Chapter 3: cause of climate change Flashcards
Climate changes whenever
the earth’s energy balance (balance between incoming radiation from the Sun and outgoing terrestrial radiation from the Earth) is disturbed.
climate forcing
Any process that can disturb the earth’s energy balance and forces the climate to change,
Climate forcing can be separated into
internal and external types
External forcing
operates from outside the Earth’s climate system, and includes changes in the global energy balance due to variations in the Earth’s orbit around the Sun, and changes in the amount of energy coming from the Sun.
Internal forcing
operates from within the climate system, for example the change in the global energy balance due to changes in the composition of the atmosphere.
Thus, external forcing is mainly because of
natural processes
internal forcing may be due to
either natural (e.g., volcanic eruptions) or anthropogenic (e.g., changes in atmospheric composition) processes
In general, climate changes prior to the Industrial Revolution can be explained by
natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations.
Recent climate changes, however, cannot be explained by natural causes alone. Research indicates that natural causes are very unlikely to explain most observed warming, especially warming since the mid‐20th century
Rather, human activities can very likely explain most of that warming
compare between models
Models that account only for the effects of natural processes are not able to explain the warming over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.
External causes include changes in
the global energy balance due to:
- orbital variations and
- changes in the solar activity
“Milankovitch Cycles (orbital variations)”
On timescales of a millennium and longer, changes in the character of the Earth’s orbit around the Sun can significantly affect the seasonal and latitudinal distribution of incoming solar energy
“Milankovitch Cycles (orbital variations)” The change in energy receipt can amount to
10% or more in certain locations
The Milankovitch Cycles force the changes between
ice age and warmer conditions on Earth, on time scales of 10,000 to 100,000 years
The last Ice Age occurred ……………….. years ago
18,000
Thus, ………………………………………… can be best explained by Milankovitch’s Astronomical theory
fluctuations in the Earth’s orbit which can upset the energy balance of the climate system
The Milankovitch’s Astronomical theory is based on
three variations in the position of Earth relative to the Sun:
- Tilt of the Earth’s axis
- Eccentricity of the Earth’s orbit
- Orbital precession
Each of these variations has its specific time period and affect the earth’s energy balance, thus forcing the climate to change.
Tilt of the Earth’s axis (Earth’s obliquity)
- Today the Earth is tilted on its rotational axis at an angle of 23.5° relative to a perpendicular to the orbital plane of Earth.
- The tilt varies between 21.5o and 24.5o over a period of 41,000 years.
- Obliquity does not influence the total amount of solar radiation received by the Earth, but it affects the distribution of insolation in space and time
Increasing tilt amplify
the strength of the seasons, especially at the poles.
- As obliquity increases, the amount of solar radiation received at high latitudes increases during summers and decreases during winters.
- Changes in obliquity have little effect at low latitudes, since the strength of the effect decrease towards the equator.
the strength of the seasons, especially at the poles.
- As obliquity increases, the amount of solar radiation received at high latitudes increases during summers and decreases during winters.
- Changes in obliquity have little effect at low latitudes, since the strength of the effect decrease towards the equator.
consequently, variations in the earth’s axial tilt affect the
strength of the latitudinal temperature gradient.
Increased tilt has the effect of
raising the annual receipt of solar energy at high latitudes, with a consequent reduction in the latitudinal temperature gradient
Eccentricity of the Earth’s orbit
The Earth’s orbit has been found to vary from being near circular (e = 0.005) to markedly elliptical (e = 0.06) over periodicities of approximately 100,000 and 413,000 years.
Variations in eccentricity influence
the total amount of solar radiation incident at the top of the Earth’s atmosphere at perihelion and aphelion.
For example, the present ellipticity of the Earth’s orbit with a 3% difference in distance causes a
6% difference in solar energy between perihelion and aphelion.
With maximum ellipticity, the energy received at ………….. would be …………….. % greater than that at ……………..
perihelion
20% to 30%
aphelion
Orbital precision (precision of equinoxes)
Due to the gravitational interaction of other planets, primarily the Moon and Jupiter, the perihelion moves in space with a consequent shifting or precession of the elliptical orbit.
This precession causes
the equinoxes to move around Earth’s orbit, completing on full 360o orbit around the Sun every 23000 years.
Effects of the precision of equinoxes
- Like obliquity, precession does not affect the total amount of solar energy received by the Earth, but only its hemispheric distribution over time.
- Due to orbital precision, after 11,000 years NHS summer occur at perihelion with the north pole is tilted towards the Sun. This results in more hotter summers.
Sunspots are
cooler areas on the sun’s surface (the photosphere), which are associated with strong local magnetic activity
faculae
associated with sunspots are the brighter and hotter areas called faculae, which increase the solar irradiance.
sunspot cycles
The periodic changes (11 or 22 years) in the sunspot number
The total solar energy flux, is related to the sunspot cycle as follows:
Internal causes include
changes in the global energy balance due to:
- volcanic activity (natural) and
- changes in atmospheric composition (natural and anthropogenic)
Volcanic activity is an example of
internal (natural) climate forcing
Explosive volcanic eruptions can
inject large quantities of dust and sulphur dioxide, in gaseous form, into the upper atmosphere, the stratosphere, where the sulphur dioxide is rapidly converted into sulphuric acid aerosols.
- Volcanic pollution (from smaller eruptions) in the lower atmosphere is removed within days by rain
- However, the volcanic dust and aerosols in the stratosphere may remain for several years, gradually spreading over much of the globe
Volcanic pollution results in
a 5 to 10% reduction in the direct solar beam, largely through scattering as a result of the highly reflective sulphuric acid aerosols.
Large eruptions, such as the Mount Pinatubo (Philippines) eruption in 1991, can bring about
a global cooling of up to 0.3°C lasting for up to 2 years.
The changing composition of the atmosphere, particularly its ……………………….. is a well‐known example of internal climate forcing
greenhouse gas content
Greenhouse gases absorb
terrestrial, long‐wave radiation from the Earth and reemit this radiation up to space and down to the surface.
• The increase in the amount of downward energy warms the surface creating the greenhouse effect.
A change in the greenhouse gas content of the atmosphere will affect
the energy balance of the climate system. For example, if the amount of carbon dioxide is increased, more out‐going radiation will be trapped in the atmosphere.
A change in the greenhouse gas content of the atmosphere will affect the energy balance of the climate system. For example, if the amount of carbon dioxide is increased, more out‐going radiation will be trapped in the atmosphere.
To restore the energy balance between energy coming from the Sun and energy leaving the Earth,
the temperature of the atmosphere rises
To restore the energy balance between energy coming from the Sun and energy leaving the Earth, the temperature of the atmosphere rises.
This enhances
the Earth’s natural greenhouse effect. Changes in the concentrations of greenhouse gases can occur in numerous ways.
- Natural changes in the carbon dioxide content of the atmosphere occurred at the end of the last Ice Age in response to the Milankovitch Cycles.
- Mankind, through energy generation, changing land use and other processes, has produced a substantial change in the atmospheric composition over most recent centuries, causing a major shift in global climate (global warming).
