We need to first identify the conceivable limits of the potential societal consequences of human-caused climate change—the best and worst outcomes that might be plausible due to our greenhouse gas emissions. These don’t have to be likely outcomes. Instead, these limits will help us capture as full a range as possible of what could happen to society. We will consider how likely different outcomes may be, to the extent that we can, in step five.
Critically, human emissions of greenhouse gases are independent of other potential warming and cooling influences on the climate system. For example, it is possible that human-caused warming could occur simultaneously with an independent cooling influence, such as a volcanic eruption, an asteroid impact, or nuclear conflict. If there were a significant cooling influence from another factor, then the warming due to our greenhouse gas emissions might conceivably help offset it. As a result, even a human-induced warming that would be highly damaging if it occurred under otherwise unchanging climate conditions might instead help keep the climate system closer to that of the 20th century.
For the best conceivable outcome let’s imagine that human-caused warming offsets what would otherwise be a catastrophic cooling of the climate system. That is, human emissions of greenhouse gases might plausibly preserve civilization as we know it by counteracting a cooling influence that would otherwise be beyond society’s capacity to handle.
While this outcome does not seem remotely likely because it depends on the co-occurrence of multiple low-probability events (e.g., an unforeseen and low-probability cooling influence that would be sufficiently large to wipe out society but not so large that human-induced warming can’t offset it), it does provide a useful limit for the best possible outcome.
The worst conceivable outcome would be for human-caused climate change to trigger the end of civilization as we know it. That is, for our greenhouse gas emissions to cause changes in climate that exceed society’s capacity to absorb.
 Note that we could plausibly expand these limits even further (e.g., to include the preservation or extinction of all life on the planet) but the preservation or destruction of civilization as we know it provides a reasonable and at least somewhat intuitive set of endmembers.
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