The planet has set a significant – and unwelcome - landmark with the concentration of heat-trapping carbon dioxide in the atmosphere passing 400 parts per million (ppm) for the first time in more than 3 million years.
While the 400 level had been reached at some measuring sites last year, Thursday saw the daily average top that rate at Hawaii's Mauna Loa station for first time.
We have no time to lose
"Humanity has never been here before," John Connor, chief executive of The Climate Institute, said in a statement. "We are in dangerous and uncharted territory, with little time to ensure a safe and sustainable future."
A report on the website of the US National Oceanic and Atmospheric Administration said the concentration of carbon dioxide had increased each year since the Mauna Loa site began tracking the greenhouse gas in 1958. In the late 1950s, the annual increase of the gas was 0.7 ppm and it has since tripled to 2.1 ppm a year during the last decade, NOAA said.
"The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration," said NOAA senior scientist Pieter Tans, according to the website.
Climate scientists in Australia say global warming resulting from the increased concentration of greenhouse gases is already contributing to the country's extreme weather conditions, including to this past summer's record heatwaves.
Since December, Australia has posted its hottest day, hottest month and hottest summer in more than a century of records.
The unusually warm conditions over the country have persisted far into autumn with the departure from the norm over the past week greater than during the blisteringly hot first week of January when national average maximums topped 39 degrees for seven consecutive days.
Nationally, the week ending May 8 saw average maximum temperatures 5.35 degrees above normal compared with 4.53 degrees during the week ending January 8, according to the Bureau of Meteorology.
Average minimum temperatures for the week were 3.43 degrees above average compared with an anomaly of 2.04 degrees during that early week in January.
"Obviously the absolute temperatures are different, so the impact is different," Karl Braganza, manager of climate monitoring at the bureau, said. "In terms of climate anomalies, it's really a standout event."
Cities such as Melbourne and Adelaide posted well above average temperatures in the past week, while Mount Wellington near Hobart on Friday smashed its May maximum temperature record by 2.4 degrees.
Sydney, meanwhile, remains on course to post days of 20 degrees or warmer until at least next Friday. If the forecasts are realised, that would make 27 consecutive days of such weather, eclipsing the previous record of 25 days this late in the season set in 1987.
The string of heatwaves have been notable in their geographic spread, with cold fronts effectively pushed south of the continent, taking the rain with them.
"You're getting most of the country under a single system," Dr Braganza said. "I'd be surprised if we've had more than one other summer, if any, where you've had a sequence of heatwaves that have done the same thing."
Carbon dioxide levels averaged about 280 ppm before the industrial revolution and temperatures have risen about 1 degree since then as the concentration of greenhouse gases has increased.
Scientists estimate that 450ppm levels of the gas will see temperatures warm by at least another degree. That concentration level may be reached by 2037, The Economist magazine reported this week.
"Today's rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended," NOAA said on its website.
''It feels like the inevitable march toward disaster,'' Maureen E. Raymo, a Columbia University earth scientist, told the New York Times.
Clock runs down
Both Labor and the Coalition are committed to reducing Australia's annual emissions by at least 5 per cent on 2000 levels by 2020, and may aim for deeper cuts if other nations set more ambitious goals.
"Because it's in our national interest to avoid further dangerous warming, Australia joined the US, China and over 170 other countries to commit to avoid a 2°C warming," Mr Connor at the Climate Institute said. "We have no time to lose."
The 400-ppm mark "is one more clear alarm bell which we ignore at great risk, because, when it comes to extreme weather and climate impacts, 'we ain't seen nothing yet'," he said.