Warmer Oceans Are Changing the Season for Harmful Algal Blooms

Abstract Achieving net-zero or negative CO₂ emissions has become a critical global objective to mitigate climate change and promote the recovery of more stable climate conditions. However, the persistence of greenhouse gas-induced warming may extend beyond expectations due to the inertial properties of the Earth system. In this study, we examine climate hysteresis using the Community Earth System Model versions CESM1.2 and CESM2.0, wherein atmospheric CO₂ concentrations or emissions are gradually increased and subsequently reduced. Utilizing a spatially explicit quantification framework, we find that approximately 89% of the Earth’s surface exhibits irreversible changes in surface temperature. These changes are particularly pronounced over the Southern Ocean, the Arctic, and the North Atlantic. A distinct cold anomaly (‘cold blob’) forms in the northern North Atlantic during the CO₂ ramp-up phase, while a warm anomaly (‘warm blob’) appears during the ramp-down phase, clearly indicating hysteretic behavior. The delayed cooling of the Southern Hemisphere, coupled with the lagged response of the Atlantic Meridional Overturning Circulation (AMOC), further contributes to pronounced hysteresis in multiple components of the climate system. These include the Intertropical Convergence Zone (ITCZ), global monsoon systems, the El Niño–Southern Oscillation (ENSO), extreme weather events, and the Indian Ocean Dipole (IOD). The study also provides mechanistic insights into the dynamic processes underlying these hysteretic responses. Speaker information Short biography Soon-Il An is a Professor of Atmospheric Sciences at Yonsei University, Seoul, and Director of the Irreversible Climate Change Research Center (2018.06–2025.02), supported by the National Research Foundation of Korea. He earned his Ph.D. from Seoul National University in 1996 and previously served at the University of Hawaii. His research encompasses climate variability, ENSO dynamics, paleoclimate, and Earth system modeling, particularly focusing on hysteresis and irreversibility under anthropogenic forcing. He has authored over 200 SCI-indexed publications and contributed as a Lead Author to the IPCC AR5. Professor An has received numerous distinctions, including the National Academy of Sciences Award, the Frontiers Planet Prize, and multiple recognitions for research and teaching excellence. He is currently a member of the CLIVAR Science Steering Group.

Abstract When thinking about human understanding, there are known knowns. These are things that we know we know. There are also known unknowns, things we do not know. Unknown unknowns are things that we don't know we don't know. Finally, there are the unknown knowns---things we do not know we know. Operational experience and the scientific literature tell us that the structure and dynamics of fronts are more complicated than our common perception of them---in other words, there are unknown knowns. Many of these unknown knowns have existed since the earliest work on fronts in the early part of the 20th century, meaning we have not evolved with the state of the science. Come prepared to have your foundations in atmospheric science challenged as I show freaks of nature (that are more common that you may realize), including cold fronts tilting forward over the warm air, cold fronts where the wind shift is not coincident with the temperature gradient, and cold fronts characterized by surface warming after frontal passage! I will also discuss the issue of how cold fronts move and whether they necessarily move faster than warm fronts. Speaker information Short biography David Schultz is Professor of Synoptic Meteorology at the University of Manchester. He has published over 200 journal articles on atmospheric phenomenon from as small as mammatus and tornadoes to as large as the global circulation during the Snowball Earth 2.5 billion years ago. His principal research interests are extratropical cyclones, fronts, severe convective storms, and the organization of precipitation, where he has helped modernize and extend the Norwegian cyclone model, the formation of occluded fronts, and airstreams such as the cold conveyor belt and sting jet. He has been critical of poor science and communication, specifically the language around instabilities and the scientific literature published on the effect of weather on the spread of COVID-19. He has received multiple teaching and supervision awards, including the 2023 Inaugural Education Award from the Royal Meteorological Society. He was the third-longest serving Chief Editor in the 151-year history of Monthly Weather Review (2008–2022) and recipient of the 2024 Cleveland Abbe Award for Distinguished Service from the American Meteorological Society. He is author of Eloquent Science: A Practical Guide to Becoming a Better Writer, Speaker, and Atmospheric Scientist, which has been translated into Chinese and published by the China Meteorological Press.

Hi everyone, We’ll meet in the Bjerknes Meeting Room on the 3rd floor on Monday 11 August, 11–12h. A program will be sent around closer to the day, but one topic will be a discussion on Bjerknes strategy. If you have anything to share with the Hazards Group, let me know in advance. I’ll send a reminder out in early August. Enjoy the summer! Stijn