Warming of 2C will push Mediterranean habitats into ‘unprecedented situation’

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.

One of the Pueblos Blancos (White Towns), Zahara de la Sierra, Spain. Credit: Terry Lawrence/iStock/Getty Images.

Pollen count

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.

‘Unprecedented situation’

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.

Glossary

RCP2.6: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP2.6 (also sometimes referred to as “RCP3-PD”) is a “peak and decline” scenario where stringent mitigation and carbon dioxide removal technologies mean atmospheric CO2 concentration peaks and then falls during this century. By 2100, CO2 levels increase to around 420ppm – around 20ppm above current levels – equivalent to 475ppm once other forcings are included (in CO2e). By 2100, global temperatures are likely to rise by 1.3-1.9C above pre-industrial levels.

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RCP2.6: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP2.6 (also sometimes referred to as “RCP3-PD”) is a “peak and decline” scenario where stringent mitigation… Read More

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.

Glossary

RCP4.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP4.5 is a “stabilisation scenario” where policies are put in place so atmospheric CO2 concentration levels off around the middle of the century, though temperatures do not stabilise before 2100. These policies include a shift to low-carbon energy technologies and the deployment of carbon capture and storage. In RCP4.5, atmospheric CO2 sits at 540ppm by 2100 – roughly 140ppm higher than now – equivalent to 630ppm once other forcings are included (in CO2e). By 2100, global temperatures are likely to rise by 2-3C above pre-industrial levels.

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RCP4.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP4.5 is a “stabilisation scenario” where policies are put in place so atmospheric CO2 concentration levels… Read More

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:

Glossary

RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP8.5 is a “very high baseline” emission scenario brought about by rapid population growth, high energy demand, fossil fuel dominance and an absence of climate change policies. This scenario is the highest of the RCPs and sees atmospheric CO2 rise to around 935ppm by 2100, equivalent to 1,370ppm once other forcings are included (in CO2e). The likely range of global temperatures by 2100 for RCP8.5 is 4.0-6.1C above pre-industrial levels. The release of the Shared Socioeconomic Pathways (SSPs) has introduced a number of additional “no-new-policy” scenarios, meaning RCP8.5 is no longer the sole option available to researchers as a high-end no-mitigation pathway.

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RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP8.5 is a “very high baseline” emission scenario brought about by rapid population growth, high energy… Read More

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:

Guiot says he hopes that both the methods and the findings of their paper will feed into the IPCC’s special report on 1.5C – the outline of which has just been agreed:

Rif Mountains landscape near the Mediterranean coast, Morocco, Africa. Credit: Lukasz Janyst/iStock/Getty Images.

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: