Chinese coal miner. China mines 4 billion
tons of coal each year, half of the world’s total. Besides contributing
to climate change, air pollution from coal burning kills roughly 0.5 to
1 million people in China each year. Photograph by Natalie Behring /
Alamy.
Part 1: What is the greenhouse effect? An accessible, scientific
introduction
Click on images to zoom. An older version of this series is also
available as a single pdf.
Introduction
Growing up, I understood that it gets warmer in
the summer because there was “excess” sunlight warming the Earth,
and cooler in winter because there was “insufficient” sunlight1There
is a subtlety here that I did not notice at the time: does it get warmer
in summer because the excess sunlight causes more warming to happen each
day than cooling happens in the night, or does it get warmer because the
days get longer and therefore the equilibrium temperature of each day is
higher than the previous? This is similar to asking on what time scale
the Earth system returns to equilibrium – yearly, or daily. The
(incomplete) answer is that the former effect is arguably dominant, as
can be seen by observing that the hottest day of the year is usually
about 1 to 2 months after summer solstice, and thus the days continue to
get hotter even when they are getting shorter. More specifically, if we
approximate the Earth’s temperature at a point as an exponential decay
to equilibrium, subject to a sinusoidal forcing, then we find that the
temperature is sinusoidal with a phase offset from the forcing such that
the tangent of the phase offset equals the ratio of the decay timescale
to the forcing period. Taking the phase offset to be 1.5 months of a 12
month year, the ratio is , so the time scale for the
Earth to reach equilibrium is about 1 forcing period, i.e., 1
year.. However I was confused that these effects happened
to cancel each other exactly: if there was any tendency for the summer
warming to exceed the winter cooling on average, however slightly, then
year after year the temperature would steadily increase unendingly. Why
doesn’t this happen? Furthermore, I did not even think to wonder what
“excess” or “insufficient” sunlight could even mean: surely any amount
of sunlight would warm the Earth – by what process was the Earth cooling
in winter?
Like many students in the US, the only time (until graduate school) I
was taught about the Earth’s climate, climate change, or the greenhouse
effect was in grade school. At that age, students are often not prepared
to engage with such questions, much less have the necessary background
in physics to answer them; and if we do not understand the processes by
which the Earth warms and cools, then we will not understand how the
greenhouse effect impacts those processes no matter how well, or poorly,
it is taught.2As an
example, here
is a lesson plan designed by NASA in which students perform an
experiment where greenhouse gases are “simulated” using plastic wrap –
ironically how greenhouses work, but having nothing to do with the
greenhouse effect. Here
is an experiment for high schoolers in which students measure the
temperature of bags with and without carbon dioxide (which is generated
in situ by an exothermic chemical reaction). Neither of these plans
contain any information related to the greenhouse effect.
Many of these students have become adults who see the words “climate
change” and “greenhouse effect” increasingly often in news media and
policy discussion, without realizing that their grasp of these concepts
is at best superficial.
The goal of this series is to better introduce the reader to the
greenhouse effect by first covering the requisite topics in
physics and then building off of it show how the greenhouse
effect occurs as a consequence. Specifically, we investigate properties
of energy, light, heat, and temperature, including the blackbody
effect, that hot objects emit light, and is how the Earth cools
off. Balancing the blackbody effect with sunlight and features of the
Earth’s atmosphere leads inevitably to the greenhouse effect, that
certain gases (called “greenhouse gases”) cause the Earth to be
warmer.
Understanding the greenhouse effect is necessary to engage with
public policy and the prospects for the climate of our future. Along the
way to this goal, we will encounter a variety of other topics, such as
geoengineering, the lifetime of different greenhouse gases in the
atmosphere, the processes that govern the structure of the atmosphere,
the temperature of black holes, and even color.
While this series is intended for a scientifically-curious reader, it
does not presume any specific advanced scientific background, and the
principles are introduced with digressions and examples to provide
context and help connect them to real-world experience. Scientific terms
are put in bold when their definition is given by the surrounding text,
and in italics otherwise; that is, terms in italics are jargon that may
have a different scientific meaning than their plain English
meaning.
Several major aspects of climate change
in this century and their relationships. Arrows indicate where one
aspect is a primary cause of another. The importance of the greenhouse
effect is that it is the main cause of global warming,
the increasing average temperature of the Earth. Some of the many
anticipated (or already realized) consequences of global warming are sea
level rise, frequent wildfires, amplified droughts and floods,
agricultural changes, deaths of coral reefs and the ecosystems that
depend on them, and more intense hurricanes/typhoons. All of these are
indirect consequences of the greenhouse effect.
In part 1 we introduce and motivate the remaining parts. Readers who
are satisfied with the summary may skip the rest of the series.
In part 2 we explain how energy enters and leaves the Earth, which
happens in the form of light. We discuss light and the different types
(that is, wavelengths) of light, and we introduce a graph called a
spectrum which shows what types of light are in a beam. We then
describe the blackbody effect, which states that all objects
emit light, and relates the amount and type of light emitted by an
object to the temperature of the object.
In part 3 we introduce the concept of an equilibrium of a
system, which could be a typical or average state for the system to be
in; it is frequently much easier to find the equilibria of a system than
to describe the exact behavior of a system as it changes over time. We
discuss equilibria in the context of temperature and calculate the
equilibrium temperature for the Earth in the absence of the
atmosphere.
In part 4 we present a simplified model of the Earth and its
atmosphere. We calculate the equilibrium temperature of the model Earth,
and see what exactly changes as we change the amount of greenhouse gas.
The finding that the temperature goes up as greenhouse gas is added is
the greenhouse effect.
In part 5 we explore the differences between the simple model and the
real-world climate of the Earth.
In appendix A we give an overview of what the atmosphere is, starting
from questions like why it is thicker on the bottom and whether it has a
top.
In appendix B we discuss the ozone layer and ozone hole. While they
have no bearing on the greenhouse effect, they are a frequent point of
confusion with the greenhouse effect. The worldwide effort to control
the ozone hole by regulating CFCs is also the only notable example of a
global agreement to address a global environmental problem, and thus
makes a useful point of comparison to the future regulation of carbon
dioxide emissions.
Summary
The Earth is warmed by visible light it receives from the
Sun, and is cooled by emitting infrared light (which is
invisible) to space. Greenhouse gases in the atmosphere are those that
absorb infrared light, but not visible light. Some of the infrared light
emitted by the Earth is absorbed by the greenhouse gases and then
re-emitted, some of which hits the Earth.
The Earth’s temperature is in balance when the total energy it
receives equals the total energy it emits. As infrared light is returned
to the Earth by greenhouse gases, the energy the Earth receives is
increased, so it must emit more energy to maintain balance. The Earth
emits more light when it is warmer. In this way adding greenhouse gases
to the atmosphere causes the Earth to become warmer.
Further reading
A very short and easy to read introduction to climate change is
What We Know About Climate Change by Kerry Emanuel. The
book is about as long as this series and is targeted to a non-scientific
audience. In addition to reviewing the material covered here (without
any equations), the book discusses what climate change is, how we know
it is happening, what the effects of climate change are, and what can be
done about climate change.
Sustainable Energy – Without the
Hot Air by David MacKay is an excellent, readable
introduction to energy in modern society, and is also targeted to
non-scientists. The book discusses the major uses of energy and the
major available sources of energy, and invites the reader to consider
how to balance these against each other in a future without fossil
fuels. The reader is empowered with the information necessary to come up
with their own energy plan, and in doing so is forced to confront with
the difficulty of finding an agreeable plan where energy production
satisfies energy demand.
In this series we use a mixture of three different scales for
describing temperature: degrees Celsius (formerly called “centigrade”),
which is the international standard temperature for day-to-day use;
degrees Fahrenheit, which is standard in the United States for
day-to-day use; and Kelvin, which is typically used within physics and
other sciences when discussing temperatures far from room temperature or
for theoretical physics. The reason for the difficulty converting
between this different scales is that Celsius and Fahrenheit do not have
zero temperature at zero degrees, but rather at -273.15 C and -459.67 F,
respectively. Indeed, both of those scales were invented and popularized
long before the scientific community reached a consensus that there was
such a thing as zero temperature (called absolute zero to avoid
confusion with other temperatures like 0 C or 0 F) or how cold it was
compared to known temperatures.
To rectify these shortcomings, Lord Kelvin invented a temperature
scale with zero temperature at zero degrees and was the first to
reliably calculate the difference between that temperature and known
temperatures, building off of contemporary work in thermodynamics by him
and other scientists. This Kelvin scale was scaled so that the
difference between two temperatures is the same number of degrees as in
the Celsius scale, so as a result one can convert from Celsius to Kelvin
by simply adding 273.15 degrees, which has the effect of shifting the
zero to the correct place. To convert between Fahrenheit and the other
two scales requires both shifting and multiplying by a constant.
Celsius and Fahrenheit continue to be more useful than Kelvin for
describing temperatures near room temperature (for which Kelvin can be
unwieldy), but are very inconvenient in the context of thermodynamics.
Climate science in general, and this series in particular, is mostly
concerned with physical processes that take place near room temperature,
but explains these processes with thermodynamics, and so uses both
Celsius and Kelvin according to what is convenient in each context.
(American meteorology continues to primarily use Fahrenheit for
historical reasons.)
, so the time scale for the
Earth to reach equilibrium is about 1 forcing period, i.e., 1
year.. However I was confused that these effects happened
to cancel each other exactly: if there was any tendency for the summer
warming to exceed the winter cooling on average, however slightly, then
year after year the temperature would steadily increase unendingly. Why
doesn’t this happen? Furthermore, I did not even think to wonder what
“excess” or “insufficient” sunlight could even mean: surely any amount
of sunlight would warm the Earth – by what process was the Earth cooling
in winter?