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Here’s How We Know 2019 Was The Second-Hottest Year Ever

This summer, we asked readers to send us their climate change questions. And they did. We received many, many, many climate change questions. So many, in fact, that we’re doing several different projects around them. You’ve seen our columns on Who’s Winning Climate Change? Today, we’re diving into the mailbag for another edition of Climate Question from an Adult – a series that will explore the business, culture and chemistry behind your most pressing questions about global warming. Have a question? Send it to us!

“What are they measuring when they talk about limiting global average temperature change to less than 2 degrees? Is it air temp? Ocean temp? Where do they take the measurements?” – Danny Kremer, Minnesota

When I started researching the answer to Danny’s question, I did not expect that it would lead to a crash course in the rapid advancement of bucket technology.

Yes, buckets. Literal buckets. The kind a bunch of sailors might put on a rope and throw over the side of a boat if they wanted to pull up some seawater and stick a thermometer in it.

When we talk about the “global average temperature” and changes to that temperature, we are talking about a calculated average that incorporates measurements taken from both the land and the sea in locations scattered all over the world.

This week, the United States’ National Climatic Data Center released its annual global climate report, which includes the latest estimate of how much hotter the planet was last year compared to the average 20th century temperature. When you see headlines that tell you 2019 was the second-hottest year on record, what you’re actually seeing is the output of a massive data collection program — one that spans not just continents and governments, but a variety of technologies and more than a century of time. Behind that single number is a huge, complex story.

And the evolution of buckets is a part of that tale. That’s because one of the ways we take the temperature of the ocean is with the help of volunteers — sailors on random merchant and naval vessels who sign up to take daily readings during their journeys and report those to the United Kingdom’s Hadley Centre for Climate Prediction and Research. This program began more than 150 years ago, as part of the British Empire’s efforts to map the warm-water currents that could efficiently carry colonialism from point A to point B.

The existence of this data is great news if you’re a scientist who wants to know how the temperature of the ocean has changed over time. Unfortunately, it also means you have to account for changes in buckets. A water sample will stay warmer in a wooden bucket than in an uninsulated canvas one, John Kennedy, a climate monitoring and research scientist at the Hadley Centre, told me. So you can’t directly compare a temperature taken in the 1880s (when wooden buckets were the norm) with one taken using the canvas buckets that became standard a few decades later. “The way we deal with buckets is that two of my former colleagues modeled how much heat a bucket loses in different weather conditions,” Kennedy said. To normalize measurements across time, scientists use that model, as well as information like ship speed and wind speed. “It has to be retested over and over again,” he told me.

And that’s just for the ocean temperatures. On the other side of the Atlantic, the U.S. government has been measuring the temperature of air since around the end of the Civil War, said Derek Arndt, chief of the climate monitoring section at the National Centers for Environmental Information, which manages the National Climatic Data Center. The measurements — originally the responsibility of surgeons at Army forts — were taken for the military and for farmers, Arndt said.

By the 1920s, that had grown into a nationwide network of volunteers — often those same farmers — who made daily records of temperature and precipitation. That network still exists today, Arndt told me, and it remains crucial, even though the National Weather Service is also taking more precise measurements more frequently, using automated weather stations.

That’s because those automated stations are mostly located at airports. When they were first set up, that meant the stations were out in the sticks. But urban sprawl has pushed the city limits — and the heat islands that accompany cities — up to the edge of the taxiway. The robots may do a better job of monitoring changes in the weather, but the human volunteers, who are still mostly located in rural areas, make sure scientists know how much of a rising temperature is due to climate change … and how much is just cities expanding.

In fact, all the measurements that go into the global average temperature have similar redundancies built in. Measuring the temperature of the sea isn’t just the responsibility of sailors with buckets — it’s also done using satellites and again using floating thermometers that bob about in the world’s oceans. Different groups of scientists study the same thing using different tools. The measurements they all produce help serve as a check on the others.

Those bobbing thermometers, for example, are part of a project called Argo, which has released thousands of these little floating labs into the sea since 2000. Besides temperature, they also collect information on the saltiness of the water and other factors that help scientists study climate change. Measurements taken by Argo floats are going to be a lot more precise than measurements taken by non-scientists on a shipping vessel, but all the Argo floats are also sourced from the same electronics supplier, said Megan Scanderbeg, a research associate at the Scripps Institution of Oceanography, which helps maintain the Argo program. If something goes wrong in the manufacturing process, all the floats could report incorrect data. And since the Argo floats are designed to ride the waves wherever the ocean takes them and not be recaptured, that sort of problem would be hard to fix. Having multiple types of measurement serves as insurance.

It also helps protect against bias. While a lot of countries, including Japan and many European nations, have measurement programs similar to those run by the U.S. and the U.K., the countries most heavily involved in collecting this data have historically been centered on the Northern Hemisphere. Even historic ocean temperature measurements taken by sailing vessels were biased towards the northern half of the globe, because that’s where the most heavily traveled trade routes were. Argo floats and satellite data helped give scientists a clearer view of what is happening all over the planet.

All of this data — ocean and air temperature measurements from satellites, robots and human volunteers — comes together at the World Meteorological Organization. It’s a branch of the United Nations that coordinates the sharing of weather and climate data between 193 member states and territories. Data from around the globe goes in — climate change updates (and daily weather forecasts) come out.

It’s also where much of the data the NCDC uses for its Global Climate Report comes from, Arndt told me. As one of the WMO’s primary archives, his team takes reams of temperature readings, runs quality control tests, checks for things like changes in the location of a temperature station that might affect its readings and processes it all into an understandable summary of what is happening as the climate changes. From this, we know 2019 was 0.95 degrees Celsius hotter than the 20th century average and 1.15 degrees hotter than the pre-industrial average. Two little numbers; awesome logistical effort.

Maggie Koerth was a senior reporter for FiveThirtyEight.