“Whenever the Labour party feels a bit sad,” according to one close observer of the current government, speaking some months ago, “they wheel out Ed Miliband to talk about rooftop solar.”

It might not have been literally true, but it often felt during Keir Starmer’s premiership as if energy secretary Miliband brought the sun. Unfailingly upbeat, with a simple message that was understandable to voters, he had a clear mission – to decarbonise the UK’s power sector by 2030 – that was seen to be progressing fast.

While the Treasury struggled with feeble growth and political gaffes such as the winter fuel payments, the NHS was disrupted by doctors’ strikes, and education laboured under dilapidated schools and discouraged teachers, Miliband’s department seemed to get stuff done at an unusual rate.

“His detractors won’t like to admit it, but Miliband has been the most successful minister under the Starmer administration,” says Ed Matthew, the UK director at the E3G thinktank. “Perhaps that is why they have attacked him so viciously.”

Rightwing media publications have published frequent editorials attacking Miliband, labelling him a “net-zero zealot” with a “messianic” devotion to climate action. Even as three intense heatwaves struck this summer and firefighters battled wildfires across the country, the calls for more drilling for the fossil fuels that are supercharging extreme weather got louder.

The fossil fuel industry, along with the Conservatives, Reform UK and sections of the Labour party, has been lobbying Andy Burnham, the new prime minister, for the Jackdaw gasfield, east of Aberdeen, and Rosebank, an oilfield west of Shetland, to be given the go-ahead. Media reports suggested Burnham was preparing to announce new drilling in the North Sea shortly after entering No 10.

Miliband may be personally disappointed if Burnham does this, having previously described Rosebank as “climate vandalism”. Miliband was also behind the push to have no new licences as a manifesto commitment.

But giving the go-ahead to Jackdaw or Rosebank would not technically breach the manifesto ban, which left a loophole for the issue of licences that were already within the planning system.

For some months, it has been clear that some such compromise might be demanded and the ground has been prepared. One close civil society observer, who did not wish to be named, said: “It is not what we wanted, but we have to get on with the rest of what’s needed.”

That bigger picture has been described as “an energy revolution”, the trajectory of which is unstoppable, whatever the changing political weather.

Matthew said: “Setting the 2030 clean power target was less about meeting the target than setting turbo boosters under the clean economy to get lift-off – which [Miliband] has achieved with record levels of renewables contracts and reforms to massively speed up deployment. This is critical to economic growth, reindustrialisation, energy security and, yes, bringing down bills. This is an energy revolution which is now inevitable and no amount of grumbling by the politicians and media barons funded by fossil fuel interests can reverse it.”

Now Miliband has moved beyond the energy department to the Foreign Office. He has always had an eye on the global aspects of this transition, acting as a powerful voice at climate summits and arguing for change. He will bring his climate knowledge and undoubted energies to bear on this role.

After all, within 72 hours of taking office in the energy department in July 2024, Miliband had lifted the ban on onshore windfarms and begun the process of consenting offshore windfarms and solar farms, while reforming a previously chaotic queue for grid connections. Auctions for low-carbon generation secured enough to supply about 23m homes, while the warm homes plan to install insulation, heat pumps and clean energy should lift about 1 million families out of fuel poverty. A new nuclear programme for small modular reactors is also under way.

Last month, the government announced that the private sector had committed about £100bn to the UK’s clean technology sector. Alongside that, there is a plan to support 400,000 people into low-carbon jobs – more than double the number employed in the rapidly declining North Sea basin.

It is curious why Miliband, a former Labour leader who lost to the Conservatives in 2015, has also been the target of unrelenting internal negative briefing. Some of it comes from his long history, stretching back to his role as one of former prime minister Gordon Brown’s proteges in the early 2000s. But other factions also pitched in.

He is popular with Labour members, which may irk some. “At a time when lots of people want to say ‘a plague on all your houses’ about politicians, Miliband stands out as a rare example of someone who’s actually made a difference,” says Craig Bennett, the chief executive of the Wildlife Trusts. “So much of that has down been down to his genuine leadership and having a real sense of purpose and vision behind it, and determination to get on with it, even when you have extraordinarily ridiculous attacks from some parts of the political spectrum.”

Attacks on Miliband from other political parties have been equally vituperative: he has been behind only Starmer, and perhaps the chancellor, Rachel Reeves, as one of the key targets of Kemi Badenoch, the Tory leader. Nigel Farage and his Reform party have vowed to make net zero “the next Brexit”. And two large unions, Unite and the GMB, launched a fierce campaign against his proposed elevation to chancellor of the exchequer, though other unions backed him.

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Ami McCarthy, the head of politics at Greenpeace UK, points to the deep pockets of vested interests and the might of the fossil fuel industry. “It’s no surprise that an agenda which challenges the profits of big oil has made him the target of coordinated smears by the industry and its allies, including the latest attacks we’ve seen this month,” they say.

Businesses, led by the CBI, pointed out that the UK’s high energy prices – about 45% above the G7 median – were damaging businesses and dampening growth, as well as raising the cost of living.

But as the CBI has made clear, net zero is not the culprit. In fact, energy prices would have been much higher without Miliband’s actions. His reforms add up to about £150 off bills for the average household, while about 6 million families on low incomes will gain about £150 more through the warm homes discount.

The problem is that people are not seeing the result in their pockets, because fossil fuel prices have risen even faster, through the compound shocks of the Ukraine invasion and the Iran war, which have sent gas and oil prices soaring. Added to this are the rules Labour inherited governing the UK’s privatised power market, which mean that the price of electricity is set by gas-fired power stations, which can be highly profitable for them but disastrous for consumers.

Miliband has been trying to wean the UK off fossil fuels, to cheap renewable energy. This will require more investment, particularly in the grid. And the government is intent on decoupling gas and electricity prices, but this is proving a tough and slow process.

If he had been made chancellor, as was rumoured at one point, Miliband could have been expected to tackle this. But John Healey, previously the defence secretary, may have less enthusiasm for that fight.

His move to the Foreign Office will prevent Miliband from following through on this project, and it remains to be seen whether his replacement, Miatta Fahnbulleh, will continue it with such enthusiasm. But it will cheer many of his international allies; the fight to limit the climate crisis is a global one, and the US departure from the field, coupled with uncertainty in Europe, and China and India’s flirtations with Russia, have left a leadership vacuum.

Mohamed Adow, the director of the Power Shift Africa thinktank, says: “On the international stage, Miliband is seen as someone who understands that ambitious climate action and economic renewal can go hand in hand. Climate diplomacy is built on relationships, trust and persistence, and countries notice when senior ministers are prepared to engage directly. Putting [him] at the heart of government would send a positive signal to the rest of the world.”

Teresa Ribera, a vice-president of the European Commission responsible for green transition, has been a close ally since Miliband began his first stint as energy secretary in 2008. “He is, by some distance, one of the most skilful and respected negotiators I have ever worked with, and one of the most honest and principled politicians I know. He commands respect in international institutions and among the world’s most influential governments, but equally among developing countries, workers, scientists and business and civil society leaders,” she told the Guardian.

This will help Britain, and the world, in his new role. “I have absolutely no doubt that he has all the qualities needed to become an outstanding foreign secretary,” says Ribera. “His experience in international climate negotiations, his understanding of the economic and social dimensions of the energy transition, and his unwavering commitment to fairness and social justice make him one of the most valuable figures not only in British politics, but in global public leadership.”