At Friday’s summit between President Vladimir Putin of Russia and José Manuel Barroso, president of the European Commission, the words “tension” and “energy” will once again be heard in the same breath. For today the biggest issue dividing Europe and Russia is natural gas, the same commodity that tied them together through even the tensest days of the cold war.

Over 40 years, the trade in gas has brought benefits to all concerned. It enabled Europe to limit its coal use and diversify its energy supplies. Natural gas has also been the major enabler in Europe for limiting carbon dioxide emissions. For Russia, long the largest supplier of gas to Europe, it has been the second largest export earner.