A heat wave is a prolonged period of excessively hot weather, The term is relative to the usual weather in the area. 
Severe heat waves can lead to deaths from hyperthermia, especially among elderly people. Heat waves are the most lethal type of weather phenomenon, overall. One public heath measure taken during heat waves is the setting-up of air-conditioned public cooling centers. 
A severe heat wave in 2003 killed tens of thousands in Europe; much of the heat was concentrated in France, where nearly 20,000 people died.

Greenhouse gases (GHG) are gaseous components of the atmosphere that contribute to the greenhouse effect. Like greenhouse glass, greenhouse gases are transparent only to some wavelengths of light. When sunlight hits the Earth, some is absorbed and re-emitted at longer wavelengths for which the greenhouse gas is opaque, hindering emission back out into space. This warms the Earth; although it is not the primary mechanism that warms greenhouses.

infos from : http://en.wikipedia.org/wiki/Greenhouse_gas