evidence: To manage the growing federal bureaucracy, presidents have gradually surrounded themselves with many layers of staff, who were eventually organized into the Executive Office of the President of the United States. Within the Executive Office, the president's innermost layer of aides, and their assistants, are located in the White House Office. [...] Presidential traditions also involve the president's role as head of government. Many outgoing presidents since James Buchanan traditionally give advice to their successor during the presidential transition.[119] Ronald Reagan and his successors have also left a private message on the desk of the Oval Office on Inauguration Day for the incoming president.[120] [...] Presidents may use their predecessors as emissaries to deliver private messages to other nations or as official representatives of the United States to state funerals and other important foreign events.[183][184] Richard Nixon made multiple foreign trips to countries including China and Russia and was lauded as an elder statesman.[185] Jimmy Carter has become a global human rights campaigner, international arbiter, and election monitor, as well as a recipient of the Nobel Peace Prize. Bill Clinton has also worked as an informal ambassador, most recently in the negotiations that led to the release of two American journalists, Laura Ling and Euna Lee, from North Korea. During his presidency, George W. Bush called on former Presidents Bush and Clinton to assist with humanitarian efforts after the 2004 Indian Ocean earthquake and tsunami. President Obama followed suit by asking Presidents Clinton and Bush to lead efforts to aid Haiti after an earthquake devastated that country in 2010.