The Earth’s climate is dynamic and ever-evolving. Flow and movement are its defining characteristics, its underlying order emergent from diverse inputs that change over time. Previous ice ages are just one example that demonstrate how fluid and adaptable climate can be, and we’ve now reached the stage where human actions have the potential to set off similar dramatic shifts in the Earth’s living conditions.
Ocean currents are one of the major determinants of weather and climatological conditions as they are experienced on land. Life as we know it and live it wouldn’t be the same without the influence of the currents, and if we alter them through our own hubris the long-term impact could be devastating.
What factors affect global climate?
The Earth features thousands of microclimates, which is a testimony to the complexity of the factors that determine temperatures, rainfall patterns, winds, humidity levels and other climatological elements.
The list of factors that affect global climate include:
- Proximity to oceans
- Prevailing winds
- Variations in the angle, intensity and duration of solar radiation
- Gravitational influences of the Sun and moon
- Plant photosynthesis
- Concentrations of greenhouse gases (which trap heat) in the atmosphere
- Atmospheric currents
- Ocean currents
While the earth’s climates are controlled by a number of factors, ocean currents and water movements play a big role. Some currents take warm water away from the equator, influencing coastal climates near the poles. Others take colder water from the poles or the deep ocean and move it towards the equator, creating cooler coastal climates.
How does the ocean influence climate?
The movement of the ocean currents has an especially significant factor in the development of the planet’s climate.
Ocean currents can and do move in all directions. But those that move north and south act like conveyor belts, transporting warm water to the polar regions and carrying cooler water back toward the equator.
As they traverse open expanses of ocean, currents generally move fast and straight. But the Earth’s land masses act as boundaries that guide the moving waters of the major currents up or down coastlines, creating smaller currents with their own unique patterns of movement (spinning clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere).
It is the former that carries warm water toward the poles and the latter that brings most of the cold water back toward the equator.
This constant cycling of water helps moderate temperatures across the planet, by creating ocean temperature variations that are less extreme than those found on land. Temperature differentials are reduced through the actions of the winds and rainfall, which originate over the ocean and lower (or raise) ground-level temperatures as they sweep across the land.
For example, the climate of coastal areas of Western Europe is a lot milder than would be expected for this altitude. This is due to the ‘Gulf Stream’, which carries warm water from the Atlantic Ocean in a northerly direction.
In addition to the influence of their currents, the oceans also affect climate by absorbing a goodly percentage of the Sun’s heat, including that which hits directly and that which is trapped by the Earth’s atmosphere.
This has helped prevent faster-rising temperatures on land, but contributed to an overall warming of the oceans that could disrupt ocean currents while creating stronger hurricanes, typhoons and tropical storms.
Another way the ocean helps control climate is through its capacity to absorb carbon dioxide, which is stored by algae, coral and sea vegetation . Up to 40 percent of the carbon dioxide emitted through the combustion of fossil fuels is absorbed by the ocean, and that has reduced the intensity of man-made global warming up to this point .
However, as climate change intensifies and the seas become warmer, their ability to soak up CO2 is expected to decline precipitously (and has already begun to do so) .
How are ocean currents formed?
Surface currents, which are defined as those that extend to a depth of 100 meters, are driven by the prevailing winds and the Earth’s rotation. But variations in water temperature (deeper waters are colder), salinity and density (water is saltier and therefore heavier and denser in the polar regions, where freezing occurs) are the primary driver of deep-water currents.
Deep ocean currents originate in the extreme Northern and Southern seas, and they carry massive quantities of colder water back toward the equator.
A lot of continents or large landmasses have also coastal currents which transport nutrients and heat, while the deep oceans have various currents which differ depending on their location and the surrounding geography.
How do ocean currents transfer heat?
The oceans directly absorb more than two-thirds of the Sun’s heat, and overall 25 percent of the planet’s global heat budget is transferred through the actions of ocean currents .
Currents that flow away from the equator are called warm currents, while those that flow toward it are known as cold currents—but in this case the words ‘warm’ and ‘cold’ are relative, it simply means they contain water with higher or lower temperatures than would be predicted by latitude alone.
Due to various factors, including evaporation and surface cooling, surface water can often become denser than the deeper water. This can create vertical, circular currents which are often referred to as “convective currents” or “convective turning.”
Often, this type of current acts in a way that it takes warm water from the equator, moves it poleward, where it cools and sinks, completing the circular current. This disperses heat from near the equator towards the poles, making some areas a lot warmer than they would otherwise be.
Without circular currents like these, may of the northern and southern extremities of human habitation would be rendered too cold to live in. Additionally, without this method of heat dispersal, areas near the equator would become a lot hotter, and perhaps even become inhabitable.
This would drastically change the world as we know it.
Thermohaline ocean circulation explained
Surface currents carry warm air to the polar regions, where the water eventually becomes cold enough to freeze. When this happens water molecules are locked into the ice but salt molecules are unaffected, and the added salt increases the density of the water just beyond and below the edge of the “freezing zone.”
This cold, dense water has a natural tendency to sink, and the surface currents flowing in behind it help drive the water deeper, creating deep cold water currents that loop back in the opposite direction, flowing toward the equator in a reversal of the movement of the surface currents.
What has just been described is the process of thermohaline circulation, which relies on density and temperature differentials to precipitate the vertical descent of the cold, salty water that comprises deep water currents .
Video animation from NASA depicting the thermohaline circulation:
Thermohaline circulation represents a crucial element in the balancing of ocean-based heat transfer, and should this process be interrupted by global warming it could have dramatic implications for the whole planet.
Climate change, the ocean currents and our uncertain future
There is an elegant balance to the complex movements of the ocean currents, mediated by multiple factors yet maintaining consistent temperature gradients that make most latitudes hospitable to life.
But anthropogenic climate change is having a profound impact on the Earth’s weather and its climatological balancing systems. A hotter planet will inevitably raise temperatures in the sea, which will cause higher sea levels, stronger winds, fiercer and more frequent hurricanes and typhoons, and greater or lesser quantities of rainfall in particular areas that may be profoundly affected by such changes.
As ocean water warms to greater depths, and temperatures climb in the polar regions, major ocean currents may slow down or even stop. This could lead to dramatic shifts in temperature everywhere: higher latitudes may be plunged into Ice-Age-like conditions, while heat waves might render equatorial regions virtually uninhabitable.
This is an extreme result and far from certain. But scientists are already observing a slowdown in a powerful Northern hemisphere ocean current system known as the Atlantic Meridional Overturning Circulation (AMOC), which helps moderate temperatures on the European continent.
This current system incorporates warm water surface currents and deep-water cold currents, and some models are predicting a 50 percent or more slowdown by the turn of the next century if greenhouse gas emissions continue at current levels . The last time this current system moved so slowly was between 10,000 and 100,000 years ago, when much of Europe and North America was covered by an impenetrable sheet of ice.
Similarly, in a study by Manabe and Stouffer (2003), the impact of increasing atmospheric carbon dioxide concentrations was analyzed. Manabe and Stouffer concluded that as CO2 concentrations increase, the Atlantic convective currents significantly decrease. With a four-fold increase in CO2, it was predicted that the current would begin to slow, and eventually stop over a period of 100 to 200 years. This would cause the climate of much of Western Europe to change, and would impact billions of people world-wide.
Worst-case scenarios can still likely be avoided, if strong action is taken to reduce carbon emissions globally. But a failure to take those actions will lead us into unchartered territory, and if that causes ocean currents to slow or shut down it could set off a chain reaction of climate change that will turn hundreds of millions of people (at least) into refugees, fleeing their homes in a desperate search for more habitable regions.
We rely on the ocean currents to survive, and if we disrupt them we do so at our own peril.