Levels of greenhouse gases continued to climb at steep rates, reaching a high not seen since well before the history of human civilization, according to NOAA data.

Why it matters: The Earth's atmosphere now contains more planet-warming gases than it has at any other point since at least about 4.3 million years ago.

• The findings may be a warning sign for the trajectory of modern-day climate change.
• During that most recent comparable warm period, known as the Pliocene epoch, sea levels were about 75 feet higher than they are today, and global average temperatures were about 7°F higher than in preindustrial times.

Inside the room: NOAA is one of the global centers that monitors greenhouse gases, chiefly carbon dioxide, methane and nitrogen oxide.

• Scientists found that 2023's rate of increase did not exceed the rate from 2022, but that it did continue the rapid rises observed during this decade.
• "As these numbers show, we still have a lot of work to do to make meaningful progress in reducing the amount of greenhouse gases accumulating in the atmosphere," NOAA Global Monitoring Laboratory director Vanda Grubišić said in a statement.

Context: Atmospheric measurements of greenhouse gases are the true test of whether and how successfully the global community is taking action to slow, stop and potentially reverse global warming.

• Despite more than three decades of worldwide negotiations to cut emissions, including a landmark agreement in Dubai last year, atmospheric concentrations keep growing. It's cranking the planetary thermostat ever higher.
• Climate change is already having major global repercussions in the form of extreme weather events, shifting ecosystems, sea level rise, coral bleaching and more.

By the numbers: The global surface concentration of CO2 during 2023 was 419.3 parts per million (ppm), which was up 2.8 ppm from 2022. This was the 12th straight year that CO2 increased by more than 2 ppm.

• Atmospheric CO2 is now more than 50% higher than during pre-industrial levels, NOAA found.
• Global CO2 emissions from fossil fuels hit a record high in 2023, studies have shown.

What they're saying: "The 2023 increase is the third-largest in the past decade, likely a result of an ongoing increase of fossil fuel CO2 emissions, coupled with increased fire emissions possibly as a result of the transition from La Niña to El Niño," said Xin Lan, a scientist who works with GML's data.

Between the lines: In addition to CO2, another concern for climate change is methane emissions. Atmospheric methane lasts in the air for far shorter timescales compared with CO2, but it is an especially potent heat-trapping agent.

• The 2023 methane growth rate that NOAA found was below recent records but still the fifth highest rate since 2007.
• Methane, which comes from microbial processes, fossil fuel operations and other sources, is now more than 160% more abundant than before the industrial revolution.
• An NOAA study published last year found that much of the recent increase in methane can be traced to livestock, agriculture, waste and wetlands, with fossil fuel infrastructure also contributing.
• However, there is still considerable uncertainty regarding the sharp increase in methane concentrations in recent years.

The bottom line: For those trying to reduce the severity of climate change, the trends are still going in the wrong direction.