"It's a bit sooner than we expected," Mr Krummel told Fairfax Media. "It just rocketed up there." Cape Grim, one of the world's most important monitoring stations for gauging changes in the atmosphere. Credit:Auscape Atmospheric readings from Cape Grim, along with two stations in Hawaii and Alaska, are closely watched as they date back decades and closely track a range of pollutants from ozone-depleting chemicals to the various greenhouse gases resulting from burning fossil fuels and clearing forests. Mr Krummel said that while mostly symbolic, the 400-ppm reading "highlights the problem of rising emissions, which are increasing more rapidly than they used to be".

A report out earlier this year from the World Meteorological Organization noted atmospheric readings of CO2 at the Mauna Loa site in Hawaii rose 3.05 ppm in 2015 alone – the biggest increase in the 56 years of research. The recent surge in CO2 levels was not unexpected because of the giant El Nino event now breaking up in the Pacific. In El Nino years, global temperatures get a kick higher and droughts tend to be worse. As a result, vegetation take out less CO2 from the atmosphere. The Climate Change Commission recommends a type of emissions trading scheme that would push up prices. Credit:Michele Mossop CSIRO's Mr Krummel said the El Nino influence was evident in Cape Grim's reading with barely a dip in the past year as might otherwise be expected in spring. "This year, it's just plateaued and now it's taken off again," he said, adding the site was "probably one of the last places on earth" to remain below 400 ppm.

Melting moments: A polar bear off the coast of Svalbard, Norway. Credit:Steven Kazlowski Sites in the northern hemisphere exceeded 400 ppm from 2012 onwards. But as the region has greater seasonal variation – mostly because there is more terrestrial vegetation – CO2 concentrations dropped back below that mark each spring. Once Cape Grim gets past a short period with CO2 levels gyrating around 400 ppm, it will need a huge global effort to curb greenhouse gas emissions to push the level back down. Sun-Herald editorial illustration from May 15. Credit:David Rowe "It's not going to go back below 400 ppm for a very long time unless we get very good at mitigation," Mr Krummel said.Global CO2 levels were running at roughly 280 ppm up until about 1850 when they started to take off. (See chart below).

Climate scientists, such as David Karoly at Melbourne University, note that when other greenhouse gases, such as methane, are included, the situation is even bleaker. The so-called carbon dioxide-equivalent level that takes in the full global warming impact is now about 485 ppm. Both 2014 and 2015 were record hot years globally in data going back about 130 years. With the effect of a strong El Nino overlaying long-term trends, this year is likely to be even hotter after a scorching start. Ed Hawkins, a climate scientist at the UK's University of Reading, has constructed the following animation showing how the world has warmed in the past 166 years.

Many regions of the world are experiencing unusual warmth for May, with parts of Alaska expected to be 15 degrees warmer than average. Even Sydney is running about 5 degrees above average so far for May and Australia on the whole is headed for close to its warmest autumn on record. The Cape Grim site, meanwhile, celebrated its 40th anniversary in March with a cloud over its future because of the CSIRO job cuts. While management has promised to maintain the facility that it runs in cooperation with the Bureau of Meteorology, the number of CSIRO staff analysing the gases collected at the site is expected to be cut by about one-third from the current tally of about 30 researchers.