Marine heatwave intensification threatens coral reef health

By Hayley Evers-King and Rob Roebeling (EUMETSAT), Ben Loveday (Innoflair) and Olivier Membrive (OSI SAF)

This summer — the summer of 2022 - the Mediterranean Sea experienced a major marine heatwave, with sea surface temperatures about 5°C warmer than average (Figure 1).

Concurrent with the past century's persistent warming of global oceans, these kind of heatwaves (periods of extreme regional ocean warming) have become more frequent and more extreme (Laufkötter et al., 2020). Marine heatwaves occur in many areas around the world, from the Pacific Ocean to the Atlantic Ocean to the Mediterranean Sea. They threaten marine biodiversity and its ecosystems (Smale et al., 2019).

One particular ecosystem impacted by marine heatwaves, are coral reefs. These heatwaves can, for example, cause coral bleaching, coral disease outbreaks, and/or algae blooms (Roberts et al., 2019). Healthy corals live in a symbiotic relationship with the microscopic algae (zooxanthellae) that live in their tissue (Wooldridge, 2013). These algae provide the coral polyps with nutrients as well as their bright colours. When sea surface temperatures become too warm this symbiotic relationship becomes disrupted and algae start leaving the coral's tissue, and corals turn white (Figure 2). Although coral can survive these so-called bleaching events, they become more stressed, more susceptible to diseases, and, on the long-term, subject to mortality (NOAA, 2021).

In its 6th Assessment Report, the United Nations' International Panel for Climate Change (IPCC, 2021) reports that ocean surface temperatures increased on average by almost 1°C globally since the start of the industrial era. Similarly, it is reported that the frequency of marine heatwaves (high confidence) as well as their intensity and lifetime (medium confidence) increased since the 1980s. In the coming century, further warming of ocean surface is projected between about 1 °C and 3 °C, depending on reduction of CO₂ emissions achieved. As the ocean surface warms up, climate scientists predict that the frequency, intensity, and duration of marine heatwaves will continue to increase, and, inevitably, create more problems for marine ecosystems in the future (IPCC, 2021). In addition, the increased absorption of atmospheric carbon dioxide into the ocean is altering the seawater chemistry through an increase of its acidity. This process of ocean acidification poses another risk to the corals, as it leads to reduced calcification rates in reef-building and reef-associated organisms (IPCC, 2021).

Observing sea surface temperatures from space

The variable drivers, and historical context for defining heatwaves regionally, mean that we need the ability to measure the range of ocean temperatures occuring all over the world, every single day over a long period in time. Only then we are able to understand what ‘normal’ temperatures look like. Thermal Infrared observations from satellite have been key for measuring sea surface temperatures over the past decades.

The Sentinel-3 satellites are the Copernicus programme's contribution to monitoring of sea surface temperatures, with the Sea and Land Surface Temperature Radiometer (SLSTR) on Sentinel-3A and B. The SLSTR provides frequent, high-quality, and high-resolution sea surface temperature data, allowing for observation at the event scale. The Sentinel-3 missions build on a long heritage of sea surface temperature missions flown by an array of international agencies, and including sensors, such as AVHRR and SEVIRI flown on EUMETSAT's Metop and MSG missions. Data from these satellites is processed and distributed to climatological data records of key ocean parameters by EUMETSAT's Ocean and Sea Ice Satellite Applications Facility (OSI SAF), and is as well used by downstream service providers such as the Copernicus Marine Service. Figure 3 shows a SLSTR sea surface temperature composite over the Great Barrier Reef during a marine heatwave day.

As useful as individual images are for measuring temperatures in real-time, it is difficult to know if a damaging marine heatwave is likely to be underway, without some historical context. For this, we have to turn towards long-term time series of measurements, i.e., “climate data records”. SLSTR on-board Sentinel-3A is also now able to function as a reference sensor, when combining multiple sea surface temperature data sets to support climate scale observations. To detect climate-related trends accurately, climate data records combining data of different sensors must consider consistency overtime. As such, they are not available in real-time. However, using many of the same principles, interim records can be created, to help detect and better understand and contextualise real-time events, such as the warm temperatures observed over the great barrier reef, in relevant context (Figure 3).

Rapid warming of Earth's oceans in the past 40 years

Sea surface temperatures show large regional variations from year-to-year, with both warmer and cooler temperatures. However, as time progressed since the 1980s, satellite-based climate data records reveal that areas with temperatures higher than the 1982-2020 mean have become more frequent, larger and warmer (Figure 4).

This can also be illustrated with so-called ‘climate stripes’ graphics that 'citizen scientists' all over the world have been using recently. As an example, the climate stripes-style graphic for the region around the Great Barrier reef (region indicated in Figure 3) illustrates the warm anomalies apparent during the years 1998, 2010, 2016, 2017, 2020 and 2021 (Figure 5).

Marine heat waves are becoming more frequent

Marine heatwave events can be identified using specific statistics, for example, when temperatures exceed a seasonal threshold, such as the 90th percentile, for a number of consecutive days (Hobday et al, 2019). Figure 6 illustrates how periods of potential marine heatwaves can be detected in records of satellite sea surface temperatures. From this figure, the 2020 and 2021 heatwaves can be clearly seen.

A study by Smale et al., (2019) found that there were over 50% more marine heatwave days per year in the period 1987-2016 compared to the number of such days in the earlier period 1925-1954. Other recent literature mentions at least 10 marine heatwaves during the period 2000-2020 (Olivier et al. (2021) and Manta et al. (2018)) as illustrated in Figure 7. The animation in Figure 8 reveals an abundant spread of areas with monthly sea surface temperature anomalies up to 6 °C. This could indicate that the actual number of marine heatwaves is much higher than the numbers reported in literature. This is confirmed by the study of (Huang, B et al., 2021), who found a strong increase in marine heatwaves over the Arctic. Their study suggests that the increasing trends of marine heatwaves in the Arctic are likely associated with the increasing surface air temperature and decreasing sea-ice concentration under the global warming environment.

Advances in ocean monitoring

New satellites and algorithms ensure continued and improved monitoring of the state and temperatures of Earth's oceans. With the launch of Sentinel-3A in 2016, a new era of ocean monitoring started for Europe. Sentinel-3A is part of the Copernicus Sentinel-3 mission that plans to operate four satellites till 2035. This mission is dedicated to delivering long-term high-quality ocean and atmospheric measurements. The satellites are capable of observing the sea surface temperatures with the Sea and Land Surface Temperature Radiometer (SLSTR), sea surface topography with the altimetry instruments, as well as ocean colour with the Ocean and Land Colour (OLCI) instrument. The fact that the Copernicus Sentinel-3 mission builds upon heritage of previous ocean observing satellites (e.g. ENVISAT), ensures continuation of long time-series, such as the sea surface temperature time-series used here. However, the improved spatial, temporal and spectral resolution, and better radiometric sensitivity allow higher quality measurements from coastal to open ocean environments. Upcoming missions such as those on the next generation of EUMETSAT's geostationary and polar orbiting mission, and the Copernicus Imaging Microwave Radiometer (CIMR), will continue and expand the global SST record.

Benefits

This Climate Use Case demonstrates how climate data records of satellite-based sea surface temperatures can provide a good basis for verifying IPCC's conclusions (IPCC, 2021) that global oceans have warmed strongly and marine heat waves have increased significantly since the 1980s. Data sources, such as those used in this case study, are vital to addressing challenges facing the oceans today. As part of the United Nations Ocean Decade, ten specific challenges are being addressed. This work, and the data underlying it, support "Challenge 2 - Protect and restore ecosystems and biodiversity". Data on marine heatwaves can help characterise the stressors facing marine ecosystems under climate changes, and contribute to decision making to protect, manage and restore those affected. A supporting Jupyter notebook allows users to access the data shown in this case study, and recreate some of the figures. The notebook is available on the EUMETSAT gitlab, along with other case study examples in the containing repository.

Data used

A satellite-based data record of reprocessed Global Ocean Sea Surface Temperature and Sea Ice, derived from (A)ATSR, SLSTR and AVHRR data over the period 1981-date, from Copernicus Marine and Environmental Monitoring Service (CMEMS) EXT that can be cited with the following DOI: https://doi.org/10.48670/moi-00169 (accessed on 2022-03-11)

A satellite-based data record of near-real-time Global Ocean Sea Surface Temperature And Sea Ice, derived from AMSR2, AVHRR, SEVIRI, VIIRS, and SLSTR data over the period 2007-date, from Copernicus Marine and Environmental Monitoring Service (CMEMS) EXT that can be cited with the following DOI: https://doi.org/10.48670/moi-00165 (accessed on 2022-03-11)

A satellite-based SLSTR level-2 NTC data record: SLSTR Level 2 Sea Surface Temperature (SST) in NTC Sentinel-3 (assessed on 2022-07-22)

Satellite-based climate data records of Sea Ice Concentration, derived from passive microwave sensor observations (SMMR, DMSP/SSM/I, DMSP/SSMIS) over the period 1979-2020, from the Ocean and Sea Ice Satellite Application Facility (OSI SAF)*) that are cited as:

• EUMETSAT Ocean and Sea Ice Satellite Application Facility, Global Sea Ice Concentration Climate Data Record v2.1 - Multi-mission, over the period 1979-2015, OSI-450, doi: 10.15770/EUM_SAF_OSI_0008 EXT, data extracted from OSI SAF FTP server (accessed 2022-09-01)
• EUMETSAT Ocean and Sea Ice Satellite Application Facility, Interim Global Sea Ice Concentration Climate Data Record v2.1 - Multi-mission, over the period 2016-onwards, OSI-430-b, doi: 10.15770/EUM_SAF_OSI_NRT_2008, data extracted from OSI SAF FTP server (accessed 2022-09-02)

A satellite-based data record of near real-time global sea surface temperatures, derived from AVHRR on Metop over the period 2011 - onwards, by the Ocean and Sea Ice Satellite Application Facility (OSI SAF) that are cited as:

• OSI SAF (2011): Full resolution L2P AVHRR Sea Surface Temperature MetaGRanules (GHRSST) - Metop, EUMETSAT SAF on Ocean and Sea Ice, DOI: 10.15770/EUM_SAF_OSI_NRT_2013. http://doi.org/10.15770/EUM_SAF_OSI_NRT_2013 (accessed 2022-09-02)

* Note: These data are input to the Global Ocean Sea Surface Temperature and Sea Ice data records of the Copernicus Marine and Environmental Monitoring Service (CMEMS).

References

Hobday, Alistair J., Lisa V. Alexander, Sarah E. Perkins, Dan A. Smale, Sandra C. Straub, Eric C.J. Oliver, Jessica A. Benthuysen, Michael T. Burrows, Markus G. Donat, Ming Feng, Neil J. Holbrook, Pippa J. Moore, Hillary A. Scannell, Alex Sen Gupta, Thomas Wernberg, 2016: A hierarchical approach to defining marine heatwaves, Progress in Oceanography, Volume 141, 2016, Pages 227-238, ISSN 0079-6611, https://doi.org/10.1016/j.pocean.2015.12.014.

Huang, B., Wang, Z., Yin, X., Arguez, A., Graham, G., Liu, C., et al., 2021: Prolonged marine heatwaves in the arctic: 1982-2020. Geophys. Res. Lett. 48:e2021GL095590. https://doi.org/10.1029/2021GL095590.

IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.

Laufkötter, C., J. Zscheischler, and T. Frölicher, 2020: High-impact marine heatwaves attributable to human-induced global warming. Science, 369(6511), 1621–1625, https://doi.org/10.1126/science.aba0690.

Manta, G., de Mello, S., Trinchin, R.,Badagian, J., & Barreiro, M., 2018. The 2017 record marine heatwave in the Southwestern Atlantic shelf. Geophysical Research Letters,45, 12,449–12,456. https://doi.org/10.1029/2018GL081070.

NOAA, 2021: What is coral bleaching? National Ocean Service website, https://oceanservice.noaa.gov/facts/coral_bleach.html, 12/01/21.

Oliver, Eric C.J., Jessica A. Benthuysen, Sofia Darmaraki, Markus G. Donat, Alistair J. Hobday, Neil J. Holbrook, Robert W. Schlegel, Alex Sen Gupta, 2021, Annual Review of Marine Science 2021 13:1, 313-342, https://doi.org/10.1146/annurev-marine-032720-095144.

Wooldridge, S. A, 2013.: Breakdown of the coral-algae symbiosis: towards formalising a linkage between warm-water bleaching thresholds and the growth rate of the intracellular zooxanthellae, Biogeosciences, 10, 1647–1658, https://doi.org/10.5194/bg-10-1647-2013, 2013.

Smale, D.A., Wernberg, T., Oliver, E.C., Thomsen, M., Harvey, B.P., Straub, S.C., Burrows, M.T., Alexander, L.V., Benthuysen, J.A., Donat, M.G. and Feng, M., 2019. Marine heatwaves threaten global biodiversity and the provision of ecosystem services. Nature Climate Change, 9(4), pp.306-312.

Roberts, S.D., Van Ruth, P.D., Wilkinson, C., Bastianello, S.S. and Bansemer, M.S., 2019. Marine heatwave, harmful algae blooms and an extensive fish kill event during 2013 in South Australia. Frontiers in Marine Science, 6, p.610.

This case study is a contribution to the United Nations Ocean Decade