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Also disputed is whether projectedworld population growth to more than 10 billion people by the year 2100 will result in a doubling or tripling of atmospheric concentrations of carbon dioxide. If such a buildup occurs, as many scientists predict, global surface temperatures could increase by anywhere from about one Celsius degree to about 4 Celsius degrees during the next century. The higher half of range involves temperature changes outside of those experienced by human civilizations since the end of the last ice age some 10,000 years ago. During the last ice age, average global temperatures were only about 5 Celsius degrees cooler than the present period.