evidence: Before humans began burning fossil fuels, naturally occurring greenhouse gases helped to make Earth’s climate habitable. Without them, the planet’s average temperature would be below freezing. So we know that even very low, natural levels of carbon dioxide and other greenhouse gases can make a huge difference in Earth’s climate. Today, CO2 levels are higher than they have been in at least 3 million years. And although they still account for only 0.04% of the atmosphere, that still adds up to billions upon billions of tons of heat-trapping gas. For example, in 2019 alone, humans dumped 36.44 billion tonnes of CO2 into the atmosphere, where it will linger for hundreds of years. So there are plenty of CO2 molecules to provide a heat-trapping blanket across the entire atmosphere. [...] Plants, the oceans, and soil are natural carbon sinks — they remove some carbon dioxide from the atmosphere and store it underground, underwater, or in roots and tree trunks. Without human activity, the vast amounts of carbon in coal, oil, and natural gas deposits would have remained stored underground and mostly separate from the rest of the carbon cycle. But by burning these fossil fuels, humans are adding a lot more carbon into the atmosphere and ocean, and the carbon sinks don’t work fast enough to clean up our mess.