I believe you are Australian?
"We" (people of the world) need to educate women in third world nations. It's a well proven fact, the more education a woman has the fewer children that women will have. It's also true those fewer children will themselves be better educated, and better fed, with better health care.
Educating the women = fewer humans born. That in turn helps with poverty.
Save the world's tropical forests? Only one way for one country to save the forests of another country....eco-tourism. If the forest suddenly becomes more valuable to a poor country because of the tourist dollars it brings in, than the slash and burn farming does, the forests will be saved. That means getting the tourist dollars to the poorest and neediest of the people in those countries. Not just the wealthy companies the come set up fancy resort hotels.
Grow enough food....the industrialized nations need to return to traditional farming methods, and not these giant mono farms, and factory raised livestock farms.
Modern farming is now completely set up to make the middle man wealthy, and keep the farmer a slave to debt he cannot possibly hope to escape. The younger generation is NOT being drawn into farming....it is not a person friendly, nor a family friendly enterprise anymore.
People need to return to smaller farms, where the livestock, and the crops exsist together, and with the manure of the livestock being used to fertilize the crops, instead of oil based chemicals.
Smaller farms produce VASTLY more food per acre than the huge mono crop farms.
To help stop the population growth in the U.S.A., we need to completely revamp the welfare system. The money should be exactly the same if you have one child or seven children. People recieving welfare (except the rare 10% who are simply too handicapped, or too mentally disabled) should work. Nobody should recieve money for nothing. They can pick up litter, mow lawns at public parks, re-roof the houses of elderly people, build wheelchair ramps, plant trees, sweep floors at the court house...whatever, does not matter. No free money. No increase in money if they make the choice of having more children. Once you have one child, it is no longer a mystery or a suprise as to how children are created.
The U.S. Government needs to STOP subsidizing CROPS. It is proven this does not work, nor help.
People need to go back to real, and honest family values. If they choose to have children, SPEND TIME WITH THEM. That means until they are 18 years of age. Teach them how to be good stewards of the land, and environment. Teach them how to have a good time without spending time and money at the mall on items they simply do not need.
There is no "single most" important factor. The two which will make the most significant differences however are the return of traditional farming, and the education of women, especially in third world countries.
Homesteading/Farming over 20 years