Mike Berners-Lee is a writer, researcher and professor in the field of sustainability and carbon footprinting. He is the founder of Small World Consulting and the author of several books including How bad are bananas? The carbon footprint of everything and There is no planet B: a handbook for the make or break years. He is a keynote speaker at the GBTA conference in Berlin on 28 February-2 March

Writer, researcher and professor Mike Berners-Lee tells BTN Europe why aviation is at 'crunch point' and how we must all take more responsibility for when and how we travel.

BTN Europe: We have a serious challenge on our hands to reduce carbon emissions globally and aviation has a big role to play in that. Where are we on that journey?

Mike Berners-Lee: Aviation¡¯s at a real crunch point because we don¡¯t know how to solve this problem. The industry is also growing. Pandemic aside, global flights were rising every year. We are seeing some technology gains ¨C more efficient aircraft ¨C but they are getting eaten up by people flying more. There¡¯s no getting around the need to be more careful about when we fly and when we don't. Nobody is saying ¡®no flying¡¯, but I think there is just a hard reality that we don't know how to put a big passenger airline up in the air on a long-haul flight without burning through something in the order of 100 tons of aviation fuel which turns itself into over three times its weight in carbon dioxide.

Climate change is a reality and it [aviation] is one of the hardest to reach sectors. It's probably the hardest to reach sector in the whole global economy. It doesn't mean to say that none of us can go on holiday and it doesn't mean to say that none of us can go on business flights, but it does mean we need to be more careful about when we decide to go.

BTNE: The global aviation industry is often cited as being responsible for around two per cent of all carbon emissions. Is that accurate?

MBL: I think it¡¯s usually underestimated by about a factor of two because those estimates don¡¯t take account of the additional impact that emissions have at high altitude. Defra, for example, in company reporting usually advocates a mark-up factor of 1.9, so that¡¯s almost double. And if you look at some countries it¡¯s bigger again. In the UK we¡¯re surrounded by sea so we fly a lot. If you take a consumption-based approach to the carbon footprint of the UK population, my estimate is more like eight per cent, but it's more like four or five per cent of global emissions.

BTNE: Do you think corporates are waking up to their environmental responsibilities?

MBL: I think people have realised that there are more situations when it really does work out better to have a virtual meeting. You save a lot of time, you save quite a bit of money, and potentially you can have five or six videoconferences in less time than it takes to have one face-to-face meeting. But there are some situations where that's not going to work and you've just got to be there in person, on the factory floor, or whatever. It's just a case of scrutinising a little more stringently whether we should fly or do things virtually. There¡¯s no getting round it ¨C it¡¯s a huge issue for the travel industry.

BTNE: Flying for business is often necessary but leisure travel is more discretionary. Should we be thinking more carefully about our personal choices?

MBL: I would like to see a culture where people are using their own consciences a bit. In the UK the average person flies short-haul roughly once a year and long-haul once every three years. But there¡¯s a huge proportion of the population that never flies, and there¡¯s some people who are really gallivanting all over the place. If those people were to be more selective, that would make a huge dent in it.

BTNE: There are three concepts we tend to hear about that could reduce aviation¡¯s carbon footprint: electric aircraft, hydrogen-powered aircraft and sustainable aviation fuels. Talk us through them.

MBL: Electric is looking like it can be good for short-haul flights, but we're talking really short. I mean domestic hops ¨C it might get us as far as London to Edinburgh. And the reason why it's only good for that is that batteries weigh a lot and you get diminishing returns. So you get to a point where you put more batteries into the plane and the plane weighs more, so it needs more power just to take off and you get to a point where you just can't get any more distance out of it. So short of a big innovation in the weight of battery energy storage ¨C which I have heard one or two whispers of ¨C it still feels a long way off.

Then there¡¯s hydrogen and using hydrogen to create ammonia. Hydrogen has got the big advantage that it is really light but it's also got a massive disadvantage that it's incredibly bulky. So even if you compress it to something like 700 bar ¨C that's taking 700 cubic metres of hydrogen and compressing it into one cubic metre ¨C you can still only get the weight up to about 70 grams a litre. And a litre will only have about something like a third of the energy in it that a litre of petrol has. Liquid hydrocarbons are different. They're compact and there's a lot of energy for the weight ¨C not quite as good as hydrogen ¨C but you can do things like poor them into the wings. But with hydrogen you have to put it into cylinders so you can't put them in the wings. And the weight of the cylinder itself is probably a lot more than the hydrogen that's in it. It's not that it's not doable in principle, but it would require a complete redesign of an aeroplane. You would look at one of these aeroplanes and it wouldn't look like anything that we understand aircraft to currently look like. The whole wing structure¡¯s different¡­ the whole thing is a different concept, so that's decades away. That's quite a pipe dream.

Using hydrogen to create ammonia [to power planes] is easier. You can get the energy density up to something like a quarter of the energy density of liquid fossil fuel. It might get you planes that can fly across the Atlantic, for example, or to California with a refuel along the way, so we're getting there, but it still requires a big technological breakthrough and a lot of R&D required to get us to that point. So that's not simple.

Now we're left with sustainable aviation fuels (SAF). On face value that looks like the simplest fix, but what do we mean by sustainable? There are two things you can do [to produce them]. One is you use biofuel you've created from biomass. The trouble with that is that it's very land intensive and we've got to feed everybody, we've got to look after biodiversity and we've got to cut carbon emissions by planting trees. It doesn't take much biofuel to really put pressure on the food system. A little bit of biofuel comes at the expense of a lot of nutrition or a lot of land.

The second way, in theory, is to have a renewable energy source such as a rack of solar panels in the desert and a process for creating aviation fuel from that. That looks to me like the best solution in the medium term, but you've still got to remember that those solar panels that have been used to create sustainable aviation fuel are solar panels that are not being used to green up the [power] grid of the country they're in.

However you look at it, we're going to have a few decades in which we have to think more carefully about flying. That absolutely doesn't mean that we can't have a thriving travel industry. And it doesn't mean we can't all get to see the world and get to have fantastic cultural experiences and have our businesses thriving with all the international contact that we need. It just means we need think a bit differently about it.

BTNE: Offsetting is often maligned. What¡¯s your assessment?

MBL: It doesn't work. It's methodologically flawed. At its worst ¨C and a lot of airlines are guilty of this ¨C it¡¯s just salving consciences in an unrealistic way and is used to encourage people to fly more while taking an action which is not meaningful.

I've written about this in reasonable detail ¨C the kind of different offsets that you can do. At one end of the spectrum is all the stuff that comes in around $3 a ton ¨C that kind of amount of money ¨C which is generally bogus. Things like making claims that you're providing an efficiency improvement that will allow someone else to cut their emissions somehow. But when you look at that, what you generally find is that when you make a person more efficient in almost anything, or a part of the economy more efficient, what you find is that one way or another, and with rebound effects, it doesn't lead to a carbon cut. Even videoconferencing outside of the pandemic, for example¡­ it's just as likely that a videoconference leads to people getting to know each other and eventually stimulating a flight. There's a whole lot of study into rebound effects and the effect of efficiency improvements and so on. So that kind of offset doesn¡¯t work at all.

So it¡¯s got to be a carbon removal instead, and then you've got all sorts of problems with lots of carbon removal schemes. First of all, are they really happening? Can you verify them? What about international standards? Supposing you can get around that and find a really good scheme that you can be sure is happening, then you've got wider environmental and social impacts to look at¡­ are you just planting monocultures of trees where there should be biodiverse savannah? You could be doing something that's good for carbon but if you look at it more holistically it's just bad for the world.

Then you¡¯ve got the high-quality nature-based solutions such as, in the UK, they'd be [planting] mixed woodland ¨C the right tree in the right place ¨C and peat restoration, and those are really good things to do. We absolutely should be doing those. But the problem is with them that all those solutions are finite. All those solutions around the world are finite. There's a limit to them all and we need to be doing them anyway.

We get into this sort of land where on the one hand it's a good thing to do them but, on the other hand, it's not useful to think of it as ¡®it's OK to have had this carbon footprint because I've offset it¡¯. All the companies that I'm advising and all the science-based targets initiatives are coming out in a similar vein as well. They are all saying there is no substitute for cutting your carbon in line with what the science says needs to be happening on a steep downward trajectory.

BTNE: Long-distance and/or high-speed rail travel is getting increasing support from some European governments. Would you like to see more of that?

MBL: Rail¡¯s brilliant. And actually it's not always just about the speed. The last time I went out to Berlin I took the train and it was great. Such a relaxing experience. I did a lot of work and it was really chilled out. And we've been on holidays with the kids where we¡¯ve taken the sleeper train out to Italy ¨C it¡¯s a really nice thing to do. The next time I go out to Berlin I think I'm going to have to fly because I'm on a tight schedule, but for trips around Europe about half the time it works for me to take the train.

BTNE: Some flights are less harmful than others so what can businesses do to reduce their emissions when they do fly?

MBL: There are few rules of thumb and the first is that full aircraft are better. It¡¯s also a good idea to have an efficient aircraft ¨C there are significant and meaningful differences between airplanes. The class of flight makes a big difference and this is a real crunch thing. You see people¡¯s jaws drop when you talk to senior business execs about this because the hard reality is that cooping yourself up in economy class, if you can bear it, has a lower carbon impact.

A world where there's no flying, though? I wouldn't want to see it. I wouldn't want to see people running countries who haven't explored the world to understand other cultures and I wouldn't want to see my kids not understanding people from other cultures. So you know there's a real value to international travel, but the fact is that if you¡¯re going to fly make sure you get a lot of value out of it.