A significant proportion of recent economic growth has relied on borrowed money, which stands today at 325% of global gross domestic product. Debt allows society to accelerate consumption, but the bill for these commitments will soon become unsustainable, as demographic changes make it more difficult to meet. Degradation of the environment results in future costs, too: either rehabilitation expenses or irreversible changes that affect living standards. Rather than reducing high borrowing levels, policy makers use financial engineering, such as quantitative easing and ultra-low or negative interest rates. A 2010 study found that European states have mortgaged themselves beyond their capacity to easily repay. Another 2010 study from the International Monetary Fund found that in the US the lifetime tax burden was positive for all ages, with the largest benefit accruing to those over age 50. But the figure for future generations is negative, meaning they will have to meet the obligations of their elders.