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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 sun’s energy away from the planet’s surface, aerosols are often considered to have a cooling effect on the earth’s 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 reflected away).
The sun gives off a huge amount of energy, some of which we see as visible sunlight. The earth’s average albedo is about .31, which means that the planet as a whole reflects away about 31% of the sun’s 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 sun’s radiation. Changes in the earth’s albedo due to the melting of snow and ice or different patterns of cloud formation could exert lasting influences on the earth’s 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 earth’s climate system.


carbon dioxide
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 planet’s surface—a process called the greenhouse effect. CO2’s molecular structure allows sunlight to penetrate the atmosphere and heat the earth’s 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.

carbon cycle
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 earth’s 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 model
Climate models are sophisticated computer simulations of the earth’s climate. These simulations combine a wide variety of data to produce projections of the earth’s climate for months or years into the future. Because there are still wide gaps in our understanding of the complexity of the earth’s climate, different models incorporating different data and assumptions produce varying projections of the planet’s future climate.

coral bleaching
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 stress—such as increases in ocean temperatures—coral 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 dioxide—meaning that higher temperatures were associated in the past with higher levels of carbon dioxide. But correlation doesn’t always mean causation: For example, height and shoe size are correlated, meaning that as a person’s height increases, it’s likely that his/her shoe size will increase also. But this doesn’t 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 factor—the person’s age.



Deforestation is the removal of the earth’s 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.


electromagnetic radiation

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 radiation—so the sun’s light penetrates the atmosphere and warms the earth, but some of the earth’s heat is trapped near the planet’s surface. (This process is called the greenhouse effect.)

energy budget
The earth’s 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 earth’s albedo and processes like the greenhouse effect affect the planet’s 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 ENSO.)
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 phenomena.


Forcings are processes that alter the earth’s 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 earth’s surface, is one example of a climate forcing.

fossil fuels
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 atmosphere.


greenhouse effect

The greenhouse effect is one of the key elements in the study of global climate change. Sunlight passes through the earth’s atmosphere and heats the planet’s surface. But not all of that heat escapes back into space—some is trapped by gases in the atmosphere. The greenhouse effect is entirely a natural consequence of the molecular structure of these gases—without the greenhouse effect, the earth’s 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 Venus’s heat is trapped by the thick, opaque atmosphere, which is why its surface broils at over 400 degrees C.

greenhouse gases
Greenhouse gases are atmospheric gases that contribute to the greenhouse effect by allowing sunlight to penetrate the atmosphere and heat the planet’s 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 earth’s atmosphere are greenhouse gases: Neither oxygen nor nitrogen (which together comprise more than 95% of the earth’s atmosphere) are greenhouse gases.

greening hypothesis
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
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, it’s 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 earth’s heat.


ice core
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. That’s why ice cores are extremely valuable sources of paleoclimate data—data on the earth’s 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 earth’s 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.


La Niña
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 years.


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 data—a single number that essentially represents the entire set. An area’s mean annual temperature, for example, is meant to characterize the overall temperature expected in that area year-round.


negative feedback
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 house’s 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 atmosphere’s stratospheric layer,
where it provides a shield against the sun’s harmful ultraviolet energy. When ozone exists closer to the planet’s 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 winter’s 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 earth’s climate.

Photosynthesis, a key element in the earth’s 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 earth’s 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 world’s ecosystems.

positive feedback
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 accurate—and where they need revision.

proxy data
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 sediments—the ages, types, and distributions of these organisms could reflect the nature of the currents that existed at the time they were deposited.


remote sensing
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 measurements.)


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 earth’s 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.

statistical significance
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 noteworthy—it is a finding that is unlikely to have occurred by chance if the theory is wrong. In other words, it’s 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 patterns—such as fluctuations in temperature over years or decades—fall under the heading of climate.

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