The Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 as an expert panel to guide the drafting of the United Nations Framework Convention on Climate Change, ratified in Rio de Janeiro in 1992. The treaty’s objective is to stabilize greenhouse gases in the atmosphere at a safe level.

The IPCC has published a series of five multi-volume climate change assessment reports, the most recent of which was completed just a few months ago, as well as a number of special reports assessing specific issues. Over time, the organization has subtly adjusted its position on the role of nuclear power as a contributor to de-carbonization goals. Here is a timeline of the IPCC’s shifting attitude toward nuclear power.

1990. The IPCC releases its First Assessment Report in preparation for the Rio Earth Summit. Notably, nuclear power is the only option included among both short-term and medium/long-term non-fossil and low-emission energy sources for electricity generation.

The report optimistically foresees Western European countries expanding their nuclear fleets to help stabilize or reduce emissions within a decade, and North American and Pacific OECD countries beginning to level off emissions by “fuel switching to nuclear” and other measures.

1995: The IPCC’s Second Assessment Report is more cautious, observing that nuclear power development had been slowing due to capital costs and public concerns about safety. Nonetheless, with a few caveats, the report affirms hopes for nuclear power as a central plank within de-carbonization plans. The report’s summary for policy makers advises: “Nuclear energy could replace baseload fossil fuel electricity generation in many parts of the world if generally acceptable responses can be found to concerns such as reactor safety, radioactive-waste transport and disposal, and nuclear proliferation.”

2001. The IPCC’s Third Assessment Report conveys deeper nervousness about the economics of building new nuclear plants. It also notes growing controversies about how and where to dispose of spent nuclear fuel, though it places hope in the development of new technologies that would reuse this fuel.

Ultimately the IPCC sees the door opening for nuclear power “in the longer run” as innovations emerge: “Fundamentally new reactor configurations may need to be developed that are based on innovative designs that integrate inherent operating safety features and waste disposal using previously generated radioactive waste as fuel and, by way of transmutation, convert nuclear waste or plutonium to less hazardous and short-lived isotopic substances.”

2007. The IPCC’s Fourth Assessment Report describes nuclear power as “an effective [greenhouse gas] mitigation option,” heralding the improved operational performance of plants and the development of new designs that promised better safety and economic performance.

Yet this report notes a geographical stratification occurring. The European Union had shunned nuclear in its “20-20-20” targets that bind countries to generate 20 percent of their energy from renewable energy resources. Instead, nuclear power was expanding in Asia and elsewhere. The IPCC report notes: “Many more reactors are either planned or proposed, mostly in China, India, Japan, Korea, Russia, South Africa and the [United States].”

2011. Between assessment reports, the IPCC publishes a special report, "Renewable Energy Sources and Climate Change Mitigation." The IPCC press office widely publicizes "the most optimistic scenario," in which nearly 80 percent of the global energy supply could be provided by renewable energy by 2150 “if backed by the right enabling public policies.” The report was written in keeping with European trends toward maximizing renewable energy.

While nuclear is included as an option in the mitigation scenarios, it is portrayed as one to be minimized, along with fossil fuels, in accordance with the European trend: “Increasing the installed capacity of [renewable energy] power plants will reduce the amount of fossil and nuclear fuels that otherwise would be needed in order to meet a given electricity demand.”

2014. The IPCC’s Fifth Assessment Report tamps down enthusiasm about renewable energy. The report candidly discusses the difficulties of spurring a renewable energy transition and integrating renewable energy. Nuclear is once again grouped with renewable energy as the key elements of a low-carbon energy system, along with carbon dioxide capture and storage (CCS). “No single mitigation option in the energy supply sector will be sufficient,” the report warns. “Achieving deep cuts [in greenhouse gas (GHG) emissions] will require more intensive use of low-GHG technologies such as renewable energy, nuclear energy, and CCS.”

Most important, the report’s scenarios show how nuclear power boosts de-carbonization efforts. To stabilize the climate at an average global surface temperature no higher than 2 degrees Celsius above the pre-industrial level, scenarios without nuclear expansion would require global energy supply to be radically curtailed below currently projected demand. With an expansion of nuclear power, however, the climate could be stabilized with far more modest efficiencies.

Looking ahead. In its 2014 report, the IPCC struck a note of urgency on the need to use all available low-carbon technologies to avert climate change. The sixth assessment report is not due until the end of the decade, and it's premature to speculate about what it will say regarding nuclear power. But trends suggest that major intergovernmental agencies increasingly view nuclear energy as an essential climate wedge within a global climate stabilization system. Last month, two OECD agencies—the International Energy Agency and the Nuclear Energy Agency—projected that nuclear power will have to double by 2050 for the world to meet the international goal of limiting warming to 2 degrees Celsius.