AGW stands for anthropogenic global warming, the increase in Earth’s average temperature caused by human activities. The planet has warmed approximately 1.44°C (2.6°F) above pre-industrial levels as of 2025, driven primarily by the release of heat-trapping gases from burning fossil fuels, agriculture, and industrial processes.
How the Enhanced Greenhouse Effect Works
Earth naturally stays warm enough to support life through a process called the greenhouse effect. About half the sun’s energy passes through the atmosphere and reaches the surface, where it’s absorbed and re-emitted as infrared heat. Certain gases in the atmosphere absorb roughly 90% of that outgoing heat and radiate it back, acting like an insulating blanket. Without this natural process, the planet would be too cold to inhabit.
The “anthropogenic” part of AGW refers to what happens when humans add extra greenhouse gases to that blanket. Since the Industrial Revolution, we’ve thickened the atmosphere’s insulating layer by burning coal, oil, and natural gas on a massive scale. The result is that more heat gets trapped and less escapes to space. Atmospheric carbon dioxide, the most important of these gases, now sits at about 430 parts per million, far above the roughly 280 ppm level that held steady for thousands of years before industrialization.
Which Gases Drive the Warming
Carbon dioxide is the dominant driver, released primarily by burning fossil fuels, manufacturing cement, and clearing forests. It lingers in the atmosphere for centuries, meaning emissions from decades ago are still trapping heat today.
Methane is a more potent heat-trapper per molecule but breaks down faster, typically within about 12 years. It comes from natural gas production and transport, livestock digestion, rice paddies, and decomposing waste in landfills. Nitrous oxide, released by agricultural fertilizers, wastewater treatment, and fossil fuel combustion, is less abundant but roughly 270 times more effective at trapping heat than carbon dioxide over a century. A fourth category, fluorinated gases, includes synthetic industrial chemicals used in refrigeration, electronics manufacturing, and other applications. These are produced in small quantities but can trap thousands of times more heat than carbon dioxide and persist in the atmosphere for centuries.
How Scientists Know It’s Human-Caused
Natural factors like volcanic eruptions, ocean circulation patterns, and fluctuations in the sun’s output do influence climate. But when researchers model Earth’s temperature using only those natural forces, the models can’t reproduce the sharp warming seen since the mid-20th century. The observed warming only matches the data when human greenhouse gas emissions are included.
Scientists also use what’s called “fingerprinting.” Human-caused warming produces a specific pattern: nights warming faster than days, the lower atmosphere warming while the upper atmosphere cools, and warming concentrated in ways that match where greenhouse gases accumulate. Solar-driven warming would warm the entire atmosphere more evenly. The patterns observed in temperature records consistently match the human-caused signature, not the solar one. Studies using statistical methods like partial correlation analysis have confirmed that anthropogenic warming has a significantly positive, multiplicative effect on extreme climate events beyond what natural variability alone would produce.
What the Warming Looks Like So Far
The global average temperature in 2025 reached an estimated 1.44°C above the 1850 to 1900 baseline, according to Berkeley Earth’s analysis. That single number masks a sharp disparity between land and ocean. Land surfaces have warmed considerably faster, hitting 2.03°C above the pre-industrial average in 2025, while ocean surfaces reached 1.03°C above baseline. This gap matters because people, crops, and ecosystems on land experience more intense heat than the global average suggests.
The warming isn’t slowing down. The IPCC has estimated the rate of human-induced warming at about 0.2°C per decade. Under current international climate pledges, global warming is expected to surpass the 1.5°C threshold that was set as an aspirational limit in the Paris Agreement, even with ambitious increases in emissions cuts after 2030.
Where the Extra Heat Goes
Most of the excess heat doesn’t stay in the air. The ocean absorbs an estimated 91% of the additional energy trapped by greenhouse gases. Over the past 50 years, more than 90% of all warming on Earth has occurred in the ocean. The upper layers (down to about 700 meters) have absorbed roughly 63% of the total stored heat since 1971, while the deep ocean below 700 meters accounts for another 30%.
This ocean absorption has consequences. Warmer water expands, which is one of the two main reasons sea levels are rising (the other being melting glaciers and ice sheets). Global sea level has risen 8 to 9 inches (21 to 24 centimeters) since 1880. The rate of rise has more than doubled, from 1.4 millimeters per year through most of the 20th century to 3.6 millimeters per year between 2006 and 2015. Warmer oceans also fuel stronger storms, disrupt marine food chains, and drive coral bleaching.
Why It Matters in Practical Terms
A degree or two of average warming translates into outsized effects at the extremes. Heatwaves become more frequent and intense. Droughts last longer in some regions while rainfall events grow heavier in others. Growing seasons shift, affecting food production. Coastal communities face accelerating flooding risk as sea levels continue climbing. Arctic sea ice is shrinking, permafrost is thawing and releasing additional stored carbon, and wildfire seasons are lengthening in many parts of the world.
The core challenge of AGW is that carbon dioxide persists in the atmosphere for hundreds of years. Even if emissions stopped entirely today, the warming already locked in would continue reshaping weather patterns, ecosystems, and coastlines for generations. The speed and scale of future changes depend largely on how quickly emissions are reduced, which is why AGW sits at the center of global energy, economic, and environmental policy debates.