You could point out that the state of the environment is desperate, but a small, peaceful protest gets very little media attention. Violent action is reported around the world, which at least convinces people to pay attention to the environment.
Now that the right to free speech is well-established, people protest every day. Peaceful protest isn't as effective, because people don't hear about it, or pay much attention if they do. [If you get the facts about the damage that has been done to the environment already and the timeline for what's happening, really convince the audience of how important the environmental situation is, you might be able to convince them that drastic measures are needed to fix this.]
Your opposition will bring up the Kyoto protocol, which has most countries working to limit greenhouse gas emissions, and say that the environment is being fixed so there's no need for violent protest. There are lots of websites that will give you information about damage to the environment (erosion, global warming, air pollution and the ozone layer, water pollution esp. by oil tankers, endangered species and extinction, etc.)
Organisations like Greenpeace have a reputation for committing property damage in protests. One argument could be that there are a lot of people who support their actions, and since you live in a democracy, their support means something.
You could also argue negligence on the government's part - "If they made and enforced laws that prevented these companies from polluting/land clearing/etc, people wouldn't have to use civil disobedience to save the planet. It's a citizen's duty to compel their government to do the right thing."
It's a hard debate topic, and a really tough legal distinction. If you start giving people the right to destroy property the courts will be overloaded with people who smashed their neighbour's gas-guzzling car. I think there are better ways to get attention than violent acts, but for the purpose of debate the above points should help.
