The Paris climate conference set the ambitious goal of finding ways to limit global warming to 1.5C, rather than the previous threshold of 2C. But what would be the difference? And how realistic is such a target? Environment 360 reports

How ambitious is the world? The Paris climate conference last December astounded many by pledging not just to keep warming “well below two degrees celsius,” but also to “pursue efforts” to limit warming to 1.5C. That raised a hugely important question: What’s the difference between a two-degree world and a 1.5-degree world?

Given we are already at one degree above pre-industrial levels, halting at 1.5C would look to be at least twice as hard as the two-degree option.

So would it be worth it? And is it even remotely achievable?

In Paris, delegates called on the UN’s Intergovernmental Panel on Climate Change (IPCC) to report on the implications of a 1.5C target. They want the job done by 2018, in time to inform renewed talks on toughening emissions targets beyond those agreed upon in Paris.

But the truth is that scientists are only now getting out of the blocks to address what a 1.5C world would look like, because until recently it sounded like a political and technological impossibility. As a commentary published online in Nature Climate Change last week warned, there is “a paucity of scientific analysis” about the consequences of pursuing a 1.5C target.

To remedy this, the paper’s researchers, led by Daniel Mitchell and others at Oxford University, called for a dedicated program of research to help inform what they described as “arguably one of the most momentous [decisions] to be made in the coming decade.” And they are on the case, with their own dedicated website and a major conference planned at Oxford in the fall.

So what is at stake? There are two issues to address. First, what would be gained by going the extra mile for 1.5? And second, what would it take to deliver?

First, the gains. According to available research, says the Oxford group, the biggest boost will not be measured in average temperatures. On its own, the difference between 1.5C and 2C is marginal. But it would have a much greater effect on the probability of extreme and destructive weather events like floods, droughts, storms, and heatwaves.

We know extreme weather is happening more often. A study last year by Erich Fischer of the Institute for Atmospheric and Climate Science in Zurich found that the risk of what was “once in a 1,000 days” hot weather has already increased fivefold. His modelling suggests that it will double again at 1.5C and double once more as we go from 1.5 to 2C. The probability of even more extreme events increases even faster.

The same will be true for droughts, says Carl-Friedrich Schleussner of the Potsdam Institute for Climate Impact Research in Germany. Last year, he reported that the extra half-degree would produce dramatic increases in the likely length of dry spells over wide areas of the globe, including the Mediterranean, Central America, the Amazon basin, and southern Africa, with resulting declines in river flows from a third to a half. Schleussner concluded that going from 1.5 to 2C “marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions.”

A few studies have tried to drill down to what the difference means for day-to-day lives. And the consequences for many will be stark. At two degrees, parts of southwest Asia, including well-populated regions of the Persian Gulf and Yemen, may become literally uninhabitable without permanent air conditioning.

Some researchers predict a massive decline in the viability of food crops critical for human survival. The extra half-degree could cut corn yields in parts of Africa by half, says Bruce Campbell of the International Center for Tropical Agriculture. Schleussner found that even in the prairies of the US, the risk of poor corn yields would double.

Two degrees, says Johan Rockström, director of the Stockholm Resilience Center, “contains significant risks for societies everywhere; 1.5 looks much more scientifically justifiable.”

Ecosystems would feel the difference too. Take tropical coral reefs, which already regularly come under stress because of high ocean temperatures, suffering “bleaching” especially during El Niño events – as happened on the Great Barrier Reef in Australia this year. Most can now recover when the waters cool again, but today’s exceptional temperature may soon become the new normal. “Virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards,” as warming slips past 1.5C , reports Schleussner.

By some estimates, curbing warming at 1.5C could be sufficient to prevent the formation of an ice-free Arctic in summer, to save the Amazon rainforest, and to prevent the Siberian tundra from melting and releasing planet-warming methane from its frozen depths. It could also save many coastal regions and islands from permanent inundation by rising sea levels, particularly in the longer run.

In 2100, the difference in sea level rise between 1.5C and 2C would be relatively small: 40cm versus 50cm. But centuries later, as the impact of warmer air temperatures on the long-term stability of the great ice sheets of Greenland and Antarctica takes hold, it would be far greater. Michiel Schaeffer of Climate Analytics, a Berlin-based think tank, calculates that by 2300, two degrees would deliver sea level rise of 2.7 meters, while 1.5 degrees would limit the rise to 1.5 meters.

Facebook Twitter Pinterest Historical and projected changes in global temperatures from 1850 through 2100 if greenhouse gases continue to rise unchecked through the end of the century. Photograph: Ed Hawkins/US Geological Survey

It looks like 1.5C matters a great deal. So how hard would it be to keep warming to that level? After all, last year was one degree above pre-industrial levels. And at various times in the past six months, global average temperatures have sometimes gone above 1.5C.

Most researchers agree that, short of some global economic meltdown, even decade-long averaged temperatures are destined to go above 1.5C of warming by mid-century. So delivering the target by the end of the century will require drawing down temperatures by using technologies and energy systems that can extract carbon dioxide from the atmosphere on a large scale.

For some, this would be nonsensical geoengineering. Kevin Anderson, a climate scientist at the University of Manchester in the UK, writing in Nature after the Paris conference, declared “the world has just gambled its future on the appearance, in a puff of smoke, of a carbon-sucking fairy godmother.”

But it could be done. The calculations are inexact. Nobody, even now, knows quite how sensitive global temperatures are to rising concentrations of greenhouse gases in the atmosphere. But here is the task, as outlined by Joeri Rogelj, of the Austria-based International Institute for Applied Systems Analysis (IIASA), in an article in Nature Climate Change in March.

The planet’s primary thermostat is the concentration of CO2 in the atmosphere. Pre-industrial levels were 280 parts per million. We just hit 400 ppm with warming at one degree and some more in the pipeline, due to time lags. The IPCC, in its most recent report, estimated that to stop at 1.5C will mean holding concentrations to around 430 ppm.

Because much of our CO2 emissions stay in the atmosphere for centuries, that means bringing annual emissions to zero. Impossible? Maybe, but the good news is that greenhouse gas emissions actually fell in 2015 despite rising global economic activity, thanks to the growing use of renewable energy. If we could build on that and bring emissions to zero by 2050, then we might limit emissions from here on out to 800bn tons.

If we could somehow find ways to extract 500bn tons from the atmosphere, Rogelj concluded, we would likely be able to have our wish of CO2 concentrations of 430 ppm and warming capped at 1.5C. The fairy godmother would have delivered.

But how? While there are chemical processes for removing CO2 out of the air, they remain very expensive. More likely are biological methods — using plants to soak up CO2 and then preventing that CO2 from getting back into the atmosphere when the plants die or are burned.

The trick that puts a glint in the eye of some technologists and climate scientists is known by the acronym BECCS, which stands for “biomass energy, carbon capture, and storage.” The idea is to convert the world’s power stations to burning biomass, such as trees or marine algae. The industrialized production of this biomass on such a scale would accelerate the natural drawdown of CO2 by plants during photosynthesis. If the CO2 created by burning the biomass could then be captured from the stacks and buried in geological strata — the prototype technology known as carbon capture and storage — then the net effect would be a permanent extraction of CO2 from the atmosphere.

It would be the reverse of the current fossil-fuel energy system. And the more energy generated, the more CO2 would be drawn out of the air.

There are huge questions about such a strategy. Wouldn’t such a vast new industry have its own absurdly high-energy requirements, putting us back at square one?

Is there the land available to cultivate all that biomass? Would we end up chopping down forests to make room for growing the biomass, creating a massive new source of emissions? While there are back-of-the-envelope calculations, nobody has yet satisfactorily answered these questions.

Other geo-engineering options that have been proposed include fertilizing the oceans so that more algae can grow, sucking up CO2 as they do, or a terrestrial equivalent – burying charred biomass known as biochar into soils, where it could provide a kind of deep fertilizer that would turn soils into carbon-suckers over many centuries. But says IIASA’s Florian Kraxner, “Of all the ways of achieving negative emissions, BECCS seems to be the most promising.”

Is this all scientific pie in the sky? Some analysts argue that, whatever was said in Paris, there is little chance of hitting even two degrees, let alone anything tougher. David Victor, of the University of California at San Diego, for instance, wrote in Yale Environment 360 at the conclusion of the Paris agreement that “the world has dithered for too long and must now brace for the consequences. Even a realistic crash program to cut emissions will blow through 2C; 1.5C is ridiculous.”

Others say that even trying to paint a picture of what a 1.5C world would look like is a fool’s errand. Mike Hulme of King’s College London in England wrote recently that it could result in bad science, because predictions about future local climate come with such wide error bars. He wondered whether, even at the request of the Paris conference, science should be “corralled into servicing a tightly determined political agenda.”

But the Oxford team is not having such defeatism. “It is our job as scientists, first and foremost, to inform. Whether or not the information we provide makes a difference is ultimately up to others,” they say in their new paper. Moreover, they point out, “if additional research is not undertaken as a matter of urgency, there is a danger… that the 2018 special report will present all the negative economic constraints of achieving 1.5C” without reporting on the potential positive impacts of reduced extreme weather activity that such a scenario could bring.

Ultimately, this is a highly political issue about who should be in charge of setting targets: those most vulnerable nations, who led the call in Paris for a 1.5C target, or those less vulnerable nations in the rich world, who were ready to stick with two degrees? Them or us?

As Petra Tschakert of Penn State University put it in a paper last year, “danger, risk, and harm would be utterly unacceptable in a 2C warmer world, largely for ‘them’ – the mollusks, and coral reefs, and the poor and marginalized populations, not only in poor countries – even if this danger has not quite hit home yet for ‘us’.”