In 2015, the world got together and signed the Paris Agreement, pledging to avoid the worst effects of human-caused climate change by limiting warming to 1.5°C above pre-industrial levels.

Four years on and the UN Secretary-General has warned we are not on track to do so, while the carbon-tracking sensors reveal we broke yet another record over the weekend – on Saturday, the atmospheric concentration of carbon dioxide (CO₂) recorded at the Mauna Loa Observatory, Hawaii, reached 415.26 parts per million (ppm). This is the first time CO₂ levels have breached 415 ppm in the entirety of human history. 

"This is the first time in human history our planet's atmosphere has had more than 415ppm CO₂," meteorologist Eric Holthaus tweeted.

"Not just in recorded history, not just since the invention of agriculture 10,000 years ago. Since before modern humans existed millions of years ago.

"We don't know a planet like this."

Atmospheric CO₂ levels have climbed 140 ppm since the Industrial Revolution – in 1700, they were at 275 ppm – and 5 ppm in the past two years alone, making levels higher than they have been for 800,000 years or more. At this rate, some predict we will hit 1,000 ppm by the end of the century.

For the record, the last time levels were at 1,000 ppm, Antarctica had forests while annual temps in Western Europe and New Zealand were 10 to 15°C higher than they are today. This was 56 to 48 million years ago, during the early Paleogene period. 

Fast forward 45 million years and the planet was in the midst of the Pliocene epoch. Then, CO₂ levels are thought to have been between 310 to 400 ppm, CNN reports. Aka lower than what we are currently seeing. And yet, temperatures were some 2 to 3°C higher than they are today. Sea levels were 25 meters (82 feet) higher.

If we continue to burn fossil fuels and expel greenhouse gases (like CO₂) into the atmosphere at current levels, experts warn we could make Earth uninhabitable. Weather will become more extreme, the oceans will boil, and biodiversity will plummet. 

A landmark report published last year found that to avoid the most catastrophic results of climate change, we must limit warming to 1.5°C above pre-industrial levels. 

The good news is it's not too late. But we have to act now.