If we don't cut greenhouse gas emissions, we'll see more deadly heat waves, acidic oceans, and rising seas.

At this point, the planet will warm no matter what — but we can still prevent it from getting too bad.

Environmentalist and author Bill McKibben told Business Insider that without intervention, the world would be: "If not hell, then a place with a similar temperature."

The world is almost certainly going to warm past what's frequently considered a critical tipping point.

A recent study pointed out that we have just a 5% chance of keeping the planet from warming more than 2 degrees Celsius, the upper limit the Paris Agreement was designed to avoid. Beyond that threshold, many researchers say the effects of climate change — like rising sea levels, ocean acidification, and intense storms — will become significantly more concerning.

But how bad could it really get? What would the planet look like if we don't cut emissions and instead keep burning fossil fuels at the rate we are now?

Business Insider recently asked author and environmentalist Bill McKibben that question, and his description of what Earth would look like was sobering.

"If not hell, then a place with a similar temperature," he said. "We have in the Earth's geological record some sense of what happens when you run carbon levels up to the levels we’re running them now — it gets a lot hotter."

Extreme as that might sound, there's significant evidence that we're feeling the effects of climate change already. Unchecked, the planet will get far hotter by 2100 — a time that many children alive today will see.

"Huge swaths of the world will be living in places that by the end of the century will have heat waves so deep that people won't be able to deal with them, you have sea level rising dramatically, to the point that most of the world's cities are drowning, the ocean turning into a hot, sour, breathless soup as it acidifies and warms," McKibben said.

SANT SALVADOR DE GUARDIOLA, Spain A car drives on a dirt road past trees that were scorched by a forest fire near Montserrat in Sant Salvador de Guardiola, in the northeastern region of Catalonia, Spain, July 27, 2015. REUTERS/Gustau Nacarino

The evidence for how bad it could get

None of that is exaggeration. A recent study in the journal Nature Climate Change found that 30% of the world is already exposed to heat intense enough to kill people for 20 or more days each year. That temperature is defined using a heat index that takes into account temperature and humidity; above 104 degrees Farenheit (40 degrees C ), organs swell and cells start to break down.

Heat waves are the deadliest weather events most years , more so than hurricanes or tornadoes. In 2010, more than 10,000 people did in a Moscow heat wave. In 2003, some estimates say a European summer heat wave killed up to 70,000.

Even if we drastically cut emissions by 2100, the world will continue to warm due to the greenhouse gases that have already been emitted. That would cause the percentage of the world exposed to deadly heat for 20 or more days to rise to 48%. Under a scenario with zero emissions reductions from today, researchers estimate that 74% of the world will be exposed to deadly heat by the end of the century.

Our oceans are at risk, too. A draft of an upcoming US government report on climate change projects that even if emissions are cut to hit zero by 2080, we'll still see between one and four feet of sea level rise by 2100. Without the cuts, it suggests that an eight-foot rise can't be ruled out. That report also suggests that oceans are becoming more acidic faster than they have at any point in the last 66 million years. Increased acidity can devastate marine life and coral reefs, which cover less than 2% of the ocean floor but are relied upon by about 25% of marine species — including many fish that are key food sources for humans.

The key takeaway here is not that the world is doomed, however. It's that if we don't dramatically cut emissions soon, we'll put the planet on course to be a much less pleasant place.

In some ways, progress towards emissions reductions is already underway. Market trends are increasing use of renewable energy sources, political movements are pushing leaders to enact new types of policies, and legal challenges to government inaction on climate are popping up around the world. The question is whether we'll act fast enough to stave off the most dire consequences of greenhouse gas emissions.

"In order to catch up with the physics of climate change, we have to go at an exponential rate," McKibben said. "It's not as if this was a static problem. If we don't get to it very soon, we'll never get to it."