Nudge: Improving Decisions About Health, Wealth, and Happiness, and the pair are informal policy advisors to Barack Obama. Cameron included the book in a 2008 reading list for Conservative MPs. Economic policy Cameron has said that it is "essential to reduce taxes on employment and wealth creation in order to enhance our economy's competitiveness. But I don't think it's sensible today to write a Conservative budget for 2009 or 2010, with specific pledges on tax reduction." He has stated that he hoped to cut taxes and raise public spending, "as the economy grows". Air travel Cameron has expressed interest in abolishing Air Passenger Duty for those who travel abroad only rarely, while introducing "frequent flyer" taxes on those who frequently fly around the globe. Copyright Cameron intends to increase the period of copyright from 50 to 70 years, bring copyright infringing downloads under stronger legal control and require Internet Service Providers to "block access and indeed close down offending file-sharing sites". He supports music industry representatives going into schools to teach children about copyright, technology hindering copyright infringement (DRM), and encourages the music industry to exercise self-censorship on its material in return for the above music industry friendly measures. Environment Cameron has regularly stressed his green credentials since becoming leader, describing himself as "passionate about our environment." He has argued that "there is a price ... for tackling climate change" but it is a "social responsibility to the next generation". He has stated he is committed to achieving the 2010 emissions limit and has announced he would change the current Climate Change Levy to a carbon tax in order to counter global warming. Cameron proposed a Climate Change Bill which would include committing to binding annual carbon reduction targets. However, a memo that was leaked to the Labour Party suggested the binding targets proposal may be dropped, and these do not form part of the proposed Bill as of November 2006. Cameron has expressed his opposition to Green Taxes in Prime Minister's Questions in October 2013, stating that, in order to cut taxes generally, he would 'roll back' the Green Tax. The 'Big Six' energy companies are advocates of this, according to the Express, in order to avoid Labour's plans for an energy price freeze, which would 'deter much-needed investment' from the energy industry (according to The Daily Telegraph). Labour – on the other hand – implied that Cameron was