Reliable statistical data cannot be collected as there is not an internationally recognised definition for the phenomenon of climate-induced migration. In addition, estimates are also hindered by the fact that an immediate connection between the consequences of climate change and migration cannot be clearly demonstrated.
In the absence of authoritative forecasts, there is a series of estimates based on unsupported assumptions (so-called guesstimates). The figures vary depending on which climatic, demographic and social values the estimates are based on. Under favourable conditions, there may be only a slight increase in current migratory movements, but under unfavourable conditions even high estimates appear to be too low.
In 2002 the UNHCR estimated the number of people forced into migration as a result of flooding, famine and other environmental factors at 24 million and later the number of persons displaced internally as a result of natural catastrophes alone at 25 million. The German Advisory Council on Global Change (WBGU) assumes that 10-25 % of all global migratory movements are the result of climate change and its consequences; that would be the equivalent today of an absolute number of 25-60 million migrants. The United Nations University – Institute for Environment and Human Security, or UNU-EHS, in Bonn estimated the number of environmental migrants up to 2010 to be at least 50 million. The Intergovernmental Panel on Climate Change anticipates a total of up to 150 million migrants as a result of climate change by 2050. The United Kingdom's Stern Review bases its estimate on a review of a large number of studies and forecasts and concludes that there are likely to be 200 million environmental migrants by 2050. The figures of Oxford professor Norman Myers are also widespread; he anticipates more than 200 million environmental migrants by 2050.