Temperatures measured on land and at sea for more than a century show that Earth’s globally averaged surface temperature is experiencing a long-term warming trend.
The concept of an average temperature for the entire globe may seem odd. After all, at this very moment, the highest and lowest temperatures on Earth are likely more than 55°C (100°F) apart. Temperatures vary from night to day and between seasonal extremes in the Northern and Southern Hemispheres. This means that some parts of Earth are quite cold while other parts are downright hot. To speak of the “average” temperature then may seem like nonsense. However, the concept of a global average temperature is convenient for detecting and tracking changes in Earth’s condition over time.
Explore this interactive graph: Click and drag to display different parts of the graph. To squeeze or stretch the graph in either direction, hold your Shift key down, then click and drag.
The graph shows average annual global temperatures since 1880 compared to the long-term average (1901-2000). The zero line represents the long-term average temperature for the whole planet; blue and red bars show the difference above or below average for each year.
To calculate a global average temperature, scientists begin with temperature measurements taken at locations around the globe. Because their goal is to track changes in temperature, measurements are converted from direct temperature readings to temperature anomalies-values that represent the difference between the observed temperature and the long-term average temperature for each location and date. Across inaccessible areas that have few measurements, scientists use surrounding temperatures and other information to fill in the missing values. Each value is then used to calculate a global temperature average. This process provides a consistent, reliable method for monitoring Earth’s surface temperature over time.
Though warming has not been uniform across the planet, the upward trend in the globally averaged temperature shows that more areas are warming than cooling. From 1900 through 2009 the globally averaged surface temperature rose by approximately 0.7° C (1.3°F). The rate of temperature increase has risen as well. For the last 50 years, global temperature rose at an average rate of about 0.13°C (around one-quarter degree Fahrenheit) per decade-almost twice as fast as the 0.07°C per decade increase observed over the previous half-century. In the next 20 years, scientists project that global average temperature will rise by around 0.2°C (about one-third of a degree Fahrenheit) per decade.
Hansen, James, The Elusive Absolute Surface Air Temperature, GISS Surface Temperature Analysis, Updated July 12, 2005.
Herring, David, Earth’s Temperature Tracker, NASA Earth Observatory. Published November 5, 2007.
Annual global temperature anomalies for land and ocean combined, expressed as departures from the 1901-2000 average. National Climatic Data Center.