source: P. Crutzen (1995) My life with O3, NOx and other YZOxs. Les Prix Nobel (The Nobel Prizes) 1995. Stockholm: Almqvist & Wiksell International. pp. 123-157).”
― Will Steffen, Global Change and the Earth System: A Planet Under Pressure: Executive Summary