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IPCC AR5 TS.6 Key Uncertainties

This final section of the Technical Summary provides readers with a
short overview of key uncertainties in the understanding of the climate
system and the ability to project changes in response to anthropogenic
influences. The overview is not comprehensive and does not describe in
detail the basis for these findings. These are found in the main body of
this Technical Summary and in the underlying chapters to which each
bullet points in the curly brackets.
TS.6.1 Key Uncertainties in Observation of Changes in
the Climate System
• There is only medium to low confidence in the rate of change of
tropospheric warming and its vertical structure. Estimates of tropospheric
warming rates encompass surface temperature warming
rate estimates. There is low confidence in the rate and vertical
structure of the stratospheric cooling. {2.4.4}
• Confidence in global precipitation change over land is low prior
to 1951 and medium afterwards because of data incompleteness.
{2.5.1}
• Substantial ambiguity and therefore low confidence remains in the
observations of global-scale cloud variability and trends. {2.5.6}
• There is low confidence in an observed global-scale trend in
drought or dryness (lack of rainfall), due to lack of direct observations,
methodological uncertainties and choice and geographical
inconsistencies in the trends. {2.6.2}
• There is low confidence that any reported long-term (centennial)
changes in tropical cyclone characteristics are robust, after
accounting for past changes in observing capabilities. {2.6.3}
• Robust conclusions on long-term changes in large-scale atmospheric
circulation are presently not possible because of large variability
on interannual to decadal time scales and remaining differences
between data sets. {2.7}
• Different global estimates of sub-surface ocean temperatures have
variations at different times and for different periods, suggesting
that sub-decadal variability in the temperature and upper heat
content (0 to to 700 m) is still poorly characterized in the historical
record. {3.2}
• Below ocean depths of 700 m the sampling in space and time is
too sparse to produce annual global ocean temperature and heat
content estimates prior to 2005. {3.2.4}
• Observational coverage of the ocean deeper than 2000 m is still
limited and hampers more robust estimates of changes in global
ocean heat content and carbon content. This also limits the quantification
of the contribution of deep ocean warming to sea level
rise. {3.2, 3.7, 3.8; Box 3.1}
• The number of continuous observational time series measuring the
strength of climate relevant ocean circulation features (e.g., the
meridional overturning circulation) is limited and the existing time
series are still too short to assess decadal and longer trends. {3.6}.
• In Antarctica, available data are inadequate to assess the status
of change of many characteristics of sea ice (e.g., thickness and
volume). {4.2.3}
• On a global scale the mass loss from melting at calving fronts and
iceberg calving are not yet comprehensively assessed. The largest
uncertainty in estimated mass loss from glaciers comes from the
Antarctic, and the observational record of ice–ocean interactions
around both ice sheets remains poor. {4.3.3, 4.4}
TS.6.2 Key Uncertainties in Drivers of Climate Change
• Uncertainties in aerosol–cloud interactions and the associated
radiative forcing remain large. As a result, uncertainties in aerosol
forcing remain the dominant contributor to the overall uncertainty
in net anthropogenic forcing, despite a better understanding of
some of the relevant atmospheric processes and the availability of
global satellite monitoring. {2.2, 7.3–7.5, 8.5}
• The cloud feedback is likely positive but its quantification remains
difficult. {7.2}
• Paleoclimate reconstructions and Earth System Models indicate
that there is a positive feedback between climate and the carbon
cycle, but confidence remains low in the strength of this feedback,
particularly for the land. {6.4}
TS.6.3 Key Uncertainties in Understanding the Climate
System and Its Recent Changes
• The simulation of clouds in AOGCMs has shown modest improvement
since AR4; however, it remains challenging. {7.2, 9.2.1, 9.4.1,
9.7.2}
• Observational uncertainties for climate variables other than temperature,
uncertainties in forcings such as aerosols, and limits in
process understanding continue to hamper attribution of changes
in many aspects of the climate system. {10.1, 10.3, 10.7}
• Changes in the water cycle remain less reliably modelled in both
their changes and their internal variability, limiting confidence in
attribution assessments. Observational uncertainties and the large
effect of internal variability on observed precipitation also precludes
a more confident assessment of the causes of precipitation
changes. {2.5.1, 2.5.4, 10.3.2}
• Modelling uncertainties related to model resolution and incorporation
of relevant processes become more important at regional
scales, and the effects of internal variability become more significant.
Therefore, challenges persist in attributing observed change
to external forcing at regional scales. {2.4.1, 10.3.1}
• The ability to simulate changes in frequency and intensity of
extreme events is limited by the ability of models to reliably simulate
mean changes in key features. {10.6.1}
• In some aspects of the climate system, including changes in
drought, changes in tropical cyclone activity, Antarctic warming,
Antarctic sea ice extent, and Antarctic mass balance, confidence
in attribution to human influence remains low due to modelling
uncertainties and low agreement between scientific studies.
{10.3.1, 10.5.2, 10.6.1}
TS.6.4 Key Uncertainties in Projections of Global and
Regional Climate Change
• Based on model results there is limited confidence in the predictability
of yearly to decadal averages of temperature both for the
global average and for some geographical regions. Multi-model
results for precipitation indicate a generally low predictability.
Short-term climate projection is also limited by the uncertainty in
projections of natural forcing. {11.1, 11.2, 11.3.1, 11.3.6; Box 11.1}
• There is medium confidence in near-term projections of a northward
shift of NH storm track and westerlies. {11.3.2}
• There is generally low confidence in basin-scale projections of significant
trends in tropical cyclone frequency and intensity in the
21st century. {11.3.2, 14.6.1}
• Projected changes in soil moisture and surface run off are not
robust in many regions. {11.3.2, 12.4.5}
• Several components or phenomena in the climate system could
potentially exhibit abrupt or nonlinear changes, but for many phenomena
there is low confidence and little consensus on the likelihood
of such events over the 21st century. {12.5.5}
• There is low confidence on magnitude of carbon losses through
CO2 or CH4 emissions to the atmosphere from thawing permafrost.
There is low confidence in projected future CH4 emissions
from natural sources due to changes in wetlands and gas hydrate
release from the sea floor. {6.4.3, 6.4.7}
• There is medium confidence in the projected contributions to sea
level rise by models of ice sheet dynamics for the 21st century, and
low confidence in their projections beyond 2100. {13.3.3}
• There is low confidence in semi-empirical model projections of
global mean sea level rise, and no consensus in the scientific community
about their reliability. {13.5.2, 13.5.3}
• There is low confidence in projections of many aspects of climate
phenomena that influence regional climate change, including
changes in amplitude and spatial pattern of modes of climate variability.
{9.5.3, 14.2–14.7}