Sometimes, climate-speak doesn’t match common-speak. There’s enormous fuss over global warming of 1.5° or 2°C. These numbers seem so small that perhaps the fussers have turned a molehill into a mountain. Who cares whether it’s 25°C or 26.5°C this summer? But that’s a misunderstanding. Global warming refers to higher average global temperatures, which are a different beast to daily temperatures.
“Scientists talk about a global mean temperature, and last year that temperature was 14.8°C,” says Sam Dean, a principal climate scientist at Niwa. “But nobody experiences that personally.”
The global mean is the average over the Earth’s surface – ocean and land, summer and winter, day and night, the tropics and the poles.
In contrast, we perceive local extremes: frosty mornings and roasting summer days. “If the daily maximum in Auckland in January was frequently hitting 30°C for a number of weeks, nobody would be surprised it was a record-breaking month,” says Dean. But that might reflect an average over 24 hours of 21°C, a mere 1°C above normal. In September, we would scarcely notice an average of a degree higher.
Until recently, global mean temperature was steady. “The normal annual variation measured between 1850 and 1900 was approximately 0.1 degrees,” says Dean. That timeframe is the pre-industrial estimate that global mean temperature is benchmarked against. Earth is now 1.1°C warmer than then.
“That change is 11 times greater than the [past] natural variation each year.”
The ocean has so far limited the temperature rise by absorbing 90% of the excess heat trapped in the atmosphere by greenhouse gases. It has soaked up about 40% of the carbon dioxide. But our luck may be running out.
“The ocean surface temperatures are now off the charts,” says Dean. “There’s record low sea ice levels, probably driven by those temperatures, and also increasing ocean acidification.” Melting Antarctic and Greenland ice sheets will cause much more sea level rise than the 20cm we’ve seen so far, he says.
The predicted jump to a 2°C mean increase above the pre-industrial temperature by mid-century will have dramatically more impact than recent climate-related calamities. “That means lots of places would go up much more than 2°C, especially continental land masses. Some parts of the world are already borderline habitable due to extreme heat or low rainfall. Hundreds of millions of people live in places that have a real chance of becoming uninhabitable above 2°C.”
Luckily, New Zealand is not one of them.
None of us will witness the added heat’s full consequences because it takes hundreds of years for Earth’s systems to settle into an equilibrium after a change. “There’s no analogy to what’s happening because nothing like this has ever happened before,” says Dean. “The speed of temperature change is unprecedented.”
Dean hopes a 2°C equilibrium is never reached. “It’s not a target to aim for, it’s a limit we really don’t want to exceed. Hopefully, we get to negative emissions later this century.” That will require sequestering massive amounts of CO₂ into vegetation or underground.
For now, the 1.5°C Paris Agreement target limit may be temporarily breached this year, and global commitments made under the agreement will, at best, hold the rise to 2.5°C by century’s end. Global strife is likely at that ostensibly tiny temperature rise, so taking action will be cheaper in terms of money and suffering.
“We can either reduce emissions or we can prepare for a world of greater armed conflict and disasters,” says Dean.
“But the great achievement of science is the ability to predict the future before it happens. It gives people the opportunity to make better decisions.”
