The Intergovernmental Panel on Climate Change (IPCC) was jointly established by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) in 1988. It is the United Nations body for assessing science related to climate change. Since 1988, the IPCC has released six major assessment reports in 1990 (First Assessment Report), 1995 (Second Assessment Report), 2001 (Third Assessment Report), 2007 (Assessment Report 4), 2013/14 (Assessment Report 5) and 2021/22 (Assessment Report 6). Each major assessment report has involved the contributions of thousands of scholars from around the world based on the examination of the published and gray literature on a wide range of topics related to climate change.
Each major report and the IPCC itself is subdivided into three main working groups: WG 1 focuses on the physical science of climate change, WG 2 focuses on the impacts of climate change, vulnerability and adaptation and WG 3 focuses on climate change mitigation/decarbonization. The full text of each report is usually well over a thousand pages long, but there are also synthesis reports of the entire enterprise, summary reports for policy makers and technical summary reports for each working group which are much shorter and are often cited. The IPCC also has a powerful digital visualization tool called the Interactive Atlas.
Each report has a number of chapters.
WG1 has chapters on observed climate, human influence, future scenarios, global biogeochemical cycles, short lived climate forcing, energy budget, water cycle, ocean, cryosphere and sea level change, regional climates, extreme events, and risk assessment.
WG2 has chapters on terrestrial ecosystem services, ocean ecosystem services, water, food security, cities and infrastructure, health, sustainable development, regionally focused chapters on each continent, risks across sectors, decision making and climate resilient development.
WG3 has chapters on emissions trends and drivers, mitigation pathways, demand, services and social aspects of mitigation, energy systems, agriculture, forestry, land use, urban systems, buildings, transport, industry, policy, international cooperation, finance, technology transfer, and transition to sustainable development.
In addition to the major assessment reports, the IPCC also releases some specialized reports, such as the Special Report on 1.5 C, and the Report on Short Lived Climate Forcing. One of the cornerstones of the major assessment reports are the efforts to envision different future socioeconomic scenarios. In the early reports, these were called the Special Reports on Emissions Scenarios (or SRES). In the Fifth Assessment Report of the IPCC (AR5 2013/14), the terminology and some of the framing was changed to call this endeavor the Representative Concentration Pathways (RCPs). In the Sixth Assessment Report (AR6 2021/22), the terminology and framing was changed again and this endeavor was called the Shared Socioeconomic Pathways (SSPs).
But regardless of the framing, the details and the specific terminology, each of these scenario development activities has sought to develop a number of possible imagined futures for the world based on different assumptions about population growth, economic development, energy use, technological advancement, etc.. Each of these different scenarios corresponds to differing levels and paces of GHG emissions, which in turn correspond to differing levels of radiative forcing and consequently different pathways of warming for the planet. The SSPs in AR6 are still connected to the radiative forcing metrics used in the scenarios for AR5 and correspond to radiative forcing levels between about 2 and 8.5 Wm-2 by the end of the 21st century. More information about the storylines for the scenarios is available on this webpage and this explainer. While the actual pathway of population growth, economic development, energy use, emissions, etc., that the world takes will likely be different than any of these imagined futures, the intention behind this activity is to imagine a range of future scenarios that span most possible future scenarios so that we can be prepared for the future we are facing and make some critically informed decisions about which type of pathway we should strive to choose and what some of the key tradeoffs are in the process. The radiative forcing of the SSPs, RCPs, etc. are also used as inputs in the network of global climate models through the Coupled Model Intercomparison Project (CMIP).
Over time, the conclusions of the IPCC regarding the evidence for climate change and the seriousness of the problem have grown consistently more strident. The language that is used in the high level reports and summaries is carefully structured to correspond to designated probability ranges of different outcomes. A great deal of work and collaboration goes into each report.
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