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THIS ARTICLE IS republished from The conversation under Creative Commons License.
Last week, the record for the hottest day on Earth was broken twice. Sunday, July 21, was declared the hottest day on Earth since records began, when the average surface temperature reached 17.09 degrees Celsius. The record was broken again on Monday, when Average temperatures reached 17.16 degrees Celsius.—And Tuesday was almost as hot.
The statements were made by Copernicus, the European climate change service, and made international headlines, especially in the northern hemisphere, which has been experiencing extreme summer heat.
Determining the average global temperature on a given day is a complex task involving thousands of observations using high-tech equipment and, in some cases, sophisticated computer models.
Now let’s look at how scientists measure the planet’s temperature and what these broken records mean.
How do we know it’s hot?
Global average surface temperature is the main indicator used to monitor how the climate is changing and is the measure used under the Paris Agreement.
It is derived from a combination of the average temperature of the air just above the land surface and in the upper layer of the ocean.
Several organizations develop estimates of the Earth’s mean surface temperature using a variety of methods. In addition to Copernicus, they include national organizations in it USAhe United Kingdomand Japan.
All the data sets produced by these agencies point to a very clear warming trend since 1900.
Most data sets are based directly on observed temperatures Measurements are made from weather stations on land and floats in the ocean, both with thermometers. Satellites in space are also used to obtain infrared estimates.
The advanced methods used today and the thousands of observations made mean that daily temperature data are much more accurate than in previous years. The further back in time we go, the more uncertain are the estimates of global average surface temperatures.
What is Copernicus?
He Copernicus Climate Change Service is part of the European Union’s Earth observation programme. To generate its temperature estimates, Copernicus uses not only observations, but also a computer model simulation.
The model simulates temperatures at 2 meters above the Earth’s surface around the world. The results are combined with an estimate of average ocean surface temperature derived from direct observation and satellite information.
Copernicus’ use of information from a model simulation means that its method differs slightly from other data sets. However, the method is well regarded and provides global estimates of mean surface temperatures over a couple of days.
Deciphering temperatures
We know the climate is changing at a rapid pace, but why is this record-breaking daily heat happening now?
As the graph above shows, global average surface temperature follows a distinct seasonal cycle. Temperatures in July are typically about 4 degrees Celsius higher than in January.
The difference is due to the larger land masses of North America, Europe, and Asia than those of the Southern Hemisphere.