07 December 2023

James Hansen, Pushker Kharecha, Makiko Sato

The COP28 Chairman and the United Nations Secretary General say that the goal to keep global warming below 1.5°C is alive, albeit barely, implying that the looser goal of the 2015 Paris Agreement (to keep warming well below 2°C) is still viable. We find that even the 2°C goal is dead if policy is limited to emission reductions and plausible CO₂ removal. IPCC (the Intergovernmental Panel on Climate Change, which advises the UN) has understated global warming in the pipeline and understated fossil fuel emissions in the pipeline via lack of realism in the Integrated Assessment Models that IPCC uses for climate projections. Wishful thinking as a policy approach must be replaced by transparent climate analysis, knowledge of the forcings that drive climate change, and realistic assessment of policy options. The next several years provide a narrow window of time to define actions that could still achieve a bright future for today’s young people. We owe young people the knowledge and the tools to continually assess the situation and devise and adjust the course of action.

Our approach to analysis of global climate change, as described in Global Warming in the Pipeline,¹ puts comparable emphasis on (1) Earth’s paleoclimate history, (2) global climate models (GCMs), (3) modern observations of climate processes and climate change. One purpose of the Pipeline paper was to distinguish between this approach and that of IPCC, which puts principal emphasis on GCMs. GCMs are an essential tool, but the models must be consistent with Earth’s history and the projections of future climate must employ plausible scenarios for energy use and for the climate forcings that drive climate change.

Policy implications of climate science can be grasped from a basic understanding of the human-made forcings that are driving Earth’s climate away from the relatively stable climate of the Holocene (approximately the past 10,000 years). Our task is to provide understandable quantification of climate forcings and changes that will be needed to maintain a hospitable climate. Concerned public, including policymakers, must learn to appreciate basic graphs that summarize real-world data, because these must provide the basis for policy discussion.

1. CLIMATE SCIENCE

There are two major climate forcings: human-made greenhouse gases (GHGs) and aerosols (fine airborne particles). GHGs reduce Earth’s thermal (heat) radiation to space and are the main cause of global warming. Aerosols reflect sunlight to space, mainly via their effect as condensation nuclei for clouds; more nuclei lead to smaller cloud drops and brighter, longerlived, clouds. Aerosols thus cause a global cooling that partially offsets GHG warming.

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