Why is drinking water a bigger threat than climate change?

Because as populations rise here on earth human waste is more and more likely to get into wells around the world or giardia or dysentary bacteria and protazoas. Then if the water isn't boiled or distilled or something it is going to kill more and more children before they are 5 or 10 years of age especially in most 3rd world countries. Then combined with the lack of education regarding sanitation in those 3rd world countries this becomes more of a problem. However, another way to look at it is that children who survive bad water are going to have incredible immune systems in these same 3rd world countries if the bad water doesn't kill them out right by 5 or 10 years of age.

Later: I remember watching 2 and 3 year olds in Katmandu, Nepal playing in rivers of open sewage flowing like a stream down a street and floating sticks in it with excrement floating by and thinking at the time as they waded barefoot in this: "Well. This is either going to kill these kids or it's going to make them some of the healthiest on earth with the strongest immune systems ever."

All this has likely changed now because this is now almost 35 years later but this is how it was in that Himalayan community some places back then in 1985 and 1986.

The elevation of Katmandu, Nepal is 4,593′
 And you are looking up at some of the highest peaks on earth there including Mt. Everest, so this is a very remarkable place. I hear most places still at higher altitudes than about 9000 feet still cannot have roads because they would be washed away by the monsoon rains in the summers still. So, suspension bridges to walk across and helicopter landing pads are the only way (outside of walking or trekking) into most of the Himalayas still. They are also the only way out if you get injured as well. Back in the mid 1980s I saw people being carried out of the Himalayas piggy back across suspension bridges and trails 50 to 100 miles to a hospital by the way. So, if you couldn't get help for a broken foot or toe or leg back then often you just died somewhere in the Himalayas. Because if you couldn't walk often you were just going to be dead especially if you were a foreign trekker with a backpack deep in the Himalayas.

In the 1980s the Helicopter landing pads weren't there yet.