Sometimes environmental change is subtle and slow, and there's debate about its cause - global warming is a good example. But there are cases in which a combination of natural processes and human actions are clearly responsible. One good example is a drying lake in Africa known as Lake Chad.

This freshwater lake lies in west Africa at the southern edge of the Sahara Desert, where the countries of Chad, Niger, Nigeria and Cameroon meet. Here's a black and white close-up satellite image of the lake taken in 1963 - I've outlined the lake in white. In the decade following, rainfall in that part of Africa declined significantly - likely just a natural variation in climate. By 1973 the lake had shrunk by about half - the reds and browns in this picture indicate vegetation and sand dunes in places that used to be water-covered.

In the meantime, the lake became a source of water for massive irrigation projects, further reducing its level. As a result, here's what the lake looks like recently - now just a shadow of its former self, with an an area of only about 10% of what it covered back in 1963.