This article Reprinted from 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 since records began, with the average surface temperature reaching 17.09°C. On Monday, the record was broken again, with the average temperature reaching 17.16°C. Tuesday was almost as hot.
The announcement, made by the European climate change service Copernicus, has attracted international attention, especially in the Northern Hemisphere, which is currently experiencing extremely hot summers.
Determining the Earth’s average temperature on any given day is a complex task, requiring thousands of observations using high-tech instruments and sometimes sophisticated computer models.
So let’s take a look at how scientists measure Earth’s temperatures and what these broken records mean.
How to know if it’s hot
Earth’s average surface temperature is the main indicator used to track how the climate is changing and is the measure used in the Paris Agreement.
It is derived from a combination of both the average temperature of the air just above the earth’s surface and the air over the ocean.
Estimates of the Earth’s average surface temperature are produced by several organizations using a variety of methodologies. In addition to Copernicus, these include national organizations in the United States, the United Kingdom, and Japan.
All of the data sets produced by these agencies show a very clear warming trend since 1900.
Most datasets are based on direct temperature observations from land-based weather stations and floats at sea, both of which are equipped with thermometers, and satellites in space are also used to gather infrared estimates.
With the advanced methods and thousands of observations in use today, daily temperature data are much more accurate than in the past. The further back in time we go, the more uncertain our estimates of the Earth’s average surface temperature become.
What is Copernicus?
The Copernicus Climate Change Service is part of the European Union’s Earth observation programme. Copernicus uses observations as well as computer model simulations to produce temperature estimates.
The model simulates temperatures two metres above the surface anywhere on Earth and combines the results with estimates of the average ocean surface temperature derived from direct observations and satellite information.
Copernicus uses information from model simulations, so its methodology is slightly different to other data sets, but the method is highly regarded and provides estimates of the average surface temperature across the globe within a few days.
Unlocking Temperature
We know the climate is changing rapidly, but why are we experiencing these record-breaking heatwaves now?
As the graph above shows, the Earth’s average surface temperature follows a clear seasonal cycle: July temperatures are typically about 4 degrees warmer than January.
This difference is due to the larger land area of North America, Europe, and Asia compared to the Southern Hemisphere.
