Nature loves to do things in cycles. You probably learned about some of
them in school, like the water cycle, maybe the carbon cycle, and heck,
maybe even the nitrogen cycle. What you probably can’t picture as a poster on your classroom wall is the salt cycle. And that’s because we’ve just discovered it. And we’ve also already broken it. Yay. In fact, humans are breaking the
natural flow of salt by… a lot. And in order for us to understand what
we’re breaking before we can fix it, let’s take a look at what the salt
cycle even is, and why it matters. [♪ INTRO] First of all, we are not just talking
about the salt on your kitchen table. Table salt, AKA sodium chloride,
is just one kind of salt out there. See, in chemistry, salts are
combinations of a positive ion and a negative ion that come
together to form a neutral compound. So for instance, your positive sodium
ion meets a negative chlorine ion, and they pair up and make the
salt on your potato chips. But you can also pair up two
sodium ions with one sulfate ion, since that’s got a charge of minus two. Or maybe that chlorine is more interested in doubling up and pairing with a magnesium ion. Point is, as long as the charges balance
out, there are a bunch of different ways to group up some ions to form a salt. But when we’re talking about the salt
cycle, the main players we’re interested in are sodium, potassium, calcium, magnesium,
chloride, bicarbonate, and sulfate. That being said, those ions
won’t stick together forever. Throw them into some water
and the polar water molecules pull the ions apart again, at least until
they dry out and get stuck back together. So it makes sense that salt can go through
natural cycles, if you think about it. Salt can erode out of rocks, dissolve
in water, and get washed out to sea. And once it’s there, it
doesn’t have to stay there; salt can be blown back to
land by the wind or deposited on the seafloor where it can
eventually be lifted back up and out. There’s a bit more to it than that, but the general idea is that salt
is pretty much always on the move. Which actually makes it surprising that
scientists didn’t think of it that way, at least not until pretty recently. Apparently the people who
study salt levels in the soil and the people who study salt levels in
freshwater don’t, like, talk to each other. And if you look at everything in isolation
like that, you can miss big trends in how things are moving from place to
place, and how that might be changing. So in a 2023 paper, researchers
connected those dots and made the case that the salt cycle
belongs with the water cycle and all the other cycles we think about
for how stuff moves around the planet. They also determined that
like so many cycles before it, humans have screwed it up. We’re basically moving salts
all around the cycle faster and at higher quantities than the
natural processes can cope with, and the amount of stuff we actually
use salts for is kinda staggering. Consider mining – not just rock salt,
but also stuff like potash that gets used for fertilizer, and
materials for cement and concrete. And when we mine them out
of environments, they’re… not there any more to erode out over
time and slowly wash downstream. And then when that cement and concrete weathers, it erodes all those salts into the water and soil, but much faster than would
have happened naturally. Same with the fertilizer that we chuck into fields and then runs off into the environment. Salts are even used in cleaning
supplies, like detergents, and they enter the environment via wastewater. Another big one is road salt. We’re mining out rock salt,
scattering it all over our roads, and never really cleaning it all up. Here in the US, it doesn’t
seem like anyone bothers to limit how much road salt you use. At best they make polite suggestions. That road salt can either get
washed away by melting snow and ice, or just kind of sit there where it is
and slowly soaks down into the soil. All of this essentially
takes that weathering process and puts it on ultra fast-forward. So maybe you’re thinking,
but Earth is surely very big. And we humans are very small. Can we really be disrupting
the salt cycle so much? Yes. Yes we can. Estimates suggest that in the United
States, just shy of 200 megatonnes of salt are washed into rivers and streams every year from natural, non-human related sources. But just road deicing alone produces
37.8 megatonnes of salt runoff. Add in other sources like agriculture
and you end up with 80-ish megatonnes of anthropogenic salts
entering the freshwater system, so 40% of what Mother Nature was doing on her own. Asking the natural salt cycle to deal
with that much salt is asking a lot. It’s not like we can just stop
doing some of these things. Food needs fertilizer, and
I dare you to get anywhere here in Missoula in January without road salt. And before all you locals say that
they don’t salt the road in Missoula. Yes, we do. That is a myth. But it is worth understanding their
impacts, including on ourselves. Salts blown by the wind can wreck soil fertility. And since the wind can carry things a long way, the effects aren’t localized to
whoever’s fault it was to start with. For example, a lake in Kazakhstan
called the Aral Sea started drying up in the 1960’s, and by now
it’s almost completely gone. Now, winds carry salt from
the dried-up lake bed up to 500 kilometers away into neighboring
Uzbekistan and Turkmenistan. That has negative consequences
for crops, wildlife, and human health all along the way. Or consider the effect on freshwater
lakes, where a spike in chloride – especially from road salt – can
signal a die-off of zooplankton and a corresponding algal bloom. That has negative consequences
for commercial fishing, as well as the overall health
of the lake’s ecosystem. In 2022, researchers warned that current
limits on chloride across North America and Europe aren’t strict enough
to prevent this from happening. And we humans are not immune to
the consequences of our actions. For instance, a lot of that salt
ends up in our drinking water, and we can’t always filter all of it out. And when we’re talking about sodium in particular, that can lead to high blood pressure,
especially in pregnant folks. Saltier groundwater can even
encourage radioactive elements to leach into the aquifers where we
get our water… and I don’t think I have to explain to you why you don’t
want radium in your drinking water. So, what can we do? Well, it’s pretty early days for this
research, like, it’s literally the same group of researchers in the same paper
saying “There is a salt cycle!” and also “we broke it.” According to those researchers, we need
a clearer understanding of human effects on salt transport so we can
figure out what effective limits would be for curbing our salt use. It’s also vitally important to understand
just how much salt we’re pumping into fresh water, because we… need that stuff. And we need more information on, and experts from, the global south to make sure we go
about making change in an equitable way. But I think it’s sort of cool
that we’re still discovering big, planet-wide phenomena like this. I mean, there’s a whole global
salt cycle and we just noticed? What else do we just not know about yet? Hopefully, next time we discover a
whole system about how our world works, we do so before we break the
whole thing, and not after. [♪ OUTRO]
