2020 was effectively tied with 2016 as the hottest year on record, the previous record holder. Continuing the planet’s long-term warming trend, the year’s globally averaged temperature was 1.84 degrees Fahrenheit (1.02 degrees Celsius) warmer than the baseline 1951-1980 mean, according to scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York. 2020 edged out 2016 by a very small amount, but GISS researchers say it was within the margin of error of the analysis, making the two years effectively tied.
Overall, the Earth’s average temperature has risen more than 2 degrees Fahrenheit since the 1880s, due to human human emissions of greenhouse gases including carbon dioxide and methane.
“The last seven years have been the warmest seven years on record, typifying the ongoing and dramatic warming trend,” said GISS Director Gavin Schmidt. “Whether one year is a record or not is not really that important. The important things are long-term trends. With these trends, and as the human impact on the climate increases, we have to expect that records will continue to be broken.”
The rising temperatures are causing loss of sea ice and ice sheet mass, sea level rise, longer and more intense heat waves, and shifts in plant and animal habitats.
Like all scientific data, the temperature findings contain a small amount of uncertainty — in this case, mainly due to changes in weather station locations and temperature measurement methods over time. The GISS temperature analysis is believed to be accurate to within 0.1 degrees Fahrenheit, with a 95 percent confidence level for the most recent period.
A separate, independent analysis by the National Oceanic and Atmospheric Administration (NOAA) came to a slightly different conclusion, pegging 2020 as the second-warmest year in their record, behind 2016. NOAA scientists use much of the same raw temperature data in their analysis, but have a different baseline period (1901-2000) and methodology. Unlike NASA, NOAA also does not infer temperatures in polar regions lacking observations, which accounts for much of the difference between NASA and NOAA records.
While long-term warming continues, a variety of events and factors contribute to any particular year’s average temperature. Two separate events changed the amount of sunlight reaching the earth’s surface in 2020. Australian bush fires during the first half of the year burned 46 million acres, releasing smoke and other particles more than 18 miles high in the atmosphere; this blocked sunlight and likely cooled the atmosphere slightly. In contrast, global shutdowns related to the coronavirus pandemic reduced particulate air pollution in many areas, allowing more sunlight to reach the surface; this produced a small but potentially significant warming effect. The shutdowns also appear to have reduced the carbon dioxide emissions that human activities produced last year. But since long-term warming is related to cumulative emissions, not those of any particular year, the overall effect of the 2020 drop will be minimal, say researchers.
The largest source of year-to-year variability in global temperatures typically comes from the El Niño-Southern Oscillation (ENSO), a naturally occurring cycle of heat exchange between the ocean and atmosphere. While the year has ended in a cool phase of ENSO, it started in a slightly warm phase. This marginally increased the average overall temperature. ENSO-driven cooling is expected to have a larger influence in 2021.
“The previous record warm year, 2016, received a significant boost from a strong El Niño. The lack of a similar assist from El Niño this year is evidence that the background climate continues to warm due to greenhouse gases,” Schmidt said.
The 2020 GISS values represent surface temperatures averaged over the whole globe and throughout the year. Local weather plays a role in regional temperature variations, so not every region experiences similar amounts of warming, even in a record year. According to NOAA, parts of the continental United States experienced record high temperatures in 2020, while others did not.
In the long term, parts of the globe are also warming faster than others. The warming trend is most pronounced in the Arctic; the GISS analysis shows it has been warming more than three times as fast as the rest of the globe over the past 30 years. The loss of Arctic sea ice — whose annual minimum area is declining by about 13 percent per decade — makes the region less reflective, meaning more sunlight is absorbed by the oceans, causing temperatures to rise further still. This phenomenon, known as Arctic amplification, is driving further sea ice loss, ice sheet melt and sea level rise, more intense Arctic fire seasons, and permafrost melt.
NASA’s analysis incorporates surface temperature measurements from more than 26,000 weather stations and thousands of ship- and buoy-based observations of sea-surface temperatures. Researchers analyze raw measurements using an algorithm that considers the varied spacing of temperature stations, and urban heating effects that could skew the conclusions. The result of these calculations is an estimate of the global average temperature difference from a baseline period of 1951 to 1980.
NASA also measures planetary temperatures from land, air and space with a fleet of satellites, and aircraft and ground-based observation campaigns. The satellite surface temperature record from the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA’s Aura satellite confirms the GISS results as the past seven years being the warmest on record. Satellite measurements of air temperature, sea-surface temperature and sea levels, as well as other space-based observations, also reflect a warming, changing world.
NASA’s full surface temperature data set and the complete methodology used to make the temperature calculation are at data.giss.nasa.gov/gistemp
GISS is affiliated with Columbia University’s Earth Institute and Columbia’s School of Engineering and Applied Science in New York.
Adapted from a press release by NASA.
