The greatest environmental concern should be to stop the destruction of wild habitats because that is the world itself, not just to count carbon credits but because it would be reprehensible to destroy it.
If you do consider global warming, of course there are many reasons why it's the answer there too. Here's an experiment. Find a place where there is still Rainforest but only few miles away it's been turned to desert, and measure the temperature at both places. Pick any height off the ground, from the forest floor to a reasonable height above the canopy and I bet the difference will be significant, maybe ten degrees?
When and if the leaders of human race learn to wield their responsibilities in a less selfish fashion, I would suggest that large tracts of land, particularly in Africa be protected forests.
I always feel powerless when all I can do is give a pittance of money to a charity, and when I mention anything like this, I find everybody just jostles to outdo me and say what a giver they are and put me in my place, so that's why I need to have a soapbox like this. Anyway, here's a link to a charity worth supporting: wildlifedirect.org
Today the emphasis is on making all the little people feel guilty about not being finicky enough with their light switches and about driving nasty cars, when cleaner cars have not been available because of big business interests. Those who need to carry equipment or musical instruments can not be expected to always use public transport. Train fares and power bills seem to double every year while these very businesses call themselves the saviours of the planet on television ads. When you're rich, it's very easy to pass the sin down to the criminal classes. Why do people call themselves "green"? Because it's the colour of a buck?
