Weather and the Greenhouse Effect

The greenhouse effect is an important natural phenomenon caused by some gases in the air (greenhouse gases) that have been responsible for keeping the Earth warm and suitable for life as we know it for billions of years. We hear a lot about the greenhouse effect nowadays, because human activities are adding a lot more of these greenhouse gases to the air and causing the average temperature of the Earth to increase. However, there will still be cold spells some places and hot spots elsewhere, so it is important to look at averages or conditions over a long period of time to get a true picture of what is happening to the climate.

So, is the increased warming of the Earth responsible for a more mild winter? The answer is a matter of context. A single season’s temperatures are an example of weather. Weather—short term variations in temperature, rainfall, storminess, and so on—is affected by a large number of both local and global factors that are not necessarily a result of global climate change. Longer-term variations, over decades or more, can show trends where the fingerprints of global change are evident. If a mild winter is part of a pattern of milder winters, as exemplified by these hardiness zone maps, then it is more likely that the greenhouse effect is responsible.

Hardiness zones are an indication of the coldest weather a plant will be subjected to in that geographic area. They are based on the average minimum winter temperatures (in Fahrenheit degrees for these maps) over a several-year period (1974-1986 for the 1990 map and 1986-2002 for the 2006 map). Milder winters, represented by the differences between these maps, are moving north.