Week 5 was about the impacts of climate change and global warming on natural systems – the cryosphere (ice caps to you and me) and the oceans.
Atmospheric warming increases surface melt, forming lakes on glaciers. These have lower albedo than the ice and absorb more heat. The warmer water melts down through the glacier and increases lubrication between the glacier and the ground, so that the glacier flows more quickly. This pushes more ice onto the sea in a given time period. If we assume that the rate of calving is dependent on the amount of ice on the sea, calving will occur more frequently overall.
Sea warming increases the rate at which the sea undercuts the glacier near the water line, so that the glacier no longer rests on the sea bed and, being unsupported, breaks off. The reduced contact with the sea bed also reduces the buttressing effect of the ice mass in the sea. The buttressing effect of ice floating on water is the same as the equivalent mass of water; it is the inertia of the mass that causes the buttressing (a slowing effect, unlike the buttresses of a cathedral wall, which stop movement). However, ice that is still grounded on the sea bed will have a stronger buttressing effect due to friction with the ground. If the grounding is reduced, so is the buttressing, allowing the glacier to flow more quickly.
The main impact upon the oceans is acidification caused by the increasing amounts of CO2 in the atmosphere. CO2 readily dissolves in sea water, more so at lower temperatures, i.e. nearer the poles. About a third of atmospheric CO2 dissolves into the oceans. As it does so, it reacts with water to form carbonic acid, which is unstable, dissociating into bicarbonate and hydrogen ions. The acidity of the ocean is determined by the concentration of hydrogen ions in the water. At present, the pH of sea water is about 8.1, slightly alkaline.
This process has always been going on, but recently the increase in atmospheric CO2 levels has meant an increase in the concentration of hydrogen ions and thus an increase in acidity. This is not the whole story. There is a buffering mechanism in the level of carbonate ions. Carbonate comes from weathering of rocks such as limestone and once in the sea water, it mops up spare hydrogen ions. This kept the pH of the oceans fairly constant over millions of years. The increase in hydrogen ions means more carbonate ions are used to mop it up, but the slowness of the weathering process means that carbonate ions cannot keep pace with the higher level of hydrogen ions. So the buffering is now falling short and the hydrogen ion concentration (ie acidity) is increasing.
Not only that, but the carbonate ions are reducing in concentration, meaning that less calcium carbonate is being produced. This affects the sea creatures that build their external skeletons from calcium carbonate, because that is vulnerable to dissolution into sea water if the concentration of calcium carbonate is not saturated. So the sea creatures skeletons can weaken, reducing their likelihood of survival. The increasing acidification of the sea water also affects the reproduction of sea creatures.
Will marine organisms be able to adapt to ocean acidification given the time scale for the predicted changes? There are uncertainties, but generally a timescale of decades will not be enough to allow marine organisms dependent on on calcium carbonate to evolve to adapt to increased acidity in the oceans. Some may do so, but even then there is likely to be a population crash while they adapt. Seas full of jellyfish, that’s what we’re looking at.
Increased carbon dioxide in the atmosphere is likely to lead to sea level rise. Are rising sea levels more of a threat to humanity than ocean acidification? There was a comment in the discussion thread, which I now cannot find again, that this was a very anthropocentric question. Indeed it is. If global warming hits hard and destroys civilisation, humanity and lots of other species, the earth will go on and some new life will emerge. But that’s not very interesting to me. The whole point of addressing CO2 emissions is to preserve human civilisation and improve it.
Which of the two, rising sea levels or ocean acidification, is more of a threat seems to me almost irrelevant. Both are serious. Probably neither is as serious as a shift to extremes in climate with more drought in some places, more rain in others, causing crop failures, wholesale population movements, war, famine, disease and death. Cheery outlook, isn’t it?
20th February 2014