Dan Cayan studies the extreme events that can dramatically impact coastlines
Dan Cayan is a climate researcher at Scripps Institution of Oceanography, UC San Diego, and the U.S. Geological Survey. He focuses on regional climate variability and change with an emphasis on the climate of California and the western United States. In addition to his research, Cayan advises decision-makers at local, state, and federal levels on climate issues. He has played a key role in a series of California climate change vulnerability and adaptation assessments and in directing the California-Nevada Applications Program, which conducts applied research and provides information to stakeholders. He entered the Scripps graduate program in 1972, was appointed as a staff research associate in 1977, and received a Ph.D. in oceanography from Scripps in 1990.
What does the public need to understand about how sea-level rise affects people’s lives?
One important thing is that sea-level rise is not a simple multi-year slow change. It is a dynamic phenomenon — historical experience indicates that sea level rise occurs in spurts. We are very likely going to see sea level rise over the long term but as it rises we’re going to see big events that occupy certain years. Our biggest problems here along the West Coast are in the wintertime when we get the largest storms. When we get large storms that coincide with high tides, that’s when there’s the potential for the most damage.
How has El Niño affected sea-level rise throughout history?
The most impressive symptoms of shorter-period sea-level rise along California have occurred during large El Niños. In the last 30 years, there have been two large El Niño events – in winter 1982-83 and in 1997-98. Sea level along the entire West Coast was heightened as this thermal pulse from the tropical Pacific Ocean made its way up the eastern boundary of the Pacific. Sea levels during those winters at times reached six to 12 inches above astronomical tides. Besides affecting ocean currents and causing West Coast waters to become anomalously warm, large El Niños invigorate the north Pacific storm track. Storms in the winters of 1982-83 and 1997-98 were very strong and had a very large footprint. Both these winter storm seasons were very large, but in 1998 California dodged some bullets because the largest storms did not happen to coincide with the highest tides. In contrast, in 1983 a few of the largest storms struck California during extremely high tides. Late January 1983 had some of the highest sea levels ever observed on the West Coast and those high marks still stand today.
In the next several decades, California’s greatest coastal impacts are likely to be caused by these same influences — that is an El Niño event, large storms, and high tides coming together. A background of rising sea levels caused by global warming will exacerbate these natural effects.
How has the understanding of sea-level rise and extreme events progressed?
In 1982-83, the ability to forecast El Niño was still evolving in a juvenile state. During the early phase of that event in summer of 1982, some of the tropical Pacific temperature data was actually rejected because it was thought that it was so much an outlier that it was probably an error. (It was discovered not long after that conditions were extremely warm and of course this turned out to be a massive El Niño.) In 1997, the community of climate and oceanographic scientists was more advanced and there was much greater awareness of how El Niño operated and there was better observational tracking, so there was, quite early in this event, an expectation of a large El Niño. Scripps got out in front of this, discussing the likelihood that this would be a very large event. This led to considerable drama, which played out in our offices and in the media. Part of the drama arose because the strongest Northern Hemisphere symptoms didn’t develop until January 1998, so we were on pins and needles wondering if this was going to materialize, but of course it did. Although it was a massive storm and water supply year, we were fortunate that the coastal damage was much reduced from that experienced in 1983 largely because the largest storms didn’t coincide with the highest tides.
What is the outlook for the 2014-15 El Niño?
Indications along the tropical Pacific this spring suggested this could develop into a substantial El Niño, but it’s somewhat doubtful it’s going to be one of those super El Niños. It may be a ‘respectable’ El Niño event. Since late summer, the tropical Pacific has relaxed back toward a more normal state and any El Niño this coming winter looks to be rather mild.
What is it going to take to improve forecasts of sea-level rise and extreme events?
In short, better observations, better understanding of physical processes involved in sea-level rise and coastal storm events, and better models of the various global, regional, and local components are needed to improve forecasts.
Observations, along the West Coast, over the Pacific basin and over the global ice and ocean areas are all necessary because global, regional, and local processes affect our coastal sea levels. Observations have advanced quite a lot. The ocean science community can provide a view of the whole Pacific Ocean with a much better fidelity than we could 15 years ago; part of this is because of floats and gliders that have been developed by colleagues here at Scripps. Remote sensing now provides estimates of important components such as global sea level so we have a much better ongoing description of ocean conditions. Atmospheric water vapor from satellites is an important ingredient in forecasting precipitation from “atmospheric river” storms, the major flood producers along the West Coast.
Tide gauge and coastal wave observations are really critical in understanding what’s happening locally. Having many years, or, ideally, many decades of these records is vital because the most extreme events and impacts occur infrequently and historical analogues may only be available in cases in the more distant past. A lot of these events play out in a time interval of just a few days, so having samples that continuously record at rapid intervals provides vital information. Our studies of these events are fed by observations of the hour-by-hour evolution of the ocean, atmosphere and heavy winds, precipitation, and runoff over the coastal lowlands where most of Californians live and over the uplands where we collect much of our water supply.
Another crucial element is having a dynamical framework to understand how these various processes mesh together and how they will evolve in the short- and longer-term future. For that, we have ocean and atmosphere models. What happens here on the coast is the result of phenomena that are happening over a semi-global landscape, so having models that look at the region and the broader picture over the globe is really important. Climate models have gotten more spatially defined and include more vertical levels. Most of them now account for the interaction of the ocean and the atmosphere.
What we would like are very fine-scale model simulations and projections and we would like those carried out over several decades to understand how these changes unfold. This resource has not really been at our fingertips because of the expense of running these very high-resolution simulations, but that definitely is the way the science is moving and in another generation, we’ll see vast improvements in the resolution and fidelity of the phenomena we’re looking at.
How much can decision-makers at various levels of government rely on your projections?
The information we are providing is state of the art. Observations and models indicate that global sea levels will continue to rise in the future, probably at an increasingly high rate. However, there is considerable uncertainty because some of the key processes are not well observed, not completely understood, and not completely captured in existing models. Sea-level rise in the next 100 years is quite likely to reach three feet above present-day levels, but it’s possible it might only reach 1.5 feet and it’s also possible it could rise to five feet. So there is an envelope of potential sea-level rise that decision-makers need to understand and plan for, weighing options of risk and expense to mitigate that risk. A confusing aspect is that over the last several years, since our large El Niño in 1998, the West Coast has experienced essentially zero sea-level rise. On the other hand, in the western part of the Pacific basin, sea levels have risen dramatically, at rates as high as three times the global average (as high as four inches per decade). We think that this Pacific basin imbalance is driven by winds and ocean currents, and that eventually, within the next few years to decades, the Pacific circulation will revert to a different pattern and sea-level rise along the West Coast is going to resume. But when and how fast this happens is not clear. This underscores the need for a whole-Pacific perspective, the importance of both regional and global observations, and improved modeling.
How can scientists get the message about sea-level rise and El Niño out to the public most effectively?
Sea-level rise is caused by a mixture of processes that are occurring over a range of time scales, including many-decade global warming processes to few-day storm events, so preparation for sea-level rise needs clear explanations of how this works. Decision-makers will need to plan for change and also prepare for extreme events. Because the existing knowledge about sea-level rise varies quite a lot across coastal professionals, agency officials, and the general public, I think a consortium of experts is needed to provide information in multiple ways on different levels of detail, from more basic to more expert. There are many ways to communicate and we’re working on several of them.
– Robert Monroe