THIS ARTICLE IS re-published from Conversation under Creative Commons License.
Earth’s record for hottest day was broken twice last week. Sunday, July 21, was declared the hottest day on Earth since records began, when the average surface temperature reached 17.09 degrees Celsius. On Monday, the record was broken again, when the average temperature reached 17.16 degrees Celsius—and Tuesday was almost as balmy.
The declarations were made by Copernicus, the European Union’s climate change agency. They made headlines around the world, especially in the Northern Hemisphere, which is experiencing extreme summer heat.
Determining the average global temperature on a given day is complicated. It requires thousands of observations using high-tech equipment and, in some cases, advanced computer models.
So let’s take a look at how scientists measure the planet’s temperature and what these record-breaking numbers mean.
How do we know it’s balmy
The Earth’s average global surface temperature is a key indicator for tracking climate change. It is the measure used under the Paris Agreement.
It is the result of a combination of the average air temperature just above the land surface and in the upper layer of the ocean.
Several organizations produce estimates of the Earth’s mean surface temperature using different methods. In addition to Copernicus, these include: national organizations In United Statesthis United KingdomAND Japan.
All the data sets presented by these agencies clearly show a warming trend since 1900.
Most of the data sets are based directly on observed temperatures from weather stations on land and in the ocean, both of which contain thermometers. Satellites in space are also used to collect infrared estimates.
Advanced methods now in exploit and many thousands of observations mean that daily temperature data are much more precise than in previous years. The further back in time we go, the more uncertain are estimates of global mean surface temperatures.
What is Copernicus?
This Copernicus Climate Service is part of the European Union’s Earth observation programme. To generate its temperature estimates, Copernicus uses not only observations but also computer model simulation.
The model simulates temperatures at 2 meters above the ground all over the world. The results are combined with an estimate average ocean surface temperature from direct observations and satellite information.
Copernicus’ exploit of information from model simulations means its method differs slightly from other data sets. However, the method is well-regarded and provides global estimates of mean surface temperatures over several days.
Unpacking temperatures
We know the climate is changing rapidly. But why is this record daily heat happening now?
As the graph above shows, the global average surface temperature follows a distinct seasonal cycle. July temperatures are typically about 4 degrees Celsius warmer than January.
The difference is due to the larger land areas of North America, Europe, and Asia compared to those in the Southern Hemisphere.