As we noted ahead of the declaration of the first recording of 400ppm in the southern hemisphere, the primary greenhouse gas increase carried not much more global warming significance than, say, 399 or 401 ppm.

But as with other markers - such as Australia's foreign debt passing a $1 trillion - the 400 ppm tally helps focus our attention.

"People react to these things when they see thresholds crossed," David Etheridge, a CSIRO principal research scientist, told us. Even more upbeat was Paul Fraser, the CSIRO scientist who had helped set up Cape Grim 40 years ago. He told 9news.com.au that passing the landmark would be a "psychological tipping point".

Other warning signs - such as the 1 degree warming point reached last year - did at least spur nations to agree at the Paris Climate Summit that temperature increases should be limited to 1.5-2 degrees, even if national policies made 3 degrees more likely.

And so, with CO2 concentrations marching past 400 ppm, when should we start worrying about global warming?

"About 30 years ago," is the blunt answer from David Karoly, a climate scientist with Melbourne University. That's when CSIRO and other scientists declared we had a problem.

"But we shouldn't give up, either," he told me this week. "The worry - and the [climate] action - should now be increased."

During the three decades since, we have poured trillions of dollars into fossil fuel extraction, refining and consumption and a much smaller sum into renewable energy that will have to replace coal, oil and gas - and soon.

Yet clean energy technologies continue to make rapid advances, including the latest breakthrough at the University of NSW lifting solar efficiency levels to above 34 per cent.

Worries mount

When it comes to extreme weather, there's certainly been a lot to be concerned about in 2016.

Each of the last 12 months is now the hottest on record for that month, with April adding the recent string of beating-by-biggest-margin months.

(See NASA's chart showing temperatures were 1.11 degrees warmer than the 1951-80 average, with large areas in the north particularly warm, while Australia had its second-warmest April.)

Among the climate-related threats, Canada's forest fire season started about a month early. One huge blaze near Alberta's tar sands forced the evacuation of about 88,000 people from the city of Fort McMurray.

The monster El Nino, riding on top of about one degree of background warming, has triggered widespread bleaching of the corals around the world, including our cherished - but apparently expendable - Great Barrier Reef.

More than half the corals in the largely pristine northern end are now dying or dead the reef's Marine Park Authority chief Russell Reichelt told the Senate earlier this month.

On the plus side

However, a dose of of warming can be welcomed where it take off the chill - as in parts of the US - or in Sydney, where summer feels like never lost of its grip even with winter just weeks away.

And all that extra CO2 is leading to a "greening" of the planet with about one-quarter of humans' carbon emissions being taken up by plants, a recent Nature paper found.

But many other changes are far from benign - at least according to the preservation of many existing eco-systems.

Less glamorous than corals, the kelp forests off eastern Tasmania are being destroyed by warm water species swept south by the strengthening and warming East Australian Current. Similarly, the massive dieback of mangrove forest in the Gulf of Carpentaria apparently linked to extreme weather got scant national media coverage.

And, as for the high Arctic where the remarkably prolonged above-average temperatures have led to record-low levels of sea ice, changes are likely to be further from our daily minds.

But less sea ice means less light reflected back to space and more heat absorbed by the oceans - and so the cycle builds and so should our concern.

Spiral warning

In case you missed it, Ed Hawkins, a climate scientist at the UK's University of Reading, came up with the illustration below to help us appreciate the spiralling warming trend since the middle of the 19th century.

By that gauge, when the temperature increase breaks past the 1.5 degrees warming - perhaps in coming months before the El Nino in the Pacific fully stops giving back heat to the atmosphere -  will be sufficiently concerned then to act?

Climate scientists, such as Professor Karoly, know globally warming limits agreed at international summits to prevent dangerous climate change - such as 1.5-2 degrees range agreed in Paris - are arbitrary in a similar way to 400 ppm.

"You'd be hard-pressed to find scientists agreeing that 1.5 degrees would be manageable for our reef," he said. "Look at what's happening to the reef this year - and that's with 1 degree warming."

Recent work by his team has identified that on current temperature and carbon emissions trends, the Great Barrier Reef will be hit by bleaching events every second year by 2035.

Other eco-systems, such as alpine ones, are also being affected as species shielded from predators by cool temperatures have literally nowhere to hide.

Big one to worry about

Natural fluctuations mean temperature spikes such as we have seen over the past year can recede - at least partly.

As this year's El Nino in the Pacific lurches towards becoming a La Nina - when equatorial winds turn back to be mostly west-ward blowing and strengthen - we can expect the run of record temperature reading to be broken.

However with greenhouse gas concentrations still rising, the heat we are trapping in the Earth's biosphere - the land, air and sea - will keep on increasing.

And for those climate sceptics who thought the first decade of the 21st century marked a slowdown in temperature increases - the so-called "warming hiatus" - their case for holding off carbon emission cuts is about to get harder to make.

As Fairfax Media noted back in late 2014, a long-lived climate phenomenon named the Pacific Decadal Oscillation, has begun to switch to its positive phase. In fact, climate experts have in the past few days detected it hitting a new high.

"You're playing catch up - now the natural factors want to make the planet warmer," David Jones, head of climate analysis with the Bureau of Meteorology, said.

"Historically when you've had a positive PDO, global temperatures are higher and the rate of warming tends to be quicker."

Some scientists view PDOs as a slower version on the El Nino pattern that forms in the Pacific every three to seven years. Others see it as a separate phenomenon.

"In reality, it's probably a bit of both," Dr Jones said.

Such a PDO switch - it is an 11-year rolling average - would indicate oceans will become less of a sink for the planet's surplus heat - and may even give more of it back to the atmosphere.

That means global conditions will favour more El Nino events but also when La Ninas come, they will tend to be more extreme.

In other words, we could soon have a lot more to worry about.