The new projections, based on advanced computer models and a review of recent extreme heatwaves, are more concerning than the conclusions reached by the UN climate scientists last year. The Intergovernmental Panel on Climate Change report stated that heatwaves are likely to become more frequent and intense over land areas this century. Monthly heat extremes in the summer now affect five percent of the world's land area, a figure projected to double by 2020 and quadruple by 2040. The tropics are expected to be the most impacted by increased heatwaves, followed by 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 summers hotter than their current hottest temperatures. The IPCC attributes rising temperatures and extreme weather events to heat-trapping gases, primarily from burning fossil fuels. Nearly 200 governments have pledged to limit global warming to under 2 degrees Celsius and are working towards a deal to reduce greenhouse gas emissions by the end of 2015. Since the Industrial Revolution, global average surface temperatures have already increased by 0.8 degrees Celsius.