Some of the best learned, and closely remembered values a child takes into adult-hood are those which are gently nurtured throughout childhood.
Some ecology and environmental books can be rather disturbing for some, and perhaps 5 years old, depending on maturity of the individual child is the beginning point for looking at the more disturbing things, such as oil spills.
One of the gentlest and loveliest ways I can imagine beginning to teach your young child (2-4) about the environment is Julia Donaldson's "The Snail and the Whale". Apart from the added benefit of being so enjoyable to read, it's a rhyming book which is great for children learning pattern in sounds and words, it also looks at how small things can help mighty creatures such as whales.
In the story the whale kindly gives a lift on his tail to a snail with itchy feet! They travel around the globe to most beautiful places until the Whale gets stuck in a small bay and the tide goes out. The small people involved come to the rescue. The snail makes her way to the local school and uses her slime trail to spell out "save the whale" on the board. The children call the fire brigade and help to keep the whale cool by pouring water over him until the tide comes in and the whale can swim away to safety. Instilling an interest in wildlife and an understanding of our obligation to help other animals can't fail to be spelled out in this lovely whale like tale!
Another "small world" approach to ecology is to look at things close to home. The rainforest and deforestation is extremely important, but simple things that individuals can do such as composting or buying locally grown produce can feel extremely empowering to our children. David Bellamy, an ecologist of many years and known to children because of the programmes he's made in the UK has authored a book called "101 Ways to Save the Earth". This looks at things like, what is in your bin - looking at what could be recycled and how, looking at garden waste, composting and your bird table and how to make sure that the wildlife in your garden and its surrounds keep viable habitats with your child's help.
Another great way of making children think is "Dinosaurs and All That Rubbish", a children's book that can be read to children and read by them aged 4-10 tells the story of a man who is so disillusioned with the mess on his own planet he leaves looking for beauty elsewhere. Dinosaurs come back to life, clear up all the mess on earth and refuse to allow the man back until he promises to keep his own planet clean and tidy. Looking at the importance of not dropping rubbish and the effect of rubbish on the planet is excellently pitched for children in that pre-teen age bracket.