The map is designed to stimulate interest in, and concern for, the impacts of global climate change on the environment and society. To ensure the integrity and usefulness of this product, the researchers/writers made every effort to conform to sound scientific principles in its research and categorization.
The basic criterion for inclusion of an event or trend on the map was its consistency with global climate model projections of a world with increased levels of atmospheric greenhouse gases, as outlined in the Intergovernmental Panel on Climate Change(IPCC: 1996, 2001).
Clearly, all the processes and types of events on the map can and have occurred naturally in the past, some even during times not considered to be global warm periods. They represent complex phenomena that have multiple causal agents. Thus, while all of these are consistent with a warming climate, it is impossible to state for any single event that it is due to global warming. Part of the difficulty is that our scientific understanding of the connection between global climatic patterns and individual weather extremes is still incomplete.
Another challenge lies in the fact that extremes'by definition'are relatively rare events. This dearth of data on extremes hinders a definitive statistical detection of regularities and trends in their occurrences. Because it is so difficult to see changing frequency patterns in the time series of any one type of event, this map was produced to show the increasingly widespread geographic occurrence of climate-related hazards and trend.
We distinguish two basic kinds of hotspots on the map.
The overall geographic distribution of the hotspots is skewed towards North America and Europe. This is primarily because these regions have more readily accessible climatic data and more comprehensive programs to monitor and study environmental change. There are, then, more published studies and reports, particularly in the United States and Europe, from which to draw reliable data.
The distribution of the events may also partly reflect the disproportionate warming observed over the mid-to-high-latitude continents compared to other regions during the last century. Signals of climate change would be expected to show up first in areas with the greatest warming and in the most sensitive systems. This explanation, however, does not account for the lack of hotspots in places such as Russia, which has experienced significant warming over the last century (IPCC, 1998).
Thus, the distribution of points on "Global Warming: Early Warning Signs" should not be considered to be an all inclusive representation of the distribution of actual climate-related events. Some events and trends in more sparsely populated areas may not have been detected, or they may have been observed but not reported, or they may not have been reported in the literature sources (such as non-English journals) used to create the map.