SINGAPORE (Reuters) - Millions of people already living in poverty are on a collision course with hotter and more variable weather that threatens crop yields and livestock, a global study on climate change "hotspots" has found. The study released on Friday identifies regions where millions of people are already living close to the edge in terms of food security and then maps the expected impacts of changes in temperature, rainfall and other factors by 2050. The result is one of the most comprehensive studies of its type and highlights the countries most at risk from shorter and hotter growing seasons, changes in rainfall, entrenched poverty and growing populations. Over the coming decades, some areas may be unable to grow crops, or new types of crops will need to be planted, while more food-insecure regions could mean mass migration, increased food aid and greater trade in food to vulnerable regions. "Countries are going to have to realise they will need to invest in moving production to a different regions. I think it is as dramatic as that," said Bruce Campbell, director of the Climate Change, Agriculture and Food Security (CCAFS) programme, which produced the report. "We can tweak and we solve things for a while but I am not convinced incremental adaptation is going to work," Campbell told Reuters from Copenhagen, adding mass migration was inevitable. CCAFS is part of the Consultative Group on International Agricultural Research (CGIAR), a global grouping of research organisations funded by governments and private money. Among the countries most vulnerable were in Africa and South Asia but also parts of China and Latin America, the researchers found. They looked at a series of thresholds based on predicted changes in temperature and rainfall to see which areas became even more vulnerable than now. For example, the researchers mapped the areas where the average maximum temperature will switch from under 30 degrees Celsius (86 Fahrenheit) to more than that level. Many crops are temperature sensitive and rice, corn, wheat yields start to fall when temperatures rise above 30 degrees Celsius. The map shows large areas of Africa, Latin America and Southeast Asia will be affected. Even a 5 percent reduction in the length of the growing period for crops can have a major impact on yields, the study found.