Today I’m joining tens of thousands of people — in 15 cities around the country — rallying outside Wall Street institutions to tell President Barack Obama that it’s time to put a tiny tax on big banks. We’re calling it a Robin Hood tax because the tiny tax will hit some of the wealthiest corporations on earth, but the money it raises will mean big bucks for people and the planet. Take back our money from the banks, give it to the people. Simple enough:

In Washington, DC, where I live, we’re heading down to JP Morgan — home to CEO Jamie Dimon who just lost $2 billion on his watch to risky trades. He told members of Congress in a hearing last week that everyone should calm down, it’s just a drop in the bucket. He’s likely to stick to the same script at another hearing today.

This is exactly the problem. Wall Street fat cats don’t care if their reckless behavior makes the economy less stable. In fact, they make a lot of money when the prices of things like food and oil race up and down.

Volatility may be a cash cow for the already rich, but it undermines long-term investment in what we actually need — like clean energy. A Robin Hood tax could curb some of that risky speculation by making high-speed trading less lucrative.

That’s part of why climate activists are calling on world leaders to tax financial speculation — and why that call is reaching a crescendo at the Rio+20 Earth Summit. People who care about our children’s future are demanding that the financial sector, which made trillions from a global economy that trashed the planet, pay its fair share to heal the environment.

The other reason is that a Robin Hood tax could raise hundreds of billions of dollars each year for building a climate-friendly economy and creating good, green jobs.

Heads of State like new French President François Hollande are committed to a financial speculation tax, and to coordinating with other European countries. Now more than ever it’s time for Obama to say yes to a Robin Hood tax.