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Climate Change IPCC 2013 Update
Context - "Climate Change 2013: The Physical Science Basis" is a comprehensive assessment of the physical aspects of climate change, which puts a focus on the elements that are relevant to understand past, document current, and project future climate change.
The report covers observations of changes in all components of the climate system and assess the current knowledge of various processes of the climate system.
Direct global-scale instrumental observation of the climate began in the middle of the 19th century, and reconstruction of the climate using proxies such as tree rings or the content of sediment layers extends the record much further in the past.
The present assessment uses a new set of new scenarios to explore the future impacts of climate change under a range of different possible emission pathways.
This Digest is a faithful summary of the leading scientific consensus report produced in 2013 by the Intergovernmental Panel on Climate Change (IPCC): "Climate Change 2013: Technical
Summary" Learn more...
- Source document:IPCC (2013)
- Summary & Details: GreenFacts
1. How are uncertainties handled by the IPCC?
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Although both the body of knowledge on the climate system, and
the confidence in projections are growing, there are still many
uncertainties in climate science. An integral element of the IPCC
5th Assessment Report (AR5) is the use of a specific
uncertainty language to reflect accurately the strength of each
Where appropriate, findings are formulated as statements of fact, but
when a qualifier is needed, there are two systems that will be applied.
The first one is, the level of confidence in the validity of a
finding is based on the type, amount, quality and consistency of evidence
(e.g., data, mechanistic understanding, theory,
models, expert judgment), and on
the degree of agreement. This goes from low agreement – limited evidence
to high agreement –robust evidence
The second one is the level of likelihood of a finding, which expres
the quantified measures of uncertainty as a probability.
When confidence or likelihood terms are used in this summary, these are
indicated in italics.
The following terms have been used to indicate the assessed
Table 1.2 - Likelihood terms associated with outcomes used in the
||Likelihood of the outcome
|* Additional terms (extremely likely:
95–100% probability, more likely than not:
>50–100% probability, and extremely unlikely:
0–5% probability) may also be used when appropriate.
About as likely as not
2. What have been the observed climate changes in the last centuries?
It is certain that the global
mean surface temperature of the
earth has increased since the beginning of the instrumental record. This
warming has been about 0.85°C from 1880 to 2012 with an increase of
about 0.72°C from 1951 to 2012. Each of the last three decades has
successively been the warmest on record. They also have very
likely been the warmest in the last 800 years and likely
the warmest in the last 1400 years even if the rate of warming over the
last 15 years is smaller than the rate since the 1950s.
The upper ocean (above 700 m)
has warmed over the course of the 20th century (certain,
the ocean warmed between 700 and 2000 m (likely,
and from 3000 m to the bottom (likely, but no
significant trend was observed between 2000 m and 3000 m.
Since at least circa 1970, the planet has been in energy imbalance,
with more energy from the sun entering than exiting the top of the
atmosphere (this is what is
called “radiative forcing”) and the
warming of the oceans accounts for most (93%) of the increase in energy
On the global scale, it is not clear if there were changes in
precipitation and cloud cover, in part because there is insufficient
data. The humidity of the lower
atmosphere has very likely
increased since the 1970s, but it is not clear what changes in
precipitations this has led to.
Changes have been observed in ocean properties (warming, changes in
salinity, increase in
carbon content and acidity,
decrease in oxygen concentration) during the past forty years. The
observed patterns of changes are consistent with a response to
There is very high confidence that the Arctic
sea ice extent has
decreased over the period 1979–2012. By contrast, it is very
likely that the Antarctic sea ice extent increased between 1979 and
2012, due to a decrease in the percentage of open water within the ice
pack. There is high confidence that parts of Antarctica
floating ice shelves are undergoing substantial changes.
There is very high confidence that terrestrial
shrunk world wide in the last decades and that they will continue to melt.
Permafrost has also been
warming all around the globe. In the Northern Hemisphere, snow cover has
It is virtually certain that the rate of global
mean sea level rise has
accelerated from relatively low rates in the order of tenths of mm per
year in the past millennia to the present rates in the order of mm per
year. More specifically, the global mean sea level has risen by 0.19
[0.17 to 0.21] m over the period 1901– 2010. The current rate of global
mean sea level is, with medium confidence, unusually high in
the context of the last two millennia.
It is very likely that the number of cold days and nights has
decreased and the number of warm days and nights has increased on the
global scale. Globally, the length and frequency of warm spells and heat
waves has increased since the middle of the 20th century. It
is likely that since 1950 the number of heavy precipitation
events over land has increased in more regions than it has decreased. It
is virtually certain that the frequency and intensity of storms
in the North Atlantic have increased since the 1970s although the reasons
for these increases are debated. Changes in droughts and floods are more
variable from region to region.
In 2011, the atmospheric
concentrations of the greenhouse gases
carbon dioxide (CO2),
and nitrous oxide (N2O),
exceed the range of concentrations recorded in
ice cores, and their rate of
rise also exceeds what is known from ice cores. The main source of
CO2 is the burning of
fossil fuels and the production
of cement. About half of this carbon ends up in the
atmosphere and the rest is
taken up by plants or by the
3. What makes the climate change?
Changes in climate are due to an imbalance between the energy from the sun received
by the earth and the energy that is radiated back to space (this
imbalance is called ‘radiative forcing’).
Over the period of the industrial era, since 1750, solar and volcanic
forcings are the two dominant natural contributors to global
climate change. There is a
high confidence that solar forcing is much smaller than the
forcing caused by greenhouse (gasses). The impact of volcanic
particles is now well
understood and there is a large negative forcing for a few years after
major volcanic eruptions such as the eruption of Mount Pinatubo in
Human activity leads to change in the
atmospheric composition either
directly (via emissions of gases or
particles) or indirectly (via
Anthropogenic emissions have driven
the changes in greenhouse gas
concentrations during the industrial era (since 1750). Over the last
15 years, CO2 has been the dominant contributor to
greenhouse gases, the main
others being methane,
nitrous oxide and
halocarbons. The contribution of each gas is usually expressed in terms
of Global Warming Potential (GWP) or of Global Temperature change
Potential (GTP). On the one hand, the GWP compares a greenhouse gas to
CO2 and is
expressed as an equivalent amount of CO2. The GTP, on the other hand
estimates the temperature change caused by a specific gas, and includes
the response of the climate system.
Aerosols are tiny liquid droplets
or particles (such as dust from
volcanœs) that are in suspension in the
atmosphere. Overall, aerosols
cause a cooling of the atmosphere, but with a large range of
uncertainty, there is a high confidence, however, that aerosols
have offset a substantial portion of the forcing due to
greenhouse gases. There is
robust evidence that
land use changes, such as
deforestation, has affected
the albedo (the reflectivity of the solar
radiation), which is different
in a darker green forest than in a paler field and thus,
the energy balance. Persistent contrails from aviation also contribute
Feedback mechanisms also play an
important role in determining (future)
climate change. For
- Snow and ice albedo
feedbacks: the warmer it gets
the less snow there is, the darker and hotter the ground is, the
warmer it gets.
- There can also be feedbacks in cloud cover, although there are
still large uncertainties attached to their importance and
4. How do we study the climate system?
Understanding of the climate system results from combining
observations, theoretical studies of mechanisms and
feedback processes, and
model simulations. Compared
to the 4th assessment report published in 2007, more detailed
observations and improved climate
models now enable the attribution
of the observed climate changes to human influences in more components
of the climate system.
It is extremely likely that human activities caused more than
half of the observed increase in global average surface temperature from
1951 to 2010, with greenhouse gases
contributing a warming between 0.5°C and 1.3°C over this period.
The observed global-mean surface
temperature (GMST) has shown a much smaller increase over the past 15
years than over the past 30 to 60 years. Changes in
radiative forcings between
decades show that for the period 1998-2011 forcing was indeed two thirds
of what it had been for the 1984-1998 period. Globally, it is very
likely that over that period the climate system has continued
to accumulate energy, for instance in the form of increasing ocean heat.
However, since some data show a slowing, and some do not, whether there
has been a slowdown in that recent period remains
The most convincing way of establishing the credibility of the
models used in
climate change science is
arguably the verification of their projections. It appears that results
of projected changes in CO2, global
mean surface temperature and
global mean sea level from previous IPCC assessment reports are in
general agreement with the observed trends.
Various oceanic parameters have been monitored and modelled in terms
of their response to climate
change. It is very likely that human influence made a
substantial contribution to the upper ocean warming (the upper 700m
layer) that has been observed since the 1970s. This warming has in turn
contributed to a global sea level rise through thermal expansion. It is
estimated that most of the energy that has been added to the climate
system has been absorbed by the oceans. Likewise, changes in
salinity, oxygen content
and acidity can also be attributed
to human influence.
The reductions in Arctic sea ice
extent and Northern hemisphere snow cover extent and widespread
(retreat)- and increased surface melt of Greenland are all evidence of
overall changes in snow and ice linked to increased
radiative forcing of
The possibility of irreversible changes, the rate and
magnitude of global climate changes
is determined by radiative forcings,
climate feedbacks and storage
of energy by the climate system. For some elements of the climate
system, there is a point where an abrupt change might happen once a
threshold is reached. These abrupt changes can be irreversible –
meaning that it takes a lot longer for the system to recover than
it takes to shift to the new state – transitions to different states of
the climate system.
- Changes in the Atlantic meridional overturning circulation
(AMOC) could produce abrupt climate changes at global scale and on
the climate of Europe and North America.
- In a warming climate, the thawing of
permafrost could lead to
the release of carbon accumulated in frozen soils, leading to
CO2 and methane
concentrations, and to further warming.
- Since the growth of the ice
sheets is a very slow process, any increase in the loss
of ice, either through melt or ice outflow would be irreversible.
5. What changes are projected in the climate system in the future?
Projections of changes in the climate system are made using a range of
climate models that simulate
changes based on a set of scenarios
of anthropogenic forcings. A new
set of scenarios, the Representative Concentration
(RCPs), was used for the climate
model simulations carried out
for this assessment. These scenarios typically include economic,
demographic, energy and simple climate components. The scenarios used in
this assessment that explore what those emissions might be have
different targets in terms in radiative
forcings by 2100, which range from a ‘strong mitigation’
scenario to a scenario of
continuing growth in emissions.
Between 2012 and 2100, depending on the
scenario, Earth System
Models (ESM) results imply
cumulative fossil fuel emissions
ranging between 270 and 1685 gigatonnes of carbon
1 1 Petagram of carbon = 1 PgC = 1 Gigatonne of carbon = 1
GtC. This corresponds to 3.67 GtCO2.
In the absence of major volcanic eruptions—which would cause
significant but temporary cooling—and, assuming no significant future
long term changes in solar irradiance, it is likely that the
global mean surface temperature
will be higher by 0.3°C to 0.7°C, during the period 2016–2035 compared
to 1986–2005 (medium confidence). Global mean temperatures will
continue to rise over the 21st century under all of the
scenarios. From around the
mid-21st century, the rate of global warming begins to be
more strongly dependent on the
scenario: the likely
global-mean surface temperatures increases are projected as 0.3 to
Ocean temperatures will very likely continue to increase over
the course of the 21st century. In some regions by the end of
the century, ocean warming is projected to exceed 0.5°C to 2.5°C in the
top few hundred meters and 0.3°C to 0.7°C at a depth of about 1 km. The
sea level are also projected to continue rising, to between 0.26 to 0.81
m before the end of the 21st century. It is virtually certain
that sea level rise will continue beyond 2100, and go on for centuries
A nearly ice-free Arctic Ocean (sea
ice extent less than 1000000 km2) in September is
likely before mid-century under the highest emission
scenario, with medium
confidence. It is very likely that there will be further
shrinking and thinning of Arctic sea ice cover, and decreases of
northern high-latitude spring time snow cover and near surface
permafrost as global
mean surface temperature
As regards the potential for stabilizing the climate, such climate
stabilization can mean in practice
- to stabilize of greenhouse
gas concentrations in the
atmosphere at a level that
would prevent dangerous
anthropogenic interference with
the climate system, which is the ultimate objective of the United
Nations Framework Convention on
Climate Change (UNFCCC) ;
- to limit the global temperature increase. The most widely discussed being
2°C above pre industrial levels;
- to return the level of
below 350 ppm.
One approach to reach climate stabilization is geo-engineering,
defined as the deliberate large-scale intervention in the Earth system
to counter undesirable impacts of climate
change on the planet, such as large-scale carbon capture and
storage, or solar radiation
management through the injection of
aerosols in the
Assessing changes in climate extreme events poses unique challenges,
not just because of the rare nature of these events, but because they
invariably happen in conjunction with disruptive conditions. For the
near- and long-term, scenario’s projections confirm a clear
tendency for increases in heavy precipitation events, although with
large regional variations. For extreme events such as floods, droughts,
and cyclones there are still a lot of uncertainties when it
comes to establishing a trend of change or establishing projections.
6. What are the main uncertainties regarding climate change?
Human influence has been detected in nearly all of the major assessed
components of the climate system. Taken together, the combined evidence
increases the overall level of confidence in the attribution of observed
climate change, and reduces
the uncertainties associated with assessment based on a single climate
variable. The coherence of observed changes with simulations of
anthropogenic and natural
forcing in the physical system is remarkable. However, a series of
uncertainties remain. Understanding of the sources and means of
characterizing uncertainties in long-term large-scale projections of
climate change has not changed significantly since the previous report,
but new experiments and studies have continued to work towards a more
complete and rigorous characterization.
The ability of climate models to
simulate surface temperature has improved in many ways, but there are
still a number of uncertainties when it comes to specific elements of the
observed changes in the climate system.
Uncertainties in how aerosols
interact with clouds remains the main contributor to the uncertainty on
man-made climate change.
In some aspects of the climate system, including droughts, tropical
cyclone activity, Antarctic warming, sea
ice extent, and glacier
mass balance, the confidence remains low in attributing changes
to human influence due to modelling
uncertainties and low agreement between scientific studies.
There are also several areas in which projections of
climate change remain
difficult: the projections of global and regional climate change and
precipitation, a poleward shift of the position and strength of Northern
Hemisphere storm tracks, the trends in tropical cyclone frequency and
intensity in the 21st century, the changes in soil moisture
and surface run off the magnitude of carbon emissions to the
atmosphere from thawing
methane emissions from
natural sources such as wetlands or gas
- There is also medium confidence on how
ice sheets will affect sea
level during the 21st century, and low confidence in
model projections of
sea level rise, and no
consensus in the scientific
community about their reliability.
- Eventually, there is low confidence in projections on many
aspects of regional climate