Global warming can also lead to a deficiency in water availability; with higher temperatures and CO2 levels, plants consume more water leaving less for people. By consequence, water in rivers and streams will decline in the mid-altitude regions like Central Asia, Europe and North America. And if CO2 levels continue to rise, or even remain the same, droughts will be happening much faster and will be lasting longer. According to a study led by Professor of Water Management, Arjen Hoekstra, and made in 2016, four billion people are affected by water scarcity at least one month per year.