of affairs has been referred to as Bretton Woods II. According to Alaistair Chan, "At the heart of the imbalance is China's desire to keep the value of the yuan stable against the dollar. Usually, a rising trade surplus leads to a rising value of the currency. A rising currency would make exports more expensive, imports less so, and push the trade surplus towards balance. China circumvents the process by intervening in exchange markets and keeping the value of the yuan depressed." According to economics writer Martin Wolf, in the eight years leading up to 2007, "three-quarters of the foreign currency reserves accumulated since the beginning of time have been piled up". In contrast to the changed approach within the emerging economies, US policy makers and economists remained relatively unconcerned about BOP imbalances. In the early to mid-1990s, many free market economists and policy makers such as U.S. Treasury secretary Paul O'Neill and Fed Chairman Alan Greenspan went on record suggesting the growing US deficit was not a major concern. While several emerging economies had intervened to boost their reserves and assist their exporters from the late 1980s, they only began running a net current account surplus after 1999. This was mirrored in the faster growth for the US current account deficit from the same year, with surpluses, deficits and the associated buildup of reserves by the surplus countries reaching record levels by the early 2000s and growing year by year. Some economists such as Kenneth Rogoff and Maurice Obstfeld began warning that the record imbalances would soon need to be addressed from as early as 2001, joined by Nouriel Roubini in 2004, but it was not until about 2007 that their concerns began to be accepted by the majority of economists. Exchange rate regime Under a fixed exchange rate system, the central bank accommodates those flows by buying up any net inflow of funds into the country or by providing foreign currency funds to the foreign exchange market to match any international outflow of funds, thus preventing the funds flows from affecting the exchange rate between the country's currency and other currencies. Then the net change per year in the central bank's foreign exchange reserves is sometimes called the balance of payments surplus or deficit. Alternatives to a fixed exchange rate system include a managed float where some changes of exchange rates are allowed, or at the other extreme