What this lesson teaches
I can explain the natural causes of climate change and their feedback loops.
Syllabus 9173, Topic 3.2, 1. The Science of Climate Change. Can natural factors fully account for contemporary climate change?
Make a guess
A huge amount of meltwater pours into the North Atlantic. What happens to the sinking of surface water there?
- Nothing changes, because winds drive ocean currents.
- It weakens, because fresher water is less dense.
- It speeds up, because there is more water.
- It stops ocean currents worldwide.
Show the answer
It weakens, because fresher water is less dense.
Fresh water lowers salinity, so surface water is lighter and sinks less easily. The overturning circulation can slow.
Temperature and saltiness control how dense seawater is, and changes in where dense water sinks alter how oceans move heat.
Thermohaline circulation is the part of the deep ocean circulation driven by density. Thermo means heat and haline means salt. Colder, saltier water is denser. Near the poles, dense surface water sinks and feeds deep currents.
In the Atlantic, warm surface water carries heat north. There, it cools, becomes dense and sinks. Deep water then flows back south. This is part of the Atlantic Meridional Overturning Circulation (AMOC). Winds and mixing also help, so the simple conveyor-belt diagram is a model.
A large flow of fresh water, from melting ice or heavy rain, makes surface water less salty and less dense. Then it sinks less easily. A weaker circulation carries less heat north, so parts of the North Atlantic can cool compared with otherwise.
Put these in order
Meltwater pours into the North Atlantic. Put the chain of effects in order.
- Less surface water sinks near the poles.
- Surface water becomes less salty and less dense.
- Parts of the North Atlantic region cool compared with what they would otherwise be.
- The Atlantic overturning circulation weakens.
- Large amounts of fresh meltwater enter the North Atlantic.
Show the answer
- Large amounts of fresh meltwater enter the North Atlantic. Fresh water mixes with the surface seawater, so it becomes less salty.
- Surface water becomes less salty and less dense. Lighter water does not sink as easily into the deep ocean.
- Less surface water sinks near the poles. Sinking feeds the deep return flow, so weaker sinking slows the overturning circulation.
- The Atlantic overturning circulation weakens. A slower circulation carries less warm surface water, and less heat, northwards.
- Parts of the North Atlantic region cool compared with what they would otherwise be.
Feedbacks can make this stronger. Weaker circulation carries less salt north, so the water stays fresh and sinking stays weak. Regional cooling can also grow sea ice, which reflects sunlight and cools the area more.
Ocean cores link past circulation changes to sudden regional climate shifts. But one cool region does not mean global cooling. Future changes are uncertain.
- Thermohaline
- Ocean circulation driven by temperature and salt.
- Denser water
- Colder and saltier; sinks near the poles.
- Fresh water input
- Reduces density and can weaken sinking.
Step by step
Change density
Cooling, salt or fresh water change how heavy surface water is.
Change overturning
Sinking and deep flows adjust.
Change heat transport
Regional temperatures respond.
Worked example: Freshening in a warming world
If meltwater freshens the North Atlantic, weaker circulation can make that region warm less than the global average. So a cooler region does not disprove global warming. Heat is just spread unevenly.
Watch out for this
Thermohaline circulation is driven only by surface winds.
Density differences from temperature and salt are central. Winds and mixing also help.
Check your understanding
Why can added fresh water slow sinking near the poles?
- It warms the water, and temperature controls density.
- It lowers salinity, which makes water denser.
- It lowers salinity, which makes surface water less dense.
Show the answer
It lowers salinity, which makes surface water less dense.
Yes. Less dense water does not sink as easily.