Climate scientists reported Friday that they are more than 95 percent certain humans are influencing Earth’s climate systems and triggering warming.

They are also more than 66 percent certain that global temperatures will rise by between 1.5 and 4.5 degrees Celsius (3.6° to 8.1°F) if the levels of carbon in the atmosphere doubles above pre-industrial levels. The findings were published in the fifth assessment report of the Intergovernmental Panel on Climate Change.

The IPCC Climate Report: What Good Will It Do?

The language of probability, confidence and uncertainty scientists use to explain the links between carbon and climate can require mental gymnastics. To many of us, a probability of 66 percent may not be particularly worrisome. And by extension, the risk (which is the probability of an event happening multiplied by its consequence) of climate change might seem acceptable.

Screen grab of the GROI tool.

But is it?

The Sweden-based Global Challenges Foundation has come out with a Global Risk and Opportunity Indicator or GROI to help us better understand climate probabilities.

The GROI compares the probability of a climate change scenario against other scenarios, for example a plane crash or being struck by lightning.

For instance, at 400 parts per million of carbon, the IPCC computes a 22 percent probability we will see temperatures rise by 2 degrees Celsius (3.6°F). Scientists argue that 2 degrees Celsius is the threshold beyond which the world will careen into dangerous climate impacts. We are already hovering near 400 ppm of carbon.

To understand these numbers, consider fatal flight accidents. The probability of a plane crash is 0.0001 percent or about 30 accidents a year.

If the probability were to increase to 22 percent, there would be 6,648,000 accidents every year. That’s 759 accidents every hour.

Global Warming: Why Only 95 Percent Certainty?

That’s a really high risk few of us would be willing to accept when it comes to flying. So why are we okay with accepting the risk when it comes to the future livability of our planet?

The IPCC report states that we need to limit carbon emissions to 1000 gigatons (about 480 ppm) of carbon to have a 66 percent chance global temperatures will not rise beyond 2 degrees Celsius.

In terms of flight accidents, that probability would give us 2,483 accidents every hour.

“Being told that the chance of either of these events happening was still “very unlikely” would be little comfort in the face of such numbers,” the Global Challenges Foundation said in a press release. “Would anyone risk flying? Very unlikely. So why do we tolerate this level of risk for the planet?”

Top Image: Aircraft vapour trails near Manchester Airport. (Robert Birkby/Corbis)