IPCC, 2023: Climate Change 2023: Synthesis Report. IPCC, 184 p., doi: https://doi.org/10.59327/IPCC/AR6-9789291691647

Around 1990, the United Nations created two new organizations to address the threat of climate change: The Intergovernmental Panel on Climate Change (IPCC) and the United Nations Framework Convention on Climate Change (UNFCCC). The former aims to advance scientific knowledge about climate change and has published six major assessments every 5–6 years from 1990 to 2023. In addition to providing the global community with a consensus about scientific knowledge, the IPCC reports play a key role in the intergovernmental climate conferences held under the auspices of the UNFCCC. The main aim of these annual negotiations is to reach agreements on policies to prevent dangerous global warming. For example, the IPCC Fifth Assessment Report published in 2014 provides the scientific background for the landmark climate conference held in 2015 in Paris, where governments agreed to try to limit global warming to 1.5°C. The Sixth Assessment Report played a similar role in the 28th governmental meeting held in Doha in late 2023. The IPCC shared the 2007 Nobel Peace Prize with Al Gore for advancing our understanding of climate change. Like its predecessors, the IPCC's latest assessment produced several reports. All of these are based on a massive effort of hundreds of scientists organized into working groups on three key climate change topics: (1) the physical science basis; (2) impacts, adaptation, and vulnerability; and (3) mitigation of climate change. The reviews of the literature undertaken by the working groups are presented in three detailed reports, each thousands of pages in length, and aimed at a scientific audience.1 To make this material more accessible to a wider audience the IPCC also publishes a Synthesis Report (called "the report" below), which is the subject of this book review. It distills the key findings of the three working groups into a more manageable 184 pages and includes a 34-page Summary for Policymakers. An even more succinct six-page set of "headlines" is also available. The report contains three main sections: (1) Current status and trends, (2) Long-term climate and development futures, and (3) Near-term responses to a changing climate. Several main conclusions emerge: the earth is warming due to the emission of greenhouse gasses (GHG) from human activities; this warming has led to widespread damage to nature and people from increases in heatwaves, heavy precipitation, droughts, and tropical cyclones and reduced food and water security in mid- and low-latitude regions; vulnerable communities that have historically contributed the least to current climate change are disproportionately affected; all impacts will grow more dire as warming continues in the future; and halting or reversing climate change will require massive and immediate action to reduce GHG emissions. Furthermore, the future has become more uncertain: with every degree of warming, the risk of truly catastrophic outcomes (e.g., several meter rise in sea level, reversal of major ocean currents; accelerated warming due to the rapid loss of methane from huge frozen methane hydrate reserves stored in arctic regions; and severe disturbances of monsoons) rises exponentially. While these findings are similar to those of earlier assessments and are by now largely familiar, the additional evidence accumulated over the past decade allows these conclusions to be reached with much greater confidence and the impacts are estimated more accurately. The report also covers important new ground. In particular, substantial attention is devoted to assessing the impact of recent international promises to cut GHG emissions. In the past, most governments rejected mandatory emission cuts, thus leading to a lack of progress at earlier international conferences. However, at the 2015 Paris Conference governments agreed to voluntary action and made commitments to achieve specific reductions in their emissions. These so-called nationally determined contributions (NDC) vary by country and are the largest for the richest economies. Commitments are not legally binding or enforceable, but this agreement is nevertheless widely considered to be a crucial step forward compared to the minimal progress achieved in the past. Will these promises be sufficient to keep global warming below 1.5°C? Are the new commitments being honored? Unfortunately, the answer to these questions is no. Most telling is a new IPCC projection labeled "Implemented Policies." It estimates the future warming that would result from the implementation of all policies in effect by the end of 2020 and "extended with comparable ambition levels beyond 2030" (p. 59). In other words, this is roughly the trajectory we are on with all the promises made at or since the Paris meeting. Under this projection, the global temperature reaches a startling 3.2°C, more than double the current preferred warming limit. The report leaves no doubt that such an outcome will be disastrous. A key cause of this large overshoot is an "emission gap" between the reduction in GHG emissions needed to stay below 1.5°C and the NDCs. These promises do have an effect, but they are simply insufficient. In addition, there is an "implementation gap" as actual emissions in recent years have been substantially higher than promised. Many countries are exceeding their own voluntary goals. As a result of these two gaps, the world's GHG emission from fossil fuels reached a record high in 2023 instead of being on the sharply declining trend needed to keep long-term planetary warming below 1.5°C. It is now likely that the planet will substantially exceed this ambitious long-term goal; in fact, in 2023, this level was already reached. The IPCC's mandate is to distill and communicate the science of climate change. In practice, this effort focuses mainly on the natural sciences and the report gives little attention to the potentially important role of the social sciences. Most importantly, the discussion on what to do about global warming lacks a solid economic basis, and important insights developed by climate and environmental economists (such as Nicholas Stern, Partha DasGupta, and Nobel Laureate William Nordhaus) are neglected. The report contains lengthy analyses of the wide-ranging impacts of global warming and proposes various ways to reduce GHG emissions, but there is no discussion of how much the different future scenarios will cost. In an ideal world, the choice of how to proceed would be based on cost–benefit analyses which would identify the optimum way forward after weighing the human and environmental cost of inaction versus the cost of intervention. This option is not (yet) available because these costs are not easy to estimate, and controversy surrounds several important economic questions. Without estimates of the costs and benefits of various scenarios, it is difficult to persuade policymakers and the public that drastic, immediate, and expensive intervention is necessary. The central obstacles to progress are what economists call negative externalities which are harmful effects on third parties generated by economic activities. A variety of actors create negative externalities consisting of harmful GHG emissions: individuals driving large SUVs; rich countries whose past and current emissions are responsible for damages in poor countries; and the contemporary generation of economic actors who may be reluctant to pay for expensive mitigation measures when the benefits will largely accrue to future, presumably much richer, generations. There is also the potential for "free-riders" in the mitigation of GHG such as countries that have little incentive to reduce their own emissions when they can benefit from the reductions achieved by other countries. Negative externalities and the challenge of the free-rider problem are difficult to resolve through free-market mechanisms and may justify government actions. Some countries are already doing this through taxation or regulation, but current efforts are completely inadequate. Emitters have little incentive to change their behavior because they suffer few consequences except scorn from environmentalists. Even if economists could undertake cost–benefit analyses and agree on ways to deal with negative externalities and free rider problems there is a final obstacle: the unwillingness of many economic actors to make significant sacrifices. Government actions taken in recent years to curb GHG emissions have run into strong opposition from a range of stakeholders. In Europe, farmers block traffic on major highways to protest green policies. US consumers are reluctant to switch to electric vehicles due to their high prices and inadequate numbers of charging stations. US automakers and their unions want and may get a delay in limits on tailpipe emissions to protect their sales of profitable gas-powered cars. Carbon taxes are politically toxic. As a result, many governments have turned to massive subsidies to encourage a switch to green technologies, but these are expensive and can be economically inefficient. There is a clear disconnect between environmental goals and politically achievable interventions. The path of least resistance is to avoid upsetting voters. Hopefully, the widely reported extreme and highly damaging weather events experienced worldwide in 2023 (when the global temperature reached an all-time high record) will serve as a wake-up call and make future climate action more acceptable to a wider spectrum of constituencies. Readers of PDR will be disappointed that the IPCC has very little to say about the contribution of population trends to climate change or about its impact on demographic processes. The report makes no mention of population projections even though the growth in human numbers has been and will be a key driver of GHG emissions.2 There is no discussion of how population policies, in particular voluntary family planning programs, can have multiple benefits including slower population growth. Massive climate-induced migration and mortality are among the main reasons to be concerned about global warming, but these demographic processes are only mentioned in a few general statements. On migration, the report observes that "Climate and weather extremes are increasingly driving displacement" (p. 51) and "Reducing future risks of involuntary migration and displacement due to climate change is possible through cooperative, international efforts to enhance institutional adaptive capacity and sustainable development" (p. 107). Adverse public health effects are expected due to "an increase in heat-related human mortality and morbidity, food-borne, water-borne, and vector-borne diseases, and mental health challenges" (p. 15). No estimates of the magnitude of climate-induced migration and mortality are provided. These estimates are admittedly difficult to obtain but without them, the case for immediate and massive action is considerably weakened. None of these observations should detract from the enormous value of this report. The IPCC is an outstanding example of international collaboration to address one of the world's most difficult challenges, and it is widely and deservedly considered the ultimate authority on climate science. Its consensus reports are packed with valuable information, insights, and conclusions based on the most up-to-date science, thus making them required reading for anyone dealing with climate science or policy. However, the reports are not easy to read for nonscientists. The text is dense and concise, and often stilted; most figures and tables are crammed with information. Fortunately, the IPCC makes its findings available in a wide variety of formats (ranging from six pages of headlines, to policymaker summaries, to several thousand-page long reports) so that most potential readers can find an appropriate source.