Our EU membership offers us protection from the Tories' worst instincts. And - crucially - the EU offers us a way of working across borders to achieve social and environmental progress that would not be possible if the UK were to walk away.
I can understand why some people on the left are sceptical about the EU.
Anyone who cares about environmental and social justice only has to take a cursory glance at what's happening in our continent to realise that the EU isn't always beneficial to progressive policies here in Britain.
A closer look, however, reveals that the picture is not black and white. Whilst, at present, the EU too often reflects the current British government's zeal for privatisation and deregulation, historically there is a lot to celebrate.
Even today, our EU membership offers us protection from the Tories' worst instincts. And - crucially - the EU offers us a way of working across borders to make things better and to achieve social and environmental progress, which simply would not be possible if the UK were to walk away.
TTIP - A taste of things to come if Britain was to go it alone
One cause for scepticism is the transatlantic trade deal, TTIP. This epitomises everything that's wrong with the EU - and with neoliberal politics in general. Back room decisions, handing power to corporations, and threats to our rights at work and public services.
But those who hope that leaving the EU would make Britain's trade policy fairer are, to be frank, fooling themselves. The UK has already signed a number of bilateral deals that subject both sides to the dreaded investor state dispute mechanisms (ISDS) which allows companies to sue states for risking their 'future profits'.
Indeed the Tory Government is a major driving force for TTIP - and David Cameron is one of the deal's top cheerleaders. With MEPs and German MPs able to access TTIP documents, the buck stops with UK Ministers for the shameful situation that politicians in Westminster cannot.
If we left the EU, then we could be left with what pro-Brexit MPs describe as the ‘WTO Option'. What then? Well if recent trade deals are anything to go by, and with the Tories still in charge, we could then expect the roll out of multiple TTIPs on steroids as Britain negotiated trade deals with countries across the world.
Mounting pressure from citizens, campaigners and progressive politicians across Europe has successfully forced the EU to open up TTIP to more scrutiny. Eventually, our MEPs will vote on the final deal. If we want to stop the rot of damaging trade deals, it's our responsibility to make sure our elected representatives know that they will not get away with waving TTIP through.
The attack on Greece - right wing Governments enforcing austerity
Another concern for people on the left is Greece and the tragedy which has unfolded there. The imposition of austerity on the country - and the deep damage it's caused - was certainly enough to make me think twice.
But to lay the blame simplistically on the EU is short-sighted. The real villains are the right wing European governments at the top table. With the European Council made up of ministers from each EU country, it often simply reflects the prevailing currents in European politics.
The imposition of austerity in Greece should not be surprising when you look at Merkel's right wing government in Germany or indeed the slash and burn policies of the Tories here in Britain.
That's not to say, of course, that institutions like the European Central Bank or the EU council aren't anti-democratic - they are. But to give people a real say in Europe we need to prise open the doors of power, not throw away the one structure which has the ability to regulate the excesses of the cross-continental market.
Brexit would do nothing to help the Greek people. We need more pan-European solidarity, resistance, and collaboration - not less.
Environment - cross-border challenges need cross-border solutions
Not infrequently, I'm furious that EU environmental and climate policies don't go far enough. But it's important to remember that some of our dirtiest power stations have been closed thanks to EU directives.
Our beaches are cleaner, our air less polluted and, as Mike McCarthy put so eloquently recently, our wildlife is far safer because of EU rules. Only last week the European Parliament backed stronger action on protecting nature. To me it's obvious that being part of the EU makes sense when it comes to protecting our environment.
Pollution and environmental degradation don't respect national borders. As an environmentalist, I'd rather be working hard to make sure EU decisions deliver bolder cross-border solutions than spending the next three years scrabbling around to salvage scraps from the Nature Directives.
Doing things differently - a new EU
Ultimately, walking away from the EU would put at risk our rights at work, our environment and our ability to influence the rules that will continue to affect us. Added to that is the profound risk that Brexit poses to our multicultural and multinational society. But if we're going to stay in the EU we also need a vision of how we can do things differently.
The EU will only change if we work together with people from across the continent to make it do so.
This week activists, campaigners and politicians from across the continent gather in Berlin to discuss how we'll win Europe back for the people. For a start that means democratising the EU structures - making top meetings like the EU Council open to the public through livestreaming, giving more power to elected politicians over unelected Commissioners, and ending the culture of secrecy.
It means further clamping down on corporate lobbyists - something that's already begun in the EU and which Westminster could learn from.
And it means thinking big about EU policies that would make all of our lives better such as cross-border minimum wages (set differently for each country but ensuring that everyone earns enough to get by on where they live). Much is still to be decided - but these talks are a solid start.
On Wednesday the movement for a different kind of EU comes to Britain as a new pro-EU campaign launches in London. The progressive case for British membership of the EU, which has been side-lined for too long in a debate dominated on both sides by big business, is about to be made loud and clear.
If you care about social justice and our environment - and want to make the EU better for all of us - I urge you to join the movement to make another Europe possible.
Caroline Lucas is the Green MP for Brighton Pavilion, and former leader of the Green Party.
This article was originally published by openDemocracy under a Creative Commons Attribution-NonCommercial 4.0 International licence.