I have always felt that a rise in global temperatures was the least of our worries about increasing CO2 in the atmosphere. I’m much more concerned about how it could perturb the delicate balance that allows mammals to live, i.e. us. One of the things that could be trouble is that CO2 dissolved in water can make the oceans more acidic by forming more carbonic acid, which could make it harder for marine creatures to make shells through calcification, which in turn could have a large impact on the coral reefs and the ocean food chain.
Another thing I worry about is that our oxygen supply could decrease. Although the direct effect of converting oxygen to water and CO2 through increased combustion of fossil fuels is small, the effect on photosynthetic organisms that make our oxygen is largely unknown. I’ve actually been somewhat optimistic on this account thinking that since we are introducing more nutrients into the oceans and CO2 is increasing then perhaps phytoplankton, which make much of our oxygen and is a blanket term for photosynthetic microscopic sea organisms like cyanobacteria and dynoflagellates, might increase. However, a paper in Nature this week, says otherwise.
The paper claims that global phytoplankton numbers are declining on average at 1% per year. They used two sources of data to make their estimates. The first is a direct measurement of chlorophyll in samples of ocean water and the second is a measure of ocean transparency using a Secchi disk, which is an 8 inch disk coloured black and white that is lowered into the water until it cannot be seen. It is believed that ocean transparency is highly correlated to chlorophyll levels. Most of the measurements involve the upper 20 metres of the ocean.
They attribute this decline to global warming. In order for phytoplankton to thrive it needs sunlight, CO2 and nutrients, i.e. fertilizer. The nutrients generally come from ocean upwelling of nutrient-rich cooler water. The theory is that the increase in ocean temperature is leading to more stratification of the ocean and less mixing between surface and deeper layers. The decline in phytoplankton could have major repercussions throughout the food chain since it provides roughly half the earth’s photosynthetic generated biomass and is the primary source of food for the ocean’s food chain. For example, baleen whales feed on krill which feed on phytoplankton.
However, despite this bad news there is always hope. One is that we are spewing tons of nutrients into the oceans and causing dead zones. Surely, some of these creatures will adapt to utilize this nutrient source. Another is that the fluctuations are very large and phytoplankton levels in the Indian Ocean are actually increasing so perhaps this could happen in other oceans as well. Finally, the measurements were mostly done for the very top layer of the ocean. However, photosynthesis can extend down to 200 metres or so below the surface of the ocean. If the top layer is getting more transparent, perhaps this means there can be more light getting below where there may be more nutrients and what is happening is that phytoplankton are thriving at greater depths.