Over the last 50 years the size of the world economy, as measured by global GDP, has increased by a factor of five, even after adjusting for inflation. Global economic growth over this period has averaged about 3.3 percent real growth per year. Recent projections indicate that global economic growth will slow somewhat over the next 50 years, to between 2 and 3 percent, mainly due to slower population growth rates and longer life expectancies that reduce the share of the working-age population. But even with a 2 percent rate of real growth, the world economy would expand by a further factor of 2.7 over 50 years, and more than sevenfold over 100 years. 1 While future economic expansion will surely increase human well-being in many ways, particularly for the nearly one billion people living in absolute poverty, it is important to address the question of whether such continued growth is environmentally sustainable.