The Paris Agreement, slated to come into force on the 4 November, contains a two-pronged, long-term goal for limiting global warming – holding to “well-below 2C above pre-industrial levels” and a more ambitious aim to “limit the temperature increase to 1.5C”.
A new study, just published in Science, shows what difference the extra 0.5C of warming could make for Mediterranean habitats during this century.
A 2C temperature rise would be enough to cause shifts in Mediterranean ecosystems that are unmatched in the past 10,000 years, the study says. Only limiting warming to no more than 1.5C would keep ecosystem changes within the fluctuations of the Earth’s recent past.
And if greenhouse emissions aren’t curbed at all, the study warns that the warm forests of southern Spain and North Africa will likely be taken over by desert by the end of the century.
The starting point for the new study, led by Dr Joel Guiot, a research fellow at the European Centre for Research and Education in Environmental Geosciences (CEREGE), was to compare the past, present and projected future climate of the Mediterranean region.
For the past, the research uses the output from an earlier study, where Guiot and his colleagues reconstructed the climate of the Mediterranean region by analysing pollen grains buried in lake sediments. Guiot explains to Carbon Brief:
The researchers analysed tens of thousands of pollen samples across the Mediterranean region, building up a record of the climate going back 10,000 years – a period known as the Holocene.
Comparing past with present, the new study show that average Mediterranean temperature in the first decade of the 21st century has already exceeded the warmest century of the entire Holocene.
And over the past century, Mediterranean temperatures have risen more quickly than the global average. While the world, on average, is now 0.85C warmer than it was during 1880-1920, the Mediterranean is 1.3C warmer.
Projections suggest that the region is likely to continue to warm faster than the worldwide average over this century, the paper says.
So, what does this mean for the Mediterranean’s ecosystems? Using a computer model, the researchers simulated how past, present and future climate has affected – and will affect – the types and distribution of vegetation growing in the region.
For the future climate, the researchers use four different pathways for how levels of greenhouse gases in the atmosphere could change over the 21st century.
At the lower end are pathways “RCP2.6” and “RCP2.6L”. In the RCP2.6 scenario, global emissions are curbed to keep global average temperature rise to below 2C above pre-industrial levels. RCP2.6L is a modified version that approximates warming of 1.5C.
Their findings show that only limiting global warming to 1.5C keeps changes in Mediterranean ecosystems within the variations recorded during the Holocene.
Once the temperature rise tips over this point, the warming pushes ecosystems into new realms – even at 2C, says Guiot:
The expected shifts in vegetation are greater for the other pathways.
Under a “moderate” emissions pathway called “RCP4.5”, model projections suggest that deserts will extend further into North Africa by the end of the century. In addition, alpine forests of southern Europe would give way to hardier “sclerophyllous” plants – scrub and woodland that have small leathery leaves.
The most severe impacts would come under the “business-as-usual” pathway, known as RCP8.5, where emissions aren’t curbed, primary energy use continues to grow and global population reaches 12 billion by 2100. The paper describes the projected shifts in vegetation:
It’s worth noting that global emissions are currently tracking just above the RCP8.5 scenario.
Special report on 1.5C
The findings show the differences in impacts between 1.5C and 2C for the Mediterranean are “not harmless” says Guiot. This suggests that the more ambitious target of 1.5C in the Paris Agreement is worth the effort, he adds:
Dr Carl-Friedrich Schleussner, from the Potsdam Institute for Climate Impact Research, who authored a recent study of the differences in impacts between 1.5C and 2C, says the study shows that 1.5C is moving up the scientific agenda. He tells Carbon Brief:
And the study’s findings reinforce why research around 1.5C is so important, Schleussner adds:
Guiot, J. and Cramer, W. (2016) Climate change: The 2015 Paris Agreement thresholds and Mediterranean basin ecosystems, Science, doi:10.1126/science.aah5015
Warming of 2C will push Mediterranean habitats into 'unprecedented situation'