The Influence of Ocean Currents on Earth's Climate

Ocean currents are large, continuous movements of seawater that flow in predictable patterns across all of Earth's ocean basins. These currents are driven by several forces including differences in water temperature, salinity, and density, as well as global wind patterns and Earth's rotation. Together, these forces create a global system of circulation that transports heat energy from the equator toward the poles and redistributes cold water from the poles back toward the tropics.

Surface ocean currents are primarily driven by prevailing winds. In the Northern Hemisphere, these winds cause ocean water to circulate in large clockwise loops known as gyres. In the Southern Hemisphere, the Coriolis Effect — caused by Earth's rotation — reverses this pattern, and gyres rotate counterclockwise. Because of the Coriolis Effect, currents in the Northern Hemisphere are deflected to the right, while currents in the Southern Hemisphere are deflected to the left.

One of the most important ocean currents in the world is the Gulf Stream, a warm surface current in the North Atlantic Ocean. The Gulf Stream originates in the Gulf of Mexico, flows northward along the eastern coast of the United States, and then curves eastward toward Western Europe. Because it carries warm tropical water northward, the Gulf Stream significantly moderates the climate of Western Europe, making it considerably warmer than other regions at the same latitude. For example, London, England, located at approximately 51°N latitude, has much milder winters than regions of Canada at the same latitude, largely due to the influence of the Gulf Stream and the North Atlantic Current that extends from it.

In contrast, cold currents such as the Labrador Current flow southward from polar regions along the eastern coast of Canada. When the cold Labrador Current meets the warm Gulf Stream off the coast of Newfoundland, dense fog and stormy conditions frequently result — a direct consequence of the temperature contrast between these two current systems.

The 2024 Edition Reference Tables for Earth and Space Sciences (ESRT) includes the Surface Ocean Currents Model, which identifies all major global ocean currents by name and indicates whether each is a warm or cold current using dashed or solid arrows, respectively. Scientists and navigators have used knowledge of ocean current patterns for centuries to understand climate, plan sea voyages, and — more recently — predict the movement of pollutants and marine debris across the world's oceans.

📖 Key Vocabulary

🗺️ Surface Ocean Currents Model (ESRT)

Examine the Surface Ocean Currents Model below from the 2024 Edition Reference Tables for Earth and Space Sciences. This model shows all major global ocean currents and their directions.

Source: 2024 Edition Reference Tables for Earth and Space Sciences (ESRT), Page 20

🔍 Reading the ESRT Current Map

The Surface Ocean Currents Model uses a key to distinguish between current types:

• Dashed arrows (- - -►) = Warm currents — carry warm water from the tropics toward the poles
• Solid arrows (———►) = Cold currents — carry cold water from polar regions toward the equator

The major ocean basins each contain their own circulation patterns. Notice how currents in the Northern Hemisphere rotate clockwise and currents in the Southern Hemisphere rotate counterclockwise — this is a result of the Coriolis Effect.

📊 Key Currents to Know

✏️ Section 1 Questions — Use the ESRT Model Above

Thermohaline Circulation and Earth's Climate System

Earth's oceans are in constant motion. While surface currents are driven largely by wind, a deeper and slower system of circulation operates throughout all of the world's ocean basins. This system, known as thermohaline circulation, is driven by differences in water temperature and salinity — the two properties that determine how dense ocean water is. Because of this, scientists sometimes refer to thermohaline circulation as the global ocean conveyor belt.

The process begins in the tropics, where the Sun heats surface water, causing it to become warm and less dense. This warm surface water gradually moves toward the poles through surface currents such as the Gulf Stream. As the water approaches higher latitudes — particularly near Greenland in the North Atlantic — it releases its heat energy into the atmosphere, warming nearby landmasses. As the water cools, it becomes denser. At the same time, the formation of sea ice leaves dissolved salts behind in the surrounding ocean water, further increasing the water's salinity and density. When the water becomes dense enough, it sinks to the deep ocean in a process called downwelling.

Once it reaches the ocean floor, this cold, dense water flows slowly back toward the equator as a deep-water current. Over hundreds to thousands of years, this deep water eventually rises back toward the surface through upwelling in regions such as the Southern Ocean and the eastern Pacific. As it rises, it carries with it dissolved nutrients from the deep ocean, supporting rich marine ecosystems near the surface. The complete circuit of thermohaline circulation connects all of Earth's major ocean basins and takes approximately 1,000 to 2,000 years to complete.

Scientists are increasingly concerned about the effects of climate change on thermohaline circulation. As global temperatures rise, glaciers and ice sheets in Greenland are melting at accelerating rates, releasing large quantities of fresh water into the North Atlantic Ocean. Fresh water has lower salinity than seawater, which means it is less dense and does not sink as readily. This influx of fresh water is diluting the North Atlantic, reducing its density, and potentially disrupting the downwelling that drives the global conveyor belt. A slowdown or collapse of thermohaline circulation could have far-reaching consequences for global climate patterns, including reduced heat transport to Europe, disrupted monsoons, and altered precipitation patterns worldwide.

📖 Key Vocabulary

🗺️ Ocean Currents Cross-Section Model

The diagram below shows how surface and deep-water currents work together. Notice the key at lower left: cold currents sink and travel as deep-water currents while warm currents travel near the surface.

⚙️ How the Conveyor Belt Works

Thermohaline circulation works in a continuous loop driven by density differences:

• Step 1: The Sun heats water in the tropics. This water is warm and less dense, so it stays near the surface.
• Step 2: Warm surface water flows toward the poles (e.g., Gulf Stream flowing northward).
• Step 3: Near Greenland and the Arctic, the water cools, becomes more dense, and sinks (downwelling). It also becomes saltier as sea ice forms, leaving salt behind.
• Step 4: Cold, dense water flows along the deep ocean floor back toward the equator and eventually toward the other ocean basins.
• Step 5: Eventually the deep water warms again or rises through upwelling, completing the circuit.

This entire circuit takes approximately 1,000–2,000 years to complete!

⚠️ Climate Change and the Conveyor Belt

As shown in Diagram 2 from the ocean model, melting glaciers are adding fresh water to the North Atlantic. Fresh water is less salty = less dense = does not sink.

This is slowing down thermohaline circulation, which could have dramatic global climate consequences:

• Reduced heat transport to Northern Europe (colder winters)
• Disrupted monsoon patterns in Asia and Africa
• Reduced nutrient upwelling, harming marine food webs
• Changes to global precipitation patterns

🔬 Interactive Cross-Section Simulator

Step 1: Drag each label to its correct location on the cross-section diagram below.
Step 2: Once all labels are correct, the particle animation will unlock — then experiment with the controls!

📋 Thermohaline Circulation — Data Collection

Use the simulator above to test each scenario. Set the sliders as shown, then record your observations below.

✏️ Section 2 Questions

📖 Key Vocabulary

🦆 The Ever Laurel Incident — Reading Passage

🔍 Analyzing the Evidence

The Ever Laurel incident became one of the most important unintentional oceanographic experiments in history. Scientists tracked where the rubber ducks washed ashore to learn about global ocean current patterns.

Key findings from the bath toy tracking:

• The toys traveled from the North Pacific spill site northward and eastward, consistent with the North Pacific Current and then the Alaska Current
• Some toys circulated within the North Pacific Gyre for years before making landfall
• Toys appeared on beaches in the US, Canada, Australia, and even the UK — showing how currents transport materials globally
• The Great Pacific Garbage Patch sits in the center of the North Pacific Gyre, where converging currents trap floating debris

♻️ Solutions to Ocean Plastic Pollution

Scientists and engineers have proposed and tested multiple approaches:

• River Barriers: Funneling plastic into collection areas before it reaches the ocean — lower cost but requires manual removal and not effective for microplastics
• Ocean C-Tube Systems: Giant floating barriers that use wind and wave energy to collect debris; can capture 50% of GPGP debris in 5 years but expensive and requires frequent maintenance
• Reducing Plastic Use: Addressing the problem at its source — switching to reusable containers, banning single-use plastics
• Recycling Programs: Converting collected plastic into reusable materials

👟 Case Study: The Nike Shoe Spill of 1990

Two years before the rubber duck incident, a cargo ship triggered what would become one of the most scientifically significant accidental ocean experiments in history. On May 27, 1990, the container ship Hansa Carrier was sailing through the North Pacific Ocean near 48°N, 161°W when it encountered a severe storm. The ship lost 21 of its 40-foot cargo containers overboard. Several of those containers held a shipment of 61,820 Nike athletic shoes — approximately 30,910 pairs — destined for retailers in the United States.

Unlike many objects that sink immediately when lost at sea, Nike shoes are buoyant — their foam soles and hollow interiors trap air and keep them afloat. As a result, tens of thousands of shoes began riding the surface currents of the North Pacific. Within months, beachcombers began finding the shoes on coastlines across the Pacific Northwest. Oregon, Washington, and British Columbia all received large numbers of shoes. Some were even found as far away as the Hawaiian Islands.

The scientific value of the Nike spill became clear when an oceanographer named Curtis Ebbesmeyer and a beachcomber named Steve McLeod began collecting reports from shore finders. Crucially, Nike shoes are stamped with unique serial numbers that could be matched to shipping manifests. This allowed researchers to confirm that shoes from the Hansa Carrier had traveled hundreds of miles from the spill site and — because shoes have no keel — were significantly affected by prevailing surface winds in addition to ocean currents.

Ebbesmeyer used the shoe landing data to help build and refine computer models of North Pacific surface currents. The data confirmed the existence and general path of the North Pacific Current — the broad flow that carries water eastward from Asia toward North America. Scientists noted that objects with high windage (exposed surface area above the water) are pushed faster and in slightly different directions than objects floating mostly below the surface. This distinction — between wind-driven and current-driven transport — became a key finding that shaped how scientists model debris movement in later spills.

✏️ Section 3 Questions

👟 Nike Shoe Spill — Questions

Base your answers on the Nike shoe case study passage and data table above.