How do we know Earth is warming now?

Global average temperature since 1890 as reproduced by the NCAR/DOE Parallel Climate Model. View a recently modeled animation of Earth's warming. Movie file is large, but worth the wait. For more than 100 years, Earth's surface temperature has been monitored by a global network of land-based weather stations. These reports are supplemented by sea-surface and air temperature readings taken at points across the oceans that cover 70% of the globe. The ups and downs of air temperature are modulated by the sea, so the uppermost ocean serves as a good index of the average air temperature just above it.

Together, these data show that Earth's surface air temperature has risen more than 1.1°F (0.7°C) since the late 1800s. This warming of the average temperature around the globe has been especially sharp since the 1970s. Global models at NCAR have simulated 20th century climate and found three main factors at work:

  1. Solar activity contributed to a warming trend in global average temperature from the 1910s through 1930s.
  2. As industrial activity increased following World War II, sun-blocking sulfates and other aerosol emissions helped lead to a slight global cooling from the 1940s to 1970s.
  3. Since 1980, the rise in greenhouse gas emissions from human activity has overwhelmed the aerosol effect to produce overall global warming.

Some urban areas have also warmed due to the heat-island effect, where buildings and pavement retain more heat than undeveloped areas and cause more runoff and thus drier conditions as well. NCAR scientists and their colleagues have worked carefully to remove urban heat-island effects and other potential biases from the global record. Even after these adjustments, the rise in global temperature remains clear.

There are other signs of a warming planet. Glaciers are retreating, especially atop lower-latitude mountains. In the Arctic the thickness and extent of summer sea ice have decreased dramatically over the last 50 years, and recent modeling by NCAR scientists shows that the Arctic’s summer ice may virtually disappear by 2040. Meanwhile, snowfall over much of Antarctica is increasing, a paradoxical sign of warming temperatures in this frozen, arid land. The annual cycle of plants and migrating animals shows a lengthening of the warm season over much of the Northern Hemisphere.

Since the late 1970s, satellites have measured the temperature in a broad layer of the troposphere several miles above Earth. For years, they showed a smaller temperature rise at these heights than at ground level. It now appears that most of the disagreement was due to errors in the satellite data and how it was interpreted. While there are still differences between tropospheric and surface warming in some regions, the discrepancy is no longer apparent on a global scale, according to a 2006 U.S. Climate Change Program report.

Studies of past, present, and future climate benefit continually from these and other improvements in data gathering, computer modeling, and analysis. For example, recent research at NCAR re-examines the role of decades-long cycles of solar variation in explaining the observed warming in the first half of the 20th century.

Researchers have identified an additional impact of humanity's increasing consumption of fossil fuels: changes in seawater chemistry as the oceans absorb more carbon dioxide. These changes pose a real threat to marine organisms, including those that build coral reefs around the world.