Monday, June 3, 2024

The wickedest of all problems- V

Increasing CO2 in the atmosphere due to human activities not only affects the climate. When water and air come into contact there’s an exchange - gases from the atmosphere get absorbed by the ocean and gases dissolved in the ocean are released into the atmosphere. When carbon dioxide combines with water in the ocean it forms carbonic acid, which makes the ocean more acidic.  This is called ocean acidification which is sometimes referred to as global warming’s “equally evil twin.”

Ocean acidification is often expressed in terms of the pH of seawater.  pH is a measure of acidity or alkalinity. The pH scale runs from zero to fourteen. A pH below 7 is considered acidic, and a pH greater than 7 is considered alkaline, or basic. Average ocean water pH is currently 8.1.  For comparison, the pH of pure water is 7, and stomach acid is around 2. Prior to the Industrial Revolution, average ocean pH was about 8.2. The change might not seem like much but the pH scale is logarithmic, so a one point change on the scale means a tenfold change in concentration. 

The oceans have absorbed between a third and a half of the CO2 humans have released into the atmosphere since about 1850.  This has resulted in the acidity of the oceans increasing by 26% since about 1850, a rate of change roughly 10 times faster than any time in the last 55 million years. If greenhouse gas emissions continue as they are doing at present, the oceans will be 150 percent more acidic than they were at the start of the industrial revolution. While this helps to reduce the rate of atmospheric warming and climate change, it comes at a cost. 

The capacity of the ocean to absorb CO2 decreases as ocean acidification increases making it less effective in moderating climate change. It makes it difficult for marine calcifying organisms, such as coral and some plankton, to form shells and skeletons, and existing shells become vulnerable to dissolution. The extra hydrogen in low-pH seawater reacts with calcium carbonate, turning it into other compounds that animals can’t use to build their shells. Most surface waters will be continually corrosive within decades. The impacts of acidification will extend up the food chain to affect economic activities such as fisheries, aquaculture and tourism. 

Most species seem to be more vulnerable in their early life stages. Juvenile fish for example, may have trouble locating suitable habitat to live. Many marine fish and invertebrates spend their early lives as larvae. Larvae are very small, which makes them especially vulnerable to increased acidity. For example, sea urchin and oyster larvae will not develop properly when acidity is increased. In another example, fish larvae lose their ability to smell and avoid predators. The vulnerability of larvae means that while organisms may be able to reproduce, their offspring may not reach adulthood.  

Ocean acidification will change the makeup of microbial communities, it will alter the availability of key nutrients, like iron and nitrogen. For similar reasons, it will change the amount of light that passes through the water, and for somewhat different reasons, it will alter the way sound propagates. (In general, acidification is expected to make the seas noisier.) It seems likely to promote the growth of toxic algae. It will impact photosynthesis — many plant species will benefit from elevated CO2 levels — and it will alter the compounds formed by dissolved metals, in some cases in ways that could be poisonous.

While many species will apparently do fine, even thrive in an acidified ocean, lots of others will not. Emiliania huxleyi, for example, is a single-celled phytoplankton. It is common at certain times of year and it forms the base of many marine food chains. Limacina helicina is a species of pteropod, or “sea butterfly,” that resembles a winged snail. It lives in the Arctic and is an important food source for many much larger animals, including herring, salmon, and whales. Both of these species appear to be highly sensitive to acidification. 

As with many aspects of climate change, the speed of CO2 release is the problem. CO2 levels have changed in the geologic past, with several  episodes being severe, but none with such speed of CO2 release as currently taking place. A useful comparison can be made to alcohol. Just as it makes a big difference to your blood chemistry whether you take a month to go through a case of alcohol or an hour, it makes a big difference to marine chemistry whether carbon dioxide is added over the course of a million years or a hundred. 

If we were adding CO2 to the air more slowly, geophysical processes like the weathering of rock would come into play to counteract acidification. But things are moving too fast for such slow-acting forces to keep up. As Rachel Carson once observed,  “Time is the essential ingredient, but in the modern world there is no time.” By burning through coal and oil deposits, humans are putting carbon back into the air that has been buried underground for  hundreds of millions of years. 

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