What is known about the inertia and time
scales associated with the changes in the climate system, ecological
systems, and socio-economic sectors and their interactions?
|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
- "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,
||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
of inertia may influence decisions regarding the mitigation of, or adaptation
to, climate change.
||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.
||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
||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,