i'm guessing Greenspam is trying to say that we wouldn't need to have another 50% decrease in oil intensity if prices spiked similarly again.  because, he would argue, we can produce more GDP with less oil, so the economy would intrisically be more resistant to oil input cost increases.    

from the article, oil went from $1.80 (1970) to $39 (1981).  though this is not inflation adjusted, this would be a 20X increase in real dollar price.  This equates to about a 5X spike in constant dollars. (using rough numbers from http://www.inflationdata.com)

if we again had this increase in oil prices the effect on the US economy would be 0.50 times as much as before: the price spike is effectively halved.  Thus to respond to the shock similarly (we're calling those 30yrs "the response"), the economy would only need to reduce its oil intensity by 25% (i am assuming the economy behaves linearly: half the price spike = half the reduction in oil intensity required).  as oil intensity continues to drop, the required reduction in oil intensity, given future spikes, falls proportionally.  this would seem to be the argument greenspam is making.  

still seems like a monumental task, given the efficiency increases and fuel switching that has already taken out much slack in the system.