杏吧原创

鈥淲e have to do everything, and we have to do it immediately.鈥� As a summary of the challenge facing humanity, those words are about as pithy as it gets. They come from , a professor of climate physics at the University of Leeds, UK, and a lead author on the (IPCC). In case you missed it, it said we鈥檙e done for. Almost.

To give ourselves a better-than-evens chance of avoiding 1.5掳C of global warming 鈥� the agreed threshold of irreversible, dangerous and possibly game-over climate change 鈥� we must hit peak greenhouse gas emissions as soon as possible, and then taper them down to net zero by 2050, give or take five years.

That鈥檚 doable 鈥� just. 鈥淚t is not impossible,鈥� says , another IPCC lead author at Radboud University in the Netherlands. 鈥淲e don鈥檛 need any fancy new technologies.鈥�

But it will require 鈥渦nprecedented rates of transformation鈥�, according to the IPCC report. We must break our addiction to fossil fuels, meat, flying, reckless consumption in general, and above all confront the head-in-the-sand denial that has prevented us from making changes we knew we had to make yonks ago, but somehow kept telling ourselves weren鈥檛 necessary just yet.

So how can we do it? Think of it as a video game with seven levels of increasing difficulty 鈥� seven levels we must now play simultaneously.

There are many pathways in this giant, multidimensional, multiplayer game, although none is certain to lead to the outcome we want. We explore them here. But before we do, we need to go over some ground rules.

How to play Hitting 1.5掳C

Our overall aim is clear: limiting global warming to the magic number of 1.5掳C above pre-industrial levels. To get there, however, we need to keep another magic number in mind: how many more tonnes of carbon dioxide can we afford to dump into the atmosphere before we bankrupt the climate. Depending on what 鈥渃arbon budget鈥� we choose, we can increase our chances of winning the game at the expense of making the gameplay harder.

We might choose 770 gigatonnes. That is the level of future emissions we think gives us a 50 per cent chance of limiting warming to 1.5掳C. To tilt the odds to the tune of two-thirds in our favour, however, we must go a lot lower, down to 570 gigatonnes. More-pessimistic assessments of how much warming has already happened put these numbers considerably lower.

To put the figures in perspective, annual global carbon emissions are currently around 40 gigatonnes. So it will be game over in less than 20 years if we don鈥檛 start to play now. 鈥淒oing nothing isn鈥檛 an option,鈥� says Forster.

One catch is that unknowns such as exactly how sensitive the climate is to carbon dioxide and other greenhouse gases, especially methane, mean our carbon budget could change even as we play. To make things even hairier, about half of the budget is already spoken for, in the shape of existing or planned electricity generating plants that 鈥� unless they are retired early 鈥� will carry on burning fossil fuels for many years to come.

To give ourselves that better-than-evens chance, we must spend our carbon budget wisely throughout the game. Where we need to spend more in one level, we must cut down in another, with the aim of peaking and then tapering down to the promised land of net zero emissions by 2050.

That goal is non-negotiable. If we keep on emitting, the climate keeps on warming, and we lose the game. As the IPCC report says: 鈥淎ny scenario that fails to reduce CO₂ emissions to net zero would not limit global warming鈥�.

So, let鈥檚 play.

听

Level 1: Kill fossil fuels

A journey of a thousand miles begins with a single step. As we start playing level 1 of the game, we get an immediate boost: we have already taken that first step. In the past few years, the transition to renewable energy has begun in earnest, driven by rapid progress in ramping up wind and solar electricity generation. In 2016, a quarter of the world鈥檚 electricity needs, making up more than 10 per cent of our total energy consumption, were covered by modern renewable sources, meaning anything but the traditional burning of biomass such as wood and dung.

Most pathways to limit warming to 1.5掳C require at least 50 per cent (and preferably much more) of the world鈥檚 energy needs to come from renewables by 2050. That means the almost complete decarbonisation of electricity generation.

The winning scenarios rely heavily on wind and solar, but also hydropower, geothermal energy and biofuels. Most pathways also require us to use nuclear energy more. In some scenarios, fossil fuels can remain in the mix, but must be defanged by using carbon capture and storage (CCS) 鈥� a technology to neutralise carbon emissions that we are busy developing in level 4. Coal, which produces more carbon per unit of energy than any other fossil fuel, must cover at most 13 per cent of our overall energy needs, and preferably zero.

The transition still isn鈥檛 happening fast enough, but nobody doubts that it is eminently doable, given the right nudges from policy-makers and financiers. These include ending both direct and hidden subsidies for fossil fuels, and imposing a form of carbon pricing to reflect the environmental damage that fossil fuels cause. Whether we will get those things to help us complete this level is another question entirely.

听

Level 2: Travel light

Already things are a little hairier in level 2. Decarbonising electricity isn鈥檛 enough to reach net zero: at the same time, we must ensure that other kinds of energy-consuming activities, currently powered by dirty fuels, convert to clean electricity or otherwise clean up their act.

This pitches us straight into a big one: how we move ourselves around. More than 90 per cent of transport is powered by oil, in the form of petrol, diesel and aviation fuel, and just 3 per cent of transport-related energy comes from renewables. Transport accounts for something like a quarter of the world鈥檚 energy-related CO₂ emissions, and its contribution is growing by about 2.5 per cent a year. As the IPCC says, this presents 鈥渕ajor challenges for deep decarbonisation鈥�.

鈥淲e鈥檒l need personal sacrifices to meet our transport goals鈥�

Recognising the scale of the challenge, most 1.5-degree pathways call for only a 30 per cent reduction in transport-related emissions by 2030. But even reaching this goal means mastering multiple weapons simultaneously.

Besides switching to electric vehicles (powered, of course, by renewables), we need to improve fuel efficiency and switch from oil to the biofuels we have made land available to produce in level 5, including in aviation. Together, these can deliver 80 per cent of the required cuts.

The rest will have to be done through personal sacrifice: choosing buses and trains over cars and planes, car-sharing and even avoiding travel altogether 鈥� unless you go on foot or by bike.

听

Level 3: Rebuild everything

If level 2 sounds like a slog, level 3 is even harder. At the last count in 2014, homes, offices, shops and other buildings were responsible for 23 per cent of the world鈥檚 energy-related emissions. Most of that is indirect consumption, via electricity used for lighting, heating, cooling, elevators, office equipment and other energy-sapping services such as kettles and servers. So clever gameplay in level 1, resulting in the massive expansion of zero-carbon electricity, will give us a head start to this level.

But a third of building-related carbon emissions come directly from burning fossil fuels, mostly gas for heating and cooking. And because we鈥檙e already lagging in tackling level 2, we must give it our all in this level, reducing emissions by 80 to 90 per cent by 2050.

Simply waiting for zero-carbon electricity to arrive is not an option. Our strategy for this level must include switching to energy-efficient lighting to reduce electricity consumption, installing state-of-the-art insulation and double-glazed windows and putting in new heating and cooking systems that don鈥檛 burn fossil fuels.

To put that challenge in perspective, the London offices of New 杏吧原创 occupy one floor of a six-storey building. The heating system is gas-powered, the lighting is mostly fluorescent and air conditioning is required in the summer to maintain a comfortable temperature. Most of the staff go home to draughty old housing stock with gas-fired boilers and stoves.

Now scale these problems up to the whole of London, the whole of the UK, the whole world. According to the IPCC report, 5 per cent of buildings in rich-world OECD countries alone will need to be 鈥渆nergy refurbished鈥� every year from now on.

鈥淣ew buildings will have to emit no CO₂ at all 鈥� preferably by 2020鈥�

Even that does not get you through the level, however. New buildings will have to be constructed without emitting any carbon dioxide at all, preferably by 2020. That means shifting away from carbon-intensive concrete and steel towards carbon-neutral wood-based materials 鈥� or fundamentally recasting the way we make those materials. Which brings us neatly to the next of our parallel levels.

听

Level 4: A new industrial revolution

Now we are deep into the game, and the difficulty ratchets up another notch. Industry is staggeringly hungry for fossil fuels 鈥� more so than buildings or transport 鈥� and its emissions are growing. Producing metals, pulp and paper, chemicals, concrete and minerals uses copious amounts of coal and other fossil fuels for heat and steam. To complete this level, these and other emissions will have to be slashed, by around 80 per cent.

Our first move must be to phase out coal. The next is to vastly increase energy efficiency, and electrify wherever possible. At this point, however, our gameplay is not good enough, because we start butting up against the limits of the weapons in our existing arsenal. The IPCC report admits that it will take massive investment in R&D to roll out new, carbon-free industrial processes for making things such as cement, iron and steel. And that is simply to hit what is called the 鈥渙vershoot target鈥�, of going over 1.5掳C for a few decades before clawing our way back (see graphic, below).

The jam-tomorrow technological fix doesn鈥檛 end there. For fossil fuels that we cannot wean ourselves off, the answer is carbon capture and storage. These technologies, as yet unproven on a large scale, aim to capture CO₂ emissions and sequester them somewhere, most probably underground, so they never reach the atmosphere.

So are we doomed to fail at this level? Forster, at the University of Leeds, says not. 鈥淲e do have the technologies, but we have to make them cheaper, more affordable, and make them work at a much bigger scale.鈥� That means not just CCS, but greener processes for making aluminium, steel, cement and the rest. Again, ideas exist 鈥� to make it through this level, we need to make them reality, and fast.

听

Level 5: Reap what we sow

Level 5 is in some ways the longest and trickiest of all 鈥� a journey through forests, farms and food.

At present, land use accounts for about a quarter of carbon emissions. Getting that to net zero is a game of complex trade-offs. Land can be used as a carbon sink, but we also need it to grow food, which inevitably emits CO₂. While balancing these competing needs isn鈥檛 easy, it gives us multiple buttons to press. 鈥淭here鈥檚 a whole portfolio of actions, and if you push hard on one you can push less hard on the others,鈥� says at Duke University in North Carolina, another IPCC lead author.

To cut a very long story short, the best way forward is to intensify agriculture while reducing our consumption of foods whose production results in lots of greenhouse gas emissions, predominantly those that come from cattle. That allows us to swap pasture for forest, while finding room to grow biofuels for aviation and other forms of transport. Pasture, especially with cows on it, is a gigantic source of both CO₂ and methane. Forests, on the other hand, pull CO₂ out of the air, so reducing deforestation is also a must.

Do all that successfully by mid-century and we鈥檙e close to winning the game. Close 鈥� but no banana.

听

Level 6: Suck, not blow

Our goal of net-zero emissions doesn鈥檛 mean emitting nothing at all. Some emissions are from sources that we do not know how to eliminate, such as nitrous oxide from agricultural fertilisers, and others are too hard to get rid of entirely. To limit global warming to 1.5掳C, these remaining emissions must be balanced out by removing carbon dioxide already in the atmosphere.

This is the theme of level 6: carbon dioxide removal (CDR) technologies. They are a secret portal that allows a successful exit from the game, delivering us to net zero as well as potentially clawing back any emissions overshoot. As the IPCC says: 鈥淎ll analysed 1.5掳C-consistent pathways use CDR to some extent.鈥�

鈥淭he goal of net-zero emissions doesn鈥檛 mean emitting nothing鈥�

The technology might conjure up ideas of some miraculous machine that sucks CO₂ out of the air, but such a technology does not exist 鈥� and may never. So we have to do it using what we already know.

Planting forests is the simplest way, so our success on this level is tied up with our progress on level 5. Another approach is BECCS, or bioenergy with carbon capture and storage: you grow biofuels, burn them and then sequester the CO₂ using the CCS technologies being simultaneously developed in level 4.

Neither is a shoo-in, says the IPCC: 鈥淐DR deployed at scale is unproven and reliance on such technology is a major risk in the race to limit warming to 1.5掳C鈥�.

But it鈥檚 not the biggest. That comes in level 7.

听

Level 7: Change ourselves

Level 7 is by far the most difficult and tortuous stage of our game 鈥� because it requires game-players to confront their own weaknesses. 鈥淲e have to address demand,鈥� says Forster.

This could be the real sticking point. When we talk about power generation, transport, industry, buildings and agriculture, we are talking about our own actions. If on any given day you use an electrical appliance, spend time inside a building, use hot water, travel anywhere in a vehicle or buy or eat anything, you are contributing to the problem. Changing that means making sacrifices, starting today: driving less, flying less, consuming less meat, having fewer children (see 鈥�What you can do鈥�).

Things are starting to change, says Forster, although not fast enough. Sales of electric cars are increasing. More people are cutting down on meat. Even the US, the climate rogue nation, is waking up, says Shindell. 鈥淚 feel there is an increasing awareness among the general population. I live in North Carolina, where we鈥檝e had two huge hurricanes. People are flabbergasted when they see the Trump administration respond but refuse to acknowledge the role of climate change at the same time.鈥�

And that, as the IPCC pointed out, is what is still lacking: political will. 鈥淲e鈥檙e talking about the biggest transformation ever, and we鈥檙e expecting policy-makers to drive it,鈥� says at the Institute of Sustainable Resources at University College London. 鈥淚s that conceivable in a democracy?鈥�

Caroline Lucas, the UK鈥檚 only Green Party MP, thinks it is 鈥� but only through people power demanding the sort of changes that will see us through levels 1 to 6. 鈥淚t鈥檚 clear we鈥檙e going to need a mass movement,鈥� she says. 鈥淎s people come to see that green alternatives improve our quality of life, as well as the state of our environment, I believe we can persuade politicians to take action.鈥�

One way or another, big change is coming. It is up to us to decide in what direction. 鈥淲e鈥檝e never seen this kind of transformation in terms of change at the global scale,鈥� says , another climate scientist and IPCC author at the University of Leeds. 鈥淚t will be very difficult,鈥� says de Coninck.

Forster goes further than that. 鈥淥f course it鈥檚 impossible,鈥� he says. 鈥淏ut we have to try.鈥� Even if we go over 1.5掳C, every bit of extra warming we shave off makes the world more liveable. And if we overshoot a little, we may be able to claw our way back. 鈥淲e have an opportunity,鈥� says Forster. 鈥淣ot only to solve climate change, but also to really make the world far better.鈥� Game on.

听

This article appeared in print under the headline 鈥淗itting 1.5掳C鈥�