'There may be a limit to the capacity of the oceans to take up heat' (VIDEO)

Tell us about the life of an observational oceanographer, what you’re studying in the seas, and why.

My last major oceanographic expedition was oa German research vessel that's about 100 metres in length and can accommodate about 80 people. We left from Cape Town, up the coast of South Africa to the equator, and then ran the equator all the way to the coast of South America, and then went back to Cape Verde, which is off the coast of northern West Africa. The whole voyage took about 55 days. A typical day would involve putting instruments into the water, getting water samples, processing them, going to the next location. We were sampling every degree of latitude, which is every 60 nautical miles. 

The reason for this expedition was to understand how processes in the equatorial Atlantic contribute to the productivity of a single-cell plant, which is basically what I study, called phytoplankton. These single-cell plants can do photosynthesis, which means they take up CO2 from the atmosphere. They form the basis of the food web—essentially they are food for fish. About 30% of the tuna consumed in Europe comes from the equatorial Atlantic. So trying to understand the processes by which this very valuable protein from the sea contributes to human food security is important.

Phytoplankton have been, from the origin of earth, responsible for life on earth. By doing photosynthesis, they basically take up carbon dioxide and put out oxygen. The great oxidation event that made this earth habitable was because you had photosynthetic cyanobacteria, the oldest organisms that did photosynthesis.

More specifically for us, by doing photosynthesis, that is taking up CO2, they also control climate. In other words, they control the concentration of greenhouse gases in the atmosphere and therefore how hot the earth gets. Paleo-oceanographers have studied this and found that every time you have an increase in productivity of phytoplankton, the earth cools down.

Besides rising seas, what are the lesser known impacts of climate change on the oceans that we should worry about? 

Observational oceanographers are interested in understanding the ocean from several different angles. Physical oceanographers are trying to understand, for example, how much heat oceans can take up. We are concerned that there may be a  limit to the capacity for the ocean to take up heat.

There is also a chemical angle. Ocean acidification means that organisms that form shells, such as corals or shellfish, will have a harder time making those shells when the oceans become more acidic. And therefore there's a whole influence on the biology of the oceans.

The third one is just the impact of the heat stress itself on life in the ocean. So we are now starting to recognize that marine heat waves have become a really significant problem. 

What are the big advances in our ability to observe the ocean and expand our understanding of how marine ecosystems work?

There are robotic profilers, we call them floats, that are instruments that go from the surface of the ocean all the way down to 2,000 metres below the ocean surface and come back up to the surface, making measurements every 10 days. In the beginning, the measurements they were making were of temperature and salinity, that is the salt content of the water.

These robotic profilers have been now deployed everywhere in the ocean. There are about 6,000 of them that are going up and down, drifting with the ocean currents, but providing us this detailed information on the ocean temperature and salt content.

More recently, in the last about 15 years or so, we've also added other sensors to these robotic platforms. I have a float with additional sensors that give us concentrations of phytoplankton, that single-cell plant I was talking about. We have deployed these floats in the Amazon river outflow to try and understand how the freshwater coming out of the Amazon river is changing the phytoplankton composition in the western tropical Atlantic.

More recently, there have been serious investments by many countries to put out hundreds of these floats in the ocean. We now also have other autonomous platforms called gliders that are providing us views of the ocean. In the future, in some fashion, oceanography is going to be the use of these autonomous platforms that are potentially guided by artificial intelligence that will allow us to observe the ocean in ways that we couldn't do from just five different ships going somewhere in the ocean and making a measurement every few hours. So I believe we are at the cusp of a real revolution in our ability to observe the ocean.

What are the big advances in oceanographic research that give you hope for a resilient planet?

One strategy to reduce carbon dioxide and greenhouse gases in the atmosphere is called carbon dioxide removal, or in the case of oceans, marine carbon dioxide removal (MCDR).

I personally am trying to figure out if a nature-based solution is one way in which we can do this. For example, in the tropical Atlantic, we've seen a new phenomenon where there's a whole new population of a seaweed called sargassum, which has moved into the tropical region. This causes a major problem to the small islands of the Caribbean, where you can have tons of this material wash up on the beaches. Many of these islands are major tourist destinations, and therefore when the beaches basically are covered with this organism, they stink when they rot. I mean, that's a major problem. They release methane when they rot, which is an even more potent greenhouse gas than CO2. The removal of these organisms also ends up destroying the marine environment, the coastal zone of these islands.

One of the methods that we've been trying to figure out is whether we can sink the sargassum offshore, before they wash up on the islands, in waters that are greater than 2,000 metres deep, and thereby basically solving a series of problems. One is you prevent them from washing up on the beach. Two, by sinking this sargassum down to depths greater than 2,000 metres, you're basically taking it out of circulation for about 200 to 300 years. So this is one method of MCDR that could potentially help in moving forward.