04/03/2015 12:31 GMT | Updated 04/05/2015 06:59 BST

Are the Greens Ready for Power?

The Green Party's Conference this weekend will be the largest ever gathering of party members. When I first joined, in 1978, it had 5,000 members. Apart from a rapid increase (and equally rapid decline) shortly after the 1989 European elections, when it attracted two million votes, membership stayed around that level for decades. Now, suddenly, it has over 50,000. What has changed?

Membership growth has not resulted from a sudden burst of public interest in climate change or organic farming, nor, thankfully, an environmental disaster such as a nuclear accident. Disillusionment with the 'third party' option is clearly one factor, just as 25 years ago with the collapse of the SDP-Liberal Alliance. Likewise, the failure of Ed Miliband to present a convincing alternative to conservatism.

Both should concern Greens seeking more than a rare glimpse of electoral success. The first, because the Liberal Democrats will start to regain the centrist vote as soon as they ditch Nick Clegg - as they surely will after the election. The second, because at the heart of green politics is a critique of consumerism that presents a fundamental ideological conflict with traditional socialism, and defectors from Labour's left threaten to expose latent tension between unreconstructed socialism and genuinely innovative green politics.

Even now, recent media scrutiny indicates that the party has much to do if it is to consolidate its vote and keep hold of members. Here are some key issues that it needs to address.

First, make sure the economics stack up and don't seek to out-spend Labour. The electorate expects parties to demonstrate economic competence, even a junior government partner (and the Greens don't aspire to more than this). Economists in the party have long warned that its Citizen's Income Scheme appears unaffordable. Mistaking the cost of its social housing scheme by a factor of ten was not merely an excruciating error but indicative of a careless approach to public finances. The party cannot promise higher public spending than Labour: without consumerism there will be less economic growth, and without economic growth there will be less tax revenue for public spending.

Second, remember the basic principles of ecology. Changing the name of the Ecology Party to Green Party twenty years ago was always a risk; basic ecological principles, such as carrying capacity, are now being disregarded. For example, a strategy to reduce Britain's population to around 40million, on a strictly voluntary basis, has been superseded by one of progressively reducing Britain's border controls. Yet Britain already imports around one-half of its food and the organic methods favoured by Greens require more land. Likewise the party's recent housing brief proposes 1.25million new homes but fails to address the land requirement or mention the Green Belt. Compassion to asylum seekers is important but not everyone concerned about migration to our relatively small island is a racist.

Third, explain how sustainability differs from austerity. The Green Party was founded on the principle of 'limits to growth'. This is still its trump card: while the main parties accept the need for an 80% cut in greenhouse gas emissions by 2050, they flounder because it requires reduced consumption and radical changes in lifestyle. The Greens alone plan for this. Austerity policies imposed by Western governments differ from the frugality often advocated by Greens: austerity is frugality without social justice. Labour has failed to redistribute wealth significantly, trusting instead that economic growth will 'trickle down' to the poor. This will not be an option for Greens proposing lower overall consumption on grounds of sustainability: they must redistribute the existing cake, not make a bigger one.

Fourth, be consistent in values that prioritise future generations. With Bishops urging church members (which still number several million) to engage in politics, many might be attracted to the party's proposals on nuclear disarmament, environmental stewardship and anti-consumerism. But some will wonder why a party that claims to prioritise the rights of future generations is reluctant to regard a foetus as an unborn child. Policy targets to reduce the high incidence of abortion would indicate that it recognises the inconsistency and is serious about addressing it.

Finally, be wary of lobbyists - particularly if they offer financial support. Alarm bells should ring when trade unions start offering funds to the party, as in the case of the RMT. According to its President, a Green Party candidate, the RMT recently gave the party £7,000 in exchange for supporting rail renationalisation. Less widely publicised, it strongly opposed recent innovative proposals to refurbish Tube carriages for use on the Britain's rail network. Socialist principles were thus upheld, sustainability disregarded.

Leader Natalie Bennett often proclaims that the Green Party alone offers hope - which is why I recently rejoined. Back in the 1980s, the party proudly proclaimed that it offered a new direction in politics that was neither left not right. It should do so again. It needs to convince voters from all political backgrounds that its policies will improve their quality of life. If the Green Party only sells its message to the left, the Conservatives will be the winners.