The new computer models, based on an "exceptional number of extreme heatwaves" in the past decade, are more concerning than the conclusions of the UN panel of climate scientists last year. The Intergovernmental Panel on Climate Change report stated that heatwaves are likely to increase in length, frequency, and intensity over most land areas this century. Monthly heat extremes in summer currently affect five percent of the world's land area, but this is projected to double by 2020 and quadruple by 2040. The tropics would be most affected by increased heatwaves, followed by other regions such as the Mediterranean, Middle East, parts of western Europe, central Asia, and the United States. Without curbing greenhouse gas emissions, many regions could experience summer months hotter than their current hottest temperatures by the end of the century. The IPCC warns that heat-trapping gases from burning fossil fuels are raising temperatures and causing more severe weather events such as downpours, floods, and rising sea levels. Global governments have agreed to limit global warming to less than 2 degrees Celsius above pre-industrial times and are working towards a deal to curb greenhouse gas emissions by the end of 2015. Since the Industrial Revolution, global average surface temperatures have risen by 0.8C (1.4F).