The word “climate” is used frequently: while discussing a sharp change in the weather, the onset of an unusually cold or hot season, and how these seasons have changed in principle compared with the past century. Professionals – meteorologists and oceanologists – divide natural phenomena into different time scales of changes, of which climate events include the averaged characteristics of the atmosphere and hydrosphere over a period of several decades. The UNESCO World Meteorological Organization recommends a 30-year standard reference period for climate change assessments.
Research on global climate changes has become a priority area for world science, but it happened relatively recently. Even 40–50 years ago, climatology was just one of the particular disciplines, the current climate was considered quite stable and it seemed that its sharp global shifts relate exclusively to the past. Climatologists studied the ancient powerful glaciations, historical warming and cooling – the Atlantic, Roman and medieval climatic optima, a small ice age of the 15th – 18th centuries and others. The warming of the Arctic, which occurred over the first half of the 20th century, was considered the last significant climatic event. As for the new global changes, they, in principle, were not expected. However, in the 1980s, after a period of relative cooling, severe global warming began and is still ongoing.
The climate change problems have finally attracted increased attention of the world community, after it has been a generally accepted view that an increase in the concentration of CO2, a greenhouse gas, caused by economic activities, is the cause of warming. Greenhouse gases – water vapor, methane, carbon dioxide, ozone and a number of others – became an additional source of heat due to their infrared radiation. Environmentalists have previously accused humankind of adverse effects on the environment: cutting down forests, transforming landscapes, changing the river beds, building water storage basins, etc. However, such actions of a global scale, as climate change on the planet, have not been attributed to civilization. It should be clarified that the human impact on the Earth’s heat balance is not limited to greenhouse gas emissions: cirrus clouds caused by aircraft’s condensation trails contribute to warming while aerosol emissions and increased surface reflectivity due to land use give rise to cooling.
The climate change topic has long been part of the global economy and politics
There are also opponents of the anthropogenic impact on the global climate, who consider modern warming to be just a manifestation of natural variability, another phase of multi-year fluctuations. Both supporters and opponents of the decisive role of human activities in global warming provide quite convincing arguments in their favor.
Due to the internal complexity of the ocean – atmosphere system, which is also exposed to geological and astronomical factors, It is not yet possible to accurately explain either the current climate fluctuations or the more catastrophic changes in the past of the Earth. Nevertheless, the climate topic has long ceased to be a purely academic discussion and has become part of the global economy and politics, manifesting itself in the form of a variety of economic restrictions for countries and industries, emission credits, etc.
What key features of the ongoing climate change can be identified in the oceans, regardless of knowledge of the true causes of contemporary warming? What are their features in comparison with the ongoing processes on land? How can they affect human economic activities at sea?
Because of its greater density and heat capacity, the ocean heats up much more slowly than land, storing a large amount of heat and smoothing not only the seasonal fluctuations of incoming solar radiation, but also long-term, multi-year heat anomalies. The average temperature of the land surface and the surface layer of the atmosphere has increased by 1 °C over the past 30 years, whereas the temperature of the ocean surface has risen by a mere 0.4 ° C. In addition, the temperature on continents has been increasing, albeit to varying degrees, but everywhere, whereas in the ocean, with prevailing heating, water cooling may also occur, such as, for example, in the eastern Pacific Ocean or in Antarctic waters. One of the reasons for the more complex ocean response to external influences is the large-scale water circulation system that forms global water exchange, including the rise of cold deep waters to the sea surface.
The shrinking of sea ice extent in the Arctic Ocean is one of the most striking manifestations of global warming. Since 1985, it has shrunk by one and a half times, which opens up promising prospects for navigation along the Northern Sea Route and mineral extraction in the Arctic shelf. The rate of Arctic warming is very high, it brings us closer to the natural conditions of the time of the Vikings who freely sailed in the polar latitudes on medieval sailing ships. Most researchers consider such a high rate as a result of positive feedback, when a decrease in the ice area reduces the total albedo (reflective property of the surface), thereby continually increasing the ability of the sea surface to absorb solar heat. The same effect is most likely associated with a high warming rate in Siberia, where the aforementioned feedback relates to snow cover. In contrast to the Northern Hemisphere, there is no reduction in the sea ice extent in the Antarctic waters, and even a reverse process of a slight increase in ice cover is observed.
Another manifestation of global warming is closely associated with the ice melting process, namely, the sea level rise of almost 20 cm since 1900. The rate of sea level rise has increased in recent years and is now about 3 mm a year. It should be emphasized that the melting of continental glaciers, primarily the ice sheets of Greenland and Antarctica rather than sea ice plays key role here. It is also worth noting that more than half of the sea level increment is not due to the arrival of an additional amount of water (resulted from the melting of glaciers), but to its thermal expansion, that is, to an increase in the specific volume of water with increasing temperature. Flooding of low-lying coastal areas, which are densely populated and economically developed in many countries, will be the major negative impact of sea-level rise. There are other, less obvious consequences of the encroachment of the sea on land (transgression of the sea). When the sea level rises, even without significant coastal flooding, shoreline erosion (destruction) increases, sedimentation patterns change in river estuaries, salty seawater penetrates upstream of rivers, fishing conditions change, water supply systems are disturbed, etc.
In the course of global warming, one-way processes (temperature rise, shrinking of ice and snow cover, sea level rise) are generally prevalent across the Earth, but there are also multidirectional processes. These include the hydrological cycle (evaporation, precipitation, surface runoff) and the associated salinity of the ocean surface layer. Since the saturated vapor pressure in the air increases with increasing temperature, the amount of water vapor in the atmosphere is expected to grow everywhere with climate warming. However, this increases not only the total moisture, but also its spatial heterogeneity: precipitation grows at the equator and in middle latitudes where there is a lot of rainfall, while evaporation increases in the tropics where it reaches a maximum.
Accordingly, the ocean’s response to changes in the hydrological cycle is that in the areas of high salinity, where evaporation prevails, water becomes more saline, while the areas of low salinity, where precipitation prevails, become more desalinated.
The rate of Arctic warming is very high and it brings us closer to the natural conditions of the time of the Vikings who freely sailed in the polar latitudes on medieval sailing ships
How do global climate changes affect such characteristics of the environment important for navigation as wind speed and wave height? Unlike the land surface, where a steady decrease in wind speed is mainly observed, the average wind speed increases over many areas of the ocean. This occurs in the Southern Ocean, in the tropical zone of the Pacific Ocean (especially in the southern hemisphere), in the central Atlantic, off the western coast of North America, in the Mediterranean Sea and the Far Eastern seas. As regards the number of tropical typhoons and mid-latitude deep cyclones leading to catastrophic storms, there are no statistically significant signs of their growth. The greatest increase in the “height of significant waves” is observed in the Southern Ocean, the North Atlantic and the North Pacific: from 20 to 50 cm per decade, according to various estimates.
Could Earth somehow dispose of the excess CO2 entering the atmosphere or end up like Venus one day with a temperature of more than 400 °C? In the history of the Earth, there have repeatedly been periods with a very high content of CO2 in the atmosphere, which was slowly removed from it during the course of various biogeochemical processes. Living organisms constantly convert carbon dioxide into carbon, being part of a variety of proteins, which then goes into the soil and biogenic rocks. On land, carbon dioxide is converted to carbonate rocks during chemical weathering.
The ocean also plays an important role in absorbing excess CO2 through its complex carbonaceous carbonate system. Carbon dioxide is dissolved in seawater and, depending on the degree of saturation, can either be absorbed from the atmosphere or released into it or can be transferred by vertical circulation to the deep layers of the ocean. When CO2 is absorbed from the atmosphere, carbonic acid (H2CO3) is formed; as a result, ocean water becomes more acidic: the concentration of hydrogen ions in seawater has increased by almost 30% since the beginning of the industrial age. This reduces, inter alia, the concentration of carbonate ions CO32- in water, which are used by mollusks, corals and plankton organisms for the construction of their shells and skeletons.
In addition to the water acidification process, there are also other changes in the chemical structure of the ocean affecting the marine ecosystem and, accordingly, the development of industrial marine fisheries and aquaculture. Oxygen concentration decreases in many areas of the oceans due to several physical causes. First, the solubility of oxygen in warmer waters decreases, and second, density stratification is aggravated in warmer climates, which limits water ventilation by oxygen from the surface layers. Reduced water ventilation intensity also decreases the supply of biogenic (nutrient) substances from the deep layers, which will inevitably reduce the bioproductive areas in the oceans.
Tropical areas are becoming warmer and more saline, while the equatorial zone – warmer and fresher. Excess carbon dioxide from the surface layers descends to a depth in areas where water masses are formed: near the Antarctic coast (near-bottom waters), in the North Atlantic (deep waters), the Southern Ocean and the North Pacific (intermediate waters). Since these descending waters have recently become less saline and, accordingly, less dense, this may potentially weaken the general circulation of the oceans (the so-called “inter-oceanic conveyor”) with far-reaching climatic impacts of reduced heat exchange between latitudes. The concentration of dissolved oxygen and carbonate ions decreases, water acidity increases, and the supply of nutrients decreases worldwide due to increased stratification.
The issues raised only partially demonstrate the complexity and ambiguity of the response of the oceans to external climate impacts. The ongoing processes encompass various geospheres of the Earth, and the interaction of the whole complex of natural sciences is necessary for their full understanding.