What causes global climate change?

There is a scientific consensus that concentrations of greenhouse

gases in the atmosphere are increasing and that this is causing

global climate change. Human-driven emissions of carbon dioxide

and other greenhouse gases, as well as land-use change, are the

processes primarily responsible for the increase. Emissions of black

carbon (soot) may also be contributing to the warming. Emissions

of reflective sulfate aerosols have been associated with a net cooling

effect.

Defining Weather and Climate

Weather is the state of the atmosphere at a specific time in a

specific place. Temperature, cloudiness, humidity, precipitation, and

winds are examples of weather elements. Thunderstorms, tornadoes,

and monsoons are also part of the weather of some places

during some seasons.

Climate is defined as long-term weather patterns that describe a

region. For example, the New York metropolitan region’s climate is

temperate, with rain evenly distributed throughout the year, cold

winters, and hot summers.

Climate Variability and Climate Change

Climate variability refers to variations in the prevailing state of the

climate on all temporal and spatial scales beyond that of individual

weather events. Variability may be due to natural internal

processes within the climate system, or to variations in natural or

anthropogenic (human-driven) external forcing. Global climate

change indicates a change in either the mean state of the climate

or in its variability, persisting for several decades or longer. This

includes changes in average weather conditions on Earth, such as

a change in average global temperature, as well as changes in how

frequently regions experience heat waves, droughts, floods, storms,

and other extreme weather. It is important to note that changes in

individual weather events will potentially contribute substantially

to changes in climate variability.

Climate change could occur naturally as a result of a change in the

sun’s energy or Earth’s orbital cycle (natural climate forcing), or it

could occur as a result of persistent anthropogenic forcing, such as

the addition of greenhouse gases, sulfate aerosols, or black carbon

to the atmosphere, or through land-use change.

The Climate System and the Carbon Cycle

The climate system is driven by the sun’s energy and regulated by

natural processes and cycles in the Earth system (Figure 1). These

include the carbon cycle and greenhouse effect, orbital cycles,

ocean currents that distribute warmer and colder water around

the globe, and atmosphere-ocean interactions that moderate

temperature. Humans are principally affecting the climate system

through alterations to the carbon cycle, which regulates the flow

of carbon among living and non-living parts of the Earth system

(Figure 2). Carbon is found in all living things, in soils and rocks, in

fossil fuels, in ocean sediments and corals, and as carbon dioxide in

the atmosphere. Each of these carbon reservoirs stores a percentage

of the Earth’s total carbon and carbon moves at varying rates

among the reservoirs. In some cases, the carbon may remain in a

reservoir for millions of years, as in the case of fossil fuels before

the Industrial Revolution. As humans burn fossil fuels to produce

energy and as they clear natural ecosystems, carbon dioxide is

released into the atmosphere, where it acts as a greenhouse gas.

Greenhouse Gases

The presence of greenhouse gases in the atmosphere is a natural

component of the climate system and helps to maintain the Earth

as a habitable planet (Figure 3). Greenhouse gases are relatively

transparent to incoming solar radiation, allowing the sun’s energy

to pass through the atmosphere to the surface of the Earth. The

energy is then absorbed by the Earth’s surface, used in processes

like photosynthesis, or emitted

back to space as infrared radiation.

Some of the emitted radiation

passes through the atmosphere

and travels back to space, but

some is absorbed by greenhouse

gas molecules and then re-emitted

in all directions. The effect of this

is to warm the Earth’s surface and

the lower atmosphere. Water vapor

(H2O) and carbon dioxide (CO2)

are the two largest contributors to

the greenhouse effect. Methane

(CH4), nitrous oxide (N2O), chlorofluorocarbons

(CFCs) and other

greenhouse gases are present

only in trace amounts, but can still

have a powerful warming effect

due to their heat-trapping abilities

and their long residence time

in the atmosphere. Without the

greenhouse effect, Earth’s average

temperature would be -0.4°F

(-18°C), rather than the present 59°F (15°C).

Concentrations of greenhouse gases – and especially carbon dioxide

– have risen over the past two hundred and fifty years, largely

due to the combustion of fossil fuels for energy production. Since

the Industrial Revolution in the eighteenth century the concentration

of carbon dioxide in the atmosphere has risen from about

270 parts per million (ppm) to about 370 ppm. Concentrations of

methane have also risen due to cattle production, the cultivation

of rice, and release from landfills. Nearly one-third of human-induced

nitrous oxide emissions are a result of industrial processes

and automobile emissions.

Land-use Change

The combustion of fossil fuels is not the only anthropogenic source

of carbon dioxide. When ecosystems are altered and vegetation is

either burned or removed, the carbon stored in them is released to

the atmosphere as carbon dioxide. The principal reasons for deforestation

are agriculture and urban growth, and harvesting timber

for fuel, construction, and paper. Currently, up to a quarter of the

carbon dioxide emissions to the atmosphere can be attributed to

land-use change.

Sulfate Aerosols and Black Carbon

Sulfate aerosols and black carbon are two important additional examples

of anthropogenic forcings. Sulfate aerosols, which enter the

atmosphere naturally during volcanic eruptions, are tiny airborne

particles that reflect sunlight back to space. Industrial activity has

recently increased their concentration in the atmosphere primarily

through the burning of fossil fuels containing sulfur. Anthropogenic

emissions of sulfate aerosols

have been associated with a net

cooling effect.

Black carbon is soot generated

from industrial pollution, traffic,

outdoor fires, and the burning

of coal and biomass fuels. Black

carbon is formed by incomplete

combustion especially of coal,

diesel fuels, biofuels and outdoor

biomass burning. Soot particles

absorb sunlight, both heating the

air and reducing the amount of

sunlight reaching the ground.

Global Climate Change in

the Twentieth Century

The climate system includes a

great deal of natural variability, and

climate fluctuations have always

been part of the Earth’s 4.6 billion

year history. However, over the past

century changes in concentrations of greenhouse gases in the

atmosphere are of an unprecedented rate and magnitude. Human

population growth has led to increasing demands for energy and

land resources. Through the burning of fossil fuels to produce

energy for industrial use, transportation, and domestic power,

and through land-use change for agriculture and forest products,

humans have been altering the Earth’s energy balance. Scientists

believe that these changes may have already begun to alter the

global climate.