Earth is getting warmer because human activities, primarily burning fossil fuels, are releasing gases that trap heat in the atmosphere. Global average surface temperature has risen about 2°F (1°C) since the pre-industrial era, and atmospheric carbon dioxide hit a record 426.9 parts per million in May 2024, the highest level in millions of years. That number is climbing faster than ever, with the two-year jump from 2022 to 2024 being the largest on record.
How Greenhouse Gases Trap Heat
The sun warms Earth’s surface, and the surface radiates that energy back toward space as infrared radiation. Certain gas molecules in the atmosphere can absorb that outgoing energy instead of letting it escape. Carbon dioxide is one of the most important. When a CO2 molecule absorbs an infrared photon, it vibrates with the extra energy. It then either re-emits another infrared photon in a random direction (often back toward Earth) or collides with neighboring gas molecules and transfers the energy to them, speeding them up. Faster-moving molecules mean a warmer atmosphere. That’s the greenhouse effect in a nutshell.
Not every gas does this. Nitrogen and oxygen make up more than 90% of the atmosphere, but their simpler molecular structures can’t absorb infrared radiation. CO2, methane, and nitrous oxide have more complex shapes that allow them to vibrate at infrared wavelengths, which is why even relatively small concentrations of these gases have an outsized effect on temperature.
Which Gases Matter Most
Carbon dioxide gets the most attention because we emit enormous quantities of it and it lingers in the atmosphere for centuries. But it’s not the only heat-trapping gas. Methane is 27 to 30 times more potent than CO2 over a 100-year period, though it breaks down after roughly a decade. Nitrous oxide is 273 times more potent than CO2 and persists for more than 100 years on average. Both are released in large amounts by agriculture, fossil fuel extraction, and industrial processes.
CO2 remains the dominant driver of warming simply because of volume. The sheer quantity released each year, combined with its long atmospheric lifespan, means it accumulates steadily. The concentration has climbed from about 280 parts per million before industrialization to nearly 427 ppm today, a roughly 50% increase.
Where the Emissions Come From
Burning fossil fuels is the largest source. In the United States in 2023, petroleum accounted for 47% of energy-related CO2 emissions, natural gas contributed about 37%, and coal added another 16%. Globally, the pattern is similar: coal, oil, and natural gas combustion for electricity, transportation, heating, and manufacturing release the bulk of CO2 entering the atmosphere each year.
Land use is the other major contributor. Agriculture, forestry, and other land use accounted for 13 to 21% of total human-caused greenhouse gas emissions between 2010 and 2019, according to the IPCC’s most recent assessment. Deforestation alone is responsible for about 45% of emissions within that land-use category. When forests are cleared, the carbon stored in trees is released, and the land loses its ability to pull CO2 back out of the air.
It’s Not the Sun
A common question is whether the sun could be driving the warming. NASA tracks both solar energy output and global surface temperatures, and the two have diverged sharply since about 1980. Solar activity has been flat or slightly declining over recent decades while global temperatures have risen markedly. The sun’s output simply hasn’t changed enough to explain what’s happening. Scientists consider it extremely unlikely that the sun has caused the warming trend observed over the past half-century.
The Ocean’s Role as a Heat Sink
About 90% of the excess heat generated by the enhanced greenhouse effect has been absorbed by the ocean rather than staying in the atmosphere. Water has a high heat capacity, meaning it can soak up enormous amounts of energy. This has actually slowed the warming we feel on land, but it comes at a cost: rising ocean temperatures fuel stronger storms, accelerate ice sheet melting, cause coral bleaching, and raise sea levels through thermal expansion.
The ocean’s ability to keep absorbing heat isn’t unlimited. As water warms, it becomes less efficient at taking up both heat and CO2 from the atmosphere, which means a greater share of future emissions will stay in the air and accelerate surface warming.
Feedback Loops That Amplify Warming
Once warming starts, several natural processes push temperatures even higher. The most significant is the ice-albedo feedback. Ice and snow are bright white and reflect a large portion of incoming sunlight back to space. As warming melts Arctic sea ice, it exposes dark ocean water underneath, which absorbs far more solar energy. That extra absorbed energy causes more warming, which melts more ice, and the cycle continues.
Research using climate simulations shows that without large-scale sea ice loss, Arctic warming and its associated feedbacks are greatly reduced. In the real world, the Arctic is warming roughly two to four times faster than the global average, a phenomenon called Arctic amplification. In summer, newly exposed ocean surfaces absorb solar radiation. In winter, the heat stored in those waters is released back into the atmosphere, keeping Arctic temperatures elevated year-round.
Other feedback loops include increased water vapor (a warmer atmosphere holds more moisture, and water vapor is itself a greenhouse gas) and thawing permafrost, which releases stored methane and CO2 from previously frozen ground.
How Fast Warming Is Happening
Earth’s temperature has risen by an average of 0.11°F (0.06°C) per decade since 1850. That pace has accelerated in recent decades. The roughly 2°F (1°C) of total warming may sound modest, but it represents a massive increase in the total heat energy circulating through the atmosphere, oceans, and ice sheets. Small shifts in the global average translate into large changes in weather extremes, sea levels, and ecosystems.
At the current rate, global temperatures are projected to reach 1.5°C (2.7°F) above pre-industrial levels around 2040. The IPCC’s Sixth Assessment Report places this threshold within the near term, between 2021 and 2040. In fact, the World Meteorological Organization estimated in 2023 that there was a 66% chance of at least one individual year exceeding 1.5°C between 2023 and 2027. Crossing that line in a single year is different from a sustained average, but it signals how close we are to a benchmark that scientists have long flagged as a critical tipping point for coral reefs, coastal flooding, and extreme heat events.
The CO2 already in the atmosphere will continue warming the planet for decades regardless of what happens next, because the gas persists for so long. How much additional warming occurs depends almost entirely on how quickly emissions are reduced going forward.