6 December 2013
As the Tara Oceans Arctic Circle expedition comes to an end, with thousands of plankton samples collected and stored in Tara’s freezers, one question is constantly asked by journalists and the general public who come to visit the boat. What about climate change? Although we’re not studying this issue directly, we have indeed been focusing on the organisms at the heart of the climate machine. To understand, we must first dissect the links between oceans and carbon.
We know that the global warming occurring on Earth for the last century is largely due to the release of carbon into the atmosphere. But we need to know what carbon we are talking about. Basically, carbon is an atom (denoted as C) which may be present in different molecules, each having very different properties. In the form of CO2 (a carbon atom bonded to two oxygen atoms), for example, this is a powerful greenhouse gas that traps infrared radiation in the atmosphere, pushing up the thermometer. This is the same carbon dioxide that comes out of our lungs every time we exhale, like all animals on our planet. When we breathe, our body transforms oxygen (in reality, di-oxygen (or O2) into CO2. At the same time many organisms on Earth do exactly the reverse: with water and light, photosynthesis can provide oxygen while consuming CO2. This is the case for plants on land, but also of phytoplankton in the ocean, not to mention the many photosynthetic bacteria. But in this chemical exchange, the carbon atom does not disappear, but is incorporated into many glucose molecules, which provide energy to the body. Plankton is at the base of food chains, so carbon produced through photosynthesis gradually finds its way into all surrounding organisms. It is important to understand that the Earth is in some ways a closed circuit. In the words of Lavoisier: “Nothing is lost, nothing is created, everything is transformed.” The amount of carbon present on our planet is thus irrelevant. The question is to know what form it is in, and where. A delicate balance has been upset by human activities: carbon stored for millions of years in the form of fossil fuels such as oil is extracted in a few decades from the deep layers of the Earth, and eventually released into the atmosphere in the form of carbon dioxide. The same applies to the problems of deforestation, where the carbon in trees is released into the air once they are cut and burned. Thus, we are in the process of emptying these famous “carbon sinks”.
Oceans at the heart of climate
While the Amazon rainforest is often referred to as the “green lung”, scientists now realize that the oceans play an equally important role as carbon sinks and source of oxygen. This is what is called the “carbon pump”. First, from a purely mechanical point of view, carbon dioxide is naturally dissolved in the oceans. As we have seen, phytoplankton transforms CO2 into O2 via photosynthesis. Finally, many planktonic organisms are also able to transform CO2, not into glucose molecules, but into carbonates (commonly known as chalk). Some small unicellular protists inhabiting the oceans produce a calcareous shell that sinks to the bottom of the seas after death. The same applies to all marine organisms (miniature carbon sinks), skeletons, and waste being deposited on the ocean floor that eventually form a sediment, thus keeping carbon out of the atmosphere. Corals, too, are also carbonaceous secretions and are part of the carbon reservoir. Thus, the oceans and their inhabitants, while taking up a majority of the heat due to global warming and providing oxygen to our atmosphere, have already absorbed a third of CO2 emissions related to human activities, in the form of dissolved carbon or minerals.
A fragile equilibrium
This massive carbon sink could turn against us if the system’s equilibrium is destroyed. Many scientists fear this may happen. Global warming is beginning to show the limits of the oceanic carbon pump: higher temperatures decrease the dissolution of CO2 in water, and the oceans’ storage capacity (which is not infinite and could reach saturation) could be dramatically reduced. Worse, the sink could turn into a carbon source, becoming a real time bomb. Another consequence of the rise in temperature is shown by the migration of some planktonic species to colder areas, disrupting a fine balance which has existed for millions of years. Finally, the lastest disturbing discovery: ocean acidification. Due to the increased CO2 concentration, the oceans are becoming more acidic, with still unclear impacts on plankton and corals, but surely impairing the healthy development of a large number of species. To investigate such impacts and attempt to find solutions, we must first of all understand the mechanisms of the carbon pump: which organisms are involved, how they participate, what are the consequences of temperature increases, acidity, concentration of CO2, etc. It is quite possible that some of the answers are now in Tara’s freezers.