That title is
very misleading

There are segments of the population that will always be unable to work for whatever reason, legal, compassionate, age, disability, etc. For example, no one expects children to work or someone who is 80, that is part of that 51% figure. It's a very crude and dodgy calculation just dividing the workers vs the population and making a percentage of it. We could lower that 51% a whole lot of we sent children to work in the coal mines!
whose share of the total has more than doubled to more than 30 per cent since the late 1970s.
This increase in part time is either good or bad, someone who knows more about economics can probably shed light on it. It honestly just could represent a want for increasing flexibility on behalf of the employee. We have a lot more people in education and training than say 50 years ago, those people have to dedicate a significant portion of their time to education but they might want to work on the side. It's a reasonable explanation but doesn't mean its right (til we have evidence of course).
Even among people in work, a sizeable share isn't doing much inherently useful. Most bureaucrats could go on permanent strike without any impact on national production. Meanwhile the corporate world is teeming with overpaid staff whose contribution to their employers' tangible, saleable output is approximately zero. Some entire "private" industries, automotive springs to mind, have been kept afloat entirely by government subsidy.
That is an absolutely
huge judgment to make right there. How does he define whats useful or not? Does he have any data to back it up beyond the old tired anecdote that public servants just sit on their ass? The automotive industry generated much more than we spent subsidising it. Sure, the subsidies were large but so were the benefits. Every nation on earth subsidises their industries to a degree, we have to compete in a subsidised world.
The depressing accretion of government regulation and taxation over the course of the 20th century is partly responsible for this growing dearth of jobs. Economists say higher rates of tax, mountains of regulation and excessive minimum employment conditions stifle enterprise, the desire to work, and condemn many to the dole queue.
This is just ridiculous, stuff like this peaked around the mid to mid-late end of the decade and has been declining ever since. He says "Economists say", which economists? Who? Like his entire article, a lot of it is just the conventional wisdom people believe or is totally unsourced. Out of all the rich, western, developed nations, we're one of the lowest taxed of the lot. People can complain about taxes all they like but they don't realise just how much worse it could be (and conversely, just how good we have it).


Might come back to the rest later (10mins of internet left) but its dubious whether technology is causing massive unemployment (
http://www.slate.com/blogs/future_tense/2013/09/30/technology_isn_t_taking_all_of_our_jobs.html). A lot of the jobs around in 20-30 years wouldn't have been heard of today. In the 70s few people could imagine computer programmer was a job, IT replaced a lot of jobs for sure but it also created a lot of jobs. Even machines in places like car factories need engineers to design them, maintenance technicians, programmers. More demand for machines means they need people to work on manufacturing them and obtaining the raw materials too. It isn't such a zero sum game as he might imagine. Let's also keep in mind there are so many jobs that are simply unpleasant to do and in an ideal (almost marxist utopia i guess) no one should be doing. Firefighting is dangerous as hell, its not really a job humans should be doing. Cleaning, picking up trash, even stuff like mining even though its become comparatively technological, the isolation is usually pretty big (in particular FIFO jobs).