M5 Climatic State Limit - Hothouse Vs. Icehouse

Topic

Earth's climate system has existed in various stable climate states throughout the planet's geological history, and transitions between these states involve crossing global bifurcations within the climate system itself. The space of stable climate states includes at least three end-member climate states. The first is the greenhouse climate, such as the one that existed during the Cretaceous—roughly ninety million years ago—characterized by carbon dioxide concentrations of 1,000 to 2,000 parts per million, the absence of permanent polar ice, and polar temperatures ranging from 10 to 20 degrees Celsius. The second is the glacial climate, such as the one prevailing today during the Quaternary, featuring permanent ice sheets at both poles. The third is "Snowball Earth," a state of near-total planetary glaciation that occurred during the Proterozoic, between 700 and 635 million years ago.

The latitudinal energy balance model captures the essential physics underlying these stable climate states: at any given latitude, the temperature depends on the balance between absorbed insolation and emitted infrared radiation, combined with meridional heat transport from other latitudes. Within this model, the critical parameter is albedo: ice reflects between 60% and 80% of incoming solar radiation, whereas the ocean or exposed land reflects only 6% to 30% of that same radiation. If ice advances far enough toward the tropics—crossing a critical latitude near thirty degrees—the ice-albedo feedback becomes catastrophically positive: the further the ice advances toward the tropics, the more the planet's average albedo increases; a higher average albedo reflects more solar radiation and cools the planet further, causing the ice to advance even closer to the tropics. Once this catastrophically positive ice-albedo feedback kicks in, the planet freezes over completely, entering the "Snowball Earth" state described earlier. Escaping this Snowball Earth state requires volcanic carbon dioxide to accumulate over millions of years until it reaches a pressure close to one-tenth of current atmospheric pressure; this is because the ice covering the entire planet reflects so much solar radiation that no other mechanism can warm the planet enough to melt it.

The Paleocene-Eocene Thermal Maximum, which occurred approximately fifty-six million years ago, is the closest paleoclimatic analog to current anthropogenic climate change: a massive injection of carbon—ranging from three thousand to ten thousand gigatonnes—occurring over less than twenty thousand years caused global warming of five to eight degrees, acidified the ocean, and drove the extinction of benthic foraminifera living on the ocean floor. The Paleocene-Eocene Thermal Maximum demonstrates that the climate system can respond non-linearly to rapid carbon forcing—much like current anthropogenic climate change, which is, in essence, a carbon forcing event that is occurring very rapidly on a geological timescale. Analysis of equilibrium climate sensitivity across different climate states—the Cretaceous greenhouse climate, the warmer Pliocene climate, and the current climate—reveals that this sensitivity depends on the climate system's background state; it is not a fixed value independent of the stable climate state the planet occupies. This dependence of equilibrium climate sensitivity on the background climate state links the three end-member climate states described earlier—greenhouse climate, glacial climate, and Snowball Earth—to rapid transition events such as the Paleocene-Eocene Thermal Maximum: to understand how much the current climate system will warm in response to a specific carbon injection, it is not enough to know the magnitude of that injection; one must also know the background climate state—among these various stable states—in which the system exists when it receives that input.

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