A climate map depicts variations in climate over a geographical area. It may cover the entire planet, a single continent or a smaller region. Climate maps may show overall climate according to a classification system based on averages or a single factor, such as temperature or precipitation. They can also show seasonal variations, projected future climates or reconstructions of past climates.
Various climate classification systems can be employed to produce a world climate map defining distinct climatic regions, but the most widely used method was developed in the early 1900s by the German-Russian climatologist Wladimir Koppen. It is based on annual and monthly average temperature and precipitation, and the resulting regions correlate closely with vegetation zones. This system was frequently modified by Koppen himself and, later, by others.
The Koppen system divides climates into five main categories, denoted by upper case letters A to E. “A” represents a humid tropical climate, with high temperatures and high rainfall throughout the year; “B” is a dry climate, with low rainfall throughout the year, subdivided into type “S” which is semi-arid and type “W” which is arid; “C” is a humid mid-latitude climate; “D” is a continental climate with relatively low rainfall and large seasonal temperature variations and “E” is a polar climate, with low temperatures throughout the year. Sometimes an “H” category is added to denote a highland climate, which applies to high mountain regions such as the Himalayas.
A second, lowercase, letter was added to create subcategories based on the seasonal pattern of precipitation, for example “s” denotes a dry summer season. Not all of these sub-categories apply to every main climate type. A third lowercase letter was added to further subdivide some climate categories according to seasonal temperature patterns, for example “a” indicates a hot summer, with average temperatures above 72°F (22°C). Again, not all of these apply to every climate type. In all, this system creates 30 climate types that can be illustrated in different colors in a climate regions map.
Specific seasons can be illustrated in a climate map, showing, for example, variability in temperature or precipitation through the year. Lines can be drawn connecting points which have the same value for a given factor to produce a “contour” map showing high and low areas for this factor. For example, lines connecting points of equal temperature are called isotherms, so a map with isotherms can show at a glance the variations in temperature within a region, or the planet as a whole. A series of maps for different times of year can illustrate seasonal variations.
Concern about climate change has led to the creation of climate maps showing possible future climates. These use projections based on the estimated overall temperature increases that would result from various levels of carbon dioxide production. One such map shows the effects of an overall rise in global temperature of 39.2°F (4°C). It is thought that temperatures would increase more over land than over the oceans and that increases would be greater at high latitudes than at low latitudes. These maps can also illustrate changing geography due to sea level rise.
It is possible to reconstruct past climates using fossil and other evidence. For example, analysis of pollen grains in ancient soil samples can indicate the types of vegetation present when the soil was formed, giving a good indication of the type of climate up to a few tens of thousands of years ago. Going further back into the past, fossil evidence of plant remains, examples of glaciation — indicating a cold climate — or wind erosion — indicating a dry climate — can provide information about climates going back hundreds of millions of years, enabling past climate maps to be created.
Climate maps have even been produced for the planet Mars. They are much simpler than maps illustrating the Earth’s climate as there are no oceans and no precipitation. Nevertheless, information on topography, albedo, the presence of ice and evidence of wind speed and direction, combined with latitude, has enabled the production of a reasonably detailed climate map of the Martian surface.