Quit Worrying About Uncertainty in Sea Level Projections

Don't ask questions.  Have faith. That seems to be the message of this article. Even the  IPCC acknowledges sea level projections are associated with deep uncertainty

As ice sheets lose mass at increasing rates, scientists are growing increasingly concerned that portions of these massive reservoirs of frozen water are poised to begin irreversibly retreating [Cornford et al., 2015; DeConto et al., 2021]. To adapt to the ensuing changes along shorelines, authorities responsible for coastal planning and climate mitigation efforts need actionable sea level rise projections. However, recent studies using climate and ice sheet models are, more and more often, coming to very different conclusions about future rates of sea level rise and even about the sensitivity of ice sheets to future warming [DeConto et al., 2021; Edwards et al., 2021].

Focusing on uncertainty in model projections of long-term sea level rise is a trap we must avoid.

How can climate scientists help decisionmakers navigate vague or conflicting information to develop practical response strategies in the face of large uncertainties? One solution that may provide needed clarity is to change our emphasis from what we do not know to what we do know.

Large discrepancies among model projections of long-term sea level rise have spawned calls among the scientific community for scientists to work on reducing uncertainty. However, focusing on uncertainty is a trap we must avoid. Instead, we should focus on the adaptation decisions we can already make on the basis of current models and communicating and building confidence in models for longer-term decisions.

The Folly of Focusing on Uncertainty

Emphasizing uncertainty is misguided for two main reasons. First, a growing body of research shows that providing uncertainty estimates to decisionmakers actually decreases the usability of climate projections [Lemos and Rood, 2010]. This is partly because it isn’t always clear how best to incorporate uncertainty into planning. Do we plan for the most likely projection of sea level rise, knowing the protections we put in place may be inadequate, or do we plan for the most extreme sea level projection despite the additional cost to do so? 

The planning process is complex, with uncertainty in global sea level projections being just one of many factors decisionmakers must consider. For example, investing in protections against sea levels that won’t be experienced for 70 years may not seem pressing when people can’t leave their homes because of air quality concerns or can’t drink tap water because it is contaminated. Furthermore, future planning and infrastructure decisions must directly confront the inequitable practices that have long disadvantaged vulnerable and marginalized populations.

Planning for shorter-term sea level rise doesn’t mean ignoring the specter of more substantial sea level rise farther down the road.

Second, although models provide a murky picture of the magnitude of sea level rise that will occur by the end of the century, estimates of what will happen in the next few decades are much clearer. This clarity is important because the most pressing adaptation decisions facing communities now—related to addressing both climate vulnerabilities and historical inequities—primarily reflect needs on decadal, not centennial, timescales. So rather than stressing distant targets that are elusive and evolving, communities need help to be successful in adapting to near-term climate risks.

Planning for shorter-term sea level rise doesn’t mean ignoring the specter of more substantial sea level rise farther down the road, and there is still a need for longer-term climate and sea level projections. For example, adaptation decisions such as where to place infrastructure designed to last more than a century (e.g., new sewer lines) call for information about long-term as well as short-term change and require significant immediate costs.

But committing to adaptation measures across the board on the basis of unclear long-term projections is like planning a dinner party years in advance: It’s good to think ahead, but it might be premature to buy the groceries. Moreover, sea level rise is not like a tsunami that will suddenly inundate coastlines (although it may seem that way when sea level rise conspires with storm surges to flood communities). Rates of sea level rise, even at the extremely high end, are measured in centimeters per year. Given the reality that sea levels will rise in the near term, plans today can focus on changes expected over the next decade or two and can then be adapted as more nebulous longer-term changes come into focus.


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