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This is particularly the case given the likely cheapness and technological feasibility of adding sulfates to the stratosphere. Crutzen estimates a global annual cost of $25–50 billion, less than a twentieth of the amount spent around the world each year on defense. He suggests high-altitude balloons or artillery shells as a way of delivering the necessary sulfur budget into the stratosphere, but there may be easier methods. Simply adding some sulfur-release mechanism to the world’s 14,000-strong fleet of passenger aeroplanes, a significant proportion of which are likely to be flying in the stratosphere at any one time, would surely be a much easier task, requiring little additional effort. In addition, there is a whole variety of potential solar-radiation management technologies that could be deployed were stratospheric sulfur to fail or to misfire in some way. These include the idea of whitening low-level clouds over the sea by spraying tiny water droplets into the air, putting a few million small sunshades in space, or even painting everyone’s roofs white. Even the air capture of carbon dioxide by mechanical means, and its disposal underground, is considered by some to be geoengineering. In my view, simply knowing what we are doing means that none of our actions in the future that affect the climate can be called unwitting. Our hands are on the thermostat whether we like it or not, so sooner or later we are going to have to face up to the need to make a decision about what temperature we want our planet to be at over the longer term.

I entirely accept, however, that there is a huge qualitative difference between an intentional and an unintentional action. No legal system gives the same penalty for premeditated murder as for accidental manslaughter. Nor would one expect the same degree of praise for saving someone’s life by accident as for doing so on purpose. Accordingly, any decision on the deployment of geoengineering would need a step-change in our own systems of international policymaking and governance, a theme I will take further in the final chapter of this book. To repeat: No one could sensibly argue that solar-radiation management in any way negates the need for urgent and decisive action to reduce greenhouse gas emissions. We must reduce CO2 emissions to zero anyway over the next few decades, not just to avoid catastrophic global warming, but also to prevent serious acidification of the oceans—another planetary boundary, examined in the next chapter.

But scientific research and development of different geoengineering strategies should be stepped up and funding from world governments increased. I have no doubt that even flawed solar-radiation management with serious unintended outcomes would be much better for the planet and humanity in general than four, five, or even six degrees of global warming. Geoengineering could be an important insurance policy against this nightmare ever happening. If tipping points are crossed that appear to be driving the climate system out of our control, we will have little option but to try to cool it down. For me this is a reason for optimism, and why I believe that my children, grandchildren, and their contemporaries will never have to see the worst-case-scenario global warming I wrote about at the end of my last book, Six Degrees, because—barring some unforeseen worldwide civilizational collapse—humanity will have developed the technologies needed to avoid the holocaust of runaway global warming. Of course, curing the disease is always better than merely treating the symptoms. But doing both may be the best option of all.

BOUNDARY EIGHT

OCEAN ACIDIFICATION

I have already said a lot about human interference with the carbon cycle, because of the supreme importance of the climate change planetary boundary. But climate change has an evil twin, whose very existence was barely noted until comparatively recently, but which is now considered by the planetary boundaries expert group to be sufficiently critical to the Earth