What is the greatest risk to earths water supply?

This question is rather subjective: the answer depends on how the question is interpreted. Many people would say climate change is the greatest risk to water supply, but the question mentions 'Earth's water supply'. It is a fact that the amount of water on Earth has not changed. Climate change is shifting climatic patterns, so some areas experience more droughts, whereas others experience more floods, but the amount of water on Earth still stays the same. My answer is therefore that pollution poses the greatest risk to Earth's water supply. Pollution decreases the amount of water we can safely use without expensive treatment. Some water pollution, such as certain toxic metals, pesticides or endocrine disruptors, simply can't be detected or treated using conventional water testing and treatment. Modern civilisation is resulting in unsustainable amounts of pollution being released into our rivers and lakes, and sooner or later, a threshold of no return will be reached, where ecological collapse will mean we won't be able to rely on these water resources for fresh water anymore.    

Carbon monoxide is the answer: )

Human embrogeneis(plus i cant see the image)