In my weekend links, I highlighted a new study published in Science. It is behind a pay wall, but has been well covered by Climate Central and Climate Progress. The study provides us with a good excuse to revisit the whole topic of climate risk.
The paper in question is an empirical study of past permafrost thaw at different temperatures. To summarise the conclusion, stalactite and stalagmite growth in Siberian caves suggests that significant permafrost thaw took place 400,000 years ago when the global mean temperature was around 1.5 degrees Celsius higher than it is today. This level of specificity is new, and is important for not being model based. Incidentally, we have an accurate temperature record going back around 800,000 years through the study of ice cores.
When such studies come through, I think it always useful to place them within a risk component analysis framework. As a reminder, risk is best defined as probability times effect—or more specifically probability times net harmful effect. It is also worth recalling that we should not get sidetracked by the accusation that such studies lack certainty. The human condition is one of decision-making under uncertainty. As individuals, the only real certainty we have in our lives is death.
Keeping these points in mind, climate change can be viewed as a chain of risk components, each of which has its own probability distribution. We move from emissions, through atmospheric concentrations (the study of the airborne fraction that I blogged about here), to climate sensitivity, until finally we arrive at the net harmful effects on both individuals and societies.