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As Earth’s climate changes, one of the hardest things to figure out is exactly how the planet will change in response. And while we can’t know the future for sure, we can get a lot of good clues from the past.

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As Earth's climate changes, one of the hardest things to figure out is exactly how the planet will change in response; like how weather patterns will shift and how species may or may not adapt. And while we can't know the future for sure, we can get a lot of clues from the past.

Our planet has been through a lot of changes and understanding different chapters in its history can help us figure out what might be coming but most of those changes took place before anyone was around to record them, so it takes some serious detective work to figure out what the world was like back then. Fortunately scientists have some pretty cool tools that help them reconstruct the climate in the past.

One place scientists can look for clues is in a common mineral called Calcium Carbonate, which forms in lots of places and is especially common in the ocean. It is made of, well, calcium and carbonate ions and if conditions are right, this mineral can come together all by itself, crystals just form straight out of the water.

But more often, it's marine organisms that harvest calcium and carbonate ions and turn them into shells and skeletons. And without knowing it, these little organisms are also recording small snapshots of Earth's history. That's because nature is messy, so when calcium carbonate forms, it's rarely ever pure. It contains other elements from the environment that get locked up in its crystal structure. 

These impurities are called trace elements, and what makes them good snapshots of the past is the fact that they don't always appear in the same quantities. 

Depending on different conditions in the environment, you'll get larger or smaller numbers of impurities in the mineral. For example, scientists can see that easily in a species of zooplankton called Foraminifera, which have shells made of calcium carbonate. 

Many of these microscopic organisms live near the surface of the ocean, where the water temperature is closely tied to the temperature of the air. And scientists have observed that at times when temperatures were higher on Earth, Foraminifera had a lot more magnesium in their shells. We don't know exactly why that's the case, but it may be because at higher temperatures, the motion of the molecules becomes more energetic, and that could make it easier for magnesium to take the place of calcium in the crystal lattice.

Regardless of how it works, many experiments have shown that this relationship between the amount of magnesium in Foraminifera and the Earth's temperature is extremely consistent across time. This means that scientists can take fossilized shells whose age they already know, measure how much magnesium is in them, and estimate the local temperature at that time. 

Scientests can also get similar information from corals, who make their skeletons out of calcium carbonate, too. In corals, calcium carbonate has a different mineral structure, so it's not the right shape to capture magnesium, but it does capture the element strontium, and the amount of strontium is also related to temperature. 

Except, strontium actually has the opposite relationship to temperature. As temperatures go up, the amount of strontium in coral shells decreases. Like magnesium in Foraminifera, this relationship isn't exactly understood. But, experiments have found that it's consistent. So, it can be used to estimate temperatures in the past. 

This is extremely useful, because both Foraminifera and corals can be found all over the world. There are other ways of extracting temperature information from certain places, like, scientists can study ice cores to learn about temperature changes at the poles. But nothing is wide-ranging as the clues in calcium carbonate. 

When it comes to piecing together climate history on land, scientists typically have to resort to a different set of clues. You might have heard about scientists using the width of tree rings as a way to figure out what environmental conditions were like in the past, but trees aren't the only things that accumulate layers of climate clues. 

In caves, mineral deposits known as speleothems do sort of a similar thing. You probably know speleothems by their more common names, stalactites and stalagmites. These, along with flat slabs on the cave floor known as flowstones are the three main types of mineral deposits in caves. 

They form when running or dripping water flows travels through a cave system. And, actually, just like Foraminifera shells, they are often made of calcium carbonate, or some other mineral containing calcium. That's because water becomes slightly acidic as it picks up carbon dioxide in soil. As a result, it dissolves some of the calcium out of the limestone rock it passes. Once it gets to the cave, the water deposits some of that calcium in a mineral form. 

These mineral deposits can build up over hundreds of thousands, or even millions, of years. And each layer says something about what the rainfall was like the year it formed. This is because the hydrogen and oxygen atoms that make up water have different isotopes, meaning some of the atoms have an extra neutron or two. Those extra neutrons don't affect an atom's charge or anything, but they do change its weight. So, atoms with extra neutrons are known as heavy isotopes.

And although they're pretty rare, when a cloud gets to its breaking point, and is ready to lose some water, the first raindrops to fall are usually made of heavy isotopes.

During periods of heavy rainfall, where clouds empty out more and more water, the number of heavy isotopes gets depleted early on, and the rain contains more of those light isotopes that wre originally left behind. Since speleothems form from rainwater, you can measure the ratio of heavy oxygen isotopes to light ones and estimate how much it rained during a certain time. 

In years with heavier rain, you'll get more light isotopes. This makes speleothems great tools to study events like monsoons, and they also make it possible to connect changes in climate to local weather. Even better, since they form in caves, these features are naturally protected from erosion, so they stick around for a long time. 

Caves are generally great places to look for old climate clues because of how they're protected from erosion. And they're often home to one particular climate recording creature: the bat. 

Like corals or foraminifera, bats are natural historians. But rather than capture climate information in their skeletons, bats leave behind clues in their poop. They deposit their poop, known as guano, in the caves where they live. And over time, it preserves information about conditions in the past. 

Even better, bats tend to poop in the same place for thousands of years, so piles of bat poop are like history books full of old climate details. By analyzing old samples of bat guano, scientists can measure the levels of different elements, which can be clues to events happening at that time. 

Bat guano is often used to figure out what humans were up to in the past. Scientists were able to connect a spike in nitrogen to the introduction of manure-based fertilizer in Jamaica around 3000 B.C.E. They've also seen the industrial revolution in bat guano- which shows up as an increase in lead levels around 1760. That's because these environmental changes altered the content of the things the bats ate, and pooped. 

But in addition to revealing human history, bat guano also offers clues about climate history. For example, the levels of carbon and hydrogen isotopes can offer clues about rainfall. In a 2008 study, scientists used bat guano from the grand canyon to reconstruct the history of monsoons in the region over 20,000 years.

Carbon isotopes can also tell you something about the types of plants in the area. For instance, thanks to bat guano, scientists figured out that some of today's tropical forests in the Philippines used to be covered in dry savannas. 

Guano can be really invaluable in low-lying regions or tropical regions where sediment layers aren't as well-preserved as they are elsewhere. Unfortunately, over time, guano has been harvested as a fertilizer, so most prehistoric guano deposits have disappeared. But some still exist in remote parts of the world, where deposits could be up to hundreds of thousands of years old. And that may have a lot to tell us about how our planet has changed.