What is a temperature anomaly?
A temperature anomaly is the difference between an observed temperature and the long-term average (baseline) temperature for the same location and time period. A positive anomaly means conditions were warmer than the historical average, while a negative anomaly means conditions were cooler. This method is preferred over absolute temperatures because it better captures relative change across diverse regions. See also our Carbon Cycle Calculator.
Where does temperature anomaly data come from?
Temperature data come from thousands of manual and automated weather stations around the world, as well as ocean buoys and ships. Quality-checked land data become part of the Global Historical Climatology Network-Monthly (GHCN-M) dataset, while ocean data come from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and the Extended Reconstructed Sea Surface Temperature (ERSST) dataset. NOAA combines these into the NOAAGlobalTemp analysis.
What baseline period should I use?
The most common baseline is the 20th century average (1901–2000), used by NOAA's National Centers for Environmental Information. NASA's Goddard Institute uses 1951–1980, while the World Meteorological Organization (WMO) uses 30-year normals — currently 1991–2020. The choice of baseline affects the anomaly value but not the underlying trend, so it's important to use the same baseline when comparing datasets.
Why do climate scientists use anomalies instead of absolute temperatures?
Anomalies are more useful than absolute temperatures because they remove the effect of local climate differences. A station in Alaska and a station in Florida will have very different absolute temperatures, but their anomalies can be directly compared and averaged. This makes it possible to construct a meaningful global average from a sparse, uneven network of weather stations. You might also find our Global Warming Potential Calculator useful.
How is a global average temperature anomaly calculated?
Scientists divide the planet into a grid — NOAA uses roughly 2,592 cells — and calculate the average temperature anomaly for each grid cell. These cell anomalies are then averaged together, weighted by area, to produce a single global average temperature anomaly for the period. This calculator simplifies that process, allowing you to compute anomalies for individual locations or average a set of values.
What is considered a significant temperature anomaly?
In everyday weather, anomalies of ±1–2°C are notable but not unusual. In terms of global climate, an anomaly of +1°C above the 20th century baseline for the entire planet is considered highly significant — the current global average anomaly is approximately +1.1°C above the pre-industrial baseline. Regional anomalies can be much larger, especially in the Arctic, where warming is amplified.
What is the difference between land, ocean, and combined anomalies?
Land surfaces warm and cool more rapidly than oceans, so land-only anomalies tend to be larger in magnitude. Ocean-only anomalies are generally more moderate due to the ocean's higher heat capacity. The combined land-and-ocean anomaly is the standard metric used to track global average surface temperature change and is what NOAA publishes in its monthly climate reports.
How does El Niño or La Niña affect temperature anomalies?
El Niño events warm sea surface temperatures in the central and eastern Pacific Ocean, which boosts the global average temperature anomaly — often by +0.1 to +0.2°C above the underlying trend in a strong year. La Niña has the opposite effect, temporarily suppressing global anomalies. Record-high annual temperature anomalies frequently coincide with strong El Niño events, such as 1998 and 2016.