Politicians are known for displaying optimism in troubled global times, and Barack Obama is no exception.

This week, in an interview with Prince Harry, the former US president said he believed the problems faced by the world indeed were solvable, and he did not shirk in the face of them.

"If you had to choose a moment in human history in which you'd want to be born you'd choose today because the fact is that the world is healthier, wealthier, better educated and more tolerant, more sophisticated and less violent," Mr Obama said.

He sounded entirely presidential — and, in one sense, he still is.

The chaos that has surrounded Donald Trump's first year in office has presented his predecessor with an opportunity that no president has received since Franklin Roosevelt in the 1940s.

Mr Obama, in effect, is serving a third term as president because the wildly unpredictable Mr Trump simply isn't playing the part that Americans, and the world, have come to expect from a US leader.

Of course, Mr Obama can't actually serve another term. The 22nd Amendment to the US Constitution, ratified in 1951, limits a president to two terms.

It was enacted after Roosevelt served an unprecedented four terms, dying in office in 1945 only months after his final term began.

And Mr Obama hasn't actually anointed himself as a de facto leader.

In fact, he told Prince Harry he felt a sense of "serenity" in watching Mr Trump's inauguration, satisfied he had accomplished a series of goals such as righting the American economy after a deep recession, and establishing national health care protection.

But it is clear Mr Obama is acting in a way at odds with the understated behaviour displayed by modern former presidents who have rarely tread so soon or deeply into national and global debates.

He enjoyed his time off, but duty called

Mr Obama's unique role has taken time to evolve. After leaving the Oval Office, he set off on a long vacation which included kitesurfing with Richard Branson in the British Virgin Islands, spending time on his memoir in French Polynesia and dining out at a hip Mexican restaurant in New York with his oldest daughter Malia before she headed to Harvard University.

From late January through late March, his @barackobama Twitter account was barely active, save for a couple of re-tweets of posts from his fledgling foundation and a remembrance of the late musician Chuck Berry.

Barack Obama clearly enjoyed his time off. ( Supplied: Jack Brockway )

That changed with the London attack on March 22, in which four people were killed and more than 50 injured when an attacker drove his car into pedestrians on Westminster Bridge and then plunged onto the grounds of the Palace of Westminster.

Mr Obama tweeted:

"My heart goes out to the victims and their families in London. No act of terror can shake the strength and resilience of our British ally."

Certainly that was acceptable ex-president sympathy, and a similar response by the sitting US President towards America's closest ally might have seemed in order.

Instead Mr Trump criticised British officials over their response to terrorism, along with a fierce reaction to what has gone on at home.

He uses social media like a boss

This summer Mr Trump set social media on fire and prompted protests around the country with his failure to denounce the marches by white supremacist groups in Charlottesville, Virginia, including an incident that killed a young woman.

In what was unusual during a national crisis, he waited two days after the August marches to make any kind of statement, and then, in a rambling series of comments, he initially refused to denounce the hate groups, saying there were people on both sides who were "very violent".

By contrast, Mr Obama's response came swiftly. Only hours after the most violent incident, he posted a photo and quoted Nelson Mandela in what became the most liked tweet in the history of Twitter.

Loading

Charlottesville, in fact, appeared to rid Mr Obama of any reluctance to comment on national and international affairs, and to eliminate any deference he might have felt as an ex-president toward Mr Trump.

In December, before the Economic Club in his home town of Chicago, Mr Obama made comments that might otherwise have sent historians' eyebrows to the ceiling, save for Mr Trump's year of outrageousness.

In front of a ballroom filled with 2,800 luncheon attendees, Mr Obama gave what could easily be perceived as a warning of what Mr Trump might affect, if unstopped.

"Right now we're seeing a collision of cultures that we're not accustomed to," Mr Obama said.

"And sadly there is something in all of us that looks for simple answers when we're agitated and insecure. Far too often what we look for is asserting our superiority over someone else."

He continued:

"The one thing I always want to emphasise to people is not to take for granted the institutions and norms and values that we've built, because it's not so much that they're fragile, but they're reversible."

Later Mr Obama said: "You have to tend to this garden of democracy or things can fall apart fairly quickly. And we've seen societies where that's happens."

He went on: "Now, presumably, there was a ballroom here in Vienna in the late 1920s or '30s that looked and seemed as if it, filled with the music and art and literature that was emerging, would continue into perpetuity. And then 60 million people died. An entire world was plunged into chaos. So you got to pay attention — and vote," Mr Obama said

He's 'young and can do a lot'

As well as venues for his speeches, Mr Obama has what can be considered the ultimate keeper of his flame.

Loading

Every time a news event happens, former White House photographer Pete Souza posts a corresponding photo on his Instagram account, featuring Mr Obama in a presidential setting (Souza coincidentally is the author of a new book of Obama photos).

But there are new photos of Mr Obama too, with French President Emanuel Macron, whom Mr Obama endorsed in France's elections earlier this year.

With the Dalai Lama, who told him, "You are young and can do a lot".

And with Prince Harry, who snagged the first in-depth interview Mr Obama has given since leaving office.

Their conversation included everything from motivating young people to whether Mr Obama preferred Queen or The Queen (of course, given his interviewer, Mr Obama chose the latter).

It is widely expected the Obamas will snag their own invitation to Prince Harry's wedding next May, which may be one of the most visible events of 2018.

Doubtless, the former president and first lady will garner the kind of attention for which Mr Trump, so eager to be seen as in charge, can only hunger.

Micheline Maynard is an author, broadcaster and former New York Times business journalist.