There was a 2009 analysis in Science that found when CO2 levels were sustained in the 400 to 500 ppm range some 15 to 20 million years ago, it was 5°F to 10°F warmer globally, and seas were also 75 to 120 feet higher.

Despite the best efforts of the Trump administration to ignore or contradict scientific reality, carbon dioxide levels continue to soar far outside the bounds of what humans have ever experienced.

Monthly levels of heat-trapping CO2 peaked at nearly 410 parts per million (ppm) in May, the month that levels peak each year at NOAA’s Mauna Loa Observatory in Hawaii.

This is worrisome for several reasons. First, CO2 is the primary heat-trapping greenhouse gas humans spew into the air, mainly by burning fossil fuels. The higher CO2 levels are, the greater the warming.

Second, CO2 levels or concentrations in the atmosphere will keep rising until the world has reduced global CO2 emissions by more than 80 percent from current levels. CO2 concentrations are like the water levels in a bathtub, and annual emissions are like the flow from the faucet. The water going down the bathtub drain is like CO2 sinks such as the oceans, forests, and soils.

Third, the annual rise in CO2 levels has been speeding up, as long predicted by climate science. Indeed 2015 and 2016 were the two biggest annual jumps in CO2 levels on record.

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The last alarm bells went off this week, when scientists announced that the Halley Research Station in Antarctica, as well as a monitoring post at the geographic South Pole, both located amid the most pristine air on the planet, have now passed the 400 ppm mark.

In other words, at every location on Earth where scientists routinely monitor carbon dioxide levels, we are now entering uncharted territory for humanity.

For reference, carbon dioxide levels were at about 280 ppm at the start of the industrial revolution, when humans began burning fossil fuels for energy. They have marched upward at increasing rates ever since.

According to Pieter Tans, the lead scientist for the Global Greenhouse Gas Reference Network, 400 ppm is the highest level that carbon dioxide levels have reached in at least 4 million years.

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