New geological data indicate that marine life is somewhat resilient to warming in the tropics. Chris Fokkema, earth scientist at Utrecht University, discovered that tropical algae were largely unaffected by a number of periods of global warming of up to 1.5 degrees Celsius in the distant past. These unicellar organisms form the basis of food webs and are generally very sensitive to rising temperatures. Previous studies of periods of even greater warming showed a dramatic decline in these organisms. "Somewhere beyond those 1.5 degrees, a tipping point occurs."
Fokkema studied sediments that were recovered from the ocean floor 200 kilometres south of Ghana in the 1990s. These sediments are around 54 to 52 million years old, dating back to a period when the continents were more or less in their current positions. "So I knew I was looking at tropical material," the researcher explains. "Just like today, the tropics warmed less rapidly than the poles during various periods of global warming. But in the tropics, organisms are very sensitive to warming because they often already live close to their optimal temperature. So even a small amount of warming could have a huge effect." Just as on land, this so-called "heat stress" caused by climate change poses a major risk to life.
Jacuzzi
During an earlier period of five degrees of global warming, the PETM or Paleocene-Eocene Thermal Maximum, 56 million years ago, it became too hot in the tropics for many unicellar organisms. "The sea water temperature became almost as high as in a jacuzzi. Species diversity declined significantly at that time, and in some locations algae disappeared completely." However, Fokkema's research shows that a specific group of algae, the dinoflagellates, were able to cope well with a warming of 1.5 degrees during phases of rapid warming between 54 and 52 million years ago. He concludes that the amount of warming determined how strong the influence of climate change was on the algae.
Hopeful result
These initial signs of resilience to warming provide a valuable addition to our knowledge about the consequences of climate change. They may give an indication of what is yet to come. "Our findings support the political goal of limiting global warming to no more than 1.5 degrees and offer hope that the consequences of such warming will indeed remain somewhat limited."
Case studies
Chris Fokkema is particularly interested in the early Eocene (56-48 million years ago), a period when the Earth was about 15 °C warmer than today and CO2 concentrations were three to five times higher than they are now. On top of that, there were regular periods of additional warming similar to today's climate change. For earth scientists, these are small but interesting case studies that provide highly relevant insights for the future.
Repository
The sediment cores were taken by the Ocean Drilling Program, the predecessor of the current International Ocean Drilling Program (IODP) ( https://iodp3.org ), which is co-funded by the Dutch Research Council (NWO). They are stored in the IODP Core Repository ( https://www.marum.de/Bremer-Kernlager.html ) in Bremen, Germany. The research conducted by PhD student Chris Fokkema was funded by a grant from the European Research Council awarded to his supervisor, Appy Sluijs.
