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Why Is 1.5 Degrees the Danger Line for Global Warming?

How a seemingly small change in average temperatures could trigger lasting changes for life as we know it.


Honestly, it’s a great question.

“Why is holding global warming to 1.5 degrees Celsius such a big deal?”

Many people think of the hottest days of the summer where temperatures already hit 40 degrees (that’s 104 degrees Fahrenheit) where they live – or hotter. Another degree or two is a little bit more uncomfortable, sure, but hardly feels like the end of the world.

So how does global warming crossing the 1.5-degree line become –  as one reader put it – “an extinction-level event”?

Spoiler alert: it’s not. At least not for humans. But it is right about the point that scientists project we’ll see some of the climate impacts we already see today begin to go from bad to outright terrifying. It’s about the point where we’ll likely see many natural systems begin to cross dangerous points of no return, triggering lasting changes and transforming life as we know it.

To put it another way, we want to do everything we can to keep warming below 1.5 degrees.

To understand why, read on.

Global Warming Is about Average Temperatures

When we talk about 1.5 degrees of warming, we’re talking about the increase in the Earth’s average temperature. We measure this increase from a baseline average temperature in the mid-to-late nineteenth century – when the Industrial Revolution swung into high gear and people began burning fossil fuels on an unprecedented level, jumpstarting climate change.

The important thing to understand is that global warming that comes from burning fossil fuels is not a uniform process. Due to a host of natural factors, some areas – like the poles – are warming much faster than others. So when we talk about preventing 1.5 degrees of global warming, we’re talking about preventing a 1.5 degree increase in the Earth’s average temperature. Some places have already crossed that line.

Temperatures Will Get Much Hotter than 1.5 Degrees

Global warming reaching 1.5 degrees doesn’t mean that average temperatures in some places won’t rise significantly beyond that number. Again – it’s just the global average.

Then there’s the fact that as average temperatures rise, spikes and heatwaves will go much, much higher than just 1.5 degrees.

As the Intergovernmental Panel on Climate Change (IPCC) – basically the gold standard for climate science – reported: “Several regional changes in climate are assessed to occur with global warming up to 1.5°C compared to pre- industrial levels, including warming of extreme temperatures in many regions.”

That’s rock-star-scientist-speak for: “If global warming reaches up to 1.5 degrees, the hottest of the hot temperatures will increase and many (more) places will get dangerously hot.”

We got a preview of what “extreme temperatures in many regions” looked like in 2018.

In Pakistan, a May heatwave took temperatures above 110 degrees Fahrenheit (43.3 degrees Celsius) and cost 65 lives in one city alone.

Europe also had a taste of the new normal last summer, with temperatures soaring above 115 degrees Fahrenheit (46 degrees Celsius) in Portugal. It wasn’t just Portugal either – the same heatwave roasted countries across the continent, breaking records and costing yet more lives.

All of which is to say, 1.5 degrees is not the limit of how much hotter things will get at some points throughout the year. Far from it.

The Climate Crisis Doesn’t Start at 1.5 Degrees – It’s Already Here

Another critical thing to understand about global warming is that it’s not the case that everything up to 1.49999 degrees is rainbows and unicorns and free ice cream for everyone. (But once we cross the 1.5 degrees-line, the Four Horsemen of the Apocalypse polish off their martinis, look at each other, and say, “It’s go time.”)

That’s because the climate crisis is already here. Today. Higher temperatures are already dragging out droughts and wiping out crops. Himalayan glaciers that provide water to some 240 million people are already melting. Storms like Hurricanes Harvey, Irma, and Marie are already getting stronger and more devastating thanks to climate change. The list goes on.

All of these impacts (and so many more) involve complex systems. Some overlap. Some don’t. But what they all have in common is heat. Heat is the factor that throws natural systems with their delicate checks and balances out of whack.

The (over)simple version is that the more heat added to the Earth’s climate system, the more out of balance natural systems get. The more out of balance natural systems get the more destruction and suffering we see. And it’s almost always poor families and people of color who suffer the most.

So where does the 1.5-degree number fit in?

Well, at about 1.5 degrees of global warming is right about where there’s enough heat to push many of the natural systems that sustain us past a dangerous turning point.

Think of 1.5 degrees not as an absolute line in the sand, but as a general indicator of where many climate impacts – on balance – go from destructive to catastrophic. It’s the sign on the door that leads to somewhere very dark indeed, somewhere no one wants to go.

The Bad News: Things Get Worse above 1.5 Degrees

So here’s the bad news: Back in the fall, the IPCC (remember, our team of global rock star scientists) released a report comparing best projections for what global warming looks like at 1.5 degrees versus 2 degrees. And at 2 degrees, we start getting into scenarios that make most dystopian horror movies look like children’s coloring books.

The IPCC projects that going from 1.5 degrees of global warming to 2 degrees could mean:

  • 1.7 billion more people experience severe heatwaves at least once every five years.
  • Seas rise – on average – another 10 centimeters (almost 4 inches),.
  • Up to several hundred million more people become exposed to climate-related risks and poverty.
  • The coral reefs that support marine environments around the world could decline as much as 99 percent.
  • Global fishery catches could decline by another 1.5 million tonnes.

Bottom line: Going above 1.5 degrees of warming puts millions more at risk of potentially life-threatening heatwaves and poverty. It all but wipes out coral reefs that entire ecosystems rely on worldwide. Seas swallow even more of our cities. And that’s just for starters.

The Good News: How Much the Planet Warms Is up to Us

The same IPCC report also noted that human activities (i.e. burning fossil fuels, for the most part) have already caused about 1 degree of global warming and that with all the greenhouse gases we’ve already put in the air, average temperatures will keep rising.


But, as the report goes on, “these emissions alone are unlikely to cause global warming of 1.5°C (medium confidence).” Which is to say, we still have a shot at holding the line at 1.5 degrees and keep what is already a climate crisis affecting millions from becoming a climate catastrophe.

But we have to act and – according to the IPCC – basically cut fossil fuel emissions in half by 2030 to do it. Which means moving very, very quickly to leave oil, coal, and gas behind and accelerate the just transition to clean energy underway around the world.

It’s a big ask and a lot to accomplish in less than 12 years. But nothing less than the future of the Earth is at stake. As teenage activist (and inspiration for the school strikes for climate spreading around the world) Greta Thunberg said, “Our house is on fire.” We know what we have to do to put it out.

There is such a thing as moving too slowly.

Ready to Take the Next Step?

The COVID-19 emergency has shattered our sense of normalcy and forced us to grapple with the kind of world we want – and need – when this crisis is over.

We believe that together, we can create a world where we rapidly transition away from fossil fuels to end the climate crisis, safeguard our health, strengthen our communities’ well-being, and ensure justice for those who have been marginalized in the past.

Will you join us for an upcoming Climate Reality Leadership Corps training?

During this moment of physical distancing, we’re excited to join together and raise our voices for change. Because even with the threat of coronavirus outside our doors, we are still working to give our children a better world to live in.