Billion Years Climate Study PDF

Equilibrium States: • A system is in equilibrium if it does not change unless it is disturbed. • Equilibrium states can be stable or unstable. • A system is in a stable equilibrium when its response to a modest disturbance returns the system to its initial equilibrium state. • A system is in an unst...

Equilibrium States: • A system is in equilibrium if it does not change unless it is disturbed. • Equilibrium states can be stable or unstable. • A system is in a stable equilibrium when its response to a modest disturbance returns the system to its initial equilibrium state. • A system is in an unstable equilibrium when its response to a disturbance moves the system to a new equilibrium state. Equilibrium States: The ball analogy ¡ Balls will seek out low points on a wavy surface. ¡ Low points are analogous to stable equilibria. ¡ High points are analogous to unstable equilibria. System Disturbances: Time matters ! A perturbation acts over a brief period. e.g. volcanic eruption Sangeang Api, Indonesia, May 31, 2014 http://www.volcanodiscovery.com System Disturbances: Time matters ! A perturbation acts over a brief period. e.g. volcanic eruption System Disturbances: Time matters ! A forcing acts over a long period. • increase in solar luminosity over the last 4.6 billion yrs • cyclic changes in orbital parameters (e.g., 100 000 yrs) source : NASA, Solar Dynamics Observatory The Daisyworld Climate Hypothetical planet Only white daisies grow on our Daisyworld. Where daisies don't grow, the soil is dark and barren. Daisy coverage affects the planet’s temperature. figures on the whiteboard: 2-7 to 2-13 from textbook Threshold Behaviour As the sun brightens, daisy coverage expands. Extent of warming on Daisyworld is not eliminated but reduced. Eventually Daisyworld warms beyond the optimum for daisies. Positive feedback causes the daisy population to crash and temperatures to rise rapidly. Lessons from Daisyworld A planetary climate system is not passive in the face of internal or external forcings and perturbations. Self regulation is a property common to many natural systems with feedback loops. Abrupt transitions can take place following a seemingly small or random perturbation to a system that is approaching a threshold. Critical Thinking Problem Earth’s average temperature is determined in part by the amount of CO2 in the atmosphere, by way of the greenhouse effect. The atmospheric CO2 content may in turn be affected by photosynthetic activity of plants, which convert CO2 into plant tissue (organic carbon); however, the rate of photosynthesis depends on the amount of CO2 in the atmosphere and on global air temperature. The components of this system, atmospheric CO2 content, global temperature, and photosynthesis rate, are intimately interconnected. By increasing global photosynthesis rates, plants would tend to lower the atmospheric CO2 level. In doing so, however, the plants would tend to cool Earth. This cooling, together with the CO2 level, might tend to reduce the photosynthetic activity of plants. Relating textbook knowledge to past/current/future climate events, ongoing research, and to the Blue Planet and the Last Billion Years Threshold Behaviour As the sun brightens, daisy coverage expands. Extent of warming on Daisyworld is not eliminated but reduced. Eventually Daisyworld warms beyond the optimum for daisies. Positive feedback causes the daisy population to crash and temperatures to rise rapidly. Lessons from Daisyworld A planetary climate system is not passive in the face of internal or external forcings and perturbations. Self regulation is a property common to many natural systems with feedback loops. Abrupt transitions can take place following a seemingly small or random perturbation to a system that is approaching a threshold. Several parts of the climate system appear to be moving closer and closer to potential thresholds: can you think of any? Lessons from Daisyworld A planetary climate system is not passive in the face of internal or external forcings and perturbations. Self regulation is a property common to many natural systems with feedback loops. Abrupt transitions can take place following a seemingly small or random perturbation to a system that is approaching a threshold. Several parts of the climate system appear to be moving closer and closer to potential thresholds: Atlantic Meridional Overturning Circulation, melt of Greenland Ice sheet, Arctic Sea Ice loss, and others (Lenton et al. 2008, PNAS) . The phrase ‘tipping point’ captures the colloquial notion that ‘little things can make a big difference’, that is, at a particular moment in time, a small change can have large, long-term consequences for a system. The term ‘tipping element’ was introduced to describe large-scale subsystems (or components) of the Earth system that can be switched — under certain circumstances — into a qualitatively different state by small perturbations. These must be at least subcontinental in scale (length scale of order ~1,000 km). The tipping point is the corresponding critical point — in forcing and a feature of the system — at which the future state of the system is qualitatively altered. Lenton 2011 Thresholds can also have hysteresis, i.e., “a system has two or more alternative states and the location of thresholds depends on the direction of the transition. Once a hysteretic system transitions to an alternate state, simply bringing the driver variable back across the original threshold will not return the system to its previous state. Ratajczak et al. 2018 Two ways to shift between alternative stable states. a, If the system is on the upper branch, but close to the bifurcation point F2, a slight incremental change in conditions may bring it beyond the bifurcation and induce a catastrophic shift to the lower alternative stable state (`forward shift'). If one tries to restore the state on the upper branch by means of reversing the conditions, the system shows hysteresis. A backward shift occurs only if conditions are reversed far enough to reach the other bifurcation point, F1. b, A perturbation (arrow) may also induce a shift to the alternative stable state, if it is sufficiently large to bring the system over the border of the attraction basin (see also Fig. 3). Scheffer et al. 2001 External conditions affect the resilience of multi-stable ecosystems to perturbation. The bottom plane shows the equilibrium curve as in Fig. 2 [previous slide]. The stability landscapes depict the equilibria and their basins of attraction at five different conditions. Stable equilibria correspond to valleys; the unstable middle section of the folded equilibrium curve corresponds to a hill. If the size of the attraction basin is small, resilience is small and even a moderate perturbation may bring the system into the alternative basin of attraction. Scheffer et al. 2001 Stability landscape showing the pathway of the Earth System out of the Holocene and thus, out of the glacial–interglacial limit cycle to its present position in the hotter Anthropocene. The fork in the road in Fig. 1 is shown here as the two divergent pathways of the Earth System in the future (broken arrows). Currently, the Earth System is on a Hothouse Earth pathway driven by human emissions of greenhouse gases and biosphere degradation toward a planetary threshold at ∼2 °C (horizontal broken line at 2 °C in Fig. 1), beyond which the system follows an essentially irreversible pathway driven by intrinsic biogeophysical feedbacks. The other pathway leads to Stabilized Earth, a pathway of Earth System stewardship guided by human-created feedbacks to a quasistable, human-maintained basin of attraction. “Stability” (vertical axis) is defined here as the inverse of the potential energy of the system. Systems in a highly stable state (deep valley) have low potential energy, and considerable energy is required to move them out of this stable state. Systems in an unstable state (top of a hill) have high potential energy, and they require only a little additional energy to push them off the hill and down toward a valley of lower potential energy. Steffen et al., 2018 A schematic illustration of possible future pathways of the climate against the background of the typical glacial–interglacial cycles (Lower Left). The interglacial state of the Earth System is at the top of the glacial–interglacial cycle, while the glacial state is at the bottom. Sea level follows temperature change relatively slowly through thermal expansion and the melting of glaciers and ice caps. The horizontal line in the middle of the figure represents the preindustrial temperature level, and the current position of the Earth System is shown by the small sphere on the red line close to the divergence between the Stabilized Earth and Hothouse Earth pathways. The proposed planetary threshold at ∼2 °C above the preindustrial level is also shown. The letters along the Stabilized Earth/Hothouse Earth pathways represent four time periods in Earth’s recent past that may give insights into positions along these pathways: A, Mid-Holocene; B, Eemian; C, Mid-Pliocene; and D, MidMiocene. Their positions on the pathway are approximate only. Steffen et al., 2018