(C) PLOS One [1]. This unaltered content originally appeared in journals.plosone.org.
Licensed under Creative Commons Attribution (CC BY) license.
url:https://journals.plos.org/plosone/s/licenses-and-copyright

------------



Future loss of local-scale thermal refugia in coral reef ecosystems

['Adele M. Dixon', 'School Of Biology', 'Faculty Of Biological Sciences', 'University Of Leeds', 'Leeds', 'United Kingdom', 'Priestley International Centre For Climate', 'Piers M. Forster', 'Scott F. Heron', 'Physics']

Date: 2022-04

Thermal refugia in the future

Coral recovery following extensive thermal stress-induced mortality is spatially variable but on average is thought to require at least 10 years to re-establish coral communities [37]. To represent sites where coral communities can be maintained and/or re-established, we define thermal refugia as 1 km reef pixels with a probability of thermal stress events less than 0.1 yr-1 (one event every 10 years; Fig 1). Exposed reefs are defined as 1 km reef pixels with a probability of thermal stress events greater than 0.2 yr-1 (one event every five years). A probabilistic frequency of 0.2 yr-1 corresponds to an intolerable level of thermal stress [11, 28, 38]. All other reef pixels are described as intermediate which indicates reefs where the level of thermal stress may be too high to maintain pre-disturbance communities and coral cover, but where species with high recovery rates might proliferate.

PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 1. Probability of DHW events > 4°C-weeks, seasonal SST variability and inter-annual SST variability in 12 coral reef regions during the period 1986–2019. Outliers (>100 * interquartile range) are shown by the black dots. Thresholds for determining thermal refugia (probability of DHW events > 4°C-weeks less than 0.1 yr‒1) and exposed reefs (probability of DHW events > 4°C-weeks greater than 0.2 yr‒1) are represented by the blue and red shaded areas, respectively. Thresholds for determining high SST variability (> 0.7°C) and low SST variability (< 0.3°C) are represented by the dark and light grey shaded areas, respectively. https://doi.org/10.1371/journal.pclm.0000004.g001

Thermal stress is calculated using the cumulative thermal stress metric Degree Heating Weeks (DHW), which is the rolling 12-week sum of SST anomalies at least 1°C higher than the long-term maximum monthly mean (MMM) [39]. Thermal stress events are identified as those with a DHW value above 4°C-weeks, which is the threshold commonly used to indicate thermal stress high enough to cause significant coral bleaching and some mortality, whereas the 8°C-weeks threshold indicates severe thermal stress leading to broad-scale catastrophic coral mortality [40]. The long-term MMM calculated here is slightly higher (up to 1°C) for much of the world’s coral reefs than those calculated by previous studies (S1 Fig). We use the European Space Agency Climate Change Initiative (CCI) 5 km SST Analysis product [41] for the early part of the time series, instead of the more-commonly used NOAA Coral Reef Watch product, due to its consistency with in situ SST measurements for coral reef regions [42]. The 5 km SST is then downscaled to 1 km by replacing the CCI 5 km monthly SST climatology with that of the 1 km MUR dataset (S1 Appendix). Together, these factors result in small changes to the MMM which can then lead to larger changes in accumulated thermal stress. The 4°C-weeks threshold we use therefore indicates more severe bleaching than described in previous studies. We define low variability reefs as those with seasonal and inter-annual SST variability less than 0.3°C and high variability reefs as those with seasonal or inter-annual SST variability greater than 0.7°C [26] (Fig 1).

In the recent era (1986–2019), 84.1% of reef pixels globally are thermal refugia (Fig 2). The percentage of global thermal refugia drops to 0.2% (0–57.8%) at 1.5°C of warming, relative to pre-industrial levels, and to 0% (0–45.2%) at 2.0°C of warming (Fig 2). Only 6.8% of reef pixels are exposed in the 1986–2019 period, increasing to 90.6% (12.1–100%) and 99.7% (16.3–100%) at 1.5°C and 2.0°C of warming, respectively. At 3.0°C and 4.0°C, there are no thermal refugia and all global reef pixels are exposed (Fig 3). Coarse resolution (50 km) CMIP3 projections for the global coral reef area estimated that 100% (4°C-weeks threshold) and 89% (8°C-weeks threshold) of coral reefs will be exposed (> 0.2 yr-1) at 1.5°C of global warming [11]. Our findings provide further support that the Paris Agreement target of limiting warming to 1.5°C will not be enough to save most coral reefs [11, 28, 43]. However, by capturing fine-scale SST features that have been known to prevent bleaching mortality in the past, we locate small reef areas where the probability of thermal stress under future warming is lower than in adjacent areas.

PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 2. Global distribution of exposure category in the 1986–2019 climate and at 1.5 and 2.0°C of future global warming. Exposure categories are thermal refugia (probability of DHW events > 4°C-weeks less than 0.1 yr‒1), intermediate (probability of DHW events > 4°C-weeks from 0.1–0.2 yr‒1) and exposed (probability of DHW events > 4°C-weeks greater than 0.2 yr‒1). Percentages indicate the regional (on map) and global (right of map) proportion of thermal refugia (blue) and exposed reefs (red). The 12 coral reef regions are outlined in light blue. The base map is made with Natural Earth. https://doi.org/10.1371/journal.pclm.0000004.g002

PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 3. Probability of DHW events > 4°C-weeks across 12 coral reef regions under 1.5, 2.0, 3.0 and 4.0°C of global warming relative to pre-industrial levels. Thresholds for determining thermal refugia (probability of DHW events > 4°C-weeks less than 0.1 yr‒1) and exposed reefs (probability of DHW events > 4°C-weeks greater than 0.2 yr‒1) are represented by the blue and red shaded areas, respectively. https://doi.org/10.1371/journal.pclm.0000004.g003

We find thermal refugia in all 12 coral reef regions in the 1986–2019 climate (Fig 2). At 1.5°C, thermal refugia are only present in two coral reef regions (Fig 2): Polynesia and the Coral Triangle. For most coral reef areas, current thermal refugia are not projected to remain so. Many known upwelling areas in Oman [7, 44], Colombia [7], Indonesia (Lesser Sunda) [9] and the Caribbean [8] are projected to have no thermal refugia remaining at 1.5°C of warming (Fig 2). The exception is in the East Indian Ocean Sumatra-Java upwelling region, which has some thermal refugia remaining at 1.5°C of warming. While upwelling areas can provide respite from coral bleaching and mortality in the present-day climate, local upwelling is only enough to mitigate thermal stress on coral reefs in very rare cases and under the smallest projected change in future warming. Similarly, there are no thermal refugia at 1.5°C of global warming in areas with high currents known to influence bleaching dynamics in the past, such as Panama, Florida [8] and Lesser Sunda, Indonesia [9]. Some small reef areas influenced by upwelling or high currents in Lesser Sunda and Oman are rated intermediate for exposure at 1.5°C of warming rather than exposed, but they are not thermal refugia given our refugia criteria. Similar patterns emerge when using an 8°C-weeks threshold to define thermal refugia, with a slightly slower decline to 0% thermal refugia (S2 Fig).

Bleaching risk is heavily influenced by inter-annual and seasonal SST variability [26, 45]. Here, we find that areas with moderate to high inter-annual variability have a lower bleaching risk with future warming (S3 Fig) because cooler years, influenced by natural climate variability, provide respite between thermal stress events [26]. For example, the probability of thermal stress events > 4°C-weeks is lower along the Sumatra-Java coast, resulting in small areas of thermal refugia at 1.5°C of warming with some intermediate reefs remaining at 2.0°C of warming in West Sumatra. This pattern most likely arises from positive Indian Ocean Dipole events that drive upwelling that results in cold SST anomalies along the Sumatra-Java coastline [18], which may provide respite from future warming in Sumatra facilitating coral reef recovery. However, some CMIP6 models simulate more regular Indian Ocean Dipole events during the historical period compared to observations, indicating that this cool respite might be less frequent than projected here [18]. Furthermore, upwelling is associated with the transport of nutrients to surface waters which can have harmful effects on coral reef ecosystems [46]. Thermal refugia in South Sumatra are associated with bay areas influenced by river input which also contribute high nutrient loading [47], potentially exacerbated by increased extreme rainfall with future warming and land use change [48].

The probability of thermal stress events > 4°C-weeks is lower in the Polynesia region under future warming than in other coral reef regions (Fig 3). The region has the highest number of thermal refugia at 1.5°C of global warming (Fig 2). CMIP6 models simulate relatively low rates of future warming in the southern Pacific compared to the rest of the world [49, 50]. Weakening of equatorial trade winds due to global warming will slow ocean circulation and equatorial upwelling [51] causing less warm water being transported away from the equator resulting in higher rates of warming in the equatorial Pacific compared to regions off the equator (e.g. French Polynesia). However, rates of warming in the southern Pacific are uncertain. SST warming rates in the tropical Pacific are affected by long-standing climate model biases in oceanographic SST features (e.g. the equatorial cold tongue bias [52]) although this bias is reduced in CMIP6 compared to CMIP5 [50].

High latitude reefs are among the first areas to lose thermal refugia under future global warming (Fig 2). These regions are characterised by high seasonal variability (Fig 1). We find that reef pixels with high seasonal SST variability have a larger increase in the probability of thermal stress between the 1986–2019 climate and 1.5°C of warming (S4 Fig). Chronic warming in highly seasonally variable regions results in summer temperatures exceeding thermal stress thresholds annually under small changes in global mean temperature [26]. High latitude reefs may therefore provide a thermal refugia for range shifting corals adapted to warmer baseline temperatures [53] but are unlikely to provide a thermal refugia for the species currently living there, unless they are able to sufficiently increase their thermal tolerance under the highly variable environmental conditions.

[END]

[1] Url: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000004

(C) Plos One. "Accelerating the publication of peer-reviewed science."
Licensed under Creative Commons Attribution (CC BY 4.0)
URL: https://creativecommons.org/licenses/by/4.0/


via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/plosone/

You are viewing proxied material from magical.fish. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.