So important is the movie's message, that distributor National Geographic made it freely available for anyone to stream on YouTube for a limited time.
We were skeptical at first, but it's well worth taking 95 minutes out of your day for. In the film, DiCaprio travels the world, from Greenland to India, looking at the effect climate change has already had on the planet, and the efforts that are underway to put the brakes on it.
"I just want to know how far we've gone, how much damage we've done, and if there's anything we can do to stop it," DiCaprio says at the start of the film.
It's not an easy watch. DiCaprio and director Fisher Stevens look at the statistics and the real-world impact of rising temperatures, from dying coral reefs to sinking cities - it's a whistle-stop tour of the damage that we're doing right now to the planet.
DiCaprio speaks to scientists and experts all across the globe about these negative effects and some of the reasons why our world is warming up, including the use of fossil fuels, the impact of the processed meat industry, and deforestation.
There are some recognisable names along the way: Elon Musk pops in to talk about Tesla's batteries and the first Gigafactory built to produce them, while The Revenant director Alejandro Iñárritu explains how he had to move his filming unit halfway around the world just to find some snow cover.
President Obama features too, speaking about the Paris climate agreement.
You'll see Canadian Arctic fisherman and hunter Jake Awa talk about how he's seen Baffin Island go from huge blocks of hard blue ice to a thin layer of "ice cream-type" ice that melts much faster.
Conservationist Farwiza Farhan explains how palm oil plantations are ruining the rainforests of Indonesia, and environmental scientist Johan Rockström discusses how a 3-4°C global temperature rise will make many parts of the world uninhabitable.
What stands out most from the movie, besides the catastrophic effects that could be coming our way if immediate action isn't taken, is just how widespread and complicated the issue is. From the foods you choose at the supermarket to the political mix of the US Senate, so many factors play into the fate of the planet we live on.
And countries all have their own issues: while places like the US are trying to switch from fossil fuels to renewable energy, developing nations such as India are still working on getting hundreds of millions of people access to reliable electricity for the first time - and coal, not renewable energy, is currently the cheapest option.
As DiCaprio admits in the film, he's used up a lot more carbon-producing energy than a lot of the humans on the planet - each of us is in some ways responsible for the mess we're in.
As you'll see, there's a lot of bad news in Before The Flood, from melting ice to rising temperatures to rising sea levels, but it's not a completely bleak picture - if we take immediate action, there is a way forward.
"If we keep pushing, keep prodding, and most importantly keep educating the public there's no reason why we can't solve this problem in time," Obama tells DiCaprio at the White House.
The full film is no longer available for free on YouTube, but you can see clips and find out more about where and when it's screening here.