Climate change is about pollution and that is easy enough to quantify with a glance skywards or taking a deep breath on a city street. The world's water crisis is not so simple for the amateur environmentalist to measure, though. Turn on a tap, and out it pours in a clean, uninterrupted stream, prompting the question: Crisis - what crisis?
Such evidence makes the alarm from scientists about water scarcity hard to swallow. They generally say that unless we change our ways, by 2025 more than half of the world's nations will face fresh water stress or shortages. By 2050, three-quarters of the global population may not have enough for its daily needs.
The convenience of city living easily shields us from such matters. We do not realise how fortunate we are; that one in three people on Planet Earth - two billion or so - live in water-stressed regions. They often do not have the convenience of plumbing, instead relying on streams and wells for their water. Global warming, pollution and population growth means that what they - and the rest of us - have is becoming increasingly rare.
During last century, the world population tripled, but water usage grew seven-fold. The number of people is expected to increase by three billion to nine billion by 2050, meaning we will need twice as much water. Earth's water supplies are finite; it is anyone's guess as to where the extra billions of litres that will be needed are to come from.
Governments are well aware of the risk of conflict over a lack of water. The precious liquid is as much part of the difficulties between Israel and its neighbours as religion and the right of sovereignty. But ensuring that there is enough is only part of the problem: it also has to be clean.
Every eight seconds, a child somewhere in the world dies from drinking dirty water. Half of hospital beds are occupied by someone with a water-borne illness. Contaminated water is implicated in 80 per cent of all sicknesses and diseases.
These facts and figures have comparatively little meaning to Hong Kong, a developed society awash at this time of year in sub-tropical rain. Yet we are in a prime position to help. The solution lies in food.
That was made clear to me on Wednesday by the deputy director-general of the Sri Lanka-based International Water Management Institute, David Molden. His organisation in May jointly released a study with the Stockholm International Water Institute and the UN's Food and Agriculture Organisation that concluded between 30 per cent and 50 per cent of the world's food was being wasted. Apart from people being simply wasteful at home and in restaurants, this was down to inefficient farming, storage and packaging. Those who know about restaurant buffets are well aware of the problem; fully half of the food ends up uneaten and in the garbage.
Water is the central ingredient in food production. Researchers, in coming up with the formula for calculating the water footprint for the world's nations (to be found at www.waterfootprint.org ), revealed some eye-opening facts. A kilogram of beef requires 16,000 litres of water to produce; a cup of coffee, 140 litres; a cup of tea, 35 litres; and a hamburger, 2,500 litres. The US by far uses more water per capita than any other country, with 2,500 cubic metres a year; China's is 702 cubic metres; and Japan's, 1,150 cubic metres. The growing affluence of people in developing countries like China and India means higher meat consumption and consequently, a larger water footprint.
There are many complicated reasons as to why the world is running out of fresh water, but it is clear that changing the way we view food - from farmer to fork - will have a considerable impact on resources. Dr Molden estimated that even if we were five per cent less wasteful with what we eat, the water savings would be substantial.
Never was there a better excuse to welcome rising food prices, go on a diet or consider vegetarianism.
Peter Kammerer is the Post's foreign editor