According to a recent National Oceanic and Atmospheric Administration (NOAA) report, the last time carbon dioxide levels were this high was 3 million years ago "when temperature was 2°–3°C (3.6°–5.4°F) higher than during the pre-industrial era, and sea level was 15–25 meters (50–80 feet) higher than today."

That period, the Pilocene Era, is unrecognizable from today. Giant camels walked around on the ice-free land above the Arctic Circle, as NBC News reported. Earth first passed the 400 million parts per million of CO2 in the atmosphere in 2013. Rather than take it as a dire warning, we have become inured to that level of concentration and have seen it rise slightly in subsequent years. In 2018, the concentration was 407.4 parts per million (ppm), according to NOAA. This year, CO2 concentrations are predicted to peak at 417 ppm, according to NBC News.

That level will put humans in new, unfamiliar territory. "For millions of years, we haven't had an atmosphere with a chemical composition as it is right now," said Martin Siegert, co-director of the Grantham Institute at Imperial College London, to NBC News. In fact, just two weeks ago, on Feb. 10, NOAA's Mauna Loa Observatory, an atmospheric baseline station in Hawaii, recorded the daily average of CO2 levels on as 416.08 parts per million, according to Common Dreams. Carbon Dioxide concentrations are an effective measure of how many fossil fuels we are burning. Coal and crude oil contain carbon that plants have pulled out of the atmosphere through photosynthesis over millions of years. However, in short order, human activity has returned that trapped carbon back into the atmosphere, as NOAA reported. "We've done in a little more than 50 years what the earth naturally took 10,000 years to do," said Siegert to NBC News.

Elevated concentrations of carbon dioxide are a hallmark of the climate crisis since they are associated with higher temperatures, melting ice and sea level rise, among other effects. University of Exeter geography professor Richard Betts, head of the climate impacts division at the UK's national weather service, expects this year's CO2 concentrations to be 10 percent higher than normal, with one or two percent of that carbon rise attributed to the Australia wildfires, as NBC News reported. The fires, which raged for nearly five months, released about 900 million tons of carbon dioxide into the atmosphere.