How is the Mediterranean changing? 40 years of data reveal climate change impact

Spanish scientists and researchers from Europe and the US compiled data from 46 oceanographic campaigns carried out between 1976 and 2018 to study changes in the Mediterranean and the impact of climate change. Temperature, oxygen, pH, carbon and even the age of water masses. More than 40 years of measurements now make it possible to look back and study how the Mediterranean is being transformed and to better understand what changes the climate crisis is bringing about in its waters. Researchers at the La Coruña and Balearic oceanographic centres of the Spanish Institute of Oceanography (IEO-CSIC), together with scientists from European and US institutions, have compiled data from 46 oceanographic campaigns carried out between 1976 and 2018 across virtually the whole of the Mediterranean. The result is Carimed (source in Spanish), a new database that makes it possible to compare measurements taken over four decades and track the evolution of indicators such as temperature, oxygen or pH. Until now, part of that information was scattered across different archives and had been collected during campaigns conducted at different times and using different methods. Comparing today's Mediterranean with that of four decades ago can help to distinguish its natural variability from changes that persist over time and to better understand how it is responding to climate change. According to the IEO-CSIC, (source in Spanish) the data will make it possible to track these long-term trends and gain a better understanding of how a sea that is particularly sensitive to warming and to other environmental disturbances is evolving. Recovering the past to understand the present "A considerable part of the data was retrieved from fragmented historical records or archives held locally, thereby bringing together measurements that were previously not accessible," explains Marta Álvarez, a chemical oceanographer at the La Coruña Oceanographic Centre and coordinator of the research group, in comments released (source in Spanish) by the IEO-CSIC. "Another part comes," Álvarez adds, "from international campaigns on board national and international research vessels that involved staff from the IEO's INOCEN group." Recovering the data, however, was only the first step. There was another challenge: how do you compare a measurement taken in the 1970s with another taken decades later? Methods, instruments and survey conditions have changed over time and could introduce differences that do not necessarily mean that the Mediterranean itself has changed. This is why the researchers reviewed and adjusted the measurements to identify possible differences arising from the way they were collected. That allows them to pinpoint, with greater confidence, the changes that really persist over the years. What can 40 years of data reveal? The Mediterranean is facing transformations that go beyond rising temperatures. The study itself highlights warming, acidification and the loss of oxygen among the main changes affecting the oceans, changes that can alter how they function and their ecosystems. This is precisely why it is important to have observations that stretch back several decades. Looking at that long-term evolution can help to detect changes that would be hard to spot from isolated measurements. To ensure that the measurements are comparable, the scientists subjected them to thorough quality control tailored to the specific characteristics of the Mediterranean. Put simply, the aim is to avoid a change in the way something is measured being mistaken for a real change in the sea. "Thanks to the rigorous 2QC process systematic biases in biogeochemical variables were minimised and a more consistent dataset was obtained," explains Maribel García-Ibáñez, a researcher at the Balearic Oceanographic Centre. What can all this information be used for now? Among other things, it makes it possible to see how processes linked to the acidification of the Mediterranean, the carbon stored in its waters or the renewal of deep waters have evolved over decades. The data can also be used to improve the models scientists use to try to understand how the Mediterranean will evolve under a changing climate. Carimed is also open to the scientific community and will be able to incorporate new observations in future, expanding a snapshot of the Mediterranean that makes it possible to connect its past with the changes it is undergoing today.