Carbon dioxide in the Earth’s atmosphere has hit levels not seen in at least four million years, with scientists warning that the effects of climate change will only worsen in Australia and around the world without swift action to curb emissions.

Figures released by the United States government’s National Oceanic and Atmospheric Administration’s (NOAA) Mauna Loa Atmospheric Baseline Observatory, situated on the slopes of the Mauna Loa volcano in Hawaii, show that CO2 levels in the atmosphere peaked at 421 parts per million in May – 50 per cent higher than before the industrial revolution began in around 1760.

Pre-industrially, CO2 levels were “consistently around 280 parts per million for almost 6,000 years of human civilisation,” NOAA officials said in a statement on Friday.

Since then humans have generated more than 1.3 trillion tonnes of CO2 pollution, “much of which will continue to warm the atmosphere for thousands of years”, they said.

CO2 levels are “now comparable to the Pliocene Climatic Optimum, between 4.1 and 4.5 million years ago”, the statement said, a time during which sea levels were between five to 25 metres higher, and “studies show that large forests occupied today’s Arctic tundra”.

While NOAA started measuring atmospheric CO2 at its Mauna Loa observatory in 1958, ice core data collected by Scripps Institution of Oceanography is used to measure CO2 levels dating back to pre-industrial times.

The ice core records show that atmospheric CO2 was approximately 278 parts per million between 1750-1800.

“The science is irrefutable: humans are altering our climate in ways that our economy and our infrastructure must adapt to,” said NOAA Administrator Rick Spinrad.

“We can see the impacts of climate change around us every day.”