There is strong international consensus that the world is experiencing global climate change, that the rate of climate change is increasing, and that much of the change is due to human activities.
Increasing greenhouse gases from a variety of human activities (burning fossil fuels for heat and energy, deforestation, fertilizing crops, raising livestock, and producing some industrial products) is dramatically affecting coral reef ecosystems.
Coral reef ecosystems are also threatened by a combination of other stressors, including overfishing, coastal development, pollution, and disease. Over the last several decades, global climate change, in combination with local stressors, has resulted in major declines in coral reef ecosystems worldwide.
Global Climate Change Stressors
Global climate change stressors include:
- Increases in ocean temperatures are predicted to increase the frequency and severity of coral bleaching events in the coming decades. Coral disease outbreaks are also predicted to increase in response to warming seas.
- Rising seas linked to climate change may not pose a major threat to coral reefs, as long as sea level rises slowly enough for coral growth to keep pace. However, coral reefs may be greatly affected if catastrophic ice melting causes major increases in global sea level. Additionally, sea-level rise may cause increased sedimentation due to shoreline erosion, which could adversely affect coral reefs. Rising seas are also likely to result in inundation and coastal erosion of low-lying islands and coastal habitats such as mangroves and sea turtle nesting beaches. The ability of reefs to keep up with rising sea levels is strongly linked to ocean acidification (below).
Changes in Storm Patterns
- Global climate change is likely to drive changes in storm patterns. Future projections based on high-resolution models suggest that by 2100:
- Tropical storms globally may be more intense
- The frequency of the most intense storms may increase substantially in some areas
- The number of all tropical storms globally may decrease or change little
Tropical storms can cause extensive damage to coral reef ecosystems by causing direct damage to the reef structure and can cause increases in sedimentation and runoff from land.
Altered Oceanic Currents
- The ocean moves large amounts of heat around the planet through global ocean currents (e.g., through upwelling, downwelling, and thermohaline circulation). Ocean currents will be affected by changes in wind, precipitation, temperature, and salinity due to global climate change. Changes in ocean currents can affect the transport or retention of pollutants, movement of larvae, and temperature regimes that may impact thermally sensitive species such as corals. Many climate models predict a weakening of the thermohaline circulation due to climate change, changing large-scale circulation and climate patterns.
Changes in Precipitation
- Observations indicate that changes are occurring in the amount, intensity, frequency and type of precipitation. Precipitation patterns demonstrate large natural variability due to El Niño and changes in atmospheric circulation patterns such as the North Atlantic Oscillation. Long-term trends over the last century show significantly wetter conditions in eastern North and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. Increased water vapor in warmer climates leads to more intense precipitation events and an increased risk of both drought (where it is not raining) and floods (where it is raining). The cycle of El Niño events dramatically affects the distribution and timing of floods and droughts, particularly in the tropics and over much of the mid-latitudes of Pacific-rim countries.
Ocean Acidification: A Result of Rising CO2, Not Climate Change
Increases in atmospheric CO2 concentrations cause increases in CO2 deposition in the surface ocean, lowering the pH of seawater (making the seawater more acidic). Resulting acidification of the oceans reduces the amount of calcium carbonate available for corals and other calcifying marine organisms to build their skeletons and shells. This process is called ocean acidification and reduces a reef’s ability to grow and withstand stress.
Ocean acidification is a direct result of CO2 emissions, not climate change. Chemical changes in the oceans due to atmospheric CO2 emissions are observable now and are highly predictable. The chemical reaction leading to ocean acidification (formation of carbonic acid when CO2 dissolves in seawater) is mostly independent of climate change, so actions such as geoengineering to cool global temperatures and reductions of other greenhouse gases will not significantly slow ocean acidification. Therefore, the only way to reduce ocean acidification globally is to reduce CO2 concentrations in the atmosphere.
Natural Changes in the Ocean-Atmosphere System
In addition to climate change, natural processes operate to drive changes in the ocean-atmosphere system. For example, the El Niño Southern Oscillation (ENSO) refers to two extreme phases of a naturally occurring climate cycle (El Niño and La Niña). ENSO is one of the most dominant features of seasonal-to-interannual climate variability, and leads to anomalous warming in the eastern equatorial Pacific Ocean. Scientists are not sure whether climate change will affect the frequency or intensity of the ENSO system over the 21st century, but research suggests that ENSO could exacerbate the impacts of climate change, particularly for coral reef ecosystems and the communities that depend upon them.
Implications for Reef Resilience
Climate change is likely to increase the disturbance regime for coral reefs, and the fate of coral reef ecosystems will increasingly be determined by their potential for recovery and long-term maintenance of structure, function and goods and services – i.e. their resilience. Resilience-based management requires that management goals for coral reef ecosystems be expanded to include both ecosystem states (e.g., coral abundance, fish density) and ecosystem processes (e.g., recruitment success, algal removal rates). It is critical for managers to prioritize management efforts toward restoring and maintaining coral reef resilience. In the face of climate change, adaptive resilience-based management is likely to offer the best hope for marine ecosystems. At the same time, reef managers need to call for a reduction of atmospheric CO2 levels, as resilience alone is unlikely to be sufficient to protect coral reef ecosystems.
Video Changing Times (1:00)
Andrew Baker discusses the need to manage reefs to buy time as we experience global climate change.
Video Impacts of Climate Change (16:47)
Dr. Ove Hoegh-Guldberg talks about the impacts of climate change on marine ecosystems.