Climate
Change
Climate
change refers to the variation in the Earth's global
climate or in regional climates over time. It describes
changes in the variability or average state of the
atmosphere over time scales ranging from decades to
millions of years. These changes can be caused by
processes internal to the Earth, external forces (e.g.
variations in sunlight intensity) or, more recently,
human activities.
In
recent usage, especially in the context of environmental
policy, the term "climate change" often
refers only to changes in modern climate, including
the rise in average surface temperature known as global
warming. In some cases, the term is also used with
a presumption of human causation, as in the United
Nations Framework Convention on Climate Change (UNFCCC).
The UNFCCC uses "climate variability" for
non-human caused variations.
For
information on temperature measurements over various
periods, and the data sources available, see temperature
record. For attribution of climate change over the
past century, see attribution of recent climate change.
Climate
change factors
Climate changes reflect variations within the Earth's
atmosphere, processes in other parts of the Earth
such as oceans and ice caps, and the impact of human
activity. The external factors that can shape climate
are often called climate forcings and include such
processes as variations in solar radiation, the Earth's
orbit, and greenhouse gas concentrations.
Variations within the Earth's climate
Weather is the day-to-day state of the atmosphere,
and is a chaotic non-linear dynamical system. On the
other hand, climate the average state of weather
is fairly stable and predictable. Climate includes
the average temperature, amount of precipitation,
days of sunlight, and other variables that might be
measured at any given site. However, there are also
changes within the Earth's environment that can affect
the climate.
Glaciation
Glaciers
are recognized as one of the most sensitive indicators
of climate change, advancing substantially during
climate cooling (e.g., the Little Ice Age) and retreating
during climate warming on moderate time scales. Glaciers
grow and collapse, both contributing to natural variability
and greatly amplifying external forces. For the last
century, however, glaciers have been unable to regenerate
enough ice during the winters to make up for the ice
lost during the summer months (see glacier retreat).
The
most important climate processes of the last several
million years are the glacial and interglacial cycles
of the present ice age. Though shaped by orbital variations,
the internal responses involving continental ice sheets
and 130 m sea-level change certainly played a key
role in deciding what climate response would be observed
in most regions. Other changes, including Heinrich
events, DansgaardOeschger events and the Younger
Dryas show the potential for glacial variations to
influence climate even in the absence of specific
orbital changes
Ocean
variability
A schematic of modern thermohaline circulationOn the
scale of mere decades, climate changes can also result
from changes within the ocean/atmosphere systems.
Many climate states, most obviously El Niño
Southern oscillation, but also including the Pacific
decadal oscillation, the North Atlantic oscillation,
and the Arctic oscillation, have been recognized as
modes within the climate system, owing their existence
at least in part to different ways that heat can be
stored in the oceans and move between different reservoirs.
On longer time scales, ocean processes such as thermohaline
circulation play a key role in redistributing heat,
and could, if changed, dramatically impact climate.
The
memory of climate
More generally, most forms of internal variability
in the climate system can be recognized as a form
of hysteresis, meaning that the current state of climate
reflects not only the inputs, but also the history
of how it got there. For example, a decade of dry
conditions may cause lakes to shrink, plains to dry
up and deserts to expand. In turn, these conditions
may lead to less rainfall in the following years.
In short, climate change can be a self-perpetuating
process because different aspects of the environment
respond at different rates and in different ways to
the fluctuations that inevitably occur.
Non-climate factors driving climate change
Greenhouse gases
Carbon dioxide variations during the last 500 million
yearsCurrent studies indicate that radiative forcing
by greenhouse gases is the primary cause of global
warming. Greenhouse gases are also important in understanding
Earth's climate history. According to these studies,
the greenhouse effect, which is the warming produced
as greenhouse gases trap heat, plays a key role in
regulating Earth's temperature.
Over
the last 600 million years, carbon dioxide concentrations
have varied from perhaps >5000 ppm to less than
200 ppm, due primarily to the impact of geological
processes and biological innovations. It has been
argued (Veizer et al. 1999) that variations in greenhouse
gas concentrations over tens of millions of years
have not been well correlated to climate change, with
plate tectonics perhaps playing a more dominant role.
However, there are several examples of rapid changes
in the concentrations of greenhouse gases in the Earth's
atmosphere that do appear to correlate to strong warming,
including the PaleoceneEocene thermal maximum,
the PermianTriassic extinction event, and the
end of the Varangian snowball earth event.
During
the modern era, rising carbon dioxide levels are implicated
as the primary cause of global warming since 1950.
Plate tectonics
On the longest time scales, plate tectonics will reposition
continents, shape oceans, build and tear down mountains
and generally serve to define the stage upon which
climate exists. More recently, plate motions have
been implicated in the intensification of the present
ice age when, approximately 3 million years ago, the
North and South American plates collided to form the
Isthmus of Panama and shut off direct mixing between
the Atlantic and Pacific Oceans.
Solar variation
Variations in solar activity during the last several
centuries based on observations of sunspots and beryllium
isotopes.The sun is the ultimate source of essentially
all heat in the climate system. The energy output
of the sun, which is converted to heat at the Earth's
surface, is an integral part of shaping the Earth's
climate. On the longest time scales, the sun itself
is getting brighter with higher energy output; as
it continues its main sequence, this slow change or
evolution affects the Earth's atmosphere. Early in
Earth's history, it is thought to have been too cold
to support liquid water at the Earth's surface, leading
to what is known as the Faint young sun paradox.
On
more modern time scales, there are also a variety
of forms of solar variation, including the 11-year
solar cycle and longer-term modulations. However,
the 11-year sunspot cycle does not manifest itself
clearly in the climatological data. Solar intensity
variations are considered to have been influential
in triggering the Little Ice Age, and for some of
the warming observed from 1900 to 1950. The cyclical
nature of the sun's energy output is not yet fully
understood; it differs from the very slow change that
is occurring to the sun as it ages and evolves.
Orbital variations
In their impact on climate, orbital variations are
in some sense an extension of solar variability, because
slight variations in the Earth's orbit lead to changes
in the distribution and abundance of sunlight reaching
the Earth's surface. Such orbital variations, known
as Milankovitch cycles, are a highly predictable consequence
of basic physics due to the mutual interactions of
the Earth, its moon, and the other planets. These
variations are considered the driving factors underlying
the glacial and interglacial cycles of the present
ice age. Subtler variations are also present, such
as the repeated advance and retreat of the Sahara
desert in response to orbital precession.
Volcanism
A single eruption of the kind that occurs several
times per century can impact climate, causing cooling
for a period of a few years. For example, the eruption
of Mount Pinatubo in 1991 is barely visible on the
global temperature profile. Huge eruptions, known
as large igneous provinces, occur only a few times
every hundred million years, but can reshape climate
for millions of years and cause mass extinctions.
Initially, scientists thought that the dust emitted
into the atmosphere from large volcanic eruptions
was responsible for the cooling by partially blocking
the transmission of solar radiation to the Earth's
surface. However, measurements indicate that most
of the dust thrown in the atmosphere returns to the
Earth's surface within six months.
Attribution of recent climate change
Human influences on climate
Anthropogenic factors are acts by humans that change
the environment and influence climate. The biggest
factor of present concern is the increase in CO2 levels
due to emissions from fossil fuel combustion, followed
by aerosols (particulate matter in the atmosphere)
which exerts a cooling effect. Other factors, including
land use, ozone depletion, animal agriculture [1]
and deforestation also impact climate.
Fossil fuels
Carbon dioxide variations over the last 400,000 years,
showing a rise since the industrial revolution.Beginning
with the industrial revolution in the 1850s and accelerating
ever since, the human consumption of fossil fuels
has elevated CO2 levels from a concentration of ~280
ppm to more than 370 ppm today. These increases are
projected to reach more than 560 ppm before the end
of the 21st century. It is known that carbon dioxide
levels are substantially higher now than at any time
in the last 800,000 years [2] Along with rising methane
levels, these changes are anticipated to cause an
increase of 1.45.6 °C between 1990 and 2100
(see global warming).
Aerosols
Anthropogenic aerosols, particularly sulphate aerosols
from fossil fuel combustion, are believed to exert
a cooling influence; see graph.[2] This, together
with natural variability, is believed to account for
the relative "plateau" in the graph of 20th
century temperatures in the middle of the century.
Land use
Prior to widespread fossil fuel use, humanity's largest
impact on local climate is likely to have resulted
from land use. Irrigation, deforestation, and agriculture
fundamentally change the environment. For example,
they change the amount of water going into and out
of a given locale. They also may change the local
albedo by influencing the ground cover and altering
the amount of sunlight that is absorbed. For example,
there is evidence to suggest that the climate of Greece
and other Mediterranean countries was permanently
changed by widespread deforestation between 700 BC
and 0 BC (the wood being used for ship-building, construction
and fuel), with the result that the modern climate
in the region is significantly hotter and drier, and
the species of trees that were used for ship-building
in the ancient world can no longer be found in the
area.
A
controversial hypothesis by William Ruddiman called
the early anthropocene hypothesis suggests that the
rise of agriculture and the accompanying deforestation
led to the increases in carbon dioxide and methane
during the period 50008000 years ago. These
increases, which reversed previous declines, may have
been responsible for delaying the onset of the next
glacial period, according to Ruddimann's overdue-glaciation
hypothesis.
Animal agriculture
According to a 2006 United Nations report, animal
agriculture is responsible for 18% of the worlds
greenhouse gas emissions as measured in CO2 equivalents.
In addition to CO2 emissions, animal agriculture produces
65% percent of human-induced nitrous oxide (which
has 296 times the global warming potential of CO2)
and 37% of human-induced methane (which has 23 times
the global warming potential of CO2).
Interplay of factors
If a certain forcing (for example, solar variation)
acts to change the climate, then there may be mechanisms
that act to amplify or reduce the effects. These are
called positive and negative feedbacks. As far as
is known, the climate system is generally stable with
respect to these feedbacks: positive feedbacks do
not "run away". Part of the reason for this
is the existence of a powerful negative feedback between
temperature and emitted radiation: radiation increases
as the fourth power of absolute temperature.
However,
a number of important positive feedbacks do exist.
The glacial and interglacial cycles of the present
ice age provide an important example. It is believed
that orbital variations provide the timing for the
growth and retreat of ice sheets. However, the ice
sheets themselves reflect sunlight back into space
and hence promote cooling and their own growth, known
as the ice-albedo feedback. Further, falling sea levels
and expanding ice decrease plant growth and indirectly
lead to declines in carbon dioxide and methane. This
leads to further cooling.
Similarly,
rising temperatures caused, for example, by anthropogenic
emissions of greenhouse gases could lead to retreating
snow lines, revealing darker ground underneath, and
consequently result in more absorption of sunlight.
Water
vapor, methane, and carbon dioxide can also act as
significant positive feedbacks, their levels rising
in response to a warming trend, thereby accelerating
that trend. Water vapor acts strictly as a feedback
(excepting small amounts in the stratosphere), unlike
the other major greenhouse gases, which can also act
as forcings.
More
complex feedbacks involve the possibility of changing
circulation patterns in the ocean or atmosphere. For
example, a significant concern in the modern case
is that melting glacial ice from Greenland will interfere
with sinking waters in the North Atlantic and inhibit
thermohaline circulation. This could affect the Gulf
Stream and the distribution of heat to Europe and
the east coast of the United States.
Other
potential feedbacks are not well understood and may
either inhibit or promote warming. For example, it
is unclear whether rising temperatures promote or
inhibit vegetative growth, which could in turn draw
down either more or less carbon dioxide. Similarly,
increasing temperatures may lead to either more or
less cloud cover.[3] Since on balance cloud cover
has a strong cooling effect, any change to the abundance
of clouds also impacts climate.
In
all, it seems likely that overall climate feedbacks
are negative, as systems with overall positive feedback
are highly unstable.
Monitoring the current status of climate
Scientists use "Indicator time series" that
represent the many aspects of climate and ecosystem
status. The time history provides an historical context.
Current status of the climate is also monitored with
climate indices.
Evidence for Climatic Change
Evidence for climatic change is taken from a variety
of sources that can be used to reconstruct past climates.
Most of the evidence is indirectclimatic changes
are inferred from changes in indicators that reflect
climate, such as vegetation, dendrochronology, ice
cores, sea level change, and glacial retreat.
Pollen Analysis
Species have particular climatic requirements that
influence their geographical distributions. Each plant
species has a distinctively shaped pollen grain, and
if these fall into oxygen-free environments, such
as peat bogs, they resist decay. Changes in the pollen
found in different levels of the bog indicate, by
implication, changes in climate.
One
limitation of this method is the fact that pollen
can be transported considerable distances by wind
or sometimes by wildlife.
Beetles
Remains of beetles are common in freshwater and land
sediments. Different species of beetles tend to be
found under different climatic conditions. Knowledge
of the present climatic range of the different species,
and the age of the sediments in which remains are
found, allows past climatic conditions to be worked
out.
Glacial Geology
Advancing glaciers leave behind moraines and other
features that often have datable material in them,
recording the time when a glacier advanced and deposited
a feature. Similarly, the lack of glacier cover can
be identified by the presence of datable soil or volcanic
tephra horizons. Glaciers are considered one of the
most sensitive climate indicators by the IPCC, and
their recent observed variations provide a global
signal of climate change. See Retreat of glaciers
since 1850.
Historical Records
Historical records include cave paintings, depth of
grave digging in Greenland, diaries, documentary evidence
of events (such as 'frost fairs' on the Thames) and
evidence of areas of vine cultivation. Daily weather
reports have been kept since 1873, and the Royal Society
has encouraged the collection of data since the seventeenth
century. Parish records are often a good source of
climate data.
Examples of climate change
Climate change has continued throughout the entire
history of Earth. The field of paleoclimatology has
provided information of climate change in the ancient
past, supplementing modern observations of climate.
Obviously, these prehistoric changes are solely the
result of natural factors.
Climate
of the deep past
Faint young sun paradox
Snowball earth
Oxygen Catastrophe
Climate of the last 500 million years
Phanerozoic overview
PaleoceneEocene Thermal Maximum
Cretaceous Thermal Maximum
PermoCarboniferous Glaciation
Ice ages
Climate of recent glaciations
DansgaardOeschger event
Younger Dryas
Ice age temperatures
Recent climate
Holocene Climatic Optimum
Medieval Warm Period
Little Ice Age
Year Without a Summer
Temperature record of the past 1000 years
Global warming
Hardiness Zone Migration
Climate change and economics
Main article: Economics of global warming
There has been a debate about how climate change
could affect the world economy. In an October
29, 2006 report by the former Chief Economist
and Senior Vice-President of the World Bank Nicholas
Stern, he states that climate change could affect
growth which could be cut by one-fifth unless
drastic action is taken. (Report's stark warning
on climate)
See also
Abrupt climate change
An Inconvenient Truth
Are We Changing Planet Earth?
Climate change response
Climate Change Science Program
Climate model
Economics of global warming
Effects of global warming
Energie-Cités
Global Observation Research Initiative in Alpine Environments
Global Warming
Global warming controversy
Greenpeace
Hell and High Water
Intergovernmental Panel on Climate Change
International Environmental Law
Iron fertilization
Kyoto Protocol
Low-carbon economy
Mitigation of global warming
Permafrost
Renewable energy
Sea level rise
Timeline of environmental events
United Kingdom Climate Change Programme
Climate tourists (Credit:
Wikipedia).
Global
Warming
Environmentalists
and the environment
Al
Gore
Nature
Earth
Hour
Climate
Action Bondi
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