Compound changes in oceans alarm scientists

Climate change is nowhere more apparent in its disruptions than in the world’s oceans. The vast bodies of water that make our planet unique are currently undergoing fast and extensive transformations that are unlike anything scientists have seen before, according to a study published Tuesday in the journal Nature Climate Change.

Researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences in Beijing, Mercator Ocean International in Toulouse, France, and the Laboratoire de Météorologie Dynamique at the École Normale Supérieure in Paris created a framework and tool to standardize and assess ocean variables and figure out when those variables are changed due to the warming climate.

The scientists examined how “compound changes” — the simultaneous effects of the Earth’s oceans losing oxygen, acidifying and becoming saltier or fresher — are pushing ecosystems past the point where adaptation is possible. And different layers of the pelagic world — a place where scientists have estimated over two million species reside, with approximately only 250,000 known to humanity — are affected to various degrees.

“For example, some specific species, e.g. some kind of fish, may be okay if their living environment is only saltier, but may become more vulnerable if they are exposed to the ‘compound change,’” Zhetao Tan, lead author of the study and a researcher at the Laboratoire de Météorologie Dynamique at the École Normale Supérieure, said in an email.

“Our study also shows that these compound changes are unprecedented since the 1850s, since the Industrial Revolution — mostly due to the human-induced climate change,” he said. “In the future, based on the climate model analysis, at least in the next 100 years, we are very likely not to be able to go back to the ocean of 150 years ago, as most of the changes are irreversible.”

Tan and his colleagues noted the most drastic changes are occurring in the Atlantic and North Pacific Oceans, and the Arabian and Mediterranean Seas. And both tropical and subtropical regions are feeling the heat, especially regarding two variables: dissolved oxygen and pH levels.

“The ocean is changing so, so [much faster] than we initially thought before; many general people, or even policymakers, regard that when we talk about climate change, it is just about temperature (warming),” Tan said. “But, in fact, not only temperature, but also salinity, and dissolved oxygen, are also changing so fast, from surface to the deep ocean. The ocean is experiencing strongly compounded change multidimensionally.”

Tan said around 25% of the ocean’s upper layers have experienced alarming simultaneous changes in temperature, salinity, and oxygen, a concerning trend that needs to be continually monitored.

“These changes may weaken the ocean’s capacity to absorb carbon and heat, undermining its role as Earth’s climate regulator,” he said. “More carbon dioxide will be stored in the atmosphere or other spheres, and we may feel intense warming, and we may experience more and extreme events such as heatwaves, severe cyclones and extreme rainfall in the upcoming future.”

The researchers’ framework allowed them to identify climate change signals, based on when and where they may have long-term effects. They were also able to combine the variables as an index to support a broader understanding to help mitigate climate risk.

Tan said it’s important for everyone — not just scientists or policymakers — to educate themselves about the ocean and its need for conservation.

“People should pay more attention to marine ecosystem protection,” he said. “Under the compound changes, most ocean species cannot have enough time and ability to ‘adapt’ to ocean compound climate change.”

“They aren’t like human beings — we still have the capacity to reduce the effects of climate disasters, to adapt to climate change," Tan added.