Ocean and Climate

Every few years, around Christmas, the cool, fish-rich water off the coast of Peru turns warm, the anchovy catch fails and the rains come to the desert. Peruvian fishermen named the event El Nino, "the Child", for the Christ child (check). In the 1960s the Norwegian-American meteorologist Jacob Bjerknes showed that it was not a local oddity but one half of a vast pattern linking the whole tropical Pacific Ocean to the atmosphere above it (check). The ocean and the air are one system, and the ocean is the slower, heavier partner. This chapter looks at what that partnership means: the ocean as a store of heat and carbon, the best-known natural swing in the system, and the changes now being measured.

The Ocean as Heat Store

Because water holds so much heat for its mass (Chapter 3) and the ocean is so large, it has taken up most of the extra heat trapped by greenhouse gases. The Intergovernmental Panel on Climate Change reported that the ocean absorbed about 91 per cent of the excess heat gained by the climate system between 1971 and 2018, with the atmosphere taking only a few per cent (check). Measurements come from a global fleet of about 4,000 profiling floats, called Argo, that drift at depth, rise to the surface every ten days and radio back the temperature and salinity (check).

El Nino and La Nina

In normal conditions the trade winds blow from east to west across the tropical Pacific. They pile up warm surface water in the west, near Indonesia, and allow cold water to upwell off South America (Chapter 6). The warm west has heavy rain; the cool east is dry.

The oscillation is irregular, returning every two to seven years, and peaks late in the year (check). It is known as the El Nino-Southern Oscillation, or ENSO, because the sea-surface changes go hand in hand with a seesaw in air pressure between the eastern and western Pacific, which the British scientist Gilbert Walker recognised in the 1920s (check). The effects reach far beyond the Pacific, changing rainfall in places as distant as southern Africa and North America. They reach the UK only weakly and unreliably, so be wary of claims that a particular winter in Scotland was "because of El Nino" (check).

Carbon and Acidification

The ocean also takes up carbon dioxide from the air. It has absorbed about a quarter to a third of the CO2 that humans have emitted (check). That slows the rise of the gas in the atmosphere, but it changes the chemistry of the sea.

1. CO2 dissolves in seawater and reacts with the water to form carbonic acid.
2. The acid releases hydrogen ions, which lower the pH.
3. The extra hydrogen ions react with carbonate ions, so fewer are left for animals that build shells and skeletons of calcium carbonate: corals, oysters, mussels and some plankton.

Sea-Level Rise

Global mean sea level has risen by about 20 centimetres since 1900 (check), and the rate has increased, to about 3.7 millimetres a year in 2006 to 2018 and higher in the most recent years (check). There are two main causes:

• Thermal expansion. Warmer water takes up more room.
• Melting land ice. Glaciers and the Greenland and Antarctic ice sheets add water to the ocean. Melting sea ice that is already floating does not raise the level.

Local sea level also depends on the land itself. In Scotland the land is still rising after the weight of the ice sheets was removed at the end of the last ice age, a process called glacial isostatic adjustment, so the relative rise there has been smaller than in southern England, where the land is sinking slightly (check). Sea level also varies with the tides, currents and storm surges (Chapters 5 and 6), so a hundred years of record are needed to see the trend.

Losing Oxygen

Warm water holds less dissolved oxygen than cold water, and warming strengthens the layering that stops oxygen-rich surface water from mixing downward (Chapter 3). Studies suggest that the ocean has lost about 2 per cent of its oxygen since the 1960s (check), and that low-oxygen zones off the coasts of west Africa and the Americas are expanding. Fish and other animals avoid or die in water that is too low in oxygen, so the losses add pressure on top of warming and acidification.

Exercises