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Milakovitch Cycles | Sun Spots and Solar Cycles
MILANKOVITCH CYCLES
The Milakovitch Cycles are the eccentricity of the orbit around
the sun, the tilt of the Earth's axis, and the direction the North
Pole points. Variations in these three factors change the amount
and distribution of incoming solar radiation. Variations
in the distribution of solar radiation affect and initiate glaciations.
Climate change, and subsequent periods of glaciation is not due
to the total amount of solar energy reaching Earth. The three Milankovitch
Cycles affect the seasonality and location of
solar energy around the Earth, and therefore the contrasts between
the seasons
Orbital Parameters
The Earth's orbit varies through time.
The amount of solar radiation striking the Earth depends on the
relative position of Earth and Sun.
Natural variations in orbital parameters have an impact on the
climate of the planet. Orbital fluctuations set conditions for
the cooler and warmer periods of glacial-interglacial stages. The
orbit of the Earth is an ellipse but the Sun is not one of the
focal points. As a result the Earth is farther from the Sun (at
aphelion) at one end of the long axis and closer (at perihelion)
at the other. The length of the long axis of the ellipse varies
over time, but at present the Earth is closest to the Sun in December.
Therefore the northern winter is somewhat warmer than it would
be if the winter solstice came at the opposite end of the orbit.
Eccentricity
The Earth's orbit around the sun is not a circle, but an ellipse
and is measured by its eccentricity or how elliptical it is. Eccentricity
fluctuates between 0 and 5% ellipticity through time.
These oscillations, from more elliptic to less elliptic, are of
prime importance to glaciation in that it alters the distance from
the Earth to the Sun. When this changes, it changes the distance
the Sun's short wave radiation must travel to reach Earth's surface
at aphelion and perihelion.
The effect of this radiation variation is to change the seasonal
contrast in the northern and southern hemispheres. When the ellipse
of the orbit is high or is at its peak, the seasonality reaches
~20%. One hemisphere will have hot summers and cold winters; the
other hemisphere will have warm summers and cool winters. When
the orbit is nearly circular, both hemispheres will have similar
seasonal contrasts in temperature. In addition, the more eccentric
the orbit, the longer the length of seasons in each hemisphere
by changing the length of time between the spring (vernal) and
fall (autumnal) equinoxes
This occurs because, as the Earth orbits, it moves farther away
from the sun every 100,000 years. The amount of solar energy reaching
the Earth decreases. The amount of change in radiation is very
small, less than 0.2%, but the variation in radiation this change
causes is of such magnitude that it greatly affects the advance
and retreat of ice sheets.
Presently, we are in a period of low eccentricity (~3%) and this
gives us a seasonal change in solar energy of ~7%.
Figure 1. Variation in the eccentricity of the
Earth's orbit over the last 750,000 years (data from Berger
and Loutre, 1991).
Blue line: eccentricity
Orange line: today's values
The Milankovitch cycles may help explain the advance and retreat
of ice over periods of 10,000 to 100,000 years. They do
not explain what caused the Ice Age in the first place.
ACTIVITY 1 Questions
1. What climate mode is it in?
Obliquity
Changes in the tilt of the Earth's axis is called obliquity. The
Earth's axis is tilted with respect to its orbit around the sun.
It is the inclination of the Earth's axis in relation to its plane
of orbit around the Sun. The tilt varies from between 21.6° and
24.5° in a periodic manner, therefore, changing the inclination
(angle between earth's rotational axis and orbital plane) of the
spin axis, up to 3o.
Because of the periodic variations of this angle, the severity
of the Earth's seasons changes at high latitudes and in the length
of the winter dark period at the poles. With less axial tilt the
Sun's solar radiation is more evenly distributed between winter
and summer. With the small tilt, the winters tend to be milder
and the summers cooler. The smaller the tilt, the less seasonal
variation there is between summer and winter at middle and high
latitudes. Changes in tilt have very little effect on low latitudes.
However, less tilt also increases the difference in radiation
that is revived between the equatorial and polar regions. This
would lead to more glaciation as because it would promote the growth
of ice sheets due to a warmer winter, in which warmer air would
be able to hold more moisture, and subsequently produce a greater
amount of snowfall. In addition, summer temperatures would be cooler,
resulting in less melting of the winter's accumulation.
The effects of tilt on the amount of solar radiation reaching
the Earth are closely linked to the effects of precession. Variation
in these two factors causes radiation changes of up to 15% at high
latitude. Radiation variation of this magnitude greatly influences
the growth and melting of ice sheets.
At present, axial tilt is in the middle of its range.
Figure 2. Variation in the tilt of the Earth's
axis over the last 750,000 years (data from Berger
and Loutre, 1991)
Blue line: traces the tilt.
Orange line: today's value.
ACTIVITY 2 Questions
1. What climate mode is it in?
Precession
The wobbling of the Earth's axis - precession of the Equinoxes
- is like a spinning top that is running-down, wobbling from side
to side. This wobbling is caused by the gravitational pull. In
the case of the top, it is the gravitational pull of the earth
and in the case of the Earth it is the gravitational pull of the
sun, moon, and planets that occurs on a cycle of 22,000 years.
A difference in distance between the Sun and Earth of only 3
% occurs between aphelion (farthest point from Earth to Sun) and
perihelion (closest point). This 3 % difference means that Earth
experiences a 6 % increase in received solar energy in January
than in July.
This 6 % range of variability is not always the case, however.
When the Earth's orbit is most elliptical or eccentric, the amount
of solar energy received at the perihelion would be in the range
of 20 to 30% percent more than at aphelion. Most certainly these
continually altering amounts of received solar energy around the
globe result in prominent changes in the Earth's climate and glacial
regimes. At present the orbital eccentricity is nearly at
the minimum of its cycle. Earth is closest to the Sun in January
and farther away in July. Earth is at perihelion very close to
the winter solstice.
The precession of Earth wobbles from pointing at Polaris (North
Star) to pointing at the star Vega. When this shift to the axis
pointing at Vega occurs, Vega would then be considered the North
Star. Due to precession, the reverse will be true in ~11,000 years.
This will give the Northern Hemisphere more severe winters.
Twice a year the sun is positioned directly over the equator.
These times are called equinoxes.
Currently the equinoxes occur on approximately March 21 and September
21.
These variations become more of an importance because of Earth's
characteristics. The Earth has an asymmetric distribution of land
masses at the moment, with all continents except for Antarctica
located in the Northern Hemisphere. Due to the wobble a climatically
significant alteration must take place. When the axis is tilted
towards Vega the positions of the Northern Hemisphere winter and
summer solstices will coincide with the aphelion and perihelion,
respectively. This means that the Northern Hemisphere will experience
winter when the Earth is furthest from the Sun and summer when
the Earth is closest to the Sun. This coincidence will result in
greater seasonal contrasts. Backward calculations of these
cyclic changes have shown that solar heat maxima and minima occurred
during the Pleistocene. Variation in the Earth's orbit through
time causes changes in the amount and distribution of sunlight
(and other solar radiation) reaching the Earth's surface. These
changes are thought to affect the development of ice sheets.
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Precession of the equinox over the last 750,000 years.
The precession is expressed as the longitude of the perihelion
from the vernal equinox (data are from Berger
and Loutre,1991).
Blue line: the precession
Orange line: today's value.
ACTIVITY 3 Questions
1. What climate mode is it in?
CORRELATION OF THE MILANKOVITCH CYCLES AND PALEOCLIMATE CHANGE
These cycles help explain the advance and retreat of ice over
a period of one hundred thousand years.
Eccentricity is the orbit: round to oblique measured in %
Obliquity: axis measured in tilt ° off normal
Precession of the equinox: % distance or longitude of perihelion
ACTIVITY 4 Questions
- Describe the patterns of peaks or extremes on the graph of
each of the three cycles. What is the trend for each? Is there
a relationship between the peaks? Between their trends? Describe
them.
- Label the y scales for each of the three cycles.
- Why does the graph for Atmosphere CO2 stop at approximately
340 ybp?
- Describe the peaks or extremes on the graph for Atmospheric
CO2 and Global Ice Volume. What is the trend for each?
Is there a relationship between their peaks? Between their trends?
Describe them.
- Is there a relationship between the three cycle peaks and trends
and those of the Atmospheric CO2 and Global Ice Volume?
- What conclusion can you make?
MILANKOVITCH'S 3-CYCLE PHENOMENA WORKING IN CONCERT
Because the periods of these three phenomena have different periods, there
will only be certain times, with a period much longer than any
of the three periods, when all three work in concert to produce
an ice age, or a "hot" age.
When the Northern Hemisphere is farthest from the sun due to precession and
the greatest orbital eccentricity, the summers
are the coolest. Winters are warmest when there is minimum tilt.
When all are in the glacial mode, snow can accumulate on and cover
broad areas of North America and Europe. At present, only
precession is in the glacial mode, with tilt and eccentricity not
favorable to glaciation.
Even when all of the orbital parameters favor glaciation, the
increase in winter snowfall and decrease in summer melt would barely
be enough to trigger glaciation, not to grow large ice sheets. Ice
sheet growth requires the support of positive feedback loops. The
most obvious one is that snow and ice have a much lower albedo
than ground and vegetation. Ice masses tend to reflect more radiation
back into space, thus cooling the climate and allowing glaciers
to expand.
ACTIVITY 5 Questions
1. What climate mode is each in?
Sun Spots and Solar Cycles: Fluctuations
in Solar Energy
 
(1991)The Yohkoh spacecraft's images of the Sun's hot atmosphere
(1994)
The sun is a star and radiates its energy to the apple-peel thickness
of the atmosphere that absorbs and reflects its rays. Solar flares
appear as bright spots and creates a magnetic field. The energy
at the centre of the magnetic field is so strong that it blocks
the flow of radiation through the field and creates cooling. The
cool, dark regions are sunspots.
The sun's energy is known to fluctuate periodically with solar
flares and sunspots. There is a good correlation between Northern
Hemisphere land temperature and the sunspot cycle reflected in
records of the last 100 years. At a time of numerous sunspots,
the atmosphere was unruly. As the sunspot count dwindled the activity
diminished. Large, dark, relatively cool areas became conspicuous
in the X-ray images. These are called coronal holes, and are the
source of a fast solar wind.
A solar flare is an explosion on the Sun that happens when energy
stored in twisted magnetic fields (usually above sunspots) is suddenly
released. Flares produce a burst of radiation across the electromagnetic
spectrum, from radio waves to x-rays and gamma-rays. Scientists
classify solar flares according to their x-ray brightness in the
wavelength range 1 to 8 Angstroms. There are three categories:
- X-class flares are big; major events that can trigger planet-wide
radio blackouts and long-lasting radiation storms.
- M-class flares are medium-sized and generally cause brief radio
blackouts that affect Earth's polar regions.
- C-class flares are small with few noticeable consequences on
Earth compared to X- and M-class events. These minor radiation
storms sometimes follow an M-class flare.
Solar cycles average 11.5 years and relate to total energy output
of sun. Lower temperatures in the late nineteenth and early twentieth
centuries coincided with relatively long sunspot cycle (11.5 years).
From 1910 the length of sunspot cycle decreased erratically to
9.7 years by 1989, Northern Hemisphere land and sea temperatures
increased erratically by 0.5oC in that interval. Variations in
the sun's radiation affects Earth's climate. Solar cycles are
used to forecast seasonal climate forcing in W/m2.
The green row lists solar influences at the top of the earth's
atmosphere. The blue blocks list known immediate effects of solar
radiation and its variations. These effects have not been fully
quantified. The blocks in red list possible secondary and tertiary
effects. These effects have not been investigated in any depth.
Several investigations have shown positive correlations of solar
influences on climate change. The hydrological system is a major
player in the weather system because 70% of the Earth's surface
is covered by water.
SOLAR CYCLES
Fluctuations in Solar Energy
A change of one year in the length of the solar cycle brings springtime
one week earlier or later. From the historical data (narrow line)
some researchers saw fluctuations that closely matched their reconstruction
(thick line) of the speeding and slowing of the Sun's magnetic
motor in past centuries (the cycle length).
ACTIVITY 6 Questions
- What is the relationship between temperature and springtime
arrivals?
- Describe the patterns of fluctuation between early and late
springtimes up to 1985. Is there a trend?
- In 1600 and 1611, what is the length of the cycle?
- Describe the patterns of fluctuations and length of the solar
cycles up to 1985. Is there a trend? When did it change?
- Predict springtime arrival for 2000.
SOLAR IRRADIANCE, AND TEMPERTURE
A causal relationship between sunspot activity, solar irradiance,
and temperature has been difficult to establish evidence of a
solar warming. The first clear sign that the Sun was responsible
for recent changes in climate came from the close match between
changes in the length of the sunspot cycle (thick line) and mean
temperatures on land in the northern hemisphere (narrow line).
ACTIVITY 7 Questions
- Explain the pattern of solar cycle lengths from 1860 to 1985.
- Explain the pattern of temperature anomaly (change from normal) in the
Northern Hemisphere from 1860 to 1985.
- Compare the two patterns. Is there a correlation?
- What is the trend? Is there a predictable pattern?
- What predictions for solar cycles and temperatures can you
make for 2000, 2020, 2040 ?
SOLAR FLARES
Class
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Peak (W/m2) between 1 and 8 Angstroms
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B
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I < 10-6
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C
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10-6 < = I < 10-5
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M
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10-5 < = I < 10-4
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X
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I > = 10-4
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This figure shows a series of solar flares detected by NOAA satellites
in July 2000: Scientists classify solar flares according to their
x-ray brightness in the wavelength range 1 to 8 Angstroms. There
are three categories and each x-ray flare has nine subdivisions
ranging from 1 - 9: C1 - C9, M1 - M9, and X1 - X9.
The frequency of flares coincides with the Sun's 11-year cycle.
When the solar cycle is at a minimum, active regions are small
and rare and few solar flares are detected. These increase in number
as the Sun approaches the maximum part of its cycle.
ACTIVITY 8 Questions
- In this figure, the three indicated flares registered (from
left to right) X2, M5, and X6.
- Which x-ray flare do you think triggered a radiation storm
around Earth nicknamed the Bastille Day event? Why?
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