Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.3.5 Synthesis of Evidence for Past Changes ===

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([[#2.3|Section 2.3]] has assessed the observational evidence for changes in key indicators across the atmosphere, cryosphere, ocean and biosphere starting, where applicable, from paleoclimate proxy records and coming up to the present day. This synthesis serves as an assessment of the evidence for change across the climate system as represented by the instrumental record and its unusualness in the longer-term context. Building upon previous sections assessing the observational evidence for each key indicator individually, this section integrates the evidence across multiple indicators to arrive at a holistic and robust final assessment.

Climate has varied across a broad range of timescales (Figure 2.34). During the Cenozoic Era temperatures generally decreased over tens of millions of years, leading to the development of ice sheets. During the last two million years, climate has fluctuated between glacials and interglacials. Within the current Holocene interglacial and, with increasing detail in the CE, it is possible to reconstruct a history both of more indicators of the climate system and, with increasing fidelity, the rates of change. Solely for the last 150 years or so are instrumental observations of globally distributed climate indicators available. However, only since the late 20th century have observational systems attained essentially global monitoring capabilities. The direct observations point unequivocally to rapid change across many indicators of the climate system since the mid-19th century. These are all consistent in indicating a world that has warmed rapidly.

Assessing the long-term context of recent changes is key to understanding their potential importance and implications. The climate system consists of many observable aspects that vary over a very broad range of timescales. Some biogeochemical indicators of change such as atmospheric CO <sub>2</sub> concentrations <sub></sub> and ocean pH have shifted rapidly and CO <sub>2</sub> concentrations <sub></sub> are currently at levels unseen in at least 800 kyr (the period of continuous polar ice-core records) and ''very likely'' for millions of years. The GMST in the past decade is ''likely'' warmer than it has been on a centennially-averaged basis in the CE and ''more likely than not'' since the peak of the LIG. Many more integrative components of the climate system (e.g., glaciers, GMSL) are experiencing conditions unseen in millennia, whereas the most slowly responding components (e.g., ice-sheet extent, permafrost, tree line) are at levels unseen in centuries (''high confidence''). The rate at which several assessed climate indicators (e.g., GMSL, OHC, GSAT) have changed over recent decades is highly unusual in the context of preceding slower changes during the current post-glacial period (''high confidence'').

In summary, directly observed changes in the atmosphere, ocean, cryosphere and biosphere represent unequivocal evidence of a warming world. Key climate indicators are now at levels not experienced for centuries to millennia. Since the late 19th century many indicators of the global climate system have changed at a rate unprecedented over at least the last two thousand years.

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[[File:3453e57fb1f33e44b1faadf8425fd5f6 IPCC_AR6_WGI_Figure_2_34.png]]

'''Figure 2.34 |''' '''Selected large-scale climate indicators during paleoclimate and recent reference periods of the Cenozoic Era.''' Values are based upon assessments carried out in this chapter, with ''confidence'' levels ranging from ''low'' to ''very high'' . Refer to Cross-Chapter Box 2.1 for description of paleoclimate reference periods and [[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] for recent reference periods. Values are reported as either the ''very likely'' range (x to y), or best estimates from beginning to end of the reference period with no stated uncertainty (x → y), or lowest and highest values with no stated uncertainty (x ~ y). Temperature is global mean surface temperature. Glacier extent is relative and colour scale is inverted so that more extensive glacier extent is intuitively blue.

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