Last Monday Was the Hottest Day on Record

THIS ARTICLE IS republished from The Conversation under a Creative Commons license.

In the past week, Earth’s record for the hottest day was broken twice. Sunday, July 21, was declared Earth’s hottest day since records began, when average surface temperature reached 17.09 degrees Celsius. On Monday the record was broken again, when average temperatures reached 17.16 degrees Celsius—and Tuesday was almost as hot.

The declarations were made by Copernicus, the European climate change service. They made international headlines—especially in the northern hemisphere, which has been experiencing extreme summer heat.

Determining the global average temperature on any given day is complex. It involves thousands of observations using high-tech equipment and, in some cases, sophisticated computer models.

So let’s take at look at how scientists take the planet’s temperature, and what these broken records mean.

How We Know It’s Hot

The global average surface temperature is the main indicator used to track how the climate is changing, and is the measure used under the Paris Agreement.

It is derived from a combination of both the average temperature of air just above the land surface and in the upper layer of the ocean.

Several organizations develop estimates of Earth’s average surface temperature using a variety of methods. Aside from Copernicus, they include national organizations in the United States, the United Kingdom, and Japan.

All datasets produced by these agencies point to a very clear warming trend since 1900.

Most datasets are based on directly observed temperatures from weather stations on land and floats on the ocean, both of which contain thermometers. Satellites in space are also used to gather infrared estimates.

The advanced methods used today, and the many thousands of observations, mean daily temperature data is far more accurate than in years past. The further back in time we go, the more uncertain the estimates of global average surface temperatures.

What Is Copernicus?

The Copernicus Climate Change Service is part of the European Union’s Earth observation program. To generate its temperature estimates, Copernicus uses not just observations, but a computer model simulation.

The model simulates temperatures at 2 meters above the land surface everywhere across the globe. The results are combined with an estimate of oceans’ average surface temperature derived from direct observation and satellite information.

Copernicus’ use of information from a model simulation means its method differs slightly from other datasets. However, the method is well regarded and provides global estimates of average surface temperatures within a couple of days.

Unpicking the Temperatures

We know the climate is changing at a rapid pace. But why is this record daily heat occurring now?

As the graph above shows, the global average surface temperature follows a distinct seasonal cycle. Temperatures in July are typically about 4 degrees Celsius higher than in January.

The difference comes down to the larger land masses of North America, Europe, and Asia, as compared to those in the southern hemisphere.