This is the tale of two ancient cities and the trees that determined their destinies.

In 3,000 BC Uruk was more densely populated than modern day New York City. [75.000 people in 6 km2]

This crowded capital had to continually expand their irrigation system to feed its growing population.

2,500 years later in Sri Lanka, the city of Anuradhapura had a similar problem.

They were also growing constantly, and like Uruk, their city relied heavily on an elaborate irrigation system.

As Uruk grew, its farmers began chopping down trees to make space for more crops.

In Anuradhapura, however, trees were sacred.

Their city housed an offshoot of the Bodhi tree under which Buddha himself was said to have attained enlightenment.

Religious reverence slowed farmer's axes and even led the city to plant additional trees in urban parks.

Initially, Uruk's expansion worked well.

But without trees to filter their water supply, Uruk's irrigation system became contaminated.

Evaporating water left mineral deposits, which rendered the soil too salty for agriculture.

Conversely, Anuradhapura's irrigation system was designed to work in concert with the surrounding forest.

Their city eventually grew to more than twice Uruk's population, and today, Anuradhapura still cares for a tree planted over 2,000 years ago.

We may think of nature as being unconnected to our urban spaces, but trees have always been an essential part of successful cities.

Trees act like a natural sponge, absorbing storm water runoff before releasing it back into the atmosphere.

The webs of their roots protect against mudslides while allowing soil to retain water and filter out toxins.

Roots help prevent floods, while reducing the need for storm drains and water treatment plants.

Their porous leaves purify the air by trapping carbon and other pollutants, making them essential in the fight against climate change.

Humanity has been uncovering these arboreal benefits for centuries.

But trees aren't just crucial to the health of a city's infrastructure; they play a vital role in the health of its citizens as well.

In the 1870s, Manhattan had few trees outside the island's parks.

Without trees to provide shade, buildings absorbed up to nine times more solar radiation during deadly summer heat waves.

Combined with the period's poor sanitation standards, the oppressive heat made the city a breeding ground for bacteria like cholera.

In modern day Hong Kong, tall skyscrapers and underground infrastructure make it difficult for trees to grow.

This contributes to the city's dangerously poor air quality, which can cause bronchitis and diminished lung function.

Trees affect our mental health as well.

Research indicates that the presence of green foliage increases attention spans and decreases stress levels.

It's even been shown that hospital patients with views of brick walls recover more slowly than those with views of trees.

Fortunately, many cities are full of views like this—and that's no accident.

As early as the 18th century, city planners began to embrace the importance of urban trees.

In 1733, Colonel James Oglethorpe planned the city of Savannah, Georgia to ensure that no neighborhood was more than a two-minute walk from a park.

After World War II, Copenhagen directed all new development along five arteries—each sandwiched between a park.

This layout increased the city's resilience to pollution and natural disasters.

And urban trees don't just benefit people.

Portland's Forest Park preserves the region's natural biodiversity, making the city home to various local plants, 112 bird species, and 62 species of mammals.

No city is more committed to trees than Singapore.

Since 1967, Singapore's government has planted over 1.2 million trees, including those within 50-meter tall vertical gardens called supertrees.

These structures sustain themselves and nearby conservatories with solar energy and collected rainwater.

Trees and vegetation currently cover over 50% of Singapore's landmass, reducing the need for air conditioning and encouraging low-pollution transportation.

By 2050, it's estimated that over 65 percent of the world will be living in cities.

City planners can lay an eco-friendly foundation, but it's up to the people who live in these urban forests to make them homes for more than humans.

The time to get involved with the climate movement is now.

Don't know where to start?

We've partnered with the U.N. to create a 30-day challenge designed to teach you everything you need to know to become a part of the solution.

Start the challenge with a friend or family member today.