The high ambition of the Paris Agreement, to limit global warming to “well below 2°C”, was driven by concern over long-term sea level rise. A warmer climate inevitably means melting ice – you don’t need a computer model to predict this, it is simple common sense.
As temperatures rise, sooner or later much of the world’s glaciers will become water, which will end up in the ocean. With enough warming, ice sheets could also begin to melt irreversibly. Also, water expands as it warms. Although the full impact will take a long time – centuries or more – the implications of even only 2°C warming for low-lying coastal areas and island states are profound. This is why, in Paris, the world agreed to “pursue efforts” to go further, and limit warming to 1.5°C above pre-industrial levels.
“Pre-industrial” is not always well-defined, but is often taken as 1850-1900 since that is when accurate measurements became widespread enough to estimate global temperature change. By the 1980s, when scientists first warned about the risks of climate change, the world had already warmed by around 0.4°C. Things have accelerated since, and while year-to-year changes show downs as well as ups, the general ongoing trend is upwards. Latest data from the Met Office shows 2016 is expected to be 1.2°C above pre-industrial levels – the hottest year ever recorded.
So given this, what will a world above 1.5°C look like?
Not much different … at first
Depending on climate sensitivity and natural variability, we could conceivably see the first year above 1.5°C as early as the late 2020s – but it is more likely to be later. In any case, the first year above 1.5°C above pre-industrial temperatures will not represent what a world that warm looks like in the longer term.
During that year we’d expect some extreme weather events somewhere in the world, as happens every year. Some of these heatwaves, heavy downpours or droughts may well have become more likely as part of the changing climate. Others, however, may not have changed in likelihood. Teasing out the signal of climate change from the noise of natural variability is hard work.
Zacarias Pereira da Mata / shutterstock
But there will be some places which do not yet see major impacts in that first year, that nevertheless will have become more likely to be affected. The “loaded dice” analogy is rather clichéd, but nevertheless useful – even a pair of loaded dice will not roll a double six every time, just more often than normal dice. So while the chances of an extreme heatwave, for instance, may have increased by the time we exceed 1.5°C, it may not necessarily occur in that year.