Where does runaway collide?

A self-reinforcing coevolution between a female mating preference and the male trait it favours: because daughters tend to inherit the preference and sons the exaggerated trait, choice and ornament become genetically correlated and can escalate together, driving the trait well past its survival optimum until other forces halt it. Fisher's 1930 The Genetical Theory of Natural Selection sketched the idea as a self-accelerating process, and Lande's 1981 Models of speciation by sexual selection on polygenic traits gave it an explicit genetic model with a line of equilibria along which the runaway can slide. The referent is a positive feedback between heredity and mate choice in a breeding population, not a temperature, a planet's climate, or a price level.[1][2]

Unit

A genetic correlation between preference and trait, growing over generations

Recovery means

Natural selection against the costly trait, and the depletion of genetic variance, can arrest or reverse the escalation

Measured by

Estimated from breeding designs and quantitative-genetic covariances between preference and trait, and from selection gradients measured in the field

A self-accelerating rise in temperature, in which heat released by an exothermic reaction raises the temperature, which speeds the reaction, which releases still more heat, so the system loses thermal control unless the excess heat can be carried away fast enough. Feng and colleagues' 2018 Thermal runaway mechanism of lithium ion battery for electric vehicles traced the chain of decomposition reactions by which a lithium cell heats itself to failure. The referent is a physical energy balance in a reacting material, judged in degrees and watts, and it has nothing to do with a mating preference, a planetary atmosphere, or an economy beyond the shared shape of a positive feedback.[3]

Unit

A temperature that rises ever faster as it climbs, in degrees per unit time

Recovery means

Cooling, venting, or separating the reactants removes heat faster than the reaction makes it and returns the system to control

Measured by

Tracked by calorimetry and thermocouples as the onset temperature, the self-heating rate, and the time to thermal runaway

A self-amplifying climate feedback in which warming evaporates more water vapour, itself a greenhouse gas, which traps more heat and warms the surface further, so that once a threshold in absorbed sunlight is crossed the oceans can evaporate away entirely. Ingersoll's 1969 The Runaway Greenhouse: A History of Water on Venus set out the radiative limit at which this feedback becomes uncontrollable and tied it to the loss of Venus's water. The referent is a feedback in a planet's radiation and water budget, judged in watts per square metre and surface temperature, not a trait in a population, a battery cell, or a currency.[4]

Unit

A warming that feeds on its own water vapour, in watts per square metre

Recovery means

Below the radiative threshold the vapour feedback stays bounded and the climate re-stabilises; above it there is no stable state to return to

Measured by

Bounded by radiative-convective models as a limiting outgoing flux, and inferred from a planet's water inventory and surface temperature

A self-feeding acceleration of prices, in which rising prices lead people to expect still higher prices, so they spend money faster and demand higher wages and prices, which pushes prices up again, until a currency can shed its value almost by the day. Cagan's 1956 The Monetary Dynamics of Hyperinflation modelled the loop between the rate of price increase and the falling demand for money that drives it. The referent is a positive feedback between prices and expectations in a monetary economy, read as a percentage rate per month, not a genetic trait, a temperature, or a planet's climate.[5]

Unit

A rate of price increase that feeds on expected inflation, in percent per month

Recovery means

A credible monetary reform that stops the money supply from expanding can break the expectations loop and halt the spiral

Measured by

Read from price indices as the monthly inflation rate, and modelled against the demand for real money balances