Coral bleaching: Stony corals live in a mutualistic relationship with single-celled algae called zooxanthellae. Zooxanthellae provide corals with food through photosynthesis in exchange for a home and give corals their bright colours. Higher-than-normal sea temperatures disrupt this relationship, the zooxanthellae are expelled, and the white coral skeleton becomes visible through transparent coral tissue – hence the term ‘bleaching’. Corals can either regain their zooxanthellae and survive or die if temperature stress persists.
Thermal stress: The sea water temperature stress that triggers bleaching is measured in degree heating weeks (DHWs). One degree heating week is equal to one degree above the long-term average summer maximum temperatures for one week. Severe bleaching is common when thermal stress exceeds 8 DHWs.
Climate model projections: Coral bleaching is chief among the threats to coral reefs posed by climate change. Bleaching events are expected to become more frequent and more severe in the decades ahead. UNEP, in collaboration with NOAA, WWF, USGS and other partners, supported development of climate model projections of the timing of 2x per decade and 10x per decade (i.e. annual) occurrence of severe bleaching conditions (i.e. thermal stress >8 DHWs). Coral reef communities will start to change when bleaching becomes more frequent than 2x per decade. Corals susceptible to bleaching will be less common and the structural complexity of many coral reefs will decline. These changes will occur more rapidly if/when bleaching events begin to occur annually; att this point recovery will be limited. The projections for the two bleaching scenarios have been produced for emissions scenarios RCP8.5 and RCP4.5 (i.e. 4 projections in total). RCP8.5 is a ‘no climate policy’ scenario that assumes emissions increase unabated. At time of writing, emissions concentrations are currently greater than what RCP8.5 projected for 2016. RCP4.5 assumes emissions peak around 2040 and then decline as a result of successful implementation of climate policies.
Maps and data: The projections are unique in that they have been produced through statistical downscaling so have high-resolution (4-km) relative to raw climate model data (~1x1°). The final outputs represent the ensemble average, with 33 climate models (from IPCC CMIP5) used for RCP8.5 and 35 used for RCP4.5. The projections results are available here for all the worlds’ coral reefs, as maps (static image files, one per projection), as Google Earth layers (kmz files), and as spatial data in the form of raster grids compatible with ArcGIS (one layer package including the four projections and reef locations). Data and images can be downloaded from the Data tab.
Main findings: There is great variation in the projected timing of the onset of annual severe bleaching conditions in most coral reef regions, as well as among and even within countries. Reefs projected to experience annual severe bleaching 10 or more years later than other reefs in the same country or territory are relative refugia, which makes them conservation priorities. The projected timing of annual severe bleaching varies >10 years in 71 of the 87 countries and territories with >500km2 of reef area, indicating the projections warrant consideration in most reef areas during conservation and management planning.
The global average projected timing for the onset of annual severe bleaching (ASB) conditions is 2043. Locations projected to experience ASB after 2052 and before 2034 are relative climate winners and losers, respectively. 16 of the top 20 countries in terms of reef area have both climate winners and losers. The average year for the projected timing of ASB conditions under RCP4.5 is 11 years later than the average projected under RCP8.5, but there is considerable spatial variation. Some reefs (<10%) are projected to face ASB conditions more than 25 years later under RCP4.5 than under RCP8.5, while almost a third of reefs get less than ten years additional time before the onset of ASB conditions under RCP4.5 than under RCP8.5.
Applications: These downscaled projections can influence and guide research, conservation and management planning and policy. In conservation planning, these data can be used to identify priority areas (relative refugia) and prioritize actions to reduce stress related to human activities to support reef resilience and reduce climate vulnerability. Such actions may include establishment of marine protected areas, fisheries restrictions, or reducing pollution from land. Managers and conservationists can also use the downscaled projections in outreach campaigns to raise awareness about climate change as well as explain and support planned management actions. The downscaled projections can also inform policy decisions, especially in relation to climate change. Emissions reductions are required to prevent the great majority of coral reefs from experiencing severe bleaching conditions annually within this century, which can be used in the formulation or revision of INDCs. The downscaled projections can also be used to inform policies related to fisheries, coastal development and conservation, land-use planning including agriculture, all of which also influence reef resilience and vulnerability, as well as adaptation planning for areas or sectors dependent on reef ecosystem services. Future research projects can incorporate these data and, for example, include spatial variation in adaptive capacity when more is known about spatial variation in the ability of corals to adapt to increasing sea temperatures.
Project partners: This project was supported by funding and in-kind contributions from UNEP; NOAA NMFS PIFSC, CRCP and AOML; WWF; USGS via the PICSC; and the ERC.
Further information: Scientific publications describing project methods and results will be posted to this site in the coming months. You can request more information on these projections by sending an email to Jerker Tamelander, Head, UNEP Coral Reef Unit: firstname.lastname@example.org
A healthy reef normally has high coral cover and complex structure.
A healthy reef provides habitat for many species and supports high productivity.
Heat stress on coral reefs leads to paling and eventually to bleaching of corals.
When coral bleaching conditions persist, the coral dies and becomes overgrown with algae.
If bleaching occurs frequently the reef does not have time to recover between events.
Without coral recovery the reef loses its complexity and structure.
Without coral recovery the ability of the coral reef to provide ecosystem services is also diminished.
Well managed reefs can recover from severe bleaching, but increasing frequency of
bleaching events is putting increasing demands on reef management efforts.
IW:LEARN is the Global Environment Facility's (GEF) International Waters Learning Exchange and Resource Network. It supports knowledge sharing and contains freely available project-related information. It also promotes experience sharing and learning among GEF International Waters projects and the country officials, agencies, and partners working on them.
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