What’s the difference between 1.5 and 2 degrees of global warming?

A difference of half a degree celsius might not mean much to someone sitting in the sun or adjusting the heating in their home. But in terms of how much warming we are subjecting the planet to, it could mean many millions more people are subjected to life-threatening climate events.

If average global temperatures reach 1.5C above pre-industrial levels, we can expect the Arctic ocean to have one ice-free summer every 100 years. But if warming rises to 2C, ice-free summers in the Arctic could happen every 10 years. That’s just one example given in a recent Intergovernmental Panel on Climate Change (IPCC) report that illustrates just how significant that extra 0.5C of warming could be.

In 2015, 196 countries adopted the Paris Agreement aimed at: “Keeping a global temperature rise this century well below 2C above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5C.”

The phrase “pre-industrial levels” is significant. In its report, Global Warming of 1.5°C, the IPCC explains that the years 1850-1900 are used as the basis for comparisons with current temperatures.

The target of 1.5C is, in the words of Myles Allen, professor of Geosystem Science at the University of Oxford and a co-author of past IPCC reports, “physically, technically and economically possible”.

Writing for the Chatham House institute in 2018, Allen said: “To achieve a 1.5C world, we need to invest around 2.8% of global GDP into the energy system between now and 2050.” He went on: “Even if we weren’t aiming for 1.5C, we’d be investing around 2% of global GDP into the energy system, because the world needs energy one way or another.”

The IPCC warns that “human-induced warming reached approximately 1C (likely between 0.8C and 1.2C) above pre-industrial levels in 2017” and that we are currently experiencing a rate of warming of 0.2C per decade.

Those numbers are averages, and greater-than-average levels of warming have already occurred in parts of the world. Between 20% and 40% of the world’s population lives somewhere that experienced warming in excess of 1.5C in at least one season in the period 2006-2015, the IPCC says. Global warming is not evenly distributed.

In a report titled A Degree of Concern: Why Global Temperatures Matter, NASA says: “Temperatures increase at different speeds everywhere, with warming generally higher over land areas than oceans. The strongest warming is happening in the Arctic during its cool seasons, and in Earth’s mid-latitude regions during the warm season.”

As mentioned, the IPCC fears ice-free summers will be increasingly common in the Arctic ocean if average global warming reaches 2C. Other extreme weather events are set to follow, too, unless warming is controlled. Indeed, some are already evident.

The Australian Climate Council has outlined some of the other major climate and weather effects of 2C warming. They include a devastating loss of coral reefs, loss of habitat for many insect species and more than a third of people exposed to extreme heat at least once every five years.

Keeping the average rise at 1.5C, rather than at 2C, could mean a reduction of 50% in the number of people who experience increasing climate change-induced water stress, according to NASA. But while 184 to 270 million fewer people could be exposed to increased water scarcity by 2050 in this scenario, NASA warns there will be dangerous levels of rainfall in some parts of the world, sparking floods reminiscent of those seen across parts of Europe in mid-July.

Scientists and activists agree that avoiding the worst-case warming scenario calls for drastic action to reduce carbon emissions and, chiefly, to reengineer global energy systems. In its Fostering Effective Energy Transition 2021 report, the World Economic Forum examined the performance of 115 countries’ energy systems. Just 10% of them have made consistent improvements, the report says.

One of the chief concerns around energy system improvements is that the heaviest users of fossil fuels aren’t yet making enough progress, according to the report, which notes that out of the world’s 10 largest economies, only the UK and France feature in the top 10 for effective energy transition.