Credit: Stellarium Click image to enlarge
Neil deGrasse Tyson of the Hayden Planetarium has complained that kids growing up where he did (in the Bronx) don't even know that there are stars. This image, generated by Stellarium software, shows the dramatic difference.
Light pollution is an obvious problem for astronomers and "poses a serious threat to wildlife, having negative impacts on plant and animal physiology. Light pollution can confuse animal navigation, alter competitive interactions, change predator-prey relations, and cause physiological harm." (Wikipedia)
But it is also an incredible waste of energy and producer of CO2. Every photon reaching into space is wasted energy. In Australia, as one example,
Public lighting is the single largest source of local government's greenhouse gas emissions, typically accounting for 30 to 50% of their emissions. There are 1.94 million public lights--one for every 10 Australians--that annually cost A$210 million, use 1,035 GWh of electricity and are responsible for 1.15 million tonnes of CO2 emissions.
This is low hanging fruit, why can't we fix this?
Image originally found on PSFK