The global human population grows by a city the size of London every month. This growth is the greatest threat the planet faces.
The article below is from the excellent Resurgence magazine, issue 51 (Nov/Dec 08), pp. 14-17. It should be standard reading for everyone who cares about the environment, or who is thinking of bringing yet more children into the world (click on the images to enlarge).
There are more facts at Wikipedia, and in the Independent article here.
Also see the website of the UK-based Optimum Population Trust who do pioneering work in this area. Sign their petition! They are concerned with the impact of population growth on the environment, and their research covers population in relation to climate change, energy, resources, biodiversity, development impacts, ageing and employment and other environmental and economic issues. They campaign for stabilisation and gradual population decrease globally and in the UK. They also have an informative blog.