A significant atmospheric monitoring station in north-west Tasmania has recorded carbon dioxide measurements above 400 parts per million for the first time.

Cape Grim, on the remote north-western tip of Tasmania, is the only station analysing baseline carbon levels in the southern hemisphere.

Scientists warned last week that the world's cleanest air was nearing the 400ppm milestone.

Lead scientist Paul Krummel, from the CSIRO, said the landmark CO2 figure was concerning.

"It's probably one of the last places on Earth to actually reach this milestone, and it just highlights the ever-increasing levels of CO2 in the atmosphere," he said.

The key impacts of carbon dioxide is warming the atmosphere and acidifying the oceans.

"Globally, this is going to contribute to rising temperatures, which we are already seeing in the atmosphere," Mr Krummel said.

"It's another milestone that we've reached where it's indicating that the health of the planet is probably in trouble."

Mr Krummel said initial analysis last year predicted Cape Grim would not record such high measurements until June or July.

"It's certainly come earlier than I expected ... most likely due to El Nino effects on the CO2," he said.

The El Nino weather pattern causes hotter and drier conditions, preventing the growth of plants that would suck up carbon dioxide.

The Cape Grim Baseline Air Pollution Station is operated by the CSIRO and the Bureau of Meteorology.

It measures the presence of greenhouse gases like carbon dioxide, methane and nitrous oxide and ozone-depleting gases like aerosols.

The station samples some of the cleanest air on the planet, derived from the Southern Ocean and unaffected by any nearby urban pollution.

When measurements began in 1976, carbon dioxide levels were slightly more than 300 parts per million.