University of New South Wales (UNSW), Australia

John Church is an Emeritus Professor in the Climate Change Research Centre, University of New South Wales. He has published across a broad range of topics in oceanography.

His area of expertise is the role of the ocean in climate, particularly anthropogenic climate change, and in understanding global and regional sea-level rise. He is the author of over 180 refereed publications, over 110 other reports and co-edited three books. He was co-convening lead author for the Chapter on Sea Level in the IPCC Third and Fifth Assessment Reports. He was awarded the 2006 Roger Revelle Medal by the Intergovernmental Oceanographic Commission, a CSIRO Medal for Research Achievement in 2006, the 2007 Eureka Prize for Scientific Research, the 2008 AMOS

R.H. Clarke Lecture, the AMOS Morton Medal in 2017, a joint winner of the BBVA Frontiers of Knowledge Climate Change Category Prize in 2019, the AAS Jaeger Medal in 2021 and the Royal Society of NSW James Cook Medal in 2022. He is an Officer of the Order of Australia, a Fellow of the Australian Academy of Science, the Australian Academy of Technology and Engineering, the American Geophysical Union, the American Meteorological Society and the Australian Meteorological and Oceanographic Society.

Axford Medal Lecture | 2 Aug (Wed) 8:15 AM – 10:00 AM | MR308 – 310

What do we really know about 20th and 21st Century Sea-Level Change?

Abstract: Accelerating sea-level rise in much of the world will result in growing impacts through the 21st century and beyond. Despite the clear identification of an accelerating rise, many uncertainties remain. Understanding historical sea-level change is a prerequisite for building confidence in useful and accurate predictions of future changes.

For many decades, our limited knowledge of the contributions to sea-level change could not explain the rise measured by coastal tide gauges – the sea level enigma. New and improved in situ and satellite observations of the ocean, improved understanding of the “solid Earth”, and better understanding and improved modelling of the climate system have helped resolve this enigma. A number of recent studies have argued that the sum of contributions from both observations and model estimates to sea-level change over the satellite era, the last half century and since 1900 adequately explains the observed sea-level rise, which means the sea-level budget is closed. The major contributions are from ocean thermal expansion and contributions from glaciers, with an accelerating ice sheet contributions over the recent decades.

Our recent work has explored the sensitivity of global and regional sea-level reconstructions to poorly known land motions and the factors causing temporal and regional variations in the rate of rise. With this knowledge, existing reconstructions of global mean sea level are mostly not significantly different to each other from 1900 to the present, both in the time- averaged rate and the temporal variability. However, while the average rate over 1900 to present is similar to that from the sum of contributions, the rate of reconstructed GMSL rise is significantly smaller/larger than the sum of contributions prior to 1940/after 1970. Why is this? What do we really know? What are potential explanations for this continuing enigma?

And what can we project about future sea level, both for the 21st century and beyond. And can we constrain projections for the 21st century and beyond?