The IPCC and the numbers a climate question uses

Prelims and MainsCurrent affairs on this: Climate science: records, attribution and tipping points

The Intergovernmental Panel on Climate Change does no research of its own: set up in 1988 by the World Meteorological Organization and the United Nations Environment Programme, it assesses published science through three Working Groups, on the physical science, on impacts and adaptation, and on mitigation, and its Summary for Policymakers is approved line by line by governments, which is why its numbers are the ones negotiations and questions use. Its Sixth Assessment Report of 2021 to 2023 found human caused warming of 1.1°C over 2011 to 2020 against 1850 to 1900, called the human cause unequivocal, and gave a remaining carbon budget of 500 billion tonnes of carbon dioxide from 2020 for an even chance of holding 1.5°C. The seventh cycle is under way. The World Meteorological Organization's State of the Global Climate and its Greenhouse Gas Bulletin give the yearly record against which those numbers are read.

• Structure: 195 member governments; Working Group I (physical science), II (impacts, adaptation and vulnerability), III (mitigation), and a Task Force on greenhouse gas inventories, whose guidelines every country's emissions report follows. An assessment cycle runs six to seven years; the sixth ended with the Synthesis Report in March 2023, and the seventh includes a special report on cities.
• The temperature record: warming is measured against 1850 to 1900, the pre industrial baseline. The Paris goal refers to a long run average, so 2024, the first calendar year about 1.5°C above the baseline and the warmest on record, did not by itself breach it; the 1.5°C goal subject explains overshoot.
• The carbon budget: about 500 billion tonnes of carbon dioxide from the start of 2020 for a 50 per cent chance at 1.5°C, 400 for a 67 per cent chance, and about 1,150 for an even chance at 2°C, against emissions of roughly 40 billion tonnes a year, which is why the 1.5°C budget runs out around 2030 on current emissions. Every 1.5°C pathway reaches net zero carbon dioxide around 2050 and relies on some carbon removal.
• The concentrations: carbon dioxide has gone from about 280 parts per million before industry to about 420, measured since 1958 at Mauna Loa as the Keeling curve; methane warms about 80 times as much as carbon dioxide over 20 years and about 28 times over 100, and lasts about 12 years in the air, which is why cutting it works fastest.
• The scenarios: the Representative Concentration Pathways of the fifth report, named for their radiative forcing in watts per square metre (2.6, 4.5, 6.0, 8.5), were joined in the sixth by the Shared Socioeconomic Pathways, which add population and growth stories, so a scenario now reads SSP1 to 2.6 or SSP5 to 8.5; the emission scenarios subject follows what the seventh cycle has changed.

Mains: The IPCC settles what is known, not what is to be done: its budgets and pathways are the shared arithmetic, and the argument at every conference is about who spends what is left of the budget, which is a question of equity the science cannot answer.

Further reading: AR6 Synthesis Report: Climate Change 2023 (IPCC) · State of the Global Climate (WMO)

See also: The 1.5°C goal and overshoot · Emission scenarios: from RCP8.5 to CMIP7 · Extreme event attribution · Climate tipping points · The 4.2 ka event · The Paris Agreement: how it works · Net zero 2070 and the National Action Plan on Climate Change