When the Paris Agreement was reached in 2015 in response to a warming world and changing climate, things quickly got mathematical.

In order to slow planetary heating caused by burning oil, coal and gas, and linked to devastating storms, flooding, drought and heat waves, countries set one number as the holy grail.

They agreed to try and limit the rise in the global average temperature to 1.5 degrees Celsius (which translates to 2.7 Fahrenheit) above pre-industrial levels. Should they fail that target, they pledged to cap the increase at a maximum of 2C (3.6F).

These differences might sound too negligible to be worth mentioning, but as scientists regularly point out, they are crucial.

In the same way that a slight increase in bodily temperature can make humans feel sick, excess heat in the atmosphere affects the planet. It intensifies the strength and likelihood of extreme weather events that touch everything from the cost of food, home insurance policies, migration, human health, and water security.

Yet in the decade since the Paris climate accord was agreed, most nations have continued to burn fossil fuels and emit greenhouse gases, such as CO2, that are causing the planet to warm. And that, in turn, means other bigger numbers are also doing the rounds.

What does 1.5 degrees Celsius mean?

The 1.5 number is not random. Striving to prevent temperatures from rising above that limit had clear reasons. The hotter the world gets, the more exposed people become to deadly heat, and nations to the risk of rising sea-levels. Likewise, ecosystems are in greater danger of collapse. For instance, the risk of irreversible losses of marine and coastal ecosystems is much higher once 1.5C is surpassed.

A report published last month by the scientific initiative World Weather Attribution and US research organization Climate Central found that since the 2015 Paris Agreement, the world has warmed 0.3C. Even those seemingly small increases translate to an extra 11 hot days per year.

Scientists and experts now broadly agree that the 1.5C goal is out of reach. "At least temporarily," according to recent UN research, which said the only way to get back on track by 2100 would be to cut emissions more than 55% within the next 15 years. This would require radical and swift reductions in greenhouse gases.

What does 2 degrees Celcius mean?

Scientists, and later politicians, had been talking about keeping temperature rises to below 2C for many years before the target became enshrined in the Paris Agreement as the goal to hit should countries miss the 1.5C mark.

Here too, however close the numbers sound, they are attached to very different realities. According to the research organization, the World Resources Institute, while 1.5C of warming will see 14% of the global population exposed to severe heat, an additional 2C will impact more than a third of all people. And at 2C of warming, between 800 million and 3 billion people worldwide would experience chronic water scarcity due to drought.

Emissions in 2030 would have to fall 25% below 2019 levels to keep the world on a 2C pathway, according to the latest UN Emissions Gap report.

Sarah Heck, an analyst at Climate Analytics, a non-profit science and policy institute, says an additional 2 degrees Celsius of heat in the atmosphere would lead to ice-free summers in the Arctic at least once a decade, as opposed to once a century in a 1.5-degree scenario.

Ice-melt doesn't only exacerbate sea-level rise, which threatens coastal communities, low-lying islands and wildlife. Thawing permafrost releases the trapped and potent greenhouse gas, methane, which causes yet more warming. 

What does 2.6 degrees Celsius mean?

Despite the goals set out in the Paris Agreement, the latest global temperature update from independent monitoring service, Climate Action Tracker, shows that under current policies to cut emissions, the world is set to warm by about 2.6C by 2100.

This level of heating could lead to the collapse of marine ecosystems and a dramatic increase in extreme weather like intense drought and rainfall. It also increases the likelihood of triggering catastrophic and, in some cases, irreversible climate "tipping points," such as major ice-sheet loss and the retreat of mountain glaciers that billions rely on for freshwater.

Extreme weather reached new heights in 2024, which was the hottest year on record. Wildfires hit the US, heat waves baked India and many other parts of the world, and the super typhoon Yagi devastated Southeast Asia.

World Weather Attribution says climate change contributed to the displacement of millions and led to the deaths of at least 3,700 people in 2024.

Climate scientist and communicator Adam Levy, who says 2.6C will be difficult for humans to cope with and adapt to, is calling on everyone to remember another number.

"The world will keep heating as long as we keep adding carbon dioxide to the atmosphere. So whatever else is going on with all the temperature limits, with all the targets, zero is the number we should always have at the back of our minds," said Levy.

Edited by: Tamsin Walker