Editing IPCC:AR6/WGI/TS (section)

==== TS.1.2.3 Understanding Climate Variability and Emerging Changes ====

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'''Observed changes in climate are unequivocal at the global scale and are increasingly apparent on regional and local spatial scales. Both the rate of long-term change and the amplitude of year-to-year variations differ between regions and across climate variables, thus influencing when changes emerge or become apparent compared to natural variations (see Emergence in Core Concepts Box). The signal of temperature change has emerged more clearly in tropical regions, where year-to-year variations tend to be small over land, than in regions with greater warming but larger year-to-year variations ( ''high confidence'' ) (Figure TS.3). Long-term changes in other variables have emerged in many regions, such as for some weather and climate extremes and Arctic sea ice area. Links to chapters 1.4.2, Cross-Chapter Box 3.1, 9.3.1, 11.3.2, 12.5.2'''
Observational datasets have been extended and improved since AR5, providing stronger evidence that the climate is changing and allowing better estimates of natural climate variability on decadal time scales. There is ''very high confidence'' that the slower rate of global surface temperature change observed over 1998–2012 compared to 1951–2012 was temporary, and was, with ''high confidence'' , induced by internal variability (particularly Pacific Decadal Variability) and variations in solar irradiance and volcanic forcing that partly offset the anthropogenic warming over this period. Global ocean heat content continued to increase throughout this period, indicating continuous warming of the entire climate system ( ''very high confidence'' ). Hot extremes also continued to increase during this period over land ( ''high confidence'' ). Even in a continually warming climate, periods of reduced and increased trends in global surface temperature at decadal time scales will continue to occur in the 21st century ( ''very high confidence'' ). Links to chapters Cross-Chapter Box 3.1, 3.3.1, 3.5.1, 4.6.2, 11.3.2

Since AR5, the increased use of ‘large ensembles’, or multiple simulations with the same climate model but using different initial conditions, supports improved understanding of the relative roles of internal variability and forced change in the climate system. Simulations and understanding of modes of climate variability, including teleconnections, have improved since AR5 ( ''medium confidence'' ), and larger ensembles allow a better quantification of uncertainty in projections due to internal climate variability. Links to chapters 1.4.2, 1.5.3, 1.5.4, 4.2, 4.4.1, Box 4.1, 8.5.2, 10.3.4, 10.4

Changes in regional climate can be detected even though natural climate variations can temporarily increase or obscure anthropogenic climate change on decadal time scales. While anthropogenic forcing has contributed to multi-decadal mean precipitation changes in several regions, internal variability can delay emergence of the anthropogenic signal in long-term precipitation changes in many land regions ( ''high confidence'' ). Links to chapters 10.4

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'''Figure TS.3 |''' '''Emergence of changes in temperature over the historical period.''' ''The intent of this figure is to show how observed changes in temperature have emerged and that the emergence pattern agrees with model simulations.'' The observed change in temperature at a global warming level of 1°C (a), and the signal-to-noise ratio (the change in temperature at a global warming level of 1°C, divided by the size of year-to-year variations, ( b) ) using data from Berkeley Earth. The right panels show the zonal means of the maps and include data from different observational datasets (red) and the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations (black, including the 5–95% range) processed in the same way as the observations. Links to chapters 1.4.2, 10.4.3

Mean temperatures and heat extremes have emerged above natural variability in almost all land regions with ''high confidence'' . Changes in temperature-related variables, such as regional temperatures, growing season length, extreme heat and frost, have already occurred, and there is ''medium confidence'' that many of these changes are attributable to human activities. Several impact-relevant changes have not yet emerged from natural variability but will emerge sooner or later in this century depending on the emissions scenario ( ''high confidence'' ). Ocean acidification and deoxygenation have already emerged over most of the global open ocean, as has a reduction in Arctic sea ice ( ''high confidence'' ). Links to chapters 9.3.1, 9.6.4, 11.2, 11.3, 12.4, 12.5, Atlas.3–Atlas.11

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