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.
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.
