The IPCC finds that our planet's climate is getting hotter due to human activities emitting Carbon Dioxide and other greenhouse gases (GHGs), with a range of consequences causing increasingly severe harm to humans and other living beings. We need to adapt to unavoidable changes whilst acting to minimise avoidable ones, most importantly by reducing the amount of CO2 and other GHGs we emit to net zero (i.e. if we emit some somewhere, we must actively remove them elsewhere). Tools we have for doing this include switching to low carbon-emitting energy sources such as renewables and nuclear, and using Carbon Capture and Storage (CCS) with carbon-emitting fuels such as fossil fuels and biomass.
The Intergovernmental Panel on Climate Change is a body of the United Nations that is dedicated to providing the world with objective, scientific information relevant to understanding the scientific basis of the risk of human-induced climate change, its natural, political, and economic impacts and risks, and possible response options. Thousands of scientists and other experts contribute on a voluntary basis to writing and reviewing reports, which are then reviewed by governments. IPCC reports contain a "Summary for Policymakers", which is subject to line-by-line approval by delegates from all participating governments. Typically, this involves the governments of more than 120 countries. The IPCC provides an internationally accepted authority on climate change, producing reports that have the agreement of leading climate scientists and consensus from participating governments. The mandate of the IPCC is to produce assessments which are “policy relevant” but not “policy prescriptive”
Amongst the various groups that make up the IPCC are its working groups which assess the scientific evidence:
The IPCC works in cycles assessing evidence and producing reports. The first was produced in 1990 and the most recent is its fifth: Assesment Report 5 (AR5), published in 2014. It is currently working on AR6, due in 2022. The reports are produced in volumes (nowadays electronically rather than on paper) corresponding to the work of Working Groups I, II and III.
Wikipedia's article on the IPCC (from which the above introduction was adapted) has a more comprehensive description of the IPCC and its work.
Working Group III finds that "The energy supply sector is the largest contributor to global GHG emissions" and that
The energy supply sector offers a multitude of options to reduce GHG emissions (robust evidence, high agreement). These options include: energy efficiency improvements and fugitive emission reductions in fuel extraction as well as in energy conversion, transmission, and distribution systems; fossil fuel switching; and low-GHG energy supply technologies such as renewable energy (RE), nuclear power, and CCS
Nuclear energy is a mature low-GHG emission source of base-load power, but its share of global electricity generation has been declining (since 1993). Nuclear energy could make an increasing contribution to low-carbon energy supply, but a variety of barriers and risks exist (robust evidence, high agreement). Its specific emissions are below 100 gCO2eq per kWh on a lifecycle basis and with more than 400 operational nuclear reactors worldwide, nuclear electricity represented 11 % of the world’s electricity generation in 2012, down from a high of 17 % in 1993. Pricing the externalities of GHG emissions (carbon pricing) could improve the competitiveness of nuclear power plants. [7.2, 7.5.4, 7.8.1, 7.12]
Barriers to and risks associated with an increasing use of nuclear energy include operational risks and the associated safety concerns, uranium mining risks, financial and regulatory risks, unresolved waste management issues, nuclear weapon proliferation concerns, and adverse public opinion (robust evidence, high agreement). New fuel cycles and reactor technologies addressing some of these issues are under development and progress has been made concerning safety and waste disposal (medium evidence, medium agreement). [7.5.4, 7.8.2, 7.9, 7.11]
Whilst the IPCC identifies "barriers to and risks associated with an increasing use of nuclear energy" it does not support those who reject either continuing to use existing nuclear or increasing its use. In Chapter 7 of their report Working Group III discusses nuclear energy in more detail — resources and their availability, costs, environmental and health effects, technical risks, public perception etc — as they do also for Renewable Energy and Carbon Capture and Storage technologies.
The IPCC's 2018 Special Report on Global Warming of 1.5 ºC (SR15), the first report in the sixth assessment (AR6), addresses the most ambitious and challenging target: of staying within 1.5C of pre-industrial temperatures.
All the scenarios which the report shows that illustrate ways of staying under 1.5C require certain amounts of nuclear energy. The scenario using least nuclear (and no CCS or BECCS) is the most optimistic in terms of "social, business and technological innovations". The most realistic in terms of societal and technological development following historical patterns – the "middle-of-the-road scenario" – requires the most.