World leaders have constantly stressed the need to limit global warming to 1.5 degrees Celsius. In the 2015 Paris Accords, countries also committed to limit global average temperature rise to less than 2 degrees above pre-industrial levels, a report by Reuters said.

What does this mean? Why does crossing the 1.5-degree mark put us at more severe risk of severe climate change effects on a global level?

So far, the world has heated to 1.1°C above pre-industrial levels, with each of the last four decades recording hotter temperatures than any decade since the mid-19th century.

Extreme weather

Climate scientist Daniela Jacob, from Germany's Climate Service Center, told Reuters "We never had such a global warming in only a few decades," explaining that "Half a degree means much more extreme weather, and it can be more often, more intense, or extended in duration."

An extreme heat event that occurred once per decade in a climate without human influence, would happen 4.1 times a decade at 1.5°C of warming, and 5.6 times at 2°C, according to the U.N. climate science panel (IPCC) cited by Reuters. If warming continued to 4°C, such events would occur 9.4 times per decade.

This means that changes in extremes will become larger, heatwaves, floods, extreme rainfall, and more intense droughts would happen more often.

Not just the weather

After the 2°C mark, the ice sheets would collapse. Instead of the few feet of sea-level rise expected by the end of the century at 1.5°C, we would witness sea levels rising up to 10 meters. 

1.5°C of warming would destroy 70% of coral reefs, whereas 2°C of warming would mean 99% of them would be destroyed.

Crops will fail, food prices would spike, hunger and famine would grow more widespread, disease-carrying mosquitoes would be able to expand across a wider range. 

"If we can keep warming below 3°C we likely remain within our adaptive capacity as a civilization, but at 2.7°C warming we would experience great hardship," says climate scientist Michael Mann, at Pennsylvania State University.