Crossing invisible thresholds leads to a Hothouse Earth---causing certain extinction

The problem, in these cases, has a name: “tipping point” [28]—it is something typical of those systems called “complex” which tend to switch rapidly from one condition to another. We will see more about these systems in the next section but, for the time being, it is enough to note that they are very common in the real world. Think of the turkey of the story, it can have two distinct and separate states: a live turkey and a dead turkey. The same two states may occur for the whole of humankind when facing climate change.

Nassim Taleb uses the term “gray swan” for extreme events or conditions that were never experienced before but that might have been statistically predicted. A good example of gray swan is the Tōhoku tsunami that hit Japan in 2011. The tsunami wave was so large that it overcame the coastal defenses built on the hypothesis that such an event was too unlikely to be considered, yet it was not impossible on the basis of the known data on the sizes of historical tsunamis. A different case is that of events totally outside the statistical distribution, true black swans or “dragon kings” as they have been termed by Sornette [29]. In these cases, the system behaves in ways just not predictable from previous historical data. No matter how many times you test a gun by having the hammer hit an empty chamber, you won’t have data about what happens when the hammer hits a live round. The transition between one state and another fits well the concept of the “Seneca Cliff” we are discussing in this book and these considerations apply also to climate predictions. In the heat of the debate, there is a point that the critics of climate science nearly always miss and that sometimes is missed also by supporters. The problem is that the current models are limited in terms of their capability of predicting extreme events. That is, they are not made to foresee the possibility of rapid, unexpected, and catastrophic variations, the concept described here in terms of the “Seneca Effect.”

We can think of plenty of ways that models cannot describe for global warming to become dangerous for humankind, actually deadly. Rising temperatures could lead to the collapse of large fractions of the Greenland and Antarctica ice sheets and that would generate truly catastrophic rises of several meters, even tens of meters, in the average sea levels. Then, of course, there is the “big one” in climate change: the tipping point that could irreversibly propel the Earth’s climate system to the condition known as “Hothouse Earth” (sometimes “Greenhouse Earth”) with average temperatures of some 5–8 °C higher than the present ones, no icecaps, no continental glaciers, a sea level rise of tens of meters, partial deoxygenation of the atmosphere. In the past, these conditions led to huge mass extinctions and, in some cases, to the near death of the whole ecosphere. The climatologist James Hansen even hinted at the possibility of a runaway greenhouse effect [30] that could lead the Earth to become a planet similar to Venus: with no life, an atmosphere composed mainly of CO2 and sulfuric acid, and temperatures in the hundreds of degrees °C (now that would be a true Seneca Cliff!). Fortunately, the current knowledge of the physics of the Earth’s atmosphere indicates that the Venus scenario is unlikely, perhaps impossible [31]. But the mere fact that these possibilities exist shows that we could be playing Russian Roulette with the Earth’s climate.