Earth's Energy Budget (Week 3) - Post 1
Earth's Energy Budget- Notes
Overview
Section 3 will follow energy
through the climate system. There are essentially three main factors to
consider: incoming energy from the Sun, reflection of solar energy, and the
greenhouse effect. We’ll have a look at Earth’s energy budget, which includes
these big three plus some additional processes, and we’ll have a look at
balances and imbalances in energy flows. Understanding solar energy,
reflection, and the greenhouse effect are crucial for designing effective
mitigation efforts.
3.1 Notes
The
Electromagnetic Spectrum
The electromagnetic
spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and
their respective wavelengths and photon
energies. This frequency range is divided into separate bands, and the electromagnetic waves within
each frequency band are called by different names. The electromagnetic waves in each of these bands have
different characteristics, such as how they are produced, how they interact
with matter, and their practical applications.
Ionizing Radiation- Gamma
rays, X-rays, and high Ultraviolet light: Their photons have enough energy to
ionize (ionization
is the process by which an atom or a molecule acquires a negative or positive
charge by gaining or losing electrons to form ions atoms) causing chemical
reactions. Exposure to these rays can be a health hazard, causing radiation
sickness, DNA damage and cancer.
Nonionizing Radiation- Visible
light wavelengths and lower: they cannot cause the effects of ionizing
radiation.
Spectroscopy- A technique that is used
to physically separate waves of different frequencies, producing a spectrum
showing the constituent frequencies. Its used to study the interactions of
electromagnetic waves with matter.
Range of
Spectrum
Wavelength ( λ )- The
spatial period of the wave, or the distance over which the wave’s shape
repeats. It’s usually determined by considering the distance between
consecutive corresponding points of the same phase (such as crests or troughs).
Frequency ( f
)- The number of occurrences of a repeating event per unit of
time.
Photon Energy (
E )- The energy carried by a single photon. Photon energy is
solely a function of the photon’s wavelength. Other factors, such as the
intensity of the radiation, do not affect photon energy.
Wavelength, Frequency, and Photon Energy
Relationships:
+Wavelength and Frequency are inversely proportional: waves
with higher frequencies have shorter wavelengths, and lower frequencies have
longer wavelengths. So, gamma rays on the far end of the EM spectrum
(wavelengths) have tiny wavelengths, while radio waves on the opposite end of
the spectrum have huge wavelengths.
+Photon Energy and Frequency are directly proportional, so
gamma ray photons have the highest energy, while radio wave photons have very
low energy. The amount of photon energy is also inversely proportional to the
wavelength.
These
relationships are illustrated within the equations below:
f= c/ λ f=
E/h
E= h*c/ λ
c = 299792458 m/s = The speed of
light in a vacuum
h =
6.62606896(33)×10−34 J·s
= 4.13566733(10)×10−15 eV·s = Planck’s Constant
Types of
Radiation
Bands of The EM Spectrum Climate Science Concerns:
Infrared Radiation-
The infrared part of
the electromagnetic spectrum covers the range from roughly 300 GHz to 400
THz (1 mm - 750 nm). It can be divided into three parts:
Far-infrared- from 300 GHz to 30 THz (1 mm – 10
μm). This radiation is typically absorbed by so-called rotational modes in
gas-phase molecules, by molecular motions in liquids, and by phonons in
solids. The water in Earth's atmosphere absorbs so strongly in this range that
it renders the atmosphere in effect opaque. However, there are certain
wavelength ranges ("windows") within the opaque range that allow
partial transmission and can be used for astronomy.
Mid-infrared- from 30 to 120 THz (10–2.5 μm). Hot objects (black-body
radiators) can radiate strongly in this range, and human skin at normal body
temperature radiates strongly at the lower end of this region. This radiation
is absorbed by molecular vibrations, where the different atoms in a molecule
vibrate around their equilibrium positions.
Near-infrared- from 120 to 400 THz (2,500–750 nm).
Physical processes that are relevant for this range are similar to those for
visible light. The highest frequencies in this region can be detected directly
by some types of photographic film, and by many types of solid state image
sensors for infrared photography and videography.
Visible Radiation-
Above infrared in frequency comes visible
light. The Sun emits
its peak power in the visible region, although integrating the entire emission
power spectrum through all wavelengths shows that the Sun emits slightly more
infrared than visible light. By definition, visible light is the part of the EM
spectrum the human eye is the most sensitive to…
The
light that excites the human visual
system is a very small portion of the electromagnetic spectrum. A rainbow shows
the optical (visible) part of the electromagnetic spectrum; infrared (if it
could be seen) would be located just beyond the red side of the rainbow with ultraviolet
appearing just beyond the violet end… Electromagnetic radiation with a wavelength between
380 nm and
760 nm (400–790 terahertz) is detected by the human eye and perceived as
visible light. Other wavelengths, especially near infrared (longer than 760 nm)
and ultraviolet (shorter than 380 nm) are also sometimes referred to as
light, especially when the visibility to humans is not relevant.
Ultraviolet Radiation-
UV is the longest wavelength radiation whose
photons are energetic enough to ionize atoms,
separating electrons from
them, and thus causing chemical
reactions. Short wavelength UV and the shorter wavelength radiation
above it (X-rays and gamma rays) are called ionizing radiation, and
exposure to them can damage living tissue, making them a health hazard. UV can
also cause many substances to glow with visible light; this is called fluorescence… At the middle range of UV, UV
rays cannot ionize but can break chemical bonds, making molecules unusually
reactive. Sunburn, for
example, is caused by the disruptive effects of middle range UV radiation on skin cells, which
is the main cause of skin cancer… The Sun emits significant UV
radiation (about 10% of its total power), including extremely short wavelength
UV that could potentially destroy most life on land (ocean water would provide
some protection for life there). However, most of the Sun's damaging UV
wavelengths are absorbed by the atmosphere before they reach the surface.
Video
Lesson- Energy from the Earth and Sun
The Sun is really hot, because its hot, it gives off lots of
energy per second, per square meter—much, much more than the Earth gives off.
Also, because its hot, the sun gives off radiation at visible wavelengths, and
the Earth, because its cooler, gives off radiation at infrared wavelengths. —
The primary reason to spend time on these relationships is so we can use them,
when we have to look the different types of energy transferring around within
Earth’s climate system.
Key Points-
+The Sun gives off shortwave (High photon energy) radiation
with a peak wavelength in the visible part of the electromagnetic spectrum.
+The Earth gives off long wave (Low photon energy) radiation
with a peak wavelength in the infrared part of the electromagnetic spectrum.
+The general relationships are that
-Hotter
objects emit more radiation than cooler objects
-Hotter
objects emit radiation at shorter wavelengths than cooler objects.
+Coming up: These differences between radiation from the Sun
and radiation from the Earth are crucial to understanding the greenhouse
effect.
3.2 Notes
Ins and
Outs of Earth’s Energy Budget:
Here we’ll examine the main processes by which energy flows
in, out, and within Earth’s climate system. We’ll spend some time with an
iconic figure showing Earth’s annual energy budget, and examine it from a stock
and flow perspective.
Learning Goals:
By the end of this section, you will be able to:
+Describe how incoming and outgoing electromagnetic radiation
interacts with Earth’s surface and its atmosphere.
+Balance energy budgets for the top of the atmosphere, the
atmosphere, and Earth’s surface.
+Predict how changes in incoming solar energy, greenhouse
gases, and albedo will affect a planet’s mean surface temperature.
Video
Lesson
*Fundamentally, any mitigation actions we choose to take have
to align with the energy flows talked about below.
Key
Points
+Shortwave (Hot) radiation from the Sun is the ultimate source
of energy for Earth’s climate system.
+About 30% of incoming solar radiation gets reflected directly
back to space.
+Earth absorbs energy from two sources: incoming solar energy
and energy re-emitted back toward the Earth by greenhouse gasses.
+Earth therefore heats up to a temperature at which it can
radiate enough energy such that its energy outflow will balance its energy
inflow.
+The
planet is currently out of balance, with inflow exceeding outflow
+Earth’s temperature will rise until inflow and outflow are
back in balance. (Amplifying Feedback Look)
Readings
Sunspots
Sunspots are darker, cooler areas
on the surface of the sun in a region called the photosphere. The photosphere
has a temperature of 5,800 degrees Kelvin. Sunspots have temperatures of about
3,800 degrees K. They look dark only in comparison with the brighter and hotter
regions of the photosphere around them.
They are caused by interactions
with the Sun's magnetic field which are not fully understood. But a sunspot is
somewhat like the cap on a soda bottle: shake it up, and you can generate a big
eruption. Sunspots occur over regions of intense magnetic activity, and when
that energy is released, solar flares and big storms called coronal mass
ejections erupt from sunspots.
(Pages
11-12)
How do we
know the current warming trend isn’t caused by the sun?
Because the Sun’s output has a
strong influence on Earth’s temperature, scientists have examined records of solar
activity to determine if changes in solar output might be responsible for the
observed global warming trend… The most direct measurements of
solar output are satellite readings, which have been available since 1979.
These satellite records show that the Sun’s output has not shown a net increase
during the past 30 years and thus cannot be responsible for the warming during
that period.
Further evidence that current
warming is not a result of solar changes can be found in the temperature trends
in the different layers of the atmosphere. These data come from two sources: weather
balloons, which have been launched twice daily from hundreds of sites worldwide
since the late 1950s, and satellites, which have monitored the temperature of
different layers of the atmosphere since the late 1970s. Both datasets have
been heavily scrutinized, and both show a warming trend in the lower layer of
the atmosphere (the troposphere) and a cooling trend in the upper layer (the
stratosphere). This is exactly the vertical pattern of temperature changes expected
from increased greenhouse gases, which trap energy closer to the Earth’s surface.
3.3 Notes
About 30% of the solar energy
that arrives at our planet gets reflected back to space nearly immediately.
Here we’ll look at some of the important reflective materials in Earth’s
climate system. Reflectivity is involved in some important feedback
loops in the climate system, and some mitigation strategies involve
deliberately influencing reflectivity.
Learning Goals
By the end of this section, you will be able to:
+Explain how reflection fits into Earth’s overall energy
budget.
+Rank different surfaces based on their reflectivity (e.g.
deserts, forests, clouds, ice, ocean water etc.).
+Predict how particular changes in the reflective properties
of Earth’s surface or atmosphere would likely affect Earth’s temperature.
+Construct feedback loops involving reflectivity.
Readings
Albedo is a measure of how
reflective a surface is. It is a measure of the proportion of the incoming
solar radiation that is reflected by the surface back into the atmosphere and
space… The
more reflective a surface is the higher the albedo value. Very white surfaces,
such as fresh snow, reflect a very high fraction of incoming radiation back to
space. Darker surfaces such as water, forests or asphalt have a much lower
albedo and more of the sun's energy is absorbed… Much of the planet each day is
covered by cloud which will have a different albedo to the surface. White
clouds reflect more sunlight than water and darker land surfaces. Tiny
particles in our atmosphere such as dust, smoke and pollen (known as
'aerosols') also affect the overall reflection of incoming solar radiation.
* In a weather and climate context we often refer to
the Earth's surface albedo for the reflectivity of the surface, and the
planetary albedo for the reflectivity of the Earth when viewed from space,
including the effect of the atmosphere, clouds and aerosols.
Albedo is a very important factor
in both weather and climate. The amount of sunlight that is absorbed or
reflected is a driving force of our weather… Changes in the albedo of
the Earth's surface, caused by changes in land surface type affect the Earth's
'energy budget'. If less energy is reflected, then there will be a warming
effect. This can lead to feedbacks, as if there is extensive snow melt, more
energy will be absorbed, leading to more surface heating and more snow melt. On
a large scale this has implications for our climate.
Tutorial
How to
calculate the overall albedo for a region?
Various parts of each region all
have a different albedo value. The total albedo value is the weighted averages
of the albedo values for each contributing part.
(Depending on the choppiness of
the ocean, it reflects between 5% and 30% of the heat energy it receives; so,
the general albedo value for the ocean is 0.10. All albedo values are expressed
between 0 and 1.)
( (area x
albedo) + (area x albedo) + (area x albedo) )/ total area = Regional Albedo
*Note-
The albedo is the amount of the sun’s energy reflected away from the surface;
an important aspect is that is changes with the angle that the sun’s rays
strike the surface. Using the ocean as an example, when the sun is high in the
sky, there is less reflection and hence reduced albedo (about 0.10), but when
the sun is low on the horizon, more of its energy is reflected by the water’s
surface and albedo is greater (up to 0.33).
Video
Lesson- Reflection of Incoming Solar Radiation
-Earth’s
reflectivity can influence and be influenced by other parts of the climate
system. -Amplifying
system- the response helps push the system in the same direction as the
original perturbation
-Reflectivity
issues in the climate- just like in other parts of the climate system, it’s
changes in the stocks of things that can influence energy flows. If the stocks
of clouds, or reflective particles, or forests, or ocean water changes, the
amount of energy reflected directly back to space will also change and will
alter earth’s energy balance.
Key Points
+Changes in Earth’s surface
toward more reflective materials promote cooling; changes toward less
reflective materials promote warming.
+On Earth’s surface, changes in
vegetation cover and land use can alter reflectivity.
+In the atmosphere, changes in
cloud cover and the stock of reflective aerosols can change Earth’s overall
reflectivity.
+Reflection is involved in
important feedback loops in the climate system.
+Feedbacks involving clouds are
not well-constrained, but recent research suggests that cloud feedback in a
warming world are likely to be net amplifying.
-Aerosols: A suspension of fine solid particles or liquid droplets in a gas
-Stratospheric sulfur aerosols are sulfur-rich particles which
exist in the stratosphere region of the Earth's atmosphere. The
layer of the atmosphere in which they exist is known as the Junge layer, or
simply the stratospheric aerosol layer.
- When
present in high levels, e.g. after a strong volcanic eruption such as Mount
Pinatubo, they produce a cooling effect, by reflecting sunlight, and
by modifying clouds as they fall out of the stratosphere. This cooling may
persist for a few years before the particles fall out.
- The
relative influence of volcanoes on the Junge layer varies considerably
according to the number and size of eruptions in any given time period, and
also of quantities of sulfur compounds released. Only stratovolcanoes
containing primarily granitic rocks
are responsible for these fluxes, as basaltic rock
erupted in shield volcanoes doesn't result in plumes
which reach the stratosphere.
- The IPCC
AR4 says explosive volcanic events are episodic, but the stratospheric
aerosols resulting from them yield substantial transitory perturbations to the
radiative energy balance of the planet, with both shortwave and longwave
effects sensitive to the microphysical characteristics of the aerosols.
-Understanding
of these aerosols comes in large part from the study of volcanic
eruptions, notably Mount
Pinatubo in the Philippines… The formation of the aerosols and their effects on the
atmosphere can also be studied in the lab. Samples of actual particles can be
recovered from the stratosphere using balloons or aircraft. Computer
models can be used to understand the behavior of aerosol particles and are
particularly useful in modelling their effect on global climate.
-General Process: It has been established
that emission of precursor gases for sulfur aerosols is the principal mechanism
by which volcanoes cause episodic global cooling. The Intergovernmental Panel on Climate Change AR4 regards stratospheric sulfate aerosols
as having a low level of scientific understanding. The aerosol particles form a whitish haze in the sky. This creates a global dimming effect, where less of the sun's radiation
is able to reach the surface of the Earth. This leads to a global cooling effect. In essence, they act as the
reverse of a greenhouse gas, which tends to allow visible light from
the sun through, whilst blocking infrared light emitted from the Earth's surface and its
atmosphere. The particles also radiate infrared energy directly, as they lose
heat into space.
- Creating
stratospheric sulfur aerosols deliberately is a proposed geoengineering technique which offers a
possible solution to some of the problems caused by global
warming. However, this will not be without side effects and it has
been suggested that the cure may be worse than the disease.
3.4 Notes
The greenhouse effect is the
reason planet Earth’s average surface temperature is above freezing. Particular
gases (greenhouse gases) in the atmosphere have chemical structures that allow
them to absorb, then re-emit infrared radiation coming from Earth’s surface.
Here we’ll examine the basics of greenhouse gases and how the greenhouse effect
works, which helps us understand why human additions of greenhouse gases to the
atmosphere cause warming. Influencing flows of greenhouse gases to and from the
atmosphere are another place to look for mitigation options.
-Learning Goals-
By the end of this section, you will be able to:
+Identify greenhouse gases; identify non-greenhouse gas air
molecules.
+Contrast the molecular structure of greenhouse gases and
non-greenhouse gases.
+Describe how greenhouse gases themselves absorb and emit
radiation, including what kinds of radiation (shortwave or longwave).
+Explain how the greenhouse effect warms Earth in terms of
energy flows.
+Describe feedbacks involving the greenhouse effect.
Reading
-The atmosphere of Earth is the layer of gases,
commonly known as air, that surrounds the planet Earth and is retained by Earth's
gravity.
- By volume, dry air contains 78.09% nitrogen,
20.95% oxygen,[2]
0.93% argon,
0.04% carbon dioxide, and small
amounts of other gases. Air also contains a variable amount of water vapor,
on average around 1% at sea level, and 0.4% over the entire atmosphere.
-The three major constituents of air, and therefore of Earth's
atmosphere, are nitrogen, oxygen, and argon. Water vapor accounts for roughly 0.25% of the
atmosphere by mass. (The concentration of water vapor (a greenhouse gas) varies
significantly from around 10 ppm by volume in the coldest portions of the
atmosphere to as much as 5% by volume in hot, humid air masses, and
concentrations of other atmospheric gases are typically quoted in terms of dry
air (without water vapor))
-Air content and atmospheric pressure vary at different
layers, and air suitable for use in photosynthesis
by terrestrial plants and breathing
of terrestrial animals is found only in Earth's troposphere
and in artificial atmospheres.
-The remaining gases are often referred to as trace gases,[6]
among which are the greenhouse gases, principally carbon dioxide,
methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many
other chemical compounds. Many substances of natural
origin may be present in locally and seasonally variable small amounts as aerosols
in an unfiltered air sample
Major constituents
of dry air, by volume
|
|||
Gas
|
Volume
|
||
Name
|
Formula
|
in ppmv
|
in %
|
N2
|
780,840
|
78.084
|
|
O2
|
209,460
|
20.946
|
|
Ar
|
9,340
|
0.9340
|
|
CO2
|
400
|
0.04
|
|
Ne
|
18.18
|
0.001818
|
|
He
|
5.24
|
0.000524
|
|
CH4
|
1.79
|
0.000179
|
|
Not
included in above dry atmosphere:
|
|||
H2O
|
10–50,000
|
0.001%–5%
|
|
(PPMV- Parts Per Million by Volume)
|
|||
Computer
Simulation Activity
In this
posting, we consider the interaction between air molecules, including Nitrogen
(N2), Oxygen (O2), Water Vapor (H2O) and Carbon Dioxide (CO2), with Photons of
various wavelengths. This may help us visualize how energy, in the form of
Photons radiated by the Sun and the Surface of the Earth, is absorbed and
re-emited by Atmospheric molecules.
Video
Lesson
-Greenhouse gasses are those that absorb and re-emit infrared radiation
coming from earth’s surface
+Greenhouse gases are those that can absorb and re-emit
infrared radiation in the wavelength range of energy emitted by Earth’s surface.
+The major constituents in our atmosphere, N2 and 02, are NOT
greenhouse gasses (because of chemical structure)
+When greenhouse gases emit infrared radiation, they emit it
in random directions.
-Some goes back toward Earth’s surface and gets re-absorbed
-Some goes toward space
-Some goes to another greenhouse gas molecule which repeats
the absorption and re-emission and sends the energy in another direction.
+Greenhouse gases slow the passage of infrared energy from
Earth’s surface to space, warming the planet.
Reading
-AIRS is the first instrument to distinguish differences in
the amount of water vapor at all altitudes within the troposphere. Using data
from AIRS, the team observed how atmospheric water vapor reacted to shifts in
surface temperatures between 2003 and 2008. By determining how humidity changed
with surface temperature, the team could compute the average global strength of
the water vapor feedback.
-“This new data set shows that as surface temperature
increases, so does atmospheric humidity,” Dessler said. “Dumping greenhouse
gases into the atmosphere makes the atmosphere more humid. And since water
vapor is itself a greenhouse gas, the increase in humidity amplifies the
warming from carbon dioxide."
-Specifically, the team found that if Earth warms 1.8 degrees
Fahrenheit, the associated increase in water vapor will trap an extra 2 Watts
of energy per square meter (about 11 square feet).
-"That number may not sound like much but add up all of
that energy over the entire Earth surface and you find that water vapor is
trapping a lot of energy," Dessler said. "We now think the water
vapor feedback is extraordinarily strong, capable of doubling the warming due
to carbon dioxide alone."
-Because the new precise observations agree with existing
assessments of water vapor's impact, researchers are more confident than ever
in model predictions that Earth's leading greenhouse gas will contribute to a
temperature rise of a few degrees by the end of the century.
-Climate models (Computer simulations of the climate system)
are designed to simulate the responses and interactions of the oceans and
atmosphere, and to account for changes to the land surface, both natural and
human-induced. They comply with fundamental laws of physics—conservation of
energy, mass, and momentum—and account for dozens of factors that influence
Earth’s climate.
-Changes to one part of the climate system can cause
additional changes to the way the planet absorbs or reflects energy. These
secondary changes are called climate
feedbacks, and they could more
than double the amount of warming caused by carbon dioxide alone. The
primary feedbacks are due to snow and ice, water vapor, clouds, and the carbon
cycle.
-Water Vapor: The
largest feedback is water vapor. Water vapor is a strong greenhouse gas. In
fact, because of its abundance in the atmosphere, water vapor causes about
two-thirds of greenhouse warming, a key factor in keeping temperatures in the
habitable range on Earth… As temperatures warm, the atmosphere becomes
capable of containing more water vapor, and so water vapor concentrations go up
to regain equilibrium. Will that trend hold as temperatures continue to warm?...
The
amount of water vapor that enters the atmosphere ultimately determines how much
additional warming will occur due to the water vapor feedback. The atmosphere
responds quickly to the water vapor feedback….The amount of water vapor
that enters the atmosphere ultimately determines how much additional warming
will occur due to the water vapor feedback. The atmosphere responds quickly to
the water vapor feedback… If this trend continues, and many models say
that it will, water vapor has the capacity to double the warming caused by
carbon dioxide alone.
-Clouds: Scientists
aren’t entirely sure where and to what degree clouds will end up amplifying or
moderating warming, but most climate
models predict a slight overall positive feedback or amplification of warming
due to a reduction in low cloud cover.
-The Carbon Cycle: For
now, primarily ocean water, and to some extent ecosystems on land, are taking
up about half of our fossil fuel and biomass burning emissions. This behavior
slows global warming by decreasing the rate of atmospheric carbon dioxide
increase, but that trend may not continue. Warmer ocean waters will hold less
dissolved carbon, leaving more in the atmosphere… The
impact of climate change on the land carbon cycle is extremely complex, but on
balance, land carbon sinks will become less efficient as plants reach
saturation, where they can no longer take up additional carbon dioxide, and
other limitations on growth occur, and as land starts to add more carbon to the
atmosphere from warming soil, fires, and insect infestations. This will result
in a faster increase in atmospheric carbon dioxide and more rapid global
warming. In some climate models, carbon cycle feedbacks from both land and
ocean add more than a degree Celsius to global temperatures by 2100.
-Emission Scenarios: Scientists
predict the range of likely temperature increase by running many possible
future scenarios through climate models… the most significant source of
uncertainty in these predictions is that scientists don’t know what choices
people will make to control greenhouse gas emissions. The higher estimates are
made on the assumption that the entire world will continue using more and more
fossil fuel per capita, a scenario scientists’ call “business-as-usual.” More
modest estimates come from scenarios in which environmentally friendly
technologies such as fuel cells, solar panels, and wind energy replace much of
today’s fossil fuel combustion. These considerations mean that
people won’t immediately see the impact of reduced greenhouse gas emissions. Even if greenhouse gas concentrations
stabilized today, the planet would continue to warm by about 0.6°C over the
next century because of greenhouses gases already in the atmosphere.
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