Editing IPCC:AR6/WGI/TS (section)

=== Selected Updates and/or New Results since AR5 ===

<div id="h2-2-siblings" class="h2-siblings"></div>

* '''Human influence''' <sup>[[#footnote-012|9]]</sup> '''on the climate system is now an established fact:''' The Fourth Assessment Report (AR4) stated in 2007 that ‘warming of the climate system is unequivocal’, and AR5 stated in 2013 that ‘human influence on the climate system is clear’. Combined evidence from across the climate system strengthens this finding. It is unequivocal that the increase of CO <sub>2</sub> , methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) in the atmosphere over the industrial era is the result of human activities and that human influence is the main driver <sup>[[#footnote-011|10]]</sup> of many changes observed across the atmosphere, ocean, cryosphere and biosphere. (Sections TS.1.2, TS.2.1 and TS.3.1)
* '''Observed global warming to date:''' A combination of improved observational records and a series of very warm years since AR5 have resulted in a substantial increase in the estimated level of global warming to date. The contribution of changes in observational understanding alone between AR5 and AR6 leads to an increase of about 0.1°C in the estimated warming since 1850–1900. For the decade 2011–2020, the increase in global surface temperature since 1850–1900 is assessed to be 1.09 [0.95 to 1.20] °C. <sup>[[#footnote-010|11]]</sup> Estimates of crossing times of global warming levels and estimates of remaining carbon budgets are updated accordingly. (Section TS.1.2, Cross-Section Box TS.1)
* '''Paleoclimate evidence:''' The AR5 assessed that many of the changes observed since the 1950s are unprecedented over decades to millennia. Updated paleoclimate evidence strengthens this assessment; over the past several decades, key indicators of the climate system are increasingly at levels unseen in centuries to millennia and are changing at rates unprecedented in at least the last 2000 years. (Box TS.2, Section TS.2)
* '''Updated assessment of recent warming:''' The AR5 reported a smaller rate of increase in global mean surface temperature over the period 1998–2012 than the rate calculated since 1951. Based on updated observational datasets showing a larger trend over 1998–2012 than earlier estimates, there is now ''high confidence'' that the observed 1998–2012 global surface temperature trend is consistent with ensembles of climate model simulations, and there is now ''very high confidence'' that the slower rate of global surface temperature increase observed over this period was a temporary event induced by internal and naturally forced variability that partly offset the anthropogenic surface warming trend over this period, while heat uptake continued to increase in the ocean. Since 2012, strong warming has been observed, with the past five years (2016–2020) being the hottest five-year period in the instrumental record since at least 1850 ( ''high confidence'' ). (Section TS.1.2, Cross-Section Box TS.1)
* '''Magnitude of climate system response:''' In this Report, it has been possible to reduce the long-standing uncertainty ranges for metrics that quantify the response of the climate system to radiative forcing, such as the equilibrium climate sensitivity (ECS) and the transient climate response (TCR), due to substantial advances (e.g., a 50% reduction in the uncertainty range of cloud feedbacks) and improved integration of multiple lines of evidence, including paleoclimate information. Improved quantification of ERF, the climate system radiative response, and the observed energy increase in the Earth system over the past five decades demonstrate improved consistency between independent estimates of climate drivers, the combined climate feedbacks, and the observed energy increase relative to AR5. (Section TS.3.2)
* '''Improved constraints on projections of future climate change:''' For the first time in an IPCC report, the assessed future change in global surface temperature is consistently constructed by combining scenario-based projections (which AR5 focused on) with observational constraints based on past simulations of warming as well as the updated assessment of ECS and TCR. In addition, initialized forecasts have been used for the period 2019–2028. The inclusion of these lines of evidence reduces the assessed uncertainty for each scenario. (Section TS.1.3, Cross-Section Box TS.1)
* '''Air quality:''' The AR5 assessed that projections of air quality are driven primarily by precursor emissions, including CH <sub>4</sub> . New scenarios explore a diversity of future options in air pollution management. The AR6 reports rapid recent shifts in the geographical distribution of some of these precursor emissions, confirms the AR5 finding, and shows higher warming effects of short-lived climate forcers in scenarios with the highest air pollution. (Sections TS.1.3 and TS.2.2, Box TS.7)
* '''Effects of short-lived climate forcers on global warming:''' The AR5 assessed the radiative forcing for emitted compounds. The AR6 has extended this by assessing the emissions-based ERFs also accounting for aerosol–cloud interactions. The best estimates of ERF attributed to sulphur dioxide (SO <sub>2</sub> ) and CH <sub>4</sub> emissions are substantially greater than in AR5, while that of black carbon is substantially reduced. The magnitude of uncertainty in the ERF due to black carbon emissions has also been reduced relative to AR5. (Section TS.3.1)
* '''Global water cycle:''' The AR5 assessed that anthropogenic influences have ''likely'' affected the global water cycle since 1960. The dedicated chapter in AR6 (Chapter 8) concludes with ''high confidence'' that human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century, with a better understanding of the response to aerosol and greenhouse gas changes. The AR6 further projects with ''high confidence'' an increase in the variability of the water cycle in most regions of the world and under all emissions scenarios. (Box TS.6)
* '''Extreme events:''' The AR5 assessed that human influence had been detected in changes in some climate extremes. A dedicated chapter in AR6 (Chapter 11) concludes that it is now an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since 1850, in particular for temperature extremes. Evidence of observed changes and attribution to human influence has strengthened for several types of extremes since AR5, in particular for extreme precipitation, droughts, tropical cyclones and compound extremes (including fire weather). (Sections TS.1.2 and TS.2.1, Box TS.10)

<span id="selected-updates-andor-new-results-since-ar5-and-sr1.5"></span>