Sea-Level Rise and Climate Change Impacts to Reefs

Science Center Objects

This study is part of the USGS Coral Reef Project.

Two simple graphs with one showing a decrease from left to right and one showing an increase from right to left.

Plots of numerical model results showing how predicted future sea-level rise will likely reduce sediment residence time (how long it stays in place, top) and increase sediment flux (how much sediment moves how quickly, bottom) on the fringing coral reef flat off south-central Molokaʻi.

The Problem

There is a growing body of evidence indicating that the rate of sea-level rise has increased relative to the past century and will continue to increase in the 21st century; that evidence has recently been summarized by the Intergovernmental Panel on Climate Change (IPCC). If all aspects of reef morphology—colony size and shape, cross-reef relief, surface rugosity, and so on—keep pace with the rising sea levels, then it is likely that changes in depth-controlled physical processes will be minimal to non-detectible. However, based on rates of vertical reef accretion in Hawaiʻi and throughout the Pacific (which are an order of magnitude smaller than predicted rates of sea-level rise), it is unlikely that reefs there and other locations will keep pace, and their inability to do so will lead to subtle but important changes in selected physical processes on some coral reefs.

Camera was positioned halfway in and out of water to show a coastal bluff with vegetation in background, coral reef underwater.

Photograph of the shallow fringing coral reef flat and adjacent land at Kaulana, Kahoʻolawe.

In addition, recent studies indicate the flux of submarine groundwater discharge from land to coral reefs in Hawaiʻi and other high islands is substantial, and often significantly colder and enriched in terrestrial-derived nutrients than surrounding seawater. Ecosystem functions of submarine groundwater discharge to coral reef ecosystems are not quantified but can be hypothesized to (1) buffer thermal stress (bleaching) in corals experiencing warming, and (2) supply nutrients to otherwise oligotrophic coastal waters. While an excess of the latter has been observed to cause complete phase shifts in the form of wholesale loss of coral and replacement by macroalgae, the role of the former has not been tested. Both may be significantly altered by impending climate change and proposed land use that alter groundwater quantity, quality, flux, composition, and fate, especially in rapidly developing areas. This effort is focused on submarine groundwater discharge, its role in shaping coral reef ecosystem structure, and the ecosystem services it provides.

The Approach

The overall objective of this research effort is to better understand how climate change may impact coral reefs. Achievement of this objective requires an understanding of the physical parameters driving change in coral reefs and the resulting ecosystem processes. The goals of this effort are to:

  1. How will reefs respond to rapid sea-level rise at a decadal time-scale? 
  2. How will increased wave energy and altered circulation across reefs affect circulation and sediment, nutrient, contaminant, and larval dynamics? 
  3. Do thresholds exist in the rate of sea-level rise that would push a reef ecosystem from a state of stability to one of net loss?
  4. How may changes in precipitation, recharge, and human-induced withdraws impact submarine groundwater discharge to the coastal zone?
  5. How will coral reefs respond to variations in submarine groundwater discharge predicted to occur due to climate change?
Map illustrations showing two scenarios with shades of color to indicate differences.

Maps of suspended-sediment concentrations under low tide conditions and high tide conditions off south-central Molokaʻi. Such comparisons over a range of water levels are useful to provide insight on how processes on reefs may respond to predicted future sea-level rise.

The approach to these interdisciplinary studies will rely on a combination of field measurements and physics-based numerical monitoring. We use a wide range of tools to try to answer these questions, including: oceanographic instruments (for example, acoustic Doppler current profilers, wave/tide gauges, temperature sensors, salinity sensors, chemical sensors) mounted on the seabed or on moorings, water-column profilers with similar suites of sensors, coral cores, geophysical water-column and sub-bottom surveys, and physics-based numerical models.