Determining whether a specific, single extreme event (like a heat wave) is due to a specific cause, such as increasing greenhouse gases, is difficult, if not impossible, for two reasons: 1) weather events are usually caused by a combination of factors and 2) a wide range of extreme events is a normal occurrence even in an unchanging climate. However, analysis of the warming observed over the past century suggests that the likelihood of some extreme events, such as heat waves, has increased due to greenhouse warming, and that the likelihood of others, such as frost or extremely cold nights, has decreased. So, a single weather event (i.e. ONE heat wave) can't be attributed to global warming. However, a consistent increase in the number of heat waves can (be attributed to global warming).