Learn science and speak logic. Many lesser creatures will thank you. If ecological rationalism sometimes shows that environmental problems are not as bad as expected, that means warnings will be all the more persuasive when genuine problems are found. Graduation from overstatement will make the environmental movement stronger, not weaker. The worst thing the environmental movement could become is another absentminded interest group stumbling along toward preconceived ends regardless of what the evidence suggests.
Skeptical debate is good for the environmental movement.
The public need not to be brainwashed into believing in ecological protection, since a clean environment is in everyone's interest. Thus the environmental movement must learn to entertain skeptical debate in a reasoned manner or will discredit itself, as all close-minded political movements eventually discredit themselves. Market forces and cost-benefit thinking aren't perfect but generally will be good for the environment. This is so if only because society may be able to afford several cost-effective conservation initiatives for the price of one poorly conceived program. Optimism not only flows from a reasoned reading of natural history, it will be an effective political tool.