I well remember my interview for a place at architecture school. As a kindly tutor leafed through my cobbled-together portfolio, on the wall I noticed a photo of a trapezoidal cabin with a whirly helical thing on top. It was, I was told, a prototype of an energy-efficient house, a concept of which I was then only dimly conscious.  That was more decades ago than I care to think, and it goes to show that green architecture is nothing new. It goes to the heart of the paradox most architects face: they tend to be hopeful, liberal types who want to change things for the better, but construction requires money and power, which are not always in the hands of the nicest people. So nice architects find themselves working for not-nice clients. Similarly with environmental matters: buildings gobble energy and resources in their construction and use, so the most ecological thing might be not to build them at all, but that would put architects out of work. So they are drawn to that conscience-salving potential oxymoron, the green building.  Just as what was once called health food has gone from muddy lentils to crisp Ottolenghi sophistication, so green architecture has been through many phases. For a while, it wore its ecology on its sleeve, sticking conspicuous turbines and ventilators on roofs, as in the large bronze chimneys over Portcullis House, the MPs' office building next to Big Ben. Now it tends to be a more technical matter, governed by the calculations of the engineering consultancies that have grown up to make buildings sustainable.  There has also been a shift in scale since my tutor's cabin. Now architects design green cities, such as Dongtan in China, or Foster and Partners' $22bn Masdar City in Abu Dhabi, which is currently on show at the Sustainable Futures exhibition at the Design Museum. But none have gone as far as the Office of Metropolitan Architecture (OMA), the practice created by Rem Koolhaas. It is proposing to redesign an entire continent – ours, Europe – along energy-saving lines. In fact, they would like to include North Africa as well. As Reinier de Graaf, the partner in charge of the proposal, says: "Megalomania is a standard part of our repertoire."  Called Roadmap 2050, it is a plan calculated to make the Ukip-ians of this world bubble and froth with rage, as it combines the belief that drastic intervention is required to mitigate climate change, with a desire to give meaning and power to the European Union. It has been commissioned by the European Climate Foundation, a philanthropic body dedicated to promoting policies that reduce greenhouse gas emissions, and it aims to show how the EU can achieve an incredible-seeming target of an 80% reduction in carbon emissions by 2050. The proposal is being considered by the EU Council of Ministers, for their possible endorsement.  The proposal's starting point is the fact that renewable energy sources such as wind and sunshine are erratic and unreliable, which means they have to be supported by other forms of power. But they are also available in different quantities in different places – wind is abundant in Britain, sun in Spain – and in different seasons. The big idea is to create a power network across the continent linking all these sources, which could then compensate for each other. If it was windless in Britain but sunny in Spain, power could travel from them to us, and vice versa.  This is a political, as well as a technical proposal. "You can use this project to create integration. It creates a very pragmatic reason to integrate," says De Graaf. It coincides with work the OMA has been doing for several years on the ways that the European Union represents itself, through their design and research subsidiary AMO, which "operates in areas beyond the traditional boundaries of architecture". Koolhaas is a member of the EU's Reflection Group, whose job is to think about what might happen a decade or two hence.  With a cheeky, provocative tone typical of OMA, they even show a map of Europe redrawn as "Eneropa", with regions defined by their energy source. Ireland and the western half of Britain become the "tidal states", while the eastern half forms part of the "isles of wind". Former Yugoslavia is miraculously reunited as "Biomassburg". Most of Portugal, Spain, Italy and Greece become "Solaria". OMA shows images of these places, like postcards from the future, with batteries of turbines, or plumes of geothermal steam.  OMA insists that its plan makes sense, even if you exclude climate issues. It has produced figures to show that the scheme would not cost all that much per head, especially when compared with road-building, war in Iraq, or bailing out bankers. They point out the benefit of reducing reliance on Middle Eastern oil and Russian gas. They argue that the economic benefits would outweigh the costs. They say that a reduction of even more than 80% could be achieved if North Africa, with all its sunshine, could be included in the grid. Their plan, they say, is "not rooted in apocalyptic hysteria", but is eminently practical.  It's a seductive proposition: go green and get richer. It is also refreshing and unusual to hear architects proposing environmental strategies that do not require the future commissioning of architects to design buildings. It also raises an obvious question: what on earth qualifies architects who spend most of their time designing museums or office buildings or Prada stores to pronounce on these subjects? This is partly answered by the fact that OMA is not acting alone, but is part of a team that includes management consultants McKinsey, energy consultants Kema and Imperial College London. But OMA still takes responsibility for the "overarching vision".  The other question is whether to believe them. OMA has over the years shown me new cities on islands off Korea, the transposition of Amsterdam Schiphol airport into the North Sea, and the redesign of the European flag of gold stars on blue into a multicoloured barcode derived from the flags of its different nations. So far all these ideas have remained on paper. Is there any reason to think the Roadmap would be different?  It is plain that their plan would need will and cohesion that has not been evident in, for example, the EU's attempts to solve the Greek debt crisis. Reinier de Graaf cites as a model President Kennedy's declaration that, before the 1960s were out, America would put men on the moon, but Herman Van Rompuy, the president of the European Union, is no Jack Kennedy. However, De Graaf argues that European countries cooperate better at a practical level than an ideological one. He also stresses that the Roadmap "doesn't require member states to give up their identities. It allows states to be themselves."  I have, frankly, no idea if by 2050 anything like this network will exist, or whether it will join the ranks of the fantastical and doomed, along with the cities teeming with autogyros imagined in the 1930s, or the 1960s' faith in the future ubiquity of hovercrafts. I doubt if anyone else knows, either. But, of all the abstract speculations about what sustainable futures might look like, there has not previously been one so tangible or engaging. Its value at the very least is to get people thinking about what, actually, we do want. OMA's Roadmap is either prophecy or provocation, but whichever way it's worth having.
Rem Koolhaas's Roadmap 2050 is a bold plan to run Europe on a grid of shared renewable energy, writes Rowan Moore