Old has its own rewards but how about the new? Specifically, 4.2 million new green jobs were predicted by the U.S. Congress of Mayors in their new report.(1)

There are now, as I write, about 3,530,000 entries in a search for '4.2 million new green jobs', e.g. "US could create 4.2 mln green jobs by 2038 - study"(2) or "Report: 4.2 million new 'green' jobs possible"(3). My own "Happiness is a Green Job (Mayors)" includes that phrase as well.(4)

Let's consider the definition of new, according to Merriam-Webster Online(5):

1 : having recently come into existence : recent , modern
2 a (i): having been seen, used, or known for a short time : novel (ii): unfamiliar
    b: being other than the former or old
3 : having been in a relationship or condition but a short time
4 a:  beginning as the resumption or repetition of a previous act or thing 
   b: made or become fresh
   c: relating to or being a new moon
5 : different from one of the same category that has existed previously
6 : of dissimilar origin and usually of superior quality
7 capitalized: MODERN; especially: having been in use after medieval times

Which definition would apply in the case of 4.2 million new green jobs? Mayors' study estimated that as of 2006 there were 751,051 Green Jobs in the U.S. economy. Their data came from the National Establishment Time Series (NETS) database by Walls&Associates with a note: "Renewable Power Generation Jobs are not available in NETS. Green Jobs totals for Metropolitan areas include all categories except Renewable Power Generation."(1) Hence, the total 2006 green jobs were actually greater than 751,051 by an unspecified amount.

The Mayor's report speculates that the current count of 750,000 -- instead of 751,051 in 2006 -- jobs amounts to less than one-half of a percent of total current jobs. Also: "The generation of 4.2 million new Green Jobs would more than quintuple the total count and could provide as much as 10% of new job growth over the next 30 years."

Green Jobs were defined as jobs that (I) were in developing new technologies to increase energy efficiency and usage of renewable resources or (II) installed existing technologies. These jobs were calculated according to their growth assumptions and various scenarios. Thus, their calculated green-job numbers, including the total 4,214,700, do not contain the 751,051 jobs. They are "new" green jobs in the sense of its meaning 1 through 5.(5)

Is it acceptable to say, the generation of 4.2 million new Green Jobs would more than quintuple the total count? Yes but it would be better to say 4,214,700 new jobs would bring the total in 2038 to more than 4,965,751, i.e. 4,214,700 plus 751,051, which is more than 6.6 times the estimated 2006 total green jobs.

However, the report reminds: "In projecting potential future Green Jobs, though, we remain conservative."

Thirty year projections are common in decision-making. Here is a glimpse of what happens inside such a study. We have to remember that the assumptions made and the scenarios chosen dictate the results obtained with usually unspecified uncertainties that are sometimes called "conservative." 

(1) http://www.usmayors.org/pressreleases/uploads/GreenJobsReport.pdf
(2) http://www.reuters.com/article/bondsNews/idUSN0229022720081002?sp=true
(4) http://www.science20.com/fine_scientist/blog/happiness_is_a_green_job_ma...
(5) http://www.merriam-webster.com/dictionary/new