Synthesis Report - Question 5

Climate Change 2001: Synthesis Report

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Box 5-1 Time scale and inertia.

The terms "time scale" and "inertia" have no generally accepted meaning across all the disciplines involved in the TAR. The following definitions are applied for the purpose of responding to this question:

  • "Time scale" is the time taken for a perturbation in a process to show at least half of its final effect. The time scales of some key Earth system processes are shown in Figure 5-1.
  • "Inertia" means a delay, slowness, or resistance in the response of climate, biological, or human systems to factors that alter their rate of change, including continuation of change in the system after the cause of that change has been removed.

These are only two of several concepts used in the literature to describe the responses of complex, non-linear, adaptive systems to external forcing.

Figure 5-1: The characteristic time scales of some key processes in the Earth system: atmospheric composition (blue), climate system (red), ecological system (green), and socio-economic system (purple). "Time scale" is defined here as the time needed for at least half of the consequences of a change in a driver of the process to have been expressed. Problems of adaptation arise when response process (such as the longevity of some plants) are much slower than driving process (the change in temperature). Inter-generational equity problems arise for all processes with time scales greater than a human generation, since a large part of the consequences of activities of a given generation will be borne by future generations.

WGI TAR Chapters 3, 4, 7, & 11, WGII TAR Chapter 5, & WGIII TAR Chapters 5, 6, & 10
5.1 This response dicusses, and gives examples of, inertia and varying time scales associated with important processes in the interacting climate, ecological, and socio-economic systems. It then discusses potentially irreversible changes -- that is, situations where parts of the climate, ecological, or socio-economic systems may fail to return to their former state within time scales of multiple human generations after the driving forces leading to change are reduced or removed. Finally, it explores how the effects of inertia may influence decisions regarding the mitigation of, or adaptation to, climate change.

5.2 Inertia is a widespread inherent characteristic of the interacting climate, ecological, and socio-economic systems. Thus some impacts of anthropogenic climate change may be slow to become apparent, and some could be irreversible if climate change is not limited in both rate and magnitude before associated thresholds, whose positions may be poorly known, are crossed.

5.3 The combined effect of the interacting inertias of the various component processes is such that stabilization of the climate and climate-impacted systems will only be achieved long after anthropogenic emissions of greenhouse gases have been reduced. The perturbation of the atmosphere and oceans, resulting from CO2 already emitted due to human activities since 1750, will persist for centuries because of the slow redistribution of carbon between large ocean and terrestrial reservoirs with slow turnover (see Figures 5-2 and 5-4). The future atmospheric concentration of CO2 is projected to remain for centuries near the highest level reached, since natural processes can only return the concentration to pre-industrial levels over geological time scales. By contrast, stabilization of emissions of shorter lived greenhouse gases such as CH4 leads, within decades, to stabilization of atmospheric concentrations. Inertia also implies that avoidance of emissions of long-lived greenhouse gases has long-lasting benefits.

WGI TAR Sections 3.2, 3.7, & 4.2, & WGI TAR Figure 9.16
5.4 The oceans and cryosphere (ice caps, ice sheets, glaciers, and permafrost) are the main sources of physical inertia in the climate system for time scales up to 1,000 years. Due to the great mass, thickness, and thermal capacity of the oceans and cryosphere, and the slowness of the heat transport process, linked ocean-climate models predict that the average temperature of the atmosphere near the Earth's surface will take hundreds of years to finally approach the new "equilibrium" temperature following a change in radiative forcing. Penetration of heat from the atmosphere into the upper "mixed layer" of the ocean occurs within decades, but transport of heat into the deep ocean requires centuries. An associated consequence is that human-induced sea-level rise will continue inexorably for many centuries after the atmospheric concentration of greenhouse gases has been stabilized. WGI TAR Sections 7.3, 7.5, & 11.5.4, & WGI TAR Figures 9.1, 9.24, & 11.16

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