What Sets Earth’s Temperature
Earth receives energy from the Sun and sends energy back to space. The temperature of the planet is set by the balance between the two. Sunlight arrives mostly as visible light, and about 30 percent of it is reflected straight back by clouds, bright surfaces such as ice, and the atmosphere. The rest is absorbed by the land, oceans and air, warming them. The warmed planet then gives off heat as infrared radiation, which is a form of light we cannot see.
If Earth had no atmosphere, a simple calculation based on this balance gives an average surface temperature well below freezing, roughly minus 18 degrees Celsius. The actual global average is considerably warmer, around 15 degrees Celsius. The difference comes from the greenhouse effect.
The Greenhouse Effect
Certain gases in the atmosphere, including water vapor, carbon dioxide, methane and nitrous oxide, absorb infrared radiation rising from the surface and radiate it in all directions, including back downward. This slows the escape of heat to space and keeps the lower atmosphere and surface warmer than they would otherwise be.
The name is a loose comparison. A garden greenhouse mainly warms because glass stops warm air from mixing with cooler outside air, which is not exactly what happens in the atmosphere, but the label has stuck.
The greenhouse effect is a natural and essential part of what makes Earth livable. The concern is not that it exists, but that human activity is increasing the concentration of some greenhouse gases and thereby strengthening it. Venus offers a vivid example of an extreme case: its dense carbon dioxide atmosphere helps keep the surface hotter than that of any other planet, although Venus is also closer to the Sun and differs from Earth in many ways, so the comparison should not be pushed too far.
Natural Influences on Climate
Climate has always changed, and several natural factors are well understood.
- The Sun’s output. The Sun’s brightness varies slightly over an 11-year cycle and over longer periods. These variations are small compared with the total energy received.
- Orbit and tilt. Slow changes in Earth’s orbit and axial tilt alter how sunlight is distributed across latitudes and seasons. These cycles are widely regarded as the pacemaker of ice ages, taking tens of thousands of years.
- Volcanic eruptions. Large eruptions can inject particles into the upper atmosphere that reflect sunlight, cooling the planet for a year or two.
- Ocean cycles. Patterns such as El Niño move heat around the oceans and affect global temperature from year to year. Because ocean currents transport so much heat, these cycles can make some years warmer or cooler than the underlying trend. This is one reason scientists look at averages over decades.
- Feedbacks. Changes can be amplified or dampened. When ice melts, darker ground or water is exposed, which absorbs more sunlight and adds to the warming. Warmer air can also hold more water vapor, itself a greenhouse gas.
Human Influences
Since the Industrial Revolution, humans have added large amounts of carbon dioxide to the atmosphere, mainly by burning coal, oil and natural gas, and by clearing forests. Methane comes from sources such as livestock, rice cultivation, fossil fuel operations and landfills. Humans also affect climate in other ways: small airborne particles from pollution can reflect sunlight and have a cooling effect locally, while changes to land surfaces alter how much sunlight is reflected.
Measurements of the air, including bubbles trapped in ancient ice, indicate that carbon dioxide was roughly 280 parts per million before industrialization and has since risen above 420 parts per million. Isotopic signatures in the carbon support the conclusion that much of the increase comes from fossil fuels. The broad scientific assessment is that human activity has warmed the planet, by an amount commonly given as a little over 1 degree Celsius compared with the late 1800s. Exact figures differ slightly between datasets and years.
The overall list of causes is covered in more depth in the discussion of climate change drivers, which also looks at the effects and responses.
How Scientists Measure Temperature Trends
Estimating a single “global temperature” is a significant statistical task, and it relies on several independent lines of evidence.
- Surface records. Thermometers on land, weather stations, ships and buoys have collected data for well over a century. Scientists correct for known issues, such as instrument changes, station relocations and the extra heat of growing cities.
- Anomalies, not absolutes. Instead of averaging raw temperatures, they calculate how much each place differs from its own long-term average. This makes it easier to combine warm and cold regions fairly.
- Satellites. Instruments in orbit infer the temperature of different layers of the atmosphere, adding to the climate record from space. They measure the radiation reaching the sensor rather than temperature directly, so interpretation is complex.
- Oceans. Floating instruments and ship measurements track ocean heat, which holds most of the extra energy trapped by greenhouse gases.
- Ice, sea level and the biosphere. Shrinking glaciers, rising seas and shifts in seasonal timing are consistent with a warming world.
- Proxies for the past. Tree rings, ice cores, corals and sediments help reconstruct temperatures before instruments existed. They carry larger uncertainties but give valuable context.
Different research groups use different methods, yet their results largely agree on the overall direction and approximate size of long-term warming.
Weather Versus Climate
A cold week or a snowy winter in one place does not contradict a rising global average. Weather describes conditions over days and in specific places, while climate describes statistical patterns over decades and large regions. A trend can be clear in the long-term data even when individual years vary, in much the same way that a few cool days do not mean summer has ended.
What Remains Uncertain
Scientists are confident that the climate is warming and that human activity plays the major role, but important questions remain about details. These include how sensitive the climate is to a given increase in carbon dioxide, how clouds will respond, how quickly ice sheets will change, and how local rainfall patterns will shift. Climate projections therefore typically come as ranges rather than single numbers, and they depend on future emissions, which are not yet known.