Earth’s Energy Balance
Earth is warmed by sunlight and cools by sending heat back to space as infrared radiation, which is invisible heat energy. Over the long run these two flows are roughly in balance, and that balance sets the planet’s average temperature. If something reduces the heat escaping to space, or increases the sunlight absorbed, the planet warms until a new balance is reached.
Climate change science is largely about what has been changing that balance and how we know.
The Greenhouse Effect
The greenhouse effect is a natural process, and life as we know it depends on it. Sunlight passes mostly through the atmosphere and warms the surface. The warm surface radiates infrared energy upward. Certain gases in the air, including water vapor, carbon dioxide, methane and nitrous oxide, absorb some of that infrared and re-emit it in all directions, including back toward the ground. This slows the escape of heat to space.
Without these gases, Earth’s average surface temperature would be well below freezing, roughly -18 degrees Celsius (about 0 degrees Fahrenheit), instead of the actual average of about 15 degrees Celsius (about 59 degrees Fahrenheit). Those figures are widely quoted approximations, but the main point is secure: the natural greenhouse effect makes the planet habitable.
The name is a loose analogy. A real greenhouse mainly warms by blocking air movement, while the atmosphere works by absorbing and re-emitting infrared radiation. The physics of that absorption comes from how molecules vibrate, which is part of the broader science of light and matter, and it was demonstrated in laboratory measurements in the 19th century.
Why Adding Greenhouse Gases Warms the Planet
Burning coal, oil and natural gas releases carbon dioxide that was stored underground for millions of years. Clearing forests and some farming practices release more. A portion of this carbon dioxide stays in the air for a very long time, and a portion is absorbed by the oceans and by plants.
More carbon dioxide means the atmosphere absorbs more of the outgoing infrared, so the planet loses heat more slowly and warms until balance is restored. Methane and nitrous oxide add to this effect as well.
Water vapor plays a special role. It is a powerful greenhouse gas, but its amount in the air mostly responds to temperature rather than driving it by itself. Warmer air can hold more vapor, which adds further warming. This is called a feedback. Scientists also study other feedbacks, such as melting snow and ice exposing darker surfaces that absorb more sunlight, and clouds, which are harder to model and remain one of the larger sources of uncertainty about how much warming will occur.
Lines of Evidence
The scientific conclusion does not rest on one measurement. Several independent kinds of evidence point the same way.
Direct measurements of carbon dioxide. Instruments have recorded rising carbon dioxide levels in the air for decades. Air bubbles trapped in ice cores show that today’s level is higher than at any time in the several hundred thousand years the cores cover.
Chemical fingerprints. Carbon from fossil fuels has a different isotope makeup than carbon from the atmosphere in general. The atmospheric shift matches what is expected from fossil fuel burning. The concept is related to atomic isotopes, forms of the same element with different masses.
Surface temperature records. Thermometer records from land stations, ships and buoys have been compiled by several separate research groups using different methods. They agree that the global average temperature has risen since the late 1800s, with most of the rise in recent decades.
Ocean heat. The oceans absorb most of the extra heat trapped by the greenhouse effect. Measurements of ocean temperature at depth show a long-term increase in stored heat.
Ice and sea level. Mountain glaciers have been shrinking in many regions, the ice sheets of Greenland and Antarctica are losing mass, and Arctic sea ice has declined. Sea level is rising, partly because water expands as it warms and partly because ice on land melts into the sea.
Where the warming occurs. If the Sun were the main cause, the whole atmosphere, including the upper layer called the stratosphere, would warm. Instead the lower atmosphere has warmed while the stratosphere has cooled, which is the pattern expected when heat is trapped closer to the surface by more greenhouse gases.
Natural causes checked. Scientists have examined changes in the Sun’s output, volcanic eruptions and natural cycles. These affect climate, but they do not account for the long-term warming seen. Over recent decades, solar output has not risen in a way that matches the warming.
Climate Is Not Weather
Weather is what the atmosphere does over hours and days, such as a storm or a cold snap. Climate is the long-term pattern, usually averaged over decades. Daily weather comes from short-lived weather systems, and a single cold winter does not cancel a long-term trend, just as a single warm day does not prove one.
Past Climates and Why They Matter
Earth’s climate has changed before, driven by shifts in its orbit, volcanic activity, changing continents and greenhouse gas levels. The study of ice ages shows that relatively modest changes in energy balance, amplified by feedbacks, can shift the planet a lot. Past changes are used to test climate models, not as evidence that natural causes must be at work today. The cause is judged by evidence for the present period, and that evidence points toward human activity.
What Remains Uncertain
Science is clear that the planet is warming and that human greenhouse gas emissions are the main cause. Less certain are the details: exactly how sensitive the climate is to a given increase in carbon dioxide, how quickly ice sheets will respond, how clouds will change, and how regional rainfall will shift. These are active areas of research, and projections are usually given as ranges rather than single values.
Choices about what to do, such as shifting toward renewable energy or changing how energy sources are used, involve economics, values and politics as well as science. This article covers only the physical understanding, which is supported by the major scientific academies and research organizations around the world.