Understanding 1.5degC: The IPCC's Forthcoming Special Report

The Intergovernmental Panel on Climate Change (IPCC) - an international body that develops non-policy prescriptive climate science assessments for decisionmakers - is currently compiling a Special Report that will provide information on what it would take to limit global warming to 1.5 degrees Celsius above pre-industrial levels. The report will also assess the climate impacts that could be avoided by keeping warming to this level, and the ways we can limit the worst impacts of climate change and adapt to the ones that are unavoidable. Report authors and government representatives will meet in Incheon, Republic of Korea from October 1-5 to review the report, with the report's Summary for Policymakers due to be released on October 7 at 9 p.m. Eastern US time (October 8 at 10:00 local time (KST)). The report is slated to come out just as nations look towards revising the commitments they made to achieve the goals of the Paris Agreement.

The Paris Agreement is a worldwide commitment adopted in 2015 under the United Nations Framework Convention on Climate Change (UNFCCC) to reduce global warming emissions and limit the increase in global temperature to well below 2degC. More specifically, the Paris Agreement includes a goal of "Holding the increase in the global average temperature to well below 2degC above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5degC above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change." Small Island Developing States that are disproportionately vulnerable to global warming were pivotal in the inclusion of the 1.5degC goal.

It has been recognized that efforts beyond those spelled out in the commitments made to meet the goals of the Paris Agreement (the Nationally Determined Contributions) will be necessary to limit global warming to 1.5degC above pre-industrial levels. As a result, policymakers are interested in what it would take to achieve this goal, as well as the benefits and tradeoffs to consider as countries look to ramp-up their commitments. (This report will fulfill an invitation made by UNFCCC member countries including the U.S. during the adoption of the Paris Agreement for, "the Intergovernmental Panel on Climate Change to provide a special report in 2018 on the impacts of global warming of 1.5 degC above pre-industrial levels and related global greenhouse gas emission pathways.") The report is thus intended to inform such deliberations and respective domestic and international climate policy.

Just preceding the invitation to the IPCC to produce the Special Report, UNFCCC member countries also decided, "to convene a facilitative dialogue among Parties in 2018 to take stock of the collective efforts of Parties in relation to progress towards the long-term goal referred to in Article 4, paragraph 1, of the Agreement and to inform the preparation of nationally determined contributions pursuant to Article 4, paragraph 8, of the Agreement." The Special Report will be an important input into this global stock-take, which will be a prominent feature of the forthcoming UNFCCC Conference of Parties (COP24) in Poland in December of this year.

In addition to its Summary for Policymakers, the report will have five underlying chapters, as well as an introductory section, several break-out boxes, and frequently asked questions. The title of the chapters will be as follows:

• Chapter 1: Framing and context
• Chapter 2: Mitigation pathways compatible with 1.5degC in the context of sustainable development
• Chapter 3: Impacts of 1.5degC global warming on natural and human systems
• Chapter 4: Strengthening and implementing the global response to the threat of climate change
• Chapter 5: Sustainable development, poverty eradication and reducing inequalities

Among other topics, the report will provide information on the global warming emissions reductions required to keep global warming to 1.5degC relative to the emissions reductions necessary to limit warming to 2degC. And it will compare the different paths nations can take to achieve these emissions reductions, including the opportunities and challenges that meeting the 1.5degC goal will present from socio-economic, technological, institutional, and environmental perspectives. The report will also assess the impacts that could be avoided if warming is kept to 1.5degC instead of 2degC, as well as the emissions reduction options relevant to keeping warming to 1.5degC and the options available to prepare for projected impacts. Furthermore, authors have been tasked with considering how to limit warming to 1.5degC together with sustainable development and poverty eradication efforts, and the implications of pursuing this goal for ethics and equity.

The report is being prepared by experts from a diverse array of countries and institutions, including from the United States. The IPCC does not conduct new science. Instead, authors reviewed the best available, peer-reviewed literature relevant for each chapter, and employed set criteria to characterize the evidence relevant to key topics, including the level of confidence, agreement, and uncertainty in the evidence base (as an example, here is a link to the criteria employed in the IPCC's Fifth Assessment Report).

The IPCC's Special Report on 1.5degC Warming has undergone a lengthy review process, including an internal review, and multiple expert and government reviews. For example, 2000 experts registered to review the First Order Draft, along with 489 government reviewers from 61 countries. The review process provides a mechanism for engaging and incorporating input from a diverse and inclusive set of experts. Authors are required to consider and respond to each comment received - responses that are then made publicly available.

The Union of Concerned Scientists will be reviewing the released documents on October 8th and posting a series of blogs that will cover key aspects of the report. Although the report is not public yet, the latest scientific literature (e.g. that assessed in the recent U.S. Climate Science Special Report) has only underscored the urgent need for action to limit the heat-trapping emissions that are fueling climate change. This new IPCC Special Report will almost certainly make this point even clearer, as it will show the world what can be avoided if we ramp down emissions now. There is too much at stake for humanity to indulge in further delay or inaction.