Behind the data: detecting marine heatwaves

Satellite instruments make it possible to spot ocean temperatures that deviate from the average and confirm long-term trends

EUMETSAT Climate Product Expert Dr Rob Roebeling explains how observations from satellite instruments reveal regional changes in sea surface temperature.

Last Updated

06 November 2023

Published on

06 November 2023

The United Nations has designated the 2020s as the Decade of Ocean Science for Sustainable Development and issued 10 challenges with the goal of achieving clean, safe, and accessible oceans by 2030. To support this goal, EUMETSAT is developing case studies that address each of the United Nations’ challenges. This article is the second in a series in which one case study author gives us the story behind the data, illuminating how satellite data from EUMETSAT and the European Union’s Copernicus programme contribute to a better understanding of the oceans.

By circulating water between the poles and tropics, oceans play a critical role in regulating global climate and marine ecosystems. This means that marine heatwaves – periods of abnormally hot ocean temperatures as compared to the average temperature – may have far-reaching effects on oceans that are already rising, becoming more acidic, and losing coral reefs and sea ice.

And this is no fleeting problem. In its most recent assessment report, the United Nations Intergovernmental Panel on Climate Change (IPCC) noted that marine heatwaves have become more frequent in the 20^th century and are expected to increase in the 21^st.

Dr Rob Roebeling, a EUMETSAT climate product expert, explains how satellite images reveal marine heatwaves in places that tend to be overlooked and how observing hotspots in oceans across the globe confirms an unsettling trend.

Hot in the Mediterranean

“What you see here are all the places where the Mediterranean Sea was more than 4°C warmer than average in August 2022,” Roebeling said. img “It’s not from one day to the next that this occurred; it’s over the whole season that this part of the ocean got warmer and this is how warm it got by 20 August. The risk of sea life not being able to cope with such high temperatures is the highest in the red area of the map, especially when this situation persists over a longer period.

“By looking at places where these red spots occur, we can detect, on a daily and monthly basis, areas where this anomalously high sea surface temperature has occurred. We often focus more attention on temperatures along the coasts, close to where people live, or in bodies of water where sea life is very active but marine heatwaves happen all over the Earth and satellite images play a crucial role in helping us to detect them.

A global trend

“This animation confirms a known trend, which is that across the globe, marine heatwaves are occurring more frequently.

The global sea surface temperature as it deviated from the average temperature during the period 1981-2020 can be seen here
Credit: Rob Roebeling, using data from the Copernicus Marine Service

“At the beginning of the animation, you see the oceans being mostly blue – this is in the 1980s. And if you go to the end, you see the oceans becoming more and more red. So we see these very dark red areas that represent marine heatwaves occurring more often.

“Across practically the whole globe we see that the ocean temperatures in 2020 were, on average, 1°C to 1.5°C warmer than they were in 1980. This may not sound like a big change in temperature but this warming represents an enormous amount of energy being absorbed into the ocean.

“This points to a global pattern of marine areas that were cooler than average in 1980 becoming warmer than average in 2020.

“I was worried when I made this animation because you read things in the newspaper but then you start looking at the data yourself and you see, wow this is a big red flag. For example, you see in 2020 that the Black Sea is super red.

“The problem isn’t just that there was a heatwave – it’s that after that, the water doesn’t cool down as easily anymore. So afterwards, this sea will stay too warm for a long time and we will have to wait and see if it will cool down anytime soon.

Essential for the future

“This is one reason why satellite data are so important. Satellite instruments, such as the Sea and Land Surface Temperature Radiometer (SLSTR) on Sentinel-3, are able to detect regional changes in sea surface temperatures. Besides this, they are also able to spot areas where the ocean is getting way too warm, such as areas experiencing a marine heatwave.

“And as more next-generation satellites are launched with even more cutting-edge instruments, it will be possible to narrow down our uncertainties and observe trends and changes more precisely, which will help us to further sharpen our understanding of the state of the climate.”

Are you an expert wanting to know more? Check out the case study where you can delve deeper into observing sea surface temperature from space and view the supporting Jupyter notebook.