Global warming
Earth science
Global warming, the phenomenon of increasing average air temperatures near the surface of Earth
over the past one to two centuries. Climate scientists have since the
mid-20th century gathered detailed observations of various weather phenomena (such as temperatures, precipitation, and storms) and of related influences on climate (such as ocean currents and the atmosphere’s chemical composition). These data indicate that Earth’s climate has changed over almost every conceivable timescale since the beginning of geologic time and that the influence of human activities since at least the beginning of the Industrial Revolution has been deeply woven into the very fabric of climate change.
Giving voice to a growing conviction of most of the scientific community, the Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment
Program (UNEP). In 2013 the IPCC reported that the interval between
1880 and 2012 saw an increase in global average surface temperature of
approximately 0.9 °C (1.5 °F). The increase is closer to 1.1 °C (2.0 °F)
when measured relative to the preindustrial (i.e., 1750–1800) mean
temperature. The IPCC stated that most of the warming observed over the
second half of the 20th century could be attributed to human activities.
It predicted that by the end of the 21st century the global mean
surface temperature would increase by 0.3 to 4.8 °C (0.5 to 8.6 °F)
relative to the 1986–2005 average. The predicted rise in temperature was
based on a range of possible scenarios that accounted for future
greenhouse gas emissions and mitigation (severity reduction) measures
and on uncertainties in the model projections. Some of the main
uncertainties include the precise role of feedback processes and the
impacts of industrial pollutants known as aerosols which may offset some warming.
Graph of the predicted increase in Earth’s average surface temperature according to a series of …Encyclopædia Britannica, Inc.
Many climate scientists
agree that significant societal, economic, and ecological damage would
result if global average temperatures rose by more than 2 °C (3.6 °F) in
such a short time. Such damage would include increased extinction of
many plant and animal species, shifts in patterns of agriculture,
and rising sea levels. The IPCC reported that the global average sea
level rose by some 19–21 cm (7.5–8.3 inches) between 1901 and 2010 and
that sea levels rose faster in the second half of the 20th century than
in the first half. It also predicted, again depending on a wide range of
scenarios, that by the end of the 21st century the global average sea
level could rise by another 26–82 cm (10.2–32.3 inches) relative to the
1986–2005 average and that a rise of well over 1 metre (3 feet) could
not be ruled out.
The scenarios referred to above depend mainly on future concentrations of certain trace gases, called greenhouse gases, that have been injected into the lower atmosphere in increasing amounts through the burning of fossil fuels for industry, transportation, and residential uses. Modern global warming is the result of an increase in magnitude of the so-called greenhouse effect, a warming of Earth’s surface and lower atmosphere caused by the presence of water vapour, carbon dioxide, methane, nitrous oxides, and other greenhouse gases. In 2014 the IPCC
reported that concentrations of carbon dioxide, methane, and nitrous
oxides in the atmosphere surpassed those found in ice cores dating back
800,000 years. Of all these gases, carbon dioxide is the most important,
both for its role in the greenhouse effect and for its role in the
human economy. It has been estimated that, at the beginning of the
industrial age in the mid-18th century, carbon dioxide concentrations in
the atmosphere were roughly 280 parts per million (ppm). By the middle
of 2014, carbon dioxide concentrations had briefly reached 400 ppm, and,
if fossil fuels continue to be burned at current rates, they are
projected to reach 560 ppm by the mid-21st century—essentially, a
doubling of carbon dioxide concentrations in 300 years.
The greenhouse effect on Earth. Some incoming sunlight is reflected by Earth’s atmosphere and …Encyclopædia Britannica, Inc.
A vigorous debate is in
progress over the extent and seriousness of rising surface
temperatures, the effects of past and future warming on human life, and
the need for action to reduce future warming and deal with its
consequences. This article provides an overview of the scientific
background and public policy debate related to the subject of global
warming. It considers the causes of rising near-surface air
temperatures, the influencing factors, the process of climate research
and forecasting, the possible ecological and social impacts of rising
temperatures, and the public policy developments since the mid-20th
century. For a detailed description of Earth’s climate, its processes,
and the responses of living things to its changing nature, see climate. For additional background on how Earth’s climate has changed throughout geologic time, see climatic variation and change. For a full description of Earth’s gaseous envelope, within which climate change and global warming occur, see atmosphere.
Climatic variation since the last glaciation
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Global warming is related
to the more general phenomenon of climate change, which refers to
changes in the totality of attributes that define climate. In addition
to changes in air temperature, climate change involves changes to precipitation patterns, winds, ocean currents,
and other measures of Earth’s climate. Normally, climate change can be
viewed as the combination of various natural forces occurring over diverse timescales. Since the advent of human civilization, climate change has involved an “anthropogenic,”
or exclusively human-caused, element, and this anthropogenic element
has become more important in the industrial period of the past two
centuries. The term global warming is used specifically to
refer to any warming of near-surface air during the past two centuries
that can be traced to anthropogenic causes.
A series of photographs of the Grinnell Glacier taken from the summit of Mount Gould in Glacier …1938-T.J. Hileman/Glacier National Park Archives, 1981 - Carl Key/USGS, 1998 - Dan Fagre/USGS, 2006 - Karen Holzer/USGS
To define the concepts of global warming and climate change properly, it is first necessary to recognize that the climate of Earth
has varied across many timescales, ranging from an individual human
life span to billions of years. This variable climate history is
typically classified in terms of “regimes” or “epochs.” For instance,
the Pleistocene glacial epoch (about 2,600,000 to 11,700 years ago) was marked by substantial variations in the global extent of glaciers and ice
sheets. These variations took place on timescales of tens to hundreds
of millennia and were driven by changes in the distribution of solar radiation
across Earth’s surface. The distribution of solar radiation is known as
the insolation pattern, and it is strongly affected by the geometry of
Earth’s orbit around the Sun and by the orientation, or tilt, of Earth’s axis relative to the direct rays of the Sun.
Worldwide, the most recent glacial period, or ice age, culminated about 21,000 years ago in what is often called the Last Glacial Maximum.
During this time, continental ice sheets extended well into the middle
latitude regions of Europe and North America, reaching as far south as
present-day London and New York City.
Global annual mean temperature appears to have been about 4–5 °C (7–9
°F) colder than in the mid-20th century. It is important to remember
that these figures are a global average. In fact, during the height of
this last ice age, Earth’s climate was characterized by greater cooling
at higher latitudes (that is, toward the poles) and relatively little
cooling over large parts of the tropical oceans (near the Equator). This
glacial interval terminated abruptly about 11,700 years ago and was
followed by the subsequent relatively ice-free period known as the Holocene Epoch.
The modern period of Earth’s history is conventionally defined as
residing within the Holocene. However, some scientists have argued that
the Holocene Epoch terminated in the relatively recent past and that
Earth currently resides in a climatic interval that could justly be
called the Anthropocene Epoch—that is, a period during which humans have
exerted a dominant influence over climate.
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Though less dramatic
than the climate changes that occurred during the Pleistocene Epoch,
significant variations in global climate have nonetheless taken place
over the course of the Holocene. During the early Holocene, roughly
9,000 years ago, atmospheric circulation and precipitation
patterns appear to have been substantially different from those of
today. For example, there is evidence for relatively wet conditions in
what is now the Sahara
Desert. The change from one climatic regime to another was caused by
only modest changes in the pattern of insolation within the Holocene
interval as well as the interaction of these patterns with large-scale
climate phenomena such as monsoons and El Niño/Southern Oscillation (ENSO).
During
the middle Holocene, some 5,000–7,000 years ago, conditions appear to
have been relatively warm—indeed, perhaps warmer than today in some
parts of the world and during certain seasons. For this reason, this
interval is sometimes referred to as the Mid-Holocene Climatic Optimum.
The relative warmth of average near-surface air temperatures at this
time, however, is somewhat unclear. Changes in the pattern of insolation
favoured warmer summers at higher latitudes in the Northern Hemisphere,
but these changes also produced cooler winters in the Northern
Hemisphere and relatively cool conditions year-round in the tropics. Any
overall hemispheric or global mean temperature changes thus reflected a
balance between competing seasonal and regional changes. In fact,
recent theoretical climate model studies suggest that global mean
temperatures during the middle Holocene were probably 0.2–0.3 °C
(0.4–0.5 °F) colder than average late 20th-century conditions.
Over
subsequent millennia, conditions appear to have cooled relative to
middle Holocene levels. This period has sometimes been referred to as
the “Neoglacial.” In the middle latitudes this cooling trend was
associated with intermittent
periods of advancing and retreating mountain glaciers reminiscent of
(though far more modest than) the more substantial advance and retreat
of the major continental ice sheets of the Pleistocene climate epoch.
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35 References found in Britannica Articles
Assorted References
- Antarctic evidence (in Antarctica: Climate)
- carbon sources and carbon sequestration (in carbon sequestration: Carbon sources and carbon sinks)
- conservation and extinction factors (in conservation (ecology): Global change)
- contemporary climatic change (in Earth (planet): The atmosphere)
- desertification (in desertification: Solutions to desertification)
- Alaskan climate (in Alaska: Climate)
- carbon cycle (in biosphere: The carbon cycle)
