Around the world, almost 7 million people move to, or are born in, cities every month.

Which can be super efficient, because when people live close to each other, it's easier

to move people, power, water, and basically everything else around.

But it's also a problem: in order to build all the buildings and infrastructure these

new city dwellers will need - the equivalent of a new New York City every single month

for the next 40 years - we're going to need a lot of concrete.

And for the health of the planet, concrete is a one-two punch.

Well, technically cement is the heavy hitter.

It's the thing that, y'know, cements the basic ingredients of concrete - sand, rocks

and water - together into a single, useful material, and it's a mean source of planet-warming

carbon dioxide.

Cement's one-two punch comes from how it's made.

First, we have to heat limestone - which typically requires us to use lots of fossil fuels, which

emit CO2.

Second, once we do that, the limestone chemically breaks down into lime, which goes into the

cement along with some other stuff, and carbon dioxide, which goes into the air.

In most industrial processes, the energy used to run machines and heat stuff up is the overwhelming

source of carbon dioxide, but with cement, when the limestone itself breaks down, it

releases even more CO2 than all the other parts put together.

All told, for every ton of cement we make, almost a ton of CO2 gets released.

Today, cement's double whammy leaves it responsible for 8% of humanity's carbon

dioxide emissions - that's more than airplanes, ships, and long distance trucking put together.

And as we keep on building a new New York City each month, the emissions from cement

are only going to go up.

So, what to do about cement?

We could build some of these new buildings out of other materials and we also could use

less cement per building.

For the cement we do use, we could try to manufacture and heat it using renewable sources.

And these things would certainly help!

But the real game-changer would be to stop using limestone itself in cement - then it

wouldn't release CO2 when it breaks down.

And there actually are alternatives out there already, some of which can reduce the amount

of limestone needed - and others that can replace limestone completely.

So far, most of these alternatives are either too expensive to make or too early in development

or are being adopted tentatively because concrete is such an important structural material.

But some alternative cements are already being used successfully in major buildings and bridges

around the world, and when we use cement with less limestone, or with no limestone at all,

as the foundation for all of our new cities, we'll reap some seriously concrete benefits.

This video was produced in collaboration with Bill and Melinda Gates, who every year write

a letter describing their philanthropic efforts.

In this year's letter, they discuss 9 surprising things they learned in 2018, and how those

surprises have prodded them into action.

In addition to being surprised by the rapid death of old-school textbooks and the fact

that toilets haven't changed in a century, they were also troubled by the environmental

impacts of global urbanization - and we were happy to help them explain one of the impacts

in a concrete way.

To read the letter yourself, click the link in the description or visit GatesLetter.com.