My interview with David MacKay has the feel of a university tutorial. Perhaps it is not surprising, since the chief scientific adviser to the Department of Energy and Climate Change is a professor of physics at Cambridge.

But the impression is reinforced in his cramped office on the sixth floor of DECC, where I negotiate piles of paperwork and shuffle furniture so we both can see his computer screen.

I have come to quiz him about DECC’s 2050 Pathway Calculator, an online tool designed to illustrate different ways to achieve Britain’s legally-binding commitment to cut emissions by 80% by mid-century, while keeping the lights on. I have done my homework, but start with a quip: what is the right answer? The professor laughs, and says any answer you like, so long as it is numerate.

“I’m really absolutely happy with any plan that adds up,” he explains. “But any plan that adds up is really challenging, so the goal of the exercise isn’t to pick a particular. It is to move the entire conversation from plans that don’t actually add up – or are a bit of wishful thinking – to move the conversation into the place that does add up, and there are choices.”

The choices in the 2050 Pathway Calculator cover which sources of energy we use – coal, gas, nuclear, renewables, biofuels – and in what proportions, along with demand-side measures, such as insulation, heating technologies and transport behaviour. It is a multiple choice exam, with four options in each case, in rising order of effort. Under ‘electrification of transport’, for instance, level 1 is a business-as-usual scenario, whereas in level 4 all cars and vans are electric by 2050. A DECC official had described the option 4 assumptions to me as “heroic”, and MacKay agrees.

“In some sectors, 4 is absolutely an Apollo programme’s worth of effort – a really hard push requiring very strong public engagement and transformation of government policy,” he says.

MacKay is well versed in the dilemmas and trade-offs inherent in our energy predicament, having written a widely-praised 2008 book, Sustainable Energy – Without the Hot Air, which analysed energy supply and consumption on a per capita basis – cleverly reducing the numbers to a comprehensible human scale.

You might even say that the book got him the job; he was encouraged to apply for the post after DECC’s top civil servant, Permanent Secretary Moira Wallace, came across it while browsing in Heffers.

While he insists that there is no single ‘right’ answer, MacKay points me to a ‘hedging scenario’, with maximum effort on all the demand-side measures, along with a strong emphasis on wind, coal with carbon capture and storage (CCS), and nuclear. This makes it look deceptively easy, lopping 92% off emissions by 2050. But the more we unpick the technological and political realities, the harder it looks to reach even the 80% target without resorting to unproven technologies or potentially unsustainable land-use.

MacKay likes the hedging scenario precisely because it over-delivers. Simply aiming for an 80% cut would be too risky, he says, because some pathways are bound to fail or under-perform. What if the Government can’t persuade people to shift to public transport, or accept more onshore wind farms? What if CCS proves uneconomic? Better to aim to over-deliver on a variety of pathways, he argues, “so that in ten or 15 years’ time, when it becomes clearer what’s happening, you’ve got a chance of ending up still on track”.

The hedging scenario could survive the total failure of any one of its three main legs of generation and still hit the 2050 target. But one of those legs involves building 90GW of new nuclear, a nine-fold increase over 40 years, raising nuclear capacity to 50% more than current peak demand. It would mean building 30 stations of 3GW, whereas the largest in Britain today, Sizewell B, is just 1.2GW.

Is any of this remotely plausible, post Fukushima? All across Europe, governments have put nuclear plans on hold, utility share prices have slumped, and in Britain, senior Liberal Democrats such as Simon Hughes and Paddy Ashdown have condemned the coalition’s nuclear policy as “unsellable”.

MacKay cautions against a knee-jerk reaction. “I would urge everyone to keep all the options on the table. I think it’s already so difficult to reach the 2050 targets, even with nuclear, that assuming that nuclear is off the table just makes the whole pressure of keeping the lights on and taking climate change action even harder.” He also insists that in Britain the industry has been “wonderfully safe”.

But why take the risk? MacKay’s model shows that nuclear power is not the determining factor in whether we hit our emissions targets. I devised a scenario based on maximum demand-side measures, renewable targets that are ambitious but politically plausible, and the lowest level of new nuclear, 90GW. Then I removed nuclear altogether, and the good news is it makes no difference to the emissions reductions. But the bad news, as MacKay points out, is that you need 47GW of back-up gas generating plant to keep the lights on when the wind does not blow.

“That corresponds to keeping going all of today’s gas-power stations and then, roughly 50% extra on top of that, and having them all mothballed and maintained so that you can use them for maybe 20 days a year as a ballpark,” MacKay explains. “Is that incredible, is it implausible? There’s a substantial capital cost in doing that, and that should be factored into the cost of choosing a pathway like this.”

DECC intends to include costs in an updated version of the model by the end of the year.

MacKay concedes that we could ditch nuclear and still hit our climate targets, but argues that the ramifications of that choice would be much greater than simply building lots more gas-fired capacity.

“People might say we really don’t want nuclear and we do want lots of wind farms on-shore and off-shore and we want to pay for them, and we want the balancing services, and we want to pay for the back-up gas-power stations, and we want to insulate our buildings really well, and we want to use more public transport.

“That’s a conceivable outcome.” But not terribly likely, he seems to imply, “given how resistant people are to, say, wind farms in the landscape, and the difficulty of local planning authorities [who] don’t even want you to have double-glazing in a house in a National Park. It would require a radical transformation of public attitudes in other sectors.”

One way to dispose of nuclear without the need for vast amounts of gas-fired back-up is to assume we build large numbers of coal-fired power stations with carbon capture and storage. This reduces the amount of back-up required but, curiously, increases emissions rather than reducing them. That’s because CCS plants – if they are ever commercialised – are likely to be 90% efficient at best, meaning 10% of the emissions will still escape.

So the more CCS plants we build, the harder it becomes to hit the emissions target. In my scenario, raising CCS from option 1 (zero) to option 4 reduces the emissions cuts achieved from 77% to 72%.

Yet the world needs CCS, says MacKay, because there is just too much cheap coal available, which if burned without carbon capture would put climate targets completely out of reach. But with around a third of UK coal-fired capacity closing by 2016 in any case, the result of a long-standing EU directive, does Britain need CCS?

“I think it would be great for British industry to be the country that leads on CCS development, so I think it’s a good idea for Britain to do it.

“But do we need it for our electricity mix? Possibly not. If we have a strong effort on offshore wind and electricity storage systems and nuclear power then we can squeeze by without CCS. But I think a balanced portfolio is probably the best thing at this stage as part of a hedging strategy.”

The only way to make CCS carbon neutral is to burn a lot of wood along with the coal, which allows the system to bury CO2 captured from the atmosphere by the trees. And Professor MacKay’s hedging strategy only manages to cut emissions by 92% by devoting a land area twice the size of Wales to biofuels and biomass forestry, as well as importing large amounts of bioenergy. Remove those assumptions and the strategy fails.

Again, is this approach remotely credible? Food prices have soared in recent years, pushing millions more into hunger according to the World Bank, partly because of the effects of the relatively modest growth in biofuel production so far. And yet these plans would involve another big increase, particularly if repeated in other countries, with potentially wider impacts both at home and abroad.

On the home front, Britain’s food security could suffer. The UK already produces just 60% of its food, and devoting 10% of our land to biofuels could only increase our import dependency. But shifting from food production to biofuels at home could also lead to impacts around the world, through what’s known as indirect land use change (ILUC) – when biofuel production in one country pushes farmers in another to clear forest to make good the lost food production.

A recent report from the Institute for European Environmental Policy found that the ILUC impact of achieving the EU’s target of 10% renewable transport fuels by 2020 would require replacement food production on a land area somewhere between the size of Belgium and Ireland, equivalent to putting up to 26M additional cars on Europe’s roads. So the emissions reductions achieved by CCS biomass co-firing could well prove illusory.

MacKay defends the model’s assumptions, stressing they were based on data from the International Energy Agency about how much low grade agricultural land exists that is not being used for food production and could be converted to energy crops, and indeed the IEA recently published a report that concluded biofuels could deliver 27% of all transport fuels in 2050.

However, that would mean increasing the land devoted to energy crops from 30M to 100M hectares – an area the size of Egypt. The report acknowledged the “great uncertainty” about land-use change impacts, and stressed the need for a strong policy framework to protect food security and biodiversity. Given the experience so far, it is hard not to be sceptical about the power of governments to regulate those impacts.

“There’s a definite tension there,” MacKay concedes. “But we reckon there are land areas that aren’t being used for food, or at least aren’t being used intensively for food which would be candidate areas for energy crops.”

Without large areas of land devoted to biofuels, the only remaining way to make the model achieve the emissions target is to ramp up a deus ex machina solution called geo-sequestration. Unlike CCS for coal- and gas-fired power stations, geo-sequestration would suck CO2 directly out of the atmosphere, for burial or use in construction materials.

However, the technology is in its infancy, and if it is ever commercialised would require tens of thousands of freestanding units – each about the size of an upturned shipping container – along with vast amounts of power. These units alone would consume the output of ten Sizewell B nuclear stations, according to DECC, which would not come cheap.

MacKay says that on a 40-year view, geo-sequestration could be part of the arsenal, along with other budding technologies such as CCS and offshore floating wind turbines. But its inclusion in the model could also be seen as clutching at straws. So how confident is he that the world can actually deliver the necessary emissions cuts?

“If we keep nuclear on the table and if CCS is successful, and if we have strong action on renewables and on the demand side I think we can find cost-effective solutions”, he concludes.

“The more of those options are taken off the table, ruled out or proved not to be deliverable, the harder it gets.”

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