The remarkable ascension of Jeremy Corbyn to the Labour leadership poses a challenge for the Green Party. His rise delights us, both because it is a triumph of authenticity over cynical marketing, and because some of what he believes in, we believe in.
But we Greens are now going to have to make a strong case for sticking with us, rather than defecting to Corbyn-led Labour. We can't just appeal to "anti-austerity" voters, or talk of combining social and environmental justice, or of pursuing the common good, because Corbyn does all that too. So what is our positive case, in response to the Corbyn challenge? It is to raise a series of our own challenges for Corbyn.
And we start with this simple point: Corbyn isn't green. Like us, you may like the fact that he is Old (rather than New) Labour. It's brilliant to see him triumphing over those who have sold out Labour's heritage. The problem, though, is the same: he's still Labour.
He still proudly believes in labour-ism. This, in an era of precarious employment and of a reducing need for labour to be done (because of automation), an era when the security that citizens need should come simply from their being citizens, not from their jobs.
The Green Party doesn't believe in work for the sake of it. That is why by contrast the Green Party ultimately favours the fundamentally egalitarian measure of introducing an unconditional Citizens Income, which we would set at a level sufficient to ensure the poorest benefited.
On our housing crisis, the headlines of Corbyn's "solution" are basically: build, build, build. Greens have a much more nuanced view. The recent report from the Green House think tank, argues that: "Instead of relying on a huge and environmentally costly building programme [as Corbyn-Labour would], we should ensure that the existing housing stock is better used; control rents and increase security in the private rented sector; discourage the purchase of housing primarily as an investment; reduce regional inequalities; and provide more affordable homes."
One of the main arguments cannily used by the many — including pretty much everyone in Labour — who want to bulldoze our green belt, is that much of the land there is not high in biodiversity value: it is simply used for farming cereals, etc.
But Greens can knock this argument back, because we are serious about alternatives to industrial agriculture, especially the soil-destroying factory-farming of animals and cereals.
Unlike Corbyn, the Green Party places how we treat our land and soil centrally among our values and our policies. We would also tax the ownership of land through a Land Value Tax and would reforming land-ownership radically.
That means we outflank "Corbynomics", which basically limits economics to labour and capital. Such outdated approaches have not come to terms with planetary boundaries, with the fundamental realities, that is, of a post-growth world. They have not, in particular, reckoned with the centrality of land as a factor of production, and as a constraint.
Corbyn has cannily angled for support from Greens. He may well have some good things to say on "the environment". But what we have outlined above is why that doesn't make him an ecologically minded thinker or leader. It doesn't make him someone who has properly joined up his thinking. It doesn't make him green.
And at this turning point in human history, when as a civilisation we will decide whether or not we are serious about leaving a habitable planet to our kids or not, we need to be holistic and bold in our thinking. This meansthat achieving a just and swift transition to a post-growth society that practises one-planet-living is nothing less than essential.
For this reason, Corbyn, for all his many virtues, is in one central respect just like Tony Blair and George Osborne and many more: he is thoroughly in hock to the outdated fetish for economic growth. He even goes so far as to favour the return of coal mining in south Wales. And on the litmus-test issue for Greens of London-airports-expansion, he fails: like Sadiq Khan, Labour's Mayoral candidate, he is in favour of a second runway at Gatwick.
All the warm words of Corbynian Labour on climate mean next to nothing, so long as the Labour Party remains dogmatically committed to economic growth as its number one policy objective – for it is growth that is recklessly driving environmental degradation and driving rampant and extreme levels of inequality in our society and in our world.
In summary, our challenge for Corbyn, might be cast in this way: Will Labour oppose economic growth for the sake of it? And oppose animal testing and factory farming? Will opposition to nuclear power become Labour policy? Plus outright opposition to the EU's TTIP?
On the other hand, will Labour support CO2 targets sufficient to return us to 350ppm, support Land Value Taxation and planet-healthy organic agriculture? Obviously, not just oppose or support them in warm vague words in some speech, but in actual votes in the House of Commons, in parliamentary committees, at PMQs, in its election manifesto?
Because even if Corbyn and Watson agree, they have the virtually impossible task of getting the rest of the Parliamentary Labour Party in line. We suspect that the answer to virtually all the questions we have just asked is: No.
Therefore, we are delighted that there will now be a real opposition to the government, but it won't mean we can spend more time at home with our children. The need for the Green Party is 100% as strong and urgent as ever.
Baroness Jenny Jones is a member of the Green Party Assembly and the only Green Party life peer in House of Lords. Rupert Read is a reader in philosophy at East Anglia University chair of the Green House think tank.