Ocean acidification limits temperature-induced poleward expansion of coral habitats around Japan

Abstract

Abstract.Using results from four coupled global carbon cycle-climate models combined with in situ observations, we estimate the effects of future global warming and ocean acidification on potential habitats for tropical/subtropical and temperate coral communities in the seas around Japan. The suitability of coral habitats is classified on the basis of the currently observed regional ranges for temperature and saturation states with regard to aragonite ( arag ). We find that, under the "business as usual" SRES A2 scenario, coral habitats are projected to expand northward by several hundred kilometers by the end of this century. At the same time, coral habitats are projected to become sandwiched between regions where the frequency of coral bleaching will increase, and regions where arag will become too low to support sufficiently high calcification rates. As a result, the habitat suitable for tropical/subtropical corals around Japan may be reduced by half by the 2020s to 2030s, and is projected to disappear by the 2030s to 2040s. The habitat suitable for the temperate coral communities is also projected to decrease, although at a less pronounced rate, due to the higher tolerance of temperate corals for low arag . Our study has two important caveats: first, it does not consider the potential adaptation of the coral communities, which would permit them to colonize habitats that are outside their current range. Second, it also does not consider whether or not coral communities can migrate quickly enough to actually occupy newly emerging habitats. As such, our results serve as a baseline for the assessment of the future evolution of coral habitats, but the consideration of important biological and ecological factors and feedbacks will be required to make more accurate projections.

An improved estimation of the poleward expansion of coral habitats based on the inter-annual variation of sea surface temperatures

“…We used monthly mean SSTs in the coldest months as temperature thresholds for coral mortality to improve the model projection, because previous studies have used the decadal-mean SST in the coldest months as indices for coral habitats (Yara et al 2011(Yara et al , 2012. Monthly mean SSTs in the coldest months were calculated from observational SST data used in previous studies (Nomura 2009;Yamano and Namizaki 2009).…”

“…1). To quantitatively assess the improvement in the simulation of poleward expansion by considering coral mortality based on the inter-annual variation of SST in the coldest months, we compared the results obtained in this study with simulations using the average SST in the coldest months during a given period (the method used by Yara et al 2011Yara et al , 2012. Statistical metrics used to assess each model's performance are summarized in Tables 1 and 2.…”

“…Many studies have projected the future impacts of rising seawater temperatures on coral habitats using high-and/or low-temperature stress indices (Donner et al 2007;Donner 2009;Teneva et al 2012;van Hooidonk et al 2013;Yara et al 2009Yara et al , 2011Yara et al , 2012Yara et al , 2014. Most studies have used a high-temperature stress index such as degree heating months, which is calculated from positive anomalies relative to mean water temperatures in the warmest months, to project the area and frequency of coral bleaching in several regions (Donner et al 2007;Donner 2009;Teneva et al 2012;van Hooidonk et al 2013;Yara et al 2009Yara et al , 2014.…”

“…Most studies have used a high-temperature stress index such as degree heating months, which is calculated from positive anomalies relative to mean water temperatures in the warmest months, to project the area and frequency of coral bleaching in several regions (Donner et al 2007;Donner 2009;Teneva et al 2012;van Hooidonk et al 2013;Yara et al 2009Yara et al , 2014. Alternatively, some studies (including this one) have used low-temperature indices, which are defined from absolute water temperature values in the coldest months, based on the current distributional limits of coral reefs and corals, and have projected a poleward expansion of coral habitats (Yara et al 2009(Yara et al , 2011(Yara et al , 2012. Yara et al (2011) investigated the potential future poleward expansion of three coral communities in response to rising sea surface temperatures (SSTs) along the coasts of Japan, based on decadal-mean SSTs in the coldest months obtained from multiple climate models and simplified indices of poleward expansion.…”

The ocean carbon sink – impacts, vulnerabilities and challenges

“…Recent scenario computations with Earth system models document that a drastic reduction of CO 2 emissions is required to preserve major coral reefs during the Anthropocene (Ricke et al, 2013). However, aspects such as potential adaptation processes and migration need yet to be included in regional studies (Yara et al, 2012).…”