Carbon dioxide acts like the Earth's thermostat: The more of it in the air, the warmer the planet. In 2023, global levels of the greenhouse gas rose to 419 parts per million, about 50 percent higher than before the Industrial Revolution. That means there are about 50 percent more carbon dioxide molecules in the air than there were in 1750.

As carbon dioxide builds up in the atmosphere, it traps heat and warms the planet. Every additional amount of carbon dioxide in the atmosphere contributes to more warming, which is why climate scientists emphasize the need to reach zero emissions.

Carbon dioxide levels are currently rising at a near-record rate. Last year marked the fourth-highest annual increase in global carbon dioxide levels, according to data released earlier this month by the National Oceanic and Atmospheric Administration's Global Monitoring Laboratory. (Aatish Bhatia, more at nytimes.com)