The assessed pathways describe integrated, quantitative evolutions of all emissions over the 21st century associated with global energy and land use and the world economy. Since 2000, the estimated level of human-induced warming has been equal to the level of observed warming with a likely range of ☒0% accounting for uncertainty due to contributions from solar and volcanic activity over the historical period ( high confidence).

Accordingly, warming from pre- industrial levels to the decade 2006–2015 is assessed to be 0.87☌ ( likely between 0.75☌ and 0.99☌). For periods shorter than 30 years, warming refers to the estimated average temperature over the 30 years centred on that shorter period, accounting for the impact of any temperature fluctuations or trend within those 30 years. Unless otherwise specified, warming is expressed relative to the period 1850–1900, used as an approximation of pre-industrial temperatures in AR5. Global warming is defined in this report as an increase in combined surface air and sea surface temperatures averaged over the globe and over a 30-year period. Human-induced warming reached approximately 1☌ ( likely between 0.8☌ and 1.2☌) above pre-industrial levels in 2017, increasing at 0.2☌ ( likely between 0.1☌ and 0.3☌) per decade ( high confidence). This chapter frames the context, knowledge-base and assessment approaches used to understand the impacts of 1.5☌ global warming above pre-industrial levels and related global greenhouse gas emission pathways, building on the IPCC Fifth Assessment Report (AR5), in the context of strengthening the global response to the threat of climate change, sustainable development and efforts to eradicate poverty.

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