One of the main goals of the cruise is to monitor the uptake of CO2 from the atmosphere by the Southern Ocean. I got a question through the blog that is partially related, even if there are no hurricanes or typhoons in these areas . The explanation that follows will help us understand more about the measurements that we are making on the boat. Here is the question:
'We also read that big storms such as hurricanes and typhoons bury carbon and CO2 deep in the oceans as sediments. How does that work and is this good for the atmosphere?
The short answer is that tropical cyclones, the generic name for hurricanes and typhoons, do not have a net effect on the CO2 fluxes between the atmosphere and the oceans. I will explain below how this works. About hurricanes and typhoons, they are the same phenomena with different names according to who named them. Tropical cyclones in the western Pacific were called Typhoons by the Japanese, while those occurring in the western Atlantic and Eastern Pacific were called hurricanes by the Aztecs.
You are probably more familiar with the flux of CO2 through the surface of water than you think. You have seen it all If you have opened a soda. Think of a 2 liter soda bottle that has clear plastic so you can see what is happening inside. There are no bubbles in the soda before you open it, but as soon as you remove the cap, bubbling begins. What happened?
Carbonated drinks get their fizziness by putting high pressure CO2 on them, which makes the gas dissolve int he soda.
What determines whether gases flow in and out of the water (we will call this flux) is how much gas is in the air relative to the amount of that gas in the water. Gases go from were there is a higher amount to where there is a lower amount. This happens in your lungs, for example, where there is more oxygen on the air you breath in than in your blood, so the oxygen moves into the blood. The term 'amount' does not really work when talking with gases. We need to talk about 'partial pressure' of the gas.
To understand partial pressure we need to understand pressure. Pressure the weight of the matter that is above an area (Pressure = force / area). The atmospheric pressure on your head is the weight of all the air above you applied on the area of the top of your head. The pressure at 4000 m depth in the ocean is given by the wight of those 4000 m of water plus the weight of the air above it.
Partial pressure is used when there is a mix of gases occupying a volume. Partial pressure is defined as the pressure that only one of the gases in the mixture would have if it were left alone to occupy the same volume. The pressure of the mix of gases is given by the sum of the partial pressures of each of the gases on the mix. Let us see how this applies to the atmosphere.
The air that we breath is made out of many different gases. Most of it is Nitrogen (close to 79% by volume of the atmosphere), followed by Oxygen (20% by volume). The remaining 1% is a mix of all other gases. Now that we know the atmospheric pressure is given by the weight of the air above us, and we know the percentage of volume that Nitrogen and Oxygen take, we can say that the partial pressure of Nitrogen is 0.79 atm (atmospheres), the partial pressure of Oxygen is 0.2 atm, and the partial pressure of the other gases is 0.01 atm. They all add up to 1 atm.
The pressure above the person is given by the weight f the atmosphere. Each gas contributes with their own weight (partial pressure) to the total pressure.
Let us go back to the example of the soda bottles. We do not see any bubbles when the bottle has not been opened because the partial pressure of the gas in the bottle (CO2) is the same as the partial pressure of CO2 in the tiny amount of air inside the bottle. We say that the CO2 is in equilibrium. The moment you open the bottle, the pressure of the air above the water drops down, so the partial pressure of the CO2 on the air also drops down. The partial pressure of the CO2 in the soda is now greater than the partial pressure of the CO2 in the air, so the CO2 fluxes from the soda to the air rapidly and you see bubbles.
There is no bubbling on the closed soda because the partial pressure of CO@ (pCO2) is the same in the soda and in the air above it (left). The soda begins to outgas the CO2 when it is opened because the pCO2 on the air goes down as the pressure goes down. Now the pCO2 in teh soda is bigger than in the air.
The amount of CO2 that can be on the water also depends on the temperature of the water. More gases can dissolve in colder water than in warm water. Your soda will keep the fizzle longer when the soda is colder than when it is warmer. This means that the partial pressure of a gas can change in a liquid even if the number of molecules of that gas does not change (that is why it is better to talk in terms of partial pressure than amount). You might have opened a warm soda bottle and found yourself immersed in a soda geyser!
This is also important for the oceans. Imagine the ocean and the atmosphere are in equilibrium for CO2 (partial pressure for CO2 is the same in the water and air). When the oceans are warmed up, the partial pressure of CO2 increases in the water, even when the amount of CO2 does not change. The partial pressure of CO2 remains the same in the air, so CO2 will flow out of the water. The reverse happens when the water cools down and takes more CO2 from the atmosphere.
Cold waters, like the ones found around McMurdo Station on this picture, can dissolve more gases than warmer waters.
Another factor that modulates the CO2 fluxes is the windspeed. The gas fluxes increase as the wind speed increases. This is particularly important when hurricanes are present, since the defining characteristic for hurricanes is their strong winds. The winds need to be at least 74 miles per hour for the storm to be a hurricane.
It gets tiring to continually write the words 'partial pressure of CO2', and I imagine reading it as well, so scientists decided to use the symbol pCO2 instead. From now on, when you see pCO2 think of partial pressure of CO2. We are ready to answer in detail the question: what is the effect of hurricanes on CO2 fluxes between the air and the sea?
I found the answer, literally, at home. It turns out my wife, Dr. Galen McKInley from University of Wisconsin - Madison, published a paper with one of her students on this topic. Here is her response:
'Hurricanes have only a short-term effect on the ocean carbon cycle. Hurricanes tend to occur in fall when sea surface temperatures are high, making pCO2 higher than atmospheric, so the strong winds form the hurricane actually drive carbon out of the ocean. The winds also stir up cold water, which lowers pCO2, but the effect is smaller than the wind speed. In our paper, we found that when integrated over a year and the whole Atlantic Ocean, hurricanes do not modify the total air-sea flux in a year in a significant way. Essentially, they cause outgassing to increase temporarily, but then there is a decline in the long-term seasonal outgassing that compensates. '
'Impacts on biology are interesting. There tends to be enhanced chlorophyll in the wake of a storm, which is partially due to mixing up of chlorophyll and partially due to increased surface productivity with nutrient supply that also happens with the mixing. The impacts on carbon have not been shown to be significant.'
Galen says 'increased surface productivity' which means that phytoplankton grows more after the hurricane has passed. The strong winds of a hurricane mix up the upper layers of the ocean, bringing the nutrients that were in the lower layers, where phytoplankton can not grow because there is no sunlight, back to the surface water. I explained in the March 21 entry that bacteria below surface waters remineralize organic matter that falls from the upper layers in the form of waste and remnants, turning it into nutrients. The upper waters get fertilized when these nutrients are brought back to the surface, creating a phytoplankton bloom.
The oceans will bury the organic matter as sediment when the waste and remnants of organisms in the surface layers sink below the area where bacteria are found. So if the surface productivity increases it is likely that more of the atmospheric CO2 will end up in the sediments, but Galen found that there is no net effect on CO2 fluxes.
Galen also addresses ocean acidification:
'Ocean Acidification (not related to hurricanes, of course) is the major side-effect of ocean carbon uptake. Since preindustrial, the ocean has taken up ~50% of what people have put in the atmosphere, and each year takes up about 25% of our emissions. This is good to limit global warming, but it causes the pH of the ocean to decline significant. The impacts on ocean life are poorly understood, but are unlikely to be anything but bad. Impacts may be catastrophic for some. This is a major area of research.'
Scientists are looking into a wide array of ways to remove CO2 from the atmosphere. What you and I can do is reduce the amount of CO2 that we put in the atmosphere with our everyday choices. I encourage you, again, to visit Galen's a website on the Carbon Cycle to learn more about this topic:
http://carboncycle.aos.wisc.edu/