IPCC Sixth Assessment Report

The report assembled three assessments and a final synthesis between 2021 and 2023. Working Group I published Climate Change 2021: The Physical Science Basis, establishing the state of knowledge about the climate system and human influence.¹ Working Group II addressed impacts, adaptation and vulnerability, while Working Group III assessed mitigation of climate change.²³ The Synthesis Report, approved in March 2023, integrates their findings for policymakers.⁴

As with earlier IPCC assessments, the reports were written by international expert teams and reviewed in multiple stages. Their summaries for policymakers were approved line by line by governments, while the underlying chapters retain the scientific evidence, uncertainties and references on which the conclusions rest.

The assessment concludes that human activities have unequivocally caused global warming, with global surface temperature reaching about 1.1°C above 1850–1900 levels in 2011–2020.¹ Human influence has warmed the atmosphere, ocean and land, and has contributed to widespread and rapid changes in the cryosphere and biosphere. Each additional increment of warming intensifies multiple hazards, including heat extremes, heavy precipitation, drought in some regions, ocean warming, acidification and sea-level rise.

Some changes are effectively irreversible over centuries to millennia, particularly those involving the deep ocean, ice sheets and sea level. The report also emphasizes that climate risks interact with one another and with social inequalities; compound events can produce effects greater than those of individual hazards considered separately.

Climate change is already damaging human well-being and ecosystems, while adaptation can reduce but not eliminate the resulting risks. Working Group II documents observed losses and damages, climate-sensitive health risks, food and water insecurity, biodiversity decline, displacement and threats to infrastructure.² It identifies limits to adaptation, especially where warming, financial constraints or ecosystem degradation push societies beyond feasible responses.

Working Group III finds that limiting warming requires rapid, deep and sustained reductions in greenhouse-gas emissions, reaching net zero carbon dioxide around mid-century for 1.5°C pathways.³ Options already available include renewable electricity, energy efficiency, electrification, methane reduction, ecosystem protection and demand-side measures. Their feasibility depends on finance, institutions, equity, technology and international cooperation rather than technology alone.

The report’s less prominent contribution is its treatment of uncertainty, timing and distribution rather than a single forecast. It distinguishes between confidence in evidence, the likelihood of outcomes and the magnitude of consequences, allowing decision-makers to act even when precise regional projections remain difficult.

Its regional chapters and interactive atlas provide detailed information for places that global averages can obscure, including small islands, mountains, cities, drylands and the Arctic.¹ The synthesis also introduces the idea of “climate resilient development,” linking mitigation, adaptation, poverty reduction and sustainable development.⁴ A further distinctive feature is its attention to compound and cascading risks—for example, simultaneous heat, drought, crop failure, energy stress and health impacts—and to “overshoot” pathways in which warming temporarily exceeds a target before declining.

Anthropogenic climate change

Climate change caused by human activities, especially greenhouse-gas emissions and land-use change.

Net zero CO2

A condition in which human-caused carbon dioxide emissions are balanced by removals of carbon dioxide from the atmosphere.

Adaptation

The process of adjusting to actual or expected climate effects to moderate harm or take advantage of beneficial opportunities.

Climate-resilient development

Development that combines adaptation and mitigation with sustainable development while reducing vulnerability to climate impacts.

Overshoot

A temporary exceedance of a specified warming or concentration threshold followed by a later decline.