I don't know anything in particular about this report on water tables and prdicted grain prices. Howevor, Lestor Brown's Earth Institute comes in for sustained criticism by Lom Stromberg in his book The Skeptical Environmentalist.
The basic charge is that they put out hypothetical doomsday press releases, such as this one, which often aren't supported by any science or use extremely selective, and not always relevant, statistics. Scaremongering in other words.
Lomberg's conclusion seemed to be that the scenarios painted were possible but extremely unlikely. Also, they did not seem to learn, or even be embarassed by, previous predictions of collapse of society that failed to materilaise.
These wake up calls have been going on since the 1970's. Some are even recycled with revised dates, long after the original predicted "end of the world as we know it"
In conclusion, The Earth Institute press releases should be taken with extremely large pinches of salt. I wouldn't be basing any stock market decisionson them!
