1.5ºC will change the world: tackling climate change

Late last year, climate change delegates from 196 countries called for global warming to be capped at "well below 2ºC", with a view to limiting this figure to 1.5ºC. Yet with the world already nudging 1ºC above pre-industrial levels, the agreement has left researchers, politicians and the general public stunned - and not a little sceptical - but, for the moment, relieved.

For many years, climate researchers have worked with warming figures closer to 2ºC, running models to predict the effect of increasing temperatures on sea levels, weather events, biodiversity and many other factors. However, recent evaluation of climate models indicate that a 1.5ºC rise represents a tipping point, beyond which the future looks very bleak in some areas.

"Climate impacts are not distributed evenly over the globe and tropical regions would bear the brunt of the differences between 1.5ºC and 2ºC," says Dr Carl- Friedrich Schleussner, researcher at climate science and policy institute Climate Analytics. "In terms of extreme temperatures, in a 2ºC world we'd probably be looking into a new climate regime in these regions with up to three months of unprecedented heatwave conditions."

Schleussner's research, published in Earth System Dynamics and Nature Climate Change, indicates extreme heat would be just the beginning in a 2ºC world. Tropical coral reefs would be wiped out, while a 1.5ºC limit could still provide room for recovery. As he puts it: "The survival of tropical coral reefs is really limited to 1.5ºC. Going beyond this is likely to drastically reduce any hope of these systems surviving climate change."

At the same time, tropical agricultural production across West Africa, South-East Asia, Central and northern South America would fall, with reductions in yields of critical crops such as wheat and maize almost doubling at 2ºC compared to 1.5ºC.

"A massive question mark hangs over the tropics regarding risk to agricultural production," says Schleussner. "Higher latitudes may actually experience better conditions for farming, but for people living in the vulnerable tropical regions, the issue of food security is so important."

"Our scientific understanding has shown us how vulnerable human societies are to climate change," he adds. "Now these least-developed nations and smaller, developing states, already experiencing climate impacts, feel warming to 2ºC would simply be too much to cope with."

Yet severe effects reach beyond the Tropics. According to Schleussner, freshwater availability across the drought-prone Mediterranean and other sub-tropical regions, including South Africa and South Australia, would decrease by around 10 per cent at 1.5ºC, but drop by up to 30 per cent at 2ºC.

Meanwhile, across the globe, heatwave durations, heavy precipitation intensity and sea-level rises are all greater at the higher warming limit. For example, Schleussner estimates sea levels to rise by 50cm come 2100, relative to the year 2000, at 2ºC. This figure would, at 1.5ºC, be 10cm lower.

As the climate analyst says: "Somewhere between 1.5ºC and 2ºC we could also look towards an ice-free Arctic in September. Whether or not it's fully ice-free is not the issue; it is the scale of change that is important."

Arctic sea loss affects key circulation patterns in the atmosphere, including the jet stream in temperate regions. "We are just beginning to understand that these changes in the Arctic can have far-reaching consequences for our global climate system," warns Schleussner.

Schleussner is hardly alone in his commitment to show that a half-degree is a massive deal. Researchers at Nasa's Jet Propulsion Laboratory (JPL) point out that while global wheat and soya production rises with a 1.5ºC rise, partly thanks to favourable warming at higher latitudes, this advantage is obliterated at 2ºC.

"For these important crops, some places in the world are already close to a thermal limit," says Dave Schimel from the Carbon Cycle and Ecosystems group at JPL. "The [European Geosciences Union] study does not take into account pests and pathogens that may spread more rapidly at higher temperatures."

Meanwhile, Schimel's colleague Felix Landerer, from Sea level and Ice at JPL Science, is keen to highlight issues over sea-level rises. He points out how ice sheets are sensitive to surrounding ocean warming: "At 2ºC, we may have crossed a threshold for significantly more sea-level rise than has been indicated in this study."

He adds: "[Even if in this century] air temperatures level off, you could have committed to sea-level rise over multiple centuries... that would potentially wipe Florida off the map."

A sea-level rise threatens coastal cities around the world, but inland, many more cities face danger from the extreme heat that would accompany 2ºC warming. As Dr Daniel Mitchell, a researcher from the Environmental Change Institute, University of Oxford, says: "Cities trap heat so a half-degree increase in global average temperature could lead to a much higher increase in temperature in that city."

"This will have a significant impact on human health, especially in built-up cities such as Cairo," he adds.

Recent research from Mitchell and colleagues drew links between human mortality during heatwaves and climate change. Looking at the extreme temperatures across Europe during summer 2003, the researchers performed thousands of climate simulations to generate a detailed statistical description of the 2003 event. From here, they could assess the effect on human mortality. The results make grim reading.

In that summer, anthropogenic climate change increased the risk of heat-related mortality by a hefty 70 per cent in central Paris, and 20 per cent in London. Of the 735 summer deaths attributed to the heatwave event in central Paris that year, results indicate 506 were a result of climate change. Meanwhile, 315 heatwave-related deaths took place in London and 64 could be attributed to climate change.

"Our results show the numbers of people that die because of a climate change heatwave event are in the order of 100 to 500 in a single city," highlights Mitchell. "This is huge, especially when you scale that up to multiple cities in numerous countries."

According to the researcher, analysis to assess the effects of such events on more cities across the world is very much a work in progress. He and colleagues are currently looking at the differences in mortality between 2ºC and 1.5ºC in cities in South America as well as Thailand and Vietnam.

Lack of reliable data coming out of Africa makes solid analysis difficult, but as Mitchell adds: "You certainly can imagine that, for example, the very large cities in Africa will be hit incredibly hard."

Researchers at engineering consultancy firm Arup are also looking at the effect of extreme weather events on cities, but with an emphasis on the vulnerability of growing and ageing populations. Arup associate Polly Turton is working with organisations including University College London, King's College London, and Climate UK to combine satellite, OpenStreetMap and socioeconomic data to assess the effects, and recently revealed a heat vulnerability map of London.

Results indicate the inner-city elderly within the boroughs of Hackney, Islington and Tower Hamlets will be most vulnerable to heat-related illnesses as the capital gets hotter. These regions combine high population density with poorer-quality housing. As Turton highlights: "The less green space, open space or permeable land surface in a city, the less that city can cope with high temperatures and heavy rainfall events."

As climate change research on urban regions continues to show benefits of limiting warming to 1.5ºC, organisations are taking action. Consortia such as the C40 Cities Climate Leadership Group and 100 Resilient Cities are campaigning for this limit while sharing ideas on how to cope.

For its part, Arup has launched research into what cities can do to hit this target. Turton is eager for progress, but as she and her Arup colleague Dr Maria Sunyer Pinya point out, even the higher 2ºC target is ambitious.

"We have the urban heat island effect that is going to make the warming situation a lot worse in cities than in rural areas, so we can't just wait and see what will happen," says Pinya. "[Warming to] 1.5ºC is happening fast to our climate and there is a definite need to take climate action right now."

As the world comes to terms with trying to stop warming in its tracks at 1.5ºC, a controversial group of technologies, Negative Emissions Technologies (NETs), are rising in prominence. Taking a different tack from reducing CO2 emissions - in which options such as energy efficiency and renewable generation are used to reduce emissions to the atmosphere - NETs actually remove CO2 from the atmosphere.

According to Mitchell, "most people believe that we now need negative emissions to get to 1.5ºC, and I agree with this". Schleussner concurs: "If we don't have negative emissions available, it appears very unlikely that we will be able to hold warming below 2ºC."

Professor Pete Smith from the Institute of Biological and Environmental Sciences at the University of Aberdeen is the lead author of a recent Nature Climate Change paper that quantified the potential global impacts of different NETs on land, greenhouse gas emissions, water and more.

"I'm desperate that we tackle climate change and we're as aggressive as we can be," he says. "But we have to do it in a way that doesn't cause more environmental damage or at least comparable levels of environmental damage than we would get from going between 1.5ºC and 2ºC."

As he highlights, no NET, or combination of NETs, can be implemented to meet the less than 2ºC limit without significant impact on resources such as land, energy and water. For example, technology front-runner bio-energy with carbon capture and storage, and afforestation demand large swathes of land and water and compete with land that would otherwise be used for food production. Meanwhile, direct air capture is enormously energy-intensive. Alongside enhanced weathering, it still requires at least two decades of development to become technically viable.

"We are almost certain to need some form of negative emission technology to stabilise warming, but 'Plan A' must be to immediately and aggressively reduce greenhouse gas emissions," says Smith.

"We knew 20 years ago what we needed to do, but singularly failed to take that action," he adds. "Back then we still had the possibility to stabilise warming with just mitigation methods, but now we have to work much harder."

Necessary or not, many aren't enthused with NETs. Post-Paris, some researchers voiced concerns over a move towards 'nonsensical geoengineering'. Most notably, Kevin Anderson, deputy director of the Tyndall Centre for Climate Change Research, wrote in Nature News: "[The climate agreement] is a genuine triumph of international diplomacy... [but] the world has just gambled its future on the appearance, in a puff of smoke, of a carbon-sucking godmother."

For Schleussner, while NETs may be necessary, especially if the 1.5ºC or even 2ºC target is exceeded, more research is needed.

"By having negative CO2 emissions, you would slowly but surely decrease radiative forcing and thereby decrease temperatures again," he says. "Yet while our current understanding of the carbon cycle shows us that there is potential to reverse warming, it is not sufficient to say 'yes we can' with confidence."

He adds: "We have to communicate the uncertainties to society, as people should be aware that there's a risk that these technologies may not work as expected."

Still, as Smith comments: "I would say we know enough about this... and we have an active field of research among the earth system science community to better understand the feedback in the carbon cycle."

In light of myriad uncertainties, researchers worldwide are now calling for a dedicated programme to nail down the effects of half a degree, which will be published in an IPCC report come 2018. Mitchell, for one, has highlighted how the known scientific risks associated with the lower target are still few and far between and research is needed to fill in the gaps.

"Certainly we know that small island states will feel sea level rises... and we know that developed nations such as the US and England can respond to the half-degree difference by changing practices," he says.

"Yet over the past couple of decades, our scientific research has focused on higher greenhouse gas scenarios, and coming out of the Paris Agreement, we were asked what all the impacts are at this [lower temperature] level," he adds. "We had to say 'actually, we don't know as we haven't looked yet'."

Indeed, in his recent research, 'Realising the impact of a 1.5ºC warmer world', published in Nature Climate Change, Mitchell emphasises the importance of amending climate tools to get to grips with this issue. The researchers and colleagues use the UK Met Office Hadley Centre model, as well as additional models from research institutes around the world. However, as he points out: "We've always based our models on how much CO2 is pumped into the atmosphere, but we're now being asked for a certain temperature threshold, so we need to run our climate models differently."

Crucially, Mitchell is keen that he and other climate scientists have identified key regions that will benefit from lower global CO2 emissions and the half-degree difference in warming that should follow.

"It's so important that we look at the positive impacts of 1.5ºC," he says. "We really need to get the US on board. Countries from Europe can lead this, but if America doesn't follow then we're fighting an uphill battle."