Climate Change: 2013 IPCC Update

6. What are the main uncertainties regarding climate change?

• 6.1 Key Uncertainties in Observation of Changes in the Climate System
• 6.2 Key Uncertainties in the main drivers of Climate Change
• 6.3 Key Uncertainties in understanding the recent changes in the climate system
• 6.4 Key Uncertainties in Projections of Global and Regional 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.

From this combined evidence it is virtually certain that human influence has warmed the global climate system. Anthropogenic influence has been identified in changes in temperature near the surface of the earth, in the atmosphere and in the oceans, as well as changes in the cryosphere (glaciers, ice sheets and permafrost), the water cycle and some climate extremes. For average annual Northern Hemisphere temperatures, the period 1983–2012 was very likely the warmest 30-year period of the last 800 years (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence). This is supported by comparison of instrumental temperatures with multiple reconstructions from a variety of proxy data and statistical methods.

Over every continent except Antarctica, anthropogenic influence has likely made a substantial contribution to surface temperature increases since the mid-20^th century.

There is also strong evidence that excludes solar forcing, volcanoes, and internal variability as the strongest drivers of warming since 1950.

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. More...

6.1 Key Uncertainties in Observation of Changes in the Climate System

The ability of climate models to simulate surface temperature has improved in many ways, but there remains some problems that had been identified in the AR4.

There continues to be very high confidence that models reproduce the observed large-scale time-mean surface temperature patterns, although systematic errors of several degrees Celsius are found in some regions. There is high confidence that on the regional scale (sub-continental and smaller), time-mean surface temperature is better simulated than at the time of the AR4; however, confidence in model capability is lower than for the large scale.

Meanwhile, models continue to perform less well for precipitation than for surface temperature. There is very high confidence that the primary factor contributing to the spread in equilibrium climate sensitivity continues to be the cloud feedback. There is indeed a very high confidence that, consistent with observations, models show a strong positive correlation between tropospheric temperature and water vapour on regional to global scales,

However, the ability to simulate changes in frequency and intensity of extreme events remains limited. There are indeed a number of uncertainties when it comes to specific elements of the observed changes in the climate system:

• Low confidence in the “vertical structure”of the warming of the lower atmosphere and the cooling of the upper atmosphere.
• Incomplete data on global precipitation changes.
• Substantial ambiguity on cloud variability
• Low confidence in the observed trend in drought or dryness.
• Low confidence in long term changes in tropical cyclones.
• The large variability between years in large-scale atmospheric circulation makes the observation of patterns difficult.
• Poor characterization in the historical record of the heat content of the upper ocean (0– 700 m).
• The sampling is too sparse to estimate the annual global ocean temperature and heat content in the deeper ocean (below 700 m) before 2005 and deeper than 2000 m, observations are still limited.
• The continuous observation of ocean circulation is limited.
• On a global scale the mass loss from melting at calving of glaciers and ice sheets are not yet comprehensively assessed. The largest uncertainties come from Antarctica where sea ice observations are also inadequate.

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6.2 Key Uncertainties in the main drivers of Climate Change

• Uncertainties in how aerosols interact with clouds remains the main contributor to the uncertainty on man-made climate increase the greenhouse impact (forcing). This interaction is likely to contribute to such increase but its quantification remains difficult.
• Reconstructions of the history of earth climate and models of the climate system (Earth System Models) also indicate that there could be a correlation (called “ positive feedback”) between climate evolution and the evolution of the concentration of CO₂, but confidence remains low in the strength of this correlation.

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6.3 Key Uncertainties in understanding the recent changes in the climate system

• 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.
• Globally, uncertainties in observations and in the forcings, such as the one caused by aerosols, limits the understanding of many processes of the climate system: the simulation of clouds and the modelling of the water cycle remains difficult.

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6.4 Key Uncertainties in Projections of Global and Regional Climate Change

• There is medium confidence in the predictability of temperature resulting from the modelization both for the global average and for some geographical regions.
• Results for precipitation indicate a generally low predictability. Short-term climate projection is also limited by the low confidence in projections of natural forcing.
• There is low confidence in projections for a poleward shift of the position and strength of Northern Hemisphere storm tracks.
• There is generally low confidence in projections of significant trends in tropical cyclone frequency and intensity in the 21^st century.
• Projected changes in soil moisture and surface run off are not robust in many regions.
• Several components or phenomena of abrupt or nonlinear changes in the climate system remain difficult to predict.
• There is low confidence on magnitude of carbon emissions to the atmosphere from thawing permafrost. There is also limited confidence in projected methane emissions from natural sources such as wetlands or gas hydrates.
• There is medium confidence in how ice sheets will affect sea level during the 21^st century, and low confidence in projections beyond 2100.
• There is low confidence in model projections of sea level rise, and no consensus in the scientific community about their reliability.
• There is low confidence in projections of many aspects of regional climate change.

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