The ABCs of Ozone Depletion and Global Warming: The Wisdom of Solomon

The effort to save Earth’s protective ozone layer from the hazards of chlorofluorocarbons (CFCs) is a tale of ingenuity among scientists and industrialists, unintended consequences for Earth’s climate, and ultimately triumph. This story has a happy ending due to international environmental legislation that was responsive to emergent science related to a suite of various anthropogenic halocarbons. Following the discovery of the Antarctic Ozone Hole in 1985, Susan Solomon led several critically important papers that provided the theoretical underpinning and observational evidence showing this phenomenon was caused by the build-up of human-produced CFCs. The Montreal Protocol banned CFCs and ushered in a class of replacement compounds called hydrochlorofluorocarbons (HCFCs). Susan Solomon then quantified the ozone depletion potentials (ODPs) of HCFCs, which showed these compounds also pose a threat to the ozone layer. This led to various Amendments of the Montreal Protocol that ushered in the development of hydrofluorocarbons (HFCs) to replace HCFCs. These HFCs pose no threat to the ozone layer because they do not contain chlorine or bromine: i.e., the ODP = 0 for HFCs. However, HFCs possess large global warming potentials (GWPs) because they are long-lived and efficiently absorb thermal heat emitted by Earth’s surface in the atmospheric window. In 2005, Susan Solomon co-chaired an IPCC report, known informally as SROC (Special Report on Ozone and Climate), that examined issues related to HFCs and perfluorocarbons (PFCs) as they pertain to “Safeguarding the Ozone Layer and the Global Climate System”. The 2005 SROC report was an important stepping stone to the 2016 Kigali Amendment to the Montreal Protocol that limits future production of HFCs with high-GWPs. Currently, industry is developing a class of chemicals known as hydrofluoroolefins (HFOs) that, like HFCs, are composed only of hydrogen, fluorine and carbon atoms. The double-bonded carbon structure at the center of HFOs makes these compounds reactive in troposphere, resulting in quite low-GWPs. The Kigali Amendment will mitigate up to 0.5°C of global warming that would have otherwise occurred, had high-GWP HFCs been used to replace HCFCs.

In this presentation, we’ll provide a succinct, broad overview of the basics of ozone depletion and global warming due to halocarbons as outlined above, highlighting how environmental policy in this area has been responsive to emergent science. Of course, global warming due to greenhouse gases CO₂, CH₄, and N₂O is today’s most pressing environmental problem. Susan Solomon provided essentially important leadership in this area as Co-Chair of the fourth IPCC Working Group 1 Physical Science Basis report, which received the Nobel Peace Prize in 2007. We’ll conclude with a brief account of our group’s current research that utilizes an Empirical Model of Global Climate to quantify the probability of limiting global warming to either 1.5°C (goal of the Paris Agreement) or 2.0°C (upper limit of the Paris Agreement) relative to pre-industrial, based upon various projections of these greenhouse gas abundances from the Shared Socioeconomic Pathways (SSPs).