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Tiny particles suspended in air. Aerosols can be particles
of soot, dust, sea salt, or even microorganisms like bacteria
and viruses. Forest fires and fires from the clearing
of land for agriculture or development are major sources
of aerosols. Water can condense around particles like
these to form raindrops, snowflakes, or hailstones. Since
aerosols tend to reflect the suns energy away from
the planets surface, aerosols are often considered
to have a cooling effect on the earths climate.
A measure of the reflectivity of a surface. Albedo ranges
from 0 to 1, with 0 indicating no reflectivity (all the
energy hitting the surface is absorbed) and 1 indicating
perfect reflectivity (all energy hitting the surface is
The sun gives off a huge amount of energy, some of which
we see as visible sunlight. The earths average albedo
is about .31, which means that the planet as a whole reflects
away about 31% of the suns energy. But different
kinds of surfaces have different albedos: Clouds, snow,
ice, and deserts reflect much of the energy they receive
back into space; darker surfaces like oceans and vegetation
absorb much more of the suns radiation. Changes
in the earths albedo due to the melting of snow
and ice or different patterns of cloud formation could
exert lasting influences on the earths climate.
Human-caused. Anthropogenic sources of climate change
include things like the burning of fossil fuels. Non-anthropogenic
sources involve naturally occurring shifts in the earths
Carbon dioxide (CO2) is one of the gases produced when
fossil fuels are burned. CO2 in the atmosphere helps keep
the earth warm, because it traps heat near the planets
surfacea process called the greenhouse effect. CO2s
molecular structure allows sunlight to penetrate the atmosphere
and heat the earths surface, but prevents heat from
escaping back into space. CO2 is one of the most important
greenhouse gases, because human activity directly affects
its concentration in the atmosphere.
The carbon cycle is the global process by which the element
carbon is stored and exchanged between the air, oceans,
earth, and living things. Through photosynthesis, carbon
dioxide is removed from the air by plants and phytoplankton
and converted into living tissue. When the plants are
eaten, or when they burn or decay, the carbon is released
back into the atmosphere as carbon dioxide. Generally,
natural processes keep the amount of carbon removed from
the atmosphere and the amount returning to the atmosphere
in balance. But human activities (such as the burning
of fossil fuels) add additional carbon dioxide to the
air. The increase in atmospheric carbon dioxide contributes
to the warming of the planet through the greenhouse effect.
Climate refers to long-term patterns in the earths
weather. Tendencies for large areas of the planet to be
wet, dry, hot, or cold are examples of climate. Weather,
on the other hand, refers to short-term events, such as
daily or weekly temperature and precipitation.
Climate models are sophisticated computer simulations
of the earths climate. These simulations combine
a wide variety of data to produce projections of the earths
climate for months or years into the future. Because there
are still wide gaps in our understanding of the complexity
of the earths climate, different models incorporating
different data and assumptions produce varying projections
of the planets future climate.
Coral reefs are colonies of millions of organisms living
symbiotically with algae. (A symbiotic relationship is
one in which each partner derives some benefit from its
relationship with the other.) These reefs are the bases
for entire ecosystems in their vicinity. Under environmental
stresssuch as increases in ocean temperaturescoral
polyps may expel algae, which leads to a bleaching (or
loss of color) of the polyps. Continued stress may kill
the coral polyps and thus destroy the reef they form,
as well as the ecosystem that depends upon it. Coral bleaching
is thus seen as an early warning signal of changes in
the ocean environment.
When two kinds of data are associated, researchers say
they are correlated. In the study of climate, research
has indicated that, for example, past temperatures are
positively correlated with past levels of atmospheric
carbon dioxidemeaning that higher temperatures were
associated in the past with higher levels of carbon dioxide.
But correlation doesnt always mean causation: For
example, height and shoe size are correlated, meaning
that as a persons height increases, its likely
that his/her shoe size will increase also. But this doesnt
mean that increasing height caused the increase in shoe
size (or vice versa). In this case, increases in both
of these kinds of data are driven by increases in a third
factorthe persons age.
Deforestation is the removal of the earths forest
cover to provide space for agriculture or development.
Large deforestation projects, particularly in South America,
are often accomplished by burning huge areas of forest,
a process that releases carbon dioxide and aerosols into
the atmosphere. Because trees are a major consumer of
atmospheric carbon dioxide, extensive loss of forested
land may contribute to the greenhouse effect by limiting
the removal of carbon dioxide from the air.
The visible light we see is only a small part of the vast
spectrum of electromagnetic radiation existing all around
us. Other types of electromagnetic radiation include ultraviolet
light, infrared radiation, X rays, and radio waves. You
can think of all these forms of radiation as waves of
different lengths; for example, gamma rays have wavelengths
on the order of millionths of a meter, while radio waves
are tens of meters in length.
In studies of the climate, visible light may be referred
to as shortwave radiation, in comparison with infrared
radiation (which we feel as heat), which is called longwave
radiation due to its longer wavelength. Greenhouse gases
generally permit visible light to pass through but block
infrared radiationso the suns light penetrates
the atmosphere and warms the earth, but some of the earths
heat is trapped near the planets surface. (This
process is called the greenhouse effect.)
The earths energy budget is the overall description
of the processes that regulate the energy received and
emitted by the planet. The energy of sunlight heats the
earth, and the earth absorbs some of this energy and radiates
the rest of it back into space. Properties such as the
earths albedo and processes like the greenhouse
effect affect the planets energy budget.
El Niño/Southern Oscillation
A shift in the normal relationship between the atmosphere
and ocean in the tropical Pacific Ocean. Normally, strong
winds (called trade winds because they aided sailing ships
transporting goods) blow to the west in the Pacific, moving
warmer surface water away from North and South America.
Simultaneously, cold water from the ocean depths rises
to the surface off the west coast of South America. This
upwelling brings nutrients to the surface, supporting
fisheries and ecosystems in the area. In an El Niño
event, these trade winds die down, causing warmer surface
water to accumulate off western North and South America.
This leads to increased rainfall, storm activity, and
flooding in the Americas (especially the southwestern
United States and Peru) and drought conditions in Australia
and other areas in the western Pacific. Fisheries on the
west coasts of North and South America are also seriously
depleted during an El Niño year. (The movement
of trade winds in the Southern Hemisphere is called the
Southern Oscillation. Because El Niño events coincide
with changes in these winds, El Niño is sometimes
called an El Niño/Southern Oscillation event or
El Niño means the little one, a reference to the
infant Jesus, because the event often becomes manifest
in December. El Niño events occur, on average,
about every four years and last for a year or more. Some
climate researchers suspect that continued global warming
may increase the frequency or severity of El Niño
Forcings are processes that alter the earths energy
budget, either by affecting the amount of visible light
energy received by the earth or the amount of infrared
energy radiated by the earth back into space. Changing
the atmospheric concentration of greenhouse gases like
carbon dioxide, which traps more heat near the earths
surface, is one example of a climate forcing.
Fossil fuels are products that form underground as a result
of natural processes, such as the action of decay or heat
on buried organic compounds. For several hundred years,
people have burned these fuels (including coal, oil, and
natural gas) for heat and power. When burned, fossil fuels
release carbon dioxide and other greenhouse gases into
The greenhouse effect is one of the key elements in the
study of global climate change. Sunlight passes through
the earths atmosphere and heats the planets
surface. But not all of that heat escapes back into spacesome
is trapped by gases in the atmosphere. The greenhouse
effect is entirely a natural consequence of the molecular
structure of these gaseswithout the greenhouse effect,
the earths average temperature would be in the frigid
neighborhood of 15 degrees C. But human activities
(primarily the burning of fossil fuels) adds to the atmospheric
concentration of greenhouse gases. This traps more heat
near the surface and leads to an increase in global temperatures.
The planet Venus provides an example of an extreme greenhouse
effect: Most of Venuss heat is trapped by the thick,
opaque atmosphere, which is why its surface broils at
over 400 degrees C.
Greenhouse gases are atmospheric gases that contribute
to the greenhouse effect by allowing sunlight to penetrate
the atmosphere and heat the planets surface but
preventing some of that heat from escaping back into space.
Carbon dioxide, water vapor, and methane are among the
most important greenhouse gases. But not all components
of the earths atmosphere are greenhouse gases: Neither
oxygen nor nitrogen (which together comprise more than
95% of the earths atmosphere) are greenhouse gases.
This controversial theory suggests that increasing levels
of atmospheric carbon dioxide will not necessarily result
in serious increases in temperature because more carbon
dioxide will promote greater plant growth. In turn, the
additional vegetation will consume excess carbon dioxide
in the atmosphere. In effect, this hypothesis proposes
that natural processes will alleviate the effects of human-caused
increases in atmospheric greenhouse gases.
Heat capacity is the amount of heat required to raise
a system by 1 degree C in temperature. Water has a heat
capacity about 4 times that of air; this means that a
given amount of water needs about 4 times as much heat
to raise its temperature as that needed to raise the temperature
of an equivalent amount of air. In other words, its
much harder to change the temperature of a given amount
of water than it is to change the temperature of the same
amount of air. For this reason, weather in coastal areas
is often more moderate than weather at the same latitudes
in inland areas: The higher humidity of the air near the
oceans makes that air more resistant to temperature changes
than the drier air farther inland. The high heat capacity
of water means that the oceans are capable of storing
and transporting a large percentage of the earths
An ice core is a section of ice drilled from a glacier
or ice sheet. Ice deposits contain samples of the atmosphere
at the time the ice formed; they also record seasonal
fluctuations of temperature and dust. Thats why
ice cores are extremely valuable sources of paleoclimate
datadata on the earths climate in the distant
past. Researchers drill sections of ice cores hundreds
of meters long and then match sections at different depths
with particular eras in the earths past. These sections
can then be analyzed for clues about atmospheric gases
and temperatures from hundreds of thousands of years ago.
A measurement taken onsite. For example, in-situ measurements
of ocean temperatures might be taken by ships or networks
of buoys. Ocean temperatures and other climate data can
also be assessed by remote sensing, often accomplished
by orbiting satellites.
In a sense, La Niña is the opposite of El Niño:
In a La Niña year, ocean temperatures in the tropical
Pacific are colder than usual. This tends to inhibit storms
and precipitation in the central Pacific but causes increased
rainfall in the far western Pacific (such as Australia,
Malaysia, and the Philippines). In the United States,
La Niña is associated with colder-than-normal winters
in the north and warmer-than-normal winters in the south.
La Niña events often immediately follow El Niño
Mean is synonymous with average, the process by which
a series of values are summed and then divided by the
number of values in the set. The mean provides one kind
of measure of the central tendency of a set of dataa
single number that essentially represents the entire set.
An areas mean annual temperature, for example, is
meant to characterize the overall temperature expected
in that area year-round.
A system in which positive changes in one direction lead
to offsetting changes in the opposite direction is called
a negative feedback loop. A home thermostat is a common
example: When the temperature rises, the thermostat shuts
off the furnace, letting the house cool down. Once the
house cools below a certain point, the thermostat turns
the furnace back on and the houses temperature rises
again. Negative feedback systems often maintain values
at a relatively constant reading.
All measurements are combinations of the actual signals
being assessed and other random elements, such as measurement
error or interfering signals. Therefore, measurements
always contain some element of noise, a term used to describe
the proportion of the measured value that results from
sources other than the signal of interest.
Ozone is actually a form of oxygen, one of the most common
gases in our atmosphere. Ozone is found naturally in the
atmospheres stratospheric layer,
where it provides a shield against the suns harmful
ultraviolet energy. When ozone exists closer to the planets
surface, it forms one element in smog.
The study of the timing of natural events. Both scientists
and non-scientists have long been interested in the natural
cycles that characterize life on our planet. Such cycles
include events like the annual onset of spring, the winters
first snowfall, and the first appearance of migratory
birds flying north or south. Because the timing of these
cycles could hold great significance for societies dependent
upon agriculture or hunting, some records of events like
these go back several hundred years or more. Scientists
can use these old records to find clues about changes
in the earths climate.
Photosynthesis, a key element in the earths carbon
cycle, is a process by which land and sea plants use sunlight
to consume atmospheric carbon dioxide and build new living
tissue. Photosynthesis is the largest means of removal
of carbon dioxide from the earths atmosphere.
Oceanic microorganisms that use photosynthesis to consume
carbon dioxide dissolved in seawater. Their place at the
bottom of the oceanic food chain means that phytoplankton
are a basic element in the worlds ecosystems.
When changes in one direction lead to larger changes in
the same direction, the system is called a positive feedback
loop. Positive feedback systems tend to amplify or accelerate
the effects of initial changes, meaning that small variations
may be strengthened to cause large effects.
Prediction is one of the fundamental elements in the scientific
study of any phenomenon. Scientists observe phenomena
and then build theories that attempt to explain underlying
processes. Predictions derived from these theories are
then compared with actual events. The accuracy of these
predictions gives an indication of where the theories
are likely to be accurateand where they need revision.
Because climate involves long-term patterns, climate researchers
often seek data about what the climate was like thousands
or millions of years ago. But such data are difficult
to find (ice cores provide one source of actual prehistoric
climate data). Researchers may therefore use other kinds
of data that tell them about something related to climate.
Non-climate data analyzed for clues to climate is called
proxy data. For example, information about the past strength
of ocean currents might be gained by studying fossil microorganisms
deposited in sedimentsthe ages, types, and distributions
of these organisms could reflect the nature of the currents
that existed at the time they were deposited.
Remote sensing refers to measurements taken at sites far
from the area of interest. Satellite imagery is an important
example: Earth-orbiting satellites provide information
about ocean temperatures, snow and ice distribution, cloud
formation, and other climate indicators. (Measurements
taken at the point of interest are called in-situ or onsite
In any scientific field, a common practice is to take
measurements of a small number of items and then attempt
to generalize from those measurements to the wider world.
In the study of climate, for example, measurements of
air temperatures obviously cannot be taken at every point
in the earths atmosphere. Instead, a subset of atmospheric
locations, or sample, is measured. Picking the sample
is a critical part of any scientific inquiry, because
it must be representative of the larger group (the population)
about which the researcher wishes to draw conclusions.
In science, the word significant has a very precise statistical
meaning. Scientists make predictions based on their theories
about how things work. They then test these predictions
through observations or experiments to determine whether
the theories are accurate descriptions of underlying processes.
A significant finding in support of a theory is not merely
important or noteworthyit is a finding that is unlikely
to have occurred by chance if the theory is wrong. In
other words, its a finding that provides support
for the theory at a high level of probability.
Weather refers to short-term phenomena, such as daily
temperature, precipitation, and wind patterns. Longer-term
patternssuch as fluctuations in temperature over
years or decadesfall under the heading of climate.