Climate change: “A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use”. IPCC Glossary

The Intergovermental Panel on Climate Change is a UN organization that carries out scientific assessments on climate change.

This organization, born in 1988, carries out reports every 5-7 years aimed at collecting data on climate change to try to mitigate it, through various political choices. In this way it is possible to make the political world cooperate with science, albeit with the many misunderstandings that still exist.

From the latest Assessment Report (AR5, IPCC 2013) it is concluded that:

• ocean warming dominates the increased energy stored in the climate system with more than 90% of the energy accumulated since 1971;

• the ocean has absorbed more than 30% of carbon emissions resulting from anthropogenic activities causing the acidification of the oceans,

• it is very likely that human action has been the dominant cause of warming since the first half of the twentieth century.

It has also been shown that since the industrial revolution there has been an increase in temperature of 1.5° C due to the emission of greenhouse gases.

As can be understood, the ocean has always played a crucial role in regulating the mechanisms of the entire biosphere, not only influencing the existence of those who inhabit it.

But what is happening to our oceans today? What are the chemical-physical characteristics that allow us to determine the climate change? We have to know our problem to resolve it so we have to know the climate change if we want to mitigate it. 

TEMPERATURE

The increase in temperature is a direct consequence of the production of an excess of 90% of the heat produced that has been and continues to be accumulated in the earth system in a very short period of time, paleoclimatically speaking.

It has in fact been reported that the increase in temperature over a shorter time span, consider for example that between 1993 and 2017, was greater than that reported between 1960 and 1993. And this is not visible only on the surface, but also at greater depths up to 2000 m (according to the measurements made and the data reported).

The heat deriving from anthropogenic activity will penetrate through very specific circulation paths that see the maximum absorption of heat along the vertical of the water column in correspondence with those places where  waters are already forming as for the Antarctic Intermediate Water along the Antarctic Circumpolar Current or Nord Atlantic Deep Water precursors in the Nordic Seas. By warming the coldest waters of the planet, the other oceans will gradually undergo massive warming.

There is also a greater warming of the Atlantic waters compared to those of the Pacific or the Indian Ocean between 700-2000m. This is due to the southward transport of the North Atlantic Deep Water which are transported to this depth (700-2000 m) by the Atlantic Meridional Overturning Circulation.

But the increase in temperature cannot lead only to a mere modification of this parameter. It has an effect on:

SALINITY

Over ten-year time scales, changes will also be visible in the form of salinity imbalances. In fact, we will see a tropical and subtropical Atlantic and the Mediterranean, saltier than the Pacific and the polar Arctic, which will instead see a decrease in salinity. The latter will be due to various causes including changes in the hydrological cycle (in all its variables) and obviously the decrease in sea ice.

The warming of surface waters at mid-latitudes combined with the decrease in salinity of the waters at higher latitudes will lead to a STRATIFICATION of the surface waters of all ocean basins.

GREENHOUSE GASES

When we talk about anthropogenic flux acting on the ocean we cannot refer only to heat but we must also talk about the so-called greenhouse gases:

“Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, which absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapor (H₂O), carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), and ozone (O₃) are the primary greenhouse gases in the Earth’s atmosphere”. IPCC Glossary

Although when we think of climate change immediately we think to the temperature parameter, in reality it all started with the first greenhouse gas emissions with coal-fired power plants at the time of the industrial revolution.

Between 1994 and 2007 the global ocean accumulated an additional 30-38 Pg C of anthropogenic CO₂, which is equivalent to an air-sea CO₂ exchange of between 2.3-2.9 Pg C y-1 bringing the total inventory for the year 2007 to 150 ± 20 Pg C.

However, there is growing evidence that the ocean’s ability to accumulate CO₂ shows both temporal and regional variability, which leads to an imbalance of this system in carrying out its task. In fact, anthropogenic carbon dioxide shows a variability in penetration in a range between 0-1000 m which shows that this is also due to the different hydrological circulation patterns depending on the basin to which we refer. In general, however, the anthropogenic carbon dioxide from the atmosphere reaches a given (variable) depth in which it is stored.

The contribution of CO₂ has direct consequences on water chemistry. Various infrared studies show a consistent decrease in pH, ACIDIFICATION, in recent decades, especially in the superficial oceans.

Reductions range between 0.013–0.03 pH units/decade. Between 1991–2011, mean surface-ocean pH has declined by 0.018 ± 0.004 units/decade in 70% of ocean biomes, with the largest declines in the Indian, eastern Equatorial Pacific and the South Pacific subtropical ocean biomes.

Due to the close link between carbonate ion concentrations and pH, mean trends in the stability of mineral forms of aragonite and calcite are important for organisms such as coccolithophorids, pteropods and corals follow those of pH, with high-latitude regions most vulnerable to under-saturation due to naturally lower mean values.

OXYGEN

Surface oxygen levels are due to the balance between:

• autotrophic oxygen production during the photosynthetic process
• temperature (solubility)
• oxygen exchanges between air and water

Deeper in the water column, consumption of oxygen during respiration and redistribution by ocean circulation and mixing are dominant processes.

A warmer ocean, in this case, will lead to a reduction of oxygen in the water for 2 causes:

• lower solubility of O₂
• higher respiration rate due to the isolation of organic matter following the stratification process.

For the 0–1000 m depth stratum oxygen is assessed to have declined in a range of 0.5–3.3% between 1970 and 2010.

The regions of lowest oxygen, known as OMZs (Oxigen Minimum Zone), with oxygen levels lower than 2 mg/L, are observed to be expanding by a medium range of 3.0–8.3%.

Diverse studies have highlighted nitrogen and phosphorus limitation in the stratified tropical ocean regions accompanied by widespread iron limitation at high latitudes and in upwelling regions that typically have elevated levels of productivity. There are also cases of co-limitations between these elements.

More generally, the stratification process will trap nutrients and reduce the supply of nutrients by reducing the exchange of these along the water column between the various layers formed.

In conclusion, what I personally understood from this process is that everything is a consequence of everything. We ourselves are a consequence and we ourselves generate others. Today, unlike in different eras, we can inform ourselves, read about everything and we have the possibility to change always, from the haircut to the car in the garage.

So let’s change, let’s get informed and free ourselves from uniformity. We should understand that we have spent a lot of time doing the wrong thing, thinking our actions go unpunished. The time has therefore come to do the right thing because the world exists and will probably continue to exist, but we will not be there.

Maria Bruno

• IPCC, 2012: Glossary of terms.
• “Changing Ocean, Marine Ecosystems, and Dependent Communities” Bindoff, N.L et al, 2019. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate.