Encyclopedia
Please note this is a comparison between Version 1 by Mohamad Nor Azra and Version 2 by Lindsay Dong.
Coral reefs are vital ecosystems with high biodiversity and ecological services for coastal communities. Climate change is accelerating, with detrimental consequences on coral reefs and related communities.
- coral
- Climate change
- ocean
1. Introduction
Scleractinians, or stony corals, emerged during the Cambrian period and constructed the earliest reefs, dating to approximately 410 million years ago [1][2][3][1,2,3]. Five major coral extinctions have occurred since then, all of which have been linked to rising temperatures and higher levels of carbon dioxide in the atmosphere [2][4][2,4].
While coral reef research commenced more than 100 years ago, concern over the state of coral reefs is relatively recent, occurring only in the last four decades. In 1981, at the 4th International Coral Reef Symposium, Edgardo Gomez initiated the conversation on threats to coral reefs by presenting his concerns to the scientific community [5][6][5,6]. Coral reefs are considered an important marine resource for coastal communities, and the conference participants were mainly focused on coral reef management and environmental impacts and related fisheries activities [5][7][8][5,7,8]. In addition to the natural stresses that have always existed on coral reefs, such as storms, freshwater inundation, and seismic and volcanic events, there is growing evidence of new emerging threats potentially causing global damage to coral reefs [7][8][9][10][7,8,9,10].
At current extinction rates, it is estimated that we are commencing the sixth mass extinction event [11][12][13][11,12,13], with individual extinctions occurring approximately 1000-fold faster than the expected background extinction rate. Theoretically, species extinctions occur at a rate proportional to the rate of speciation or the creation of new species [14]. Current extinction rates are much higher than speciation rates. This is largely due to the fact of anthropogenic factors [15][16][15,16], such as habitat destruction [17], deforestation [18], pollution [7], ocean acidification, climate change resulting from greenhouse gas emissions, and overexploitation of ecological resources [19][20][21][19,20,21]. It is estimated that 75% of species will go extinct unless human pressures on the environment are scaled back soon [20][22][23][24][25][20,22,23,24,25]. Further management efforts are required to reduce the impacts of climate change and human anthropogenic stress towards coral reef communities [7].
Anthropogenic pressures on reefs have been the dominant factor damaging coral reefs through a range of stresses (Figure 1). Unsustainable land-based human activities, such as deforestation, poorly regulated agriculture, and urban/industrial development, are major contributors to the release of excessive sediments and nutrients into the environment [26][27][28][26,27,28]. Increases in human-caused greenhouse gas emissions are the primary factor in the current climatic shift around the world [29][30][29,30]. The ocean acts as a massive sink that absorbs carbon dioxide, resulting in the acidification of the oceans [31][32][31,32]. Coral bleaching and widespread damage to the coral reef ecosystem have become increasingly common because of thermal stress brought on by rising ocean temperatures [33]. Lack of food to sustain the coral reef ecosystem and disruption of larvae dispersal are both attributable to altered currents, upwelling, and/or vertical mixing brought on by changing currents and winds [34]. Storms and cyclones are “agents of mortality” on coral reefs and can have a direct impact on the structure and local distribution of coral reef assemblages, especially through the large waves they produce [35]. Reduced salinity caused by heavy rainfall and enhanced surface run-off onto nearshore reefs during cyclones, storms, and heavy precipitation rates can lead to algal blooms and other devastating results [36][37][36,37]. The rise in sea level, caused by thermal expansion and the melting of ice on land, has varied across different regions of the world over the past century, with an average increase of approximately 20 cm [38]. The sedimentary mechanisms triggered by the rising sea levels have the potential to intensify and jeopardize crucial physiological reef processes, such as photosynthesis, feeding, and recruitment, thereby posing a severe threat to coral reefs and related ecosystems, such as seagrass meadows and mangrove forests [39][40][39,40]. This threat, coupled with increasing carbon dioxide (CO2) emissions, can negatively impact these vital ecosystems. Without effective local measures and a concerted effort to reduce carbon dioxide emissions, these effects are projected to intensify, leading to an unprecedented degradation of marine biodiversity and ecological balance [41][42][41,42].
Figure 1.
Threats to coral reefs posed by factors related to climate change.
2. The Evolution of Coral Reef under Changing Climate
Coral reefs are a vital marine ecosystem service, providing high biodiversity and supporting the livelihoods of coastal communities. However, ocean warming and temperature are the largest threats to corals from anthropogenic climate change [43][192]. Between 1997 and 2018, the global average percentage of coral cover was approximately 32%, but by 2100, RCP 8.5 predicts a global decline in coral cover of 5 and 15%, equating to a relative global decline of more than 40% [44][45][115,193]. This decline is due to the fact that sea surface temperatures (SSTs) are projected to increase by more than 3 °C by the turn of the century [46][194]. These declines could have significant ecological and socioeconomic impacts, particularly in coastal communities that rely on coral reefs for food, tourism, and other ecosystem services.
For example, The Republic of Palau, a small Micronesian nation, has already experienced significant losses in coral reef cover [47][195]. Over 87% of Palau’s households are linked to coral reef-associated activities, which are critical to the country’s economic and social well-being. While tourism, particularly ecotourism, is a significant contributor to GDP growth, tax revenue, and employment, climate change-related stressors have caused a steady decline in coral reef cover. This decline has indirectly caused a major decline in tourism, threatening the country’s economic sustainability [48][196]. According to Barnett [49][197], climate change is a significant threat to food security for people in Pacific SIDS, primarily due to the decline in fisheries output resulting from the impact of climate change on total coral cover.
Apart from impacting the socioecological structure, the impact of climate change can have cascading effects on the entire reef ecosystem, affecting the abundance and diversity of other marine species that depend on corals for food and shelter. Up to 14% of species may be in imminent danger of extinction at a warming of 1.5 °C and up to 29% at a warming of 3 °C. This rise in ocean temperature will probably force coral to colonize higher latitudes that currently lack reefs [50][51][52][198,199,200]. However, various factors, including the need for a suitable substrate [53][201], connectivity to other reefs [54][202], ocean acidification [55][203], and light intensity [56][204], may outweigh the advantages of reefs as they expand to high latitudes [45][193].
