Compare the Prices of Carbon Dioxide in the US and the UK

How come the price of CO₂ is so low (less than a quarter of the UK one) in the United States?


Carbon price in 2010 (in 2005 US$)

US: $20.16 per tonne CO₂e

UK: $85.33 per tonne CO₂e

Data Source
US: U.S. Department of Energy. (2010). Social Cost of Carbon for Regulatory Impact Analysis Under Executive Order 12866. Retrieved from http://bit.ly/daAGEh
UK: Department of Energy and Climate Change. (2009). Carbon Valuation in UK Policy Appraisal: A Revised Approach. Retrieved from http://bit.ly/9xTFB1

Original numbers

US: $21.4 (in 2007 US$)

UK: £52   (in 2009 UK£)

CPI

2007/2005 in US: 1.062 (http://bit.ly/aVpy41)

2009/2005 in UK: 1.108 (http://bit.ly/aQtdnf)

Average Currency Exchange Rate in 2005 (http://bit.ly/aaIvis)

$1 = £0.550