1. Earth's Water: Where It All Is

Earth is the 'blue planet': about 71% of its surface is covered by water. With that much water, it's tempting to think we'll never run short. But look closer at where the water is, and the picture changes.

Scientists describe the water cycle in terms of reservoirs — places where water is stored:

• Oceans hold about 97% of all of Earth's water — but it is salt water, which we cannot drink or use on crops without treatment.
• That leaves only about 3% as fresh water. And most of that is frozen in ice caps and glaciers (~69%) or buried as groundwater (~30%).
• Only about 1% of the fresh water — a tiny sliver of all water — sits in the lakes, rivers, and atmosphere we most easily use.

So the 'blue planet' actually has very little easy-to-reach fresh water, which is why it must be used carefully (Lesson 6).

⚠️ Common misconception: 'Earth has plenty of water, so fresh water can't run out.' Most water is salty, and most fresh water is frozen — the usable part is small and unevenly spread around the world.

💡 Tip: remember the chain salty ocean (most) → fresh (little) → and most fresh water is frozen.