The 1870s Challenger expedition is often considered to be the foundation of the modern science of oceanography.

The expedition, led by British naturalist John Murray and Scottish naturalist Charles Wyville Thomson, covered more than 125,000 kilometers and lasted about four years. During that time, the crew sailed the HMS Challenger, a small Navy warship converted into a floating laboratory, from England across most of the world’s oceans before heading back home again.

But this expedition is notable for more than just its historical value. Data from the Challenger expedition is still useful today, helping us figure out how the climate disaster is changing the global ocean. During its voyage, the Challenger made observations of ocean depth, temperature, seawater density, biology, and the composition of the seafloor.

Many of these parameters, like seawater density and temperature, are routine measurements still being made on oceanographic ships today. Now, around 150 years later, scientists at the UK’s National Oceanography Center and the Scottish Association for Marine Science have been looking back at the Challenger data to help us understand how the climate crisis is affecting the ocean. See, while we have made great strides in oceanography since the Challenger expedition, there’s still a lot we don’t know about the ocean.

In fact, more than 80% of the ocean remains unexplored, according to some estimates. And because oceanography is still a relatively new science, we don’t have a lot of historical data that can help us look at how the ocean has changed with time. In fact, we don’t have any data for ocean salinity, the measure of how salty seawater is at a particular point, before 1950.

But while the Challenger expedition didn’t specifically measure salinity, they did measure seawater density and temperature. And temperature and salinity are two of the main contributors to density. So those researchers realized that it is possible to look back at the data from the Challenger expedition and calculate salinity from their temperature and density measurements.

Just algebra, right? You have two variables, find the third one. They combined the data from the Challenger expedition with data from a Prussian ship, the SMS Gazelle, and began piecing together the state of ocean salinity in the 1870s.

Now, for context, the Industrial Revolution began between 1760 and 1840, and atmospheric CO2 started to increase at around the same time, with records showing that CO2 in the atmosphere increased slightly during the 1800s. But atmospheric carbon dioxide really started ramping up in the 1900s, after the Challenger expedition. That means, while it’s not perfect, the Challenger data gives us a picture of what the ocean was like just before the climate crisis really kicked into gear.

And by combining our modern measurements with their historical measurements of salinity, researchers found that parts of the ocean that were already salty in 1870, like the Atlantic, have grown saltier, while parts of the ocean that were already less salty in 1870, have grown fresher. At this point, you might be asking how salinity tells us anything about how the climate has changed. It’s not like those smokestacks give off salt.

But salinity gives us information about two things. It describes how much salt is dissolved in seawater. So to change salinity, you can do one of two things: you can change the amount of salt in the water, or you can change the amount of water the salt is dissolved in.

But the overall amount of salt in the ocean hasn’t really changed over time. In fact, it would take millions of years to change sodium and chlorine levels significantly. So the changes in salinity are probably not due to more or less salt in the ocean.

Instead, these changes probably have to do with the amount of water. Specifically, they are related to the activity of the water cycle: Evaporation and precipitation. Evaporation makes seawater saltier, by removing freshwater from it.

And precipitation does the opposite. It makes seawater fresher by adding more water to the sea So in that context, the study’s results show that parts of the ocean that have historically had a lot of evaporation are seeing even more, while parts of the ocean that have always had a lot of rain have become rainier. Now, if this result could be generalized to the whole world, then it implies that areas of the world that are dry are going to get drier, and areas of the world that are wet are going to get wetter.

Sound familiar? Experts have been warning about this consequence of the climate disaster for a long time, and rainfall measurements published in the last major climate report show that it’s already getting started. This is an issue because a lot of dry areas already struggle with problems like drought and wildfires, which are only going to get worse as these areas become drier.

And wet areas historically struggle with problems like landslides, floods, erosion, extreme weather, and the spread of insect-borne diseases, which again, are only going to get worse if these areas get wetter. That doesn’t mean the situation is hopeless, though. Learning about these patterns helps us prepare for future changes, and the more we know about what’s coming, the more we can do to stop it.

Both by working to slow down the climate crisis, and by working to mitigate the worst of its effects. Thanks for watching this episode of SciShow! You can help us make even more cool videos about stuff like how 19th century data helps us fight the climate crisis.

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