The state of the ocean
0 of 4 hands-on
01 Introduction
Divers see the ocean closer than almost anyone else, and that gives us a particular responsibility. This lesson looks at what the ocean does, how it is changing and why the picture is not only bad news. Understanding the state of the sea is the starting point for everything else in this course.
Objectives
- Describe what the ocean does for people and for the planet
- Explain the three big changes affecting the sea: warming, acidification and loss of oxygen
- Explain shifting baselines and why divers can misjudge how healthy a site is
02 Watch
One ocean
4 min · Video in production
- 1
A view of Earth showing that most of its surface is ocean, with one connected body of water wrapping around the continents.
- 2
Quick scenes of what the sea provides: plankton producing oxygen, fishing boats at dawn, a coastal village, and a reef absorbing a wave.
- 3
A time-lapse of a reef changing across decades, from crowded and colourful to pale and quiet, and then a section recovering.
- 4
A diver at the surface looking down at a healthy reef, with the sound of the sea.
03 Learn
What the ocean does for us
The ocean covers about 71% of the surface of the Earth. It is not just scenery for divers. Tiny floating plants called phytoplankton produce a very large share of the oxygen we breathe, with common estimates putting it at around half. The ocean also stores huge amounts of heat and carbon dioxide, which slows the change in the air around us.
- Climate: the ocean has absorbed more than 90% of the extra heat trapped by greenhouse gases, and roughly a quarter of the carbon dioxide people release each year.
- Food: fish and other seafood are a major source of protein for billions of people.
- Livelihoods: fishing, tourism and shipping support hundreds of millions of jobs, and diving is part of that.
- Protection: reefs, mangroves and seagrass soften waves and reduce coastal flooding.
Three big changes
Most of the stress on the sea from the changing climate can be grouped into three linked problems.
| Change | What is happening | Effect on marine life |
|---|---|---|
| Warming | Seawater is absorbing most of the extra heat | Coral bleaching, animals moving towards cooler water, changes in breeding and food supply |
| Acidification | Seawater absorbs carbon dioxide and becomes slightly less alkaline | Harder for corals, shellfish and some plankton to build shells and skeletons |
| Loss of oxygen | Warmer water holds less oxygen, and nutrient pollution creates low-oxygen zones | Fish and other animals avoid or die in oxygen-poor water |
Take a guess first
Since before the industrial age, the average pH of the surface ocean has fallen by about 0.1 units. Each whole pH unit is a tenfold change in acidity. By roughly what percentage has the acidity of the ocean risen with a 0.1 fall?
Shifting baselines
Each generation tends to take the ocean as it first knew it as normal. A diver who started on a reef with few large fish may think that is how reefs are. A diver who knew the same reef decades ago would remember something very different. This is called shifting baseline syndrome, and it makes it easy to underestimate how much has been lost.
Predict first
A new diver says: "This reef has hardly any big fish, but that must just be how reefs are." What is the best response?
Where divers fit in
Divers are witnesses, ambassadors and sometimes part of the problem. We notice changes first, we tell others what we have seen, and we spend money in coastal communities that can value healthy reefs. We also touch reefs, disturb animals and fly long distances to dive. The rest of this course is about tipping that balance towards helping.
04 Explore
Try it yourself
Match each change in the sea with its main effect.
Match each change with its main effect.
- Warming
- Acidification
- Loss of oxygen
- Shifting baselines
- Protection and recovery
05 Apply
Scenario
On a dive trip, an older local fisher tells you that the shallow reef used to be full of large groupers, and now you can hardly find one. A younger diver in your group says, "It still looks great to me."
What is the most useful way to think about this?
06 Check
07 Summary
Key points
- The ocean produces oxygen, stores heat and carbon, feeds people and protects coasts.
- Warming, acidification and loss of oxygen are the three big linked changes.
- Shifting baselines hide how much has been lost, so long-term records and local memory matter.
- Recovery is possible where pressure is reduced, and divers can help.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.