2. Internal and external causes
of climate change
What makes the climate change?
Climate varies on all
timescales and space scales,
from interannual climatic variability to very long-period variations related to
the evolution of the atmosphere and changes in the lithosphere
Internal or external mechanisms that operate at different
frequencies are responsible for climatic fluctuations
Different forcing factors may operate together
i) Variations of the Earth’s
orbital parameters (eccentricity, obliquity, precession)
ii) Continental drift, mountain
building
iii) Composition of the
atmosphere
iv) Volcanic activity
i) Variations of the Earth’s
orbital parameters (eccentricity, obliquity, precession):
EXTERNAL FORCING
The position and orientation of the Earth relative to the Sun
has not been constant
Ultimate cause of glaciations in the past would be linked to
changes in the Earth’s orbital parameter (CROLL, 1867a, b; 1875; MILANKOVITCH (1941);
BERGER (1977a, 1978, 1979, 1988)
BASIC ELEMENTS OF THE EARTH’S ORBITAL MOTION AROUND THE SUN
TODAY:
a) -Earth revolves around the Sun along a slightly elliptical
path >>>> Earth closest to the Sun (perihelion) around January 3
and farthest from it (aphelion) around July 5
Results in a
difference of ~3.5% in solar radiation received
b) -Earth is tilted on its rotational axis 23.4° from a plane
perpendicular to the plane of the ecliptic (the apparent surface over which it
moves during its revolution around the Sun)
c) -The Earth axis points toward Polaris at the North Pole
***NONE OF THESE FACTORS HAVE REMAIN CONSTANT THROUGH TIME
(due to gravitational effects of the Sun, Moon, and the other planets on the Earth)
Variations have occurred in the degree of orbital eccentricity around the Sun, in the axial tilt, and in the timing of the perihelion (closest to the Sun)
with respect to seasons on the Earth (precession of the equinoxes)
a) Variations in orbital eccentricity are quasi-periodic with
an average period length of ~95,800 yr
over the past 5 million yr >>>> The orbit has varied from almost circular (no difference between
perihelion and aphelion) to maximum
eccentricity when solar radiation receipts (outside the atmosphere)
varied by ~30% between
aphelion and perihelion
***Affects the relative
intensities of the seasons
b) Changes in axial tilt are periodic with a mean period of
41,000 yr and the angle of inclination has varied from 21.8 to 24.4%
>>>> Most recent maximum is 100,000 yr ago
The angle defines the latitudes of the polar circles (Arctic
and Antarctic) and the tropics, which in turn delimit the area of daylong polar
night in the winter, and the maximum latitudes reached by the zenith sun in the
midsummer in each hemisphere
***Changes in angle have relatively little effect on
radiation receipts at low latitudes but the effect increases towards the poles: summer radiation at high
latitudes increases, but winter radiation totals decreases
c) The Earth wobbles slightly around its axis of rotation
>>>> entails changes in the seasonal timing of perihelion and
aphelion
The effect of this wobble is to change systematically the timing of the solstices and equinoxes
relative to the extreme positions the Earth occupies on its elliptical path
around the Sun (mean period of ~21,700 yr)
***11,000 yr ago, perihelion occurred when the Northern
Hemisphere was tilted towards the Sun (mid-June)
rather than in the midwinter, as is the case today
When the right combination of orbital perturbations act together, the solar insolation is
minimum at the latitudes where the ice sheets had formed >>>> These
minima coincide with the four major
periods of glaciation during the past 650,000 yr
The change in insolation is believed to be too small to cause
large temperature variations >>>> seen as a triggering mechanism for other climate parameters
***Thus the cause of the major ice age glaciations remain
unresolved
INTERLUDE
The concepts of positive and negative feedback
All components of the climate system are intimately linked or
coupled with all other components >>>> Change in one component may involve changes throughout the entire climate
system
POSITIVE FEEDBACK: when the change is
amplified >>>> The system is increasingly destabilized
EXAMPLE: Growth
of continental ice sheets
Whatever the initial perturbation of the climate system that
led to continental ice-sheet growth in the past, once snow and ice persisted
year round, the higher continental
albedo would have resulted in lower
global radiation receipts, hence lower temperature, and a more favorable environment for ice-sheet
growth
What might
cause a return towards warmer conditions?
NEGATIVE FEEDBACK: when the change is
dampened >>>> The system is increasingly re-stabilized
EXAMPLE: Reduction
of the Greenhouse effect
If temperature continues to increase, there would be more evaporation from the oceans, increased cloudiness (higher global albedo), and hence a
decrease in energy to the system
In addition, higher temperature at high latitudes, associated
with increased poleward advection of moisture, might be accompanied by more snowfall, resulting in higher continental albedo (and/or a
shorter snow-free period) and hence lower
overall energy receipts
ii) Continental drift, mountain
building
On the time
scale of tens of millions of years, the climate change are likely to be linked
to plate tectonics
Plate motions lead to cycles of ocean basin growth and destruction,
known as Wilson cycles, involving continental rifting, seafloor-spreading,
subduction, and collision
CLIMATE-TECTONIC
CONNECTIONS
a) Geographic distribution and
size of continents
Through the course of a Wilson cycle, continents collide and
split apart, mountains are uplifted and eroded, and ocean basins open and close
The redistribution and changing size and elevation of
continental land masses may have caused climate change on long time scales
Computer climate models have shown that the climate is very
sensitive to changing geography >>>> but they are not responsible
for decreasing temperatures on a global scale
Climate’s sensitivity to mountains and high plateaus show
that plateau uplift in Tibet and western North America has a small effect on
global temperature but cannot explain the magnitude of the cooling trend
Plateau uplift does, however, have a significant impact on climate,
including the diversion of North Hemisphere westerly winds and intensification
of monsoonals circulation
Monsoon system: system of low-level winds blowing into a
continent in summer and out of it in winter, controlled by atmospheric pressure
systems developed seasonally over the continent
In the winter, there is a strong outflow of dry continental
air from the north across China, Southeast Asia, India and the Middle East
***Winter monsoon=dry conditions
In the summer, warm humid air from the Indian Ocean and the
southwestern Pacific moves northward and northwestward into Asia, Passing over
India, Indochina and China
***Summer monsoon=heavy rainfall in southeastern Asia
The Himalayan Mountains enhances the monsoon in two ways:
It imposes a
barrier that blocks the northward and southward flow of moisture
It alters
the flow of upper-level winds that influence surface conditions
b) Geometry of ocean basins
What is the effect of opening and closing gateways for the
flow of ocean currents?
Redistribution of heat on the planet by changing ocean
circulation can isolate polar regions, cause the growth of ice sheets and sea
ice, and increase temperature differences between the equator and the poles
Poleward movement of warm water
to the northern edges of the Atlantic provides a flow of heat to northern
latitudes trough the Gulf Stream
This flow is absorbed by the
atmosphere and then carried eastward on prevailing westerly winds to the
European continent
***35% of the total annual insolation received by the
Atlantic ocean north of 40º latitude
This is why there is a shift of
several climate zones in Europe northward by as much as 10º latitude compare to
North America
It is possible that changes in heat transport caused by
variations in ocean gateways may have played a significant role in cooling
trends over the last 60 millions years, and, in particular, may help explain
some of the relatively sudden cooling events
c) Tectonic processes
The plates drift like broken ice on a river
Because the entire Earth is completely encased in these
plates, any tectonic motion becomes a kind of perpetual slow-motion collision
between plates
Incredible masses of rock and sediment crush together at
imperceptible speeds, these collisions causing earthquakes and volcanic
eruptions
During some of these collisions, sedimentary carbonates are
lifted and exposed at the surface