Scientists are often asked how they can predict the climate in 100 years when they cannot accurately predict the weather tomorrow. Keep in mind that weather is constantly changing, whereas climate, the long-term average of weather, undergoes more gradual change and is easier to predict. In the same way, we can state with confidence the average life expectancy of a typical female in Great Britain, but we cannot easily predict the year in which a specific woman will die.

Question:You are experiencing an unusually cold winter, which your friend suggests is clear evidence against global warming. Is he correct? Why or why not?

One unusually cold winter does not mean that the average trend is a decrease in temperature. There are always hot and cold extremes, but we need to determine average weather over long periods of time to find climate trends.