Runaway Feedback
Trajectories of the Earth System in the Anthropocene
Risk of a Hothouse Earth Pathway
Limit Cycles and Planetary Thresholds.
The trajectory of the Earth System through the Late Quaternary, particularly the Holocene, provides the context for exploring the human-driven changes of the Anthropocene and the future trajectories of the system (SI Appendix has more detail). Fig. 1 shows a simplified representation of complex Earth System dynamics, where the physical climate system is subjected to the effects of slow changes in Earth’s orbit and inclination. Over the Late Quaternary (past 1.2 million years), the system has remained bounded between glacial and interglacial extremes. Not every glacial–interglacial cycle of the past million years follows precisely the same trajectory (7), but the cycles follow the same overall pathway (a term that we use to refer to a family of broadly similar trajectories). The full glacial and interglacial states and the ca. 100,000-years oscillations between them in the Late Quaternary loosely constitute limit cycles (technically, the asymptotic dynamics of ice ages are best modeled as pullback attractors in a nonautonomous dynamical system). This limit cycle is shown in a schematic fashion in blue in Fig. 1, Lower Left using temperature and sea level as the axes. The Holocene is represented by the top of the limit cycle loop near the label A.
Our analysis suggests that the Earth System may be approaching a planetary threshold that could lock in a continuing rapid pathway toward much hotter conditions—Hothouse Earth. This pathway would be propelled by strong, intrinsic, biogeophysical feedbacks difficult to influence by human actions, a pathway that could not be reversed, steered, or substantially slowed.
Where such a threshold might be is uncertain, but it could be only decades ahead at a temperature rise of ∼2.0 °C above preindustrial, and thus, it could be within the range of the Paris Accord temperature targets.
The impacts of a Hothouse Earth pathway on human societies would likely be massive, sometimes abrupt, and undoubtedly disruptive.
