In doing research for a feature related to water, I came across an almost unbelievable statistic from an otherwise sober and respectable report. A day later, I'm still unable to comprehend the scale of it; I keep re-reading it, turning the page around, and saying out aloud, so strong is the instinct that it can't quite be true.

The report is Charting our water future: Economic frameworks to inform decision-making; it was put together by consultants McKinsey & Company on behalf of the 2030 Water Resources Group, an alliance of concerned bodies including the World Bank Group and big private interests such as the Coca-Cola Company, SAB Miller and Standard Chartered Bank.

The headline point of the report is that by 2030, unless substantial changes are made to conserve water and build new supplies, there will be a 40% gap between projected water demand from a bigger, richer global population, and "accessible, reliable" supplies.

The really astonishing statistic though was this: that 42% of all projected water demand – yes, nearly half of all the water for homes, industry and agriculture – will be required by just four countries. Four. China, India, Brazil and South Africa. Read it for yourself on page 15 of the Executive Summary. I know China and India are big, and getting bigger, fast. But still ...

To avoid future water crises, we're told that gargantuan investment is needed in water infrastructure. Some of this will be big desalination plants and dams; much of it – as McKinseys stress – will be smaller, cheaper and potentially hugely beneficial improvements to irrigation ditches and pumps, to leaking pipes and the types of seeds farmers plant.

The big question is not really what will help, but how to get it done. Water pricing is the single biggest factor that would make a difference, but few politicians will campaign, or even govern, for a mandate to put up bills because water is essential. Which is, of course, the reason why they should.

It may be true that the global water problem is made up of a series of local water issues. But if 42% of demand is in China, India, Brazil and South Africa – all of which are already suffering water stresses – then perhaps the world needs to put a much, much greater focus on these four countries. Solving shortages in Rio or Shanghai will not help farmers in Tajikistan or California, but many millions of people would benefit, and could set an example to the rest of the world.