Consider the carbon. If you fly to the People's Climate March in New York City September 21st, then you're spewing copious carbon dioxide into the atmosphere. Same is true if you're driving or even taking the train, though far less of course.
So the People's Climate March itself won't lower the amount of CO2 pollution in the atmosphere. But the sight of thousands of people at the march—including Secretary General of the United Nations Ban Ki-moon and other luminaries—might help convince world leaders that the fight against climate change has popular support. And more than 125 such leaders, including President Obama, are expected at the Secretary General's climate summit this week.
The good news is that there are steps to take that we know work to combat climate change. Whether that be international treaties like the Montreal Protocol or replacing fossil fuel burning with renewable power or even nuclear, there are policies that work if world leaders have the courage to adopt them.
So if all those marchers can convince world leaders to take action, then their one-time CO2 output will be worth it.