Image copyright Getty Images Image caption Mauna Loa instruments maintain the reference measurements of CO2 concentrations

A big spike in atmospheric carbon dioxide levels means the greenhouse gas is about to pass a symbolic threshold.

Twenty-sixteen will very likely mark the first time the concentration of CO2, as measured atop Hawaii's famous Mauna Loa volcano, has been above 400 parts per million for the entire year.

The forecast is from the UK Met Office.

It says carbon dioxide levels have seen a surge in recent months as a result of the El Niño climate phenomenon, which has warmed and dried the tropics.

These conditions not only limit the ability of forests to draw down CO2 from the atmosphere but also trigger huge fires around the globe that inject extra carbon into the air.

This means the instruments at Mauna Loa, which maintain the benchmark record of CO2, are unlikely to see any month in 2016 where the concentration dips below 400ppm (that is, 400 molecules of CO2 for every one million molecules in the atmosphere).

Media playback is unsupported on your device Media caption Richard Betts: "With El Nino we get large forest fires in the tropics"

To be clear: the value has no particular significance for the physics of the climate system; it is just a number. But it has resonance because the last time CO2 was regularly above 400ppm was three to five million years ago - before modern humans existed.

"There's nothing magical about this number," said Prof Richard Betts at the Met Office's Hadley Centre in Exeter.

"We don't expect anything suddenly to happen. It's just an interesting milestone that reminds us of our ongoing influence on the climate system," he told BBC News.

The usual trend seen at the volcano is for the CO2 levels to rise in winter months and then to fall back as the Northern Hemisphere growing season kicks in. Last year most months were above 400 with only three below.

Ordinarily, one expects the carbon dioxide concentration to rise by an average of about 2ppm per year, but the team expects a record rise in 2016 of 3.15ppm, plus or minus 0.53ppm.

The scientists used a seasonal climate model to predict sea-surface temperatures in the Eastern Pacific - where the El Niño shows itself most obviously - and then linked these to a statistical relationship with CO2 to generate a picture of what levels would probably look like across the calendar year.

This gives an average for 2016 of 404.45, with a September low of 401.48 (again with errors of plus or minus 0.53ppm).

The team has already had success in forecasting the 2016 high, recorded in May, of 407.7ppm. The group had predicted 407.57.

"It's important to note that this year's rise in CO2 is bigger than the last El Niño, in 1997/8, because human emissions have gone up by 25% since then. So, it's the natural effect on top of the increasing human effect," said Prof Betts.

For the future, it is unlikely that CO2 concentrations will come down again at Mauna Loa within at least a generation, even if the commitments to constrain emissions agreed at the Paris climate summit last year are fully implemented. The Earth system responds only very slowly to change.

"Elsewhere on the Earth's surface, we will see values below 400. At Barrow in Alaska, for example, you get a much larger seasonal cycle," the meteorological agency's head of climate impacts research explained.

Image copyright Scripps Institution of Oceanography

The CO2 time series at Mauna Loa is recorded by two independent groups. One is associated with the US National Oceanographic and Atmospheric Administration (Noaa); the other is curated by California's Scripps Institution of Oceanography.

The latter's graph of ever-rising carbon dioxide levels has become known as the "Keeling Curve", after the man who first started taking the measurements in 1958 - Charles Keeling. Back then the ppm value was around 315.

Today, Keeling's son, Ralph, continues his father's work, and is a co-author on the Met Office paper.

"Back in September last year, we suspected that we were measuring CO2 concentrations below 400 ppm for the last time," he said in a statement. "Now it is looking like this was indeed the case."

Scientists contacted by the BBC who were not connected with this study said that if there was a sharp La Niña (the opposite to El Niño), which exhibits a cooling of waters in the Eastern Pacific, it would suppress CO2 levels, but perhaps not sufficiently to take the concentration under the mark again.

"There is a possibility we may see 400ppm again but it seems unlikely," said Prof Gavin Foster from the University of Southampton.

He wrote a blog last year in which he considered the implications of returning to the climate conditions of the Pliocene - the last time Earth had 400ppm.

"The world was a different place back then - global temperatures around 3C warmer than the pre-industrial and sea-levels were around 20m higher (due to retreat/collapse of the West Antarctic and Greenland ice sheets).

"With global temperatures likely to top 1C above pre-industrial levels [in 2015] and CO2 now likely to stay above 400pppm, it appears we are well on the way to a Pliocene-like future."

The Met Office study is published in the journal Nature Climate Change.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos