When Sir David King, Tony Blair's chief scientist, wrote a guest editorial for the journal Science there was little doubt that his message was directed at the US government and in particular the advisers behind President George Bush who led America out of the Kyoto agreement to limit global warming.
The article, published on 9 January, was a frank denunciation of American policy on climate change, which Sir David said was more serious than international terrorism in terms of the potential loss of human life and damage to the environment.
It is rare for a British civil servant to be outspoken about the policy of a friendly government but Sir David, a scientist at Cambridge University, comes from a culture of academic freedom; a culture at odds with the Whitehall machinery of official secrets.
In his article, Sir David did not spell out why he thought climate change was more serious than terrorism but it was clear that he was concerned about the deaths that global warming could cause in the future and, indeed, may have already caused.
He said: "Last year, Europe experienced an unprecedented heatwave; France, alone, bearing around 15,000 excess or premature fatalities as a consequence. Although this was clearly an extreme event, when average temperatures are rising, extreme temperature events become more frequent and more serious.
"As a consequence of continued warming, millions more people around the world may in future be exposed to the risk of hunger, drought, flooding, and debilitating diseases such as malaria.
"Poor people in developing countries are likely to be most vulnerable. For instance, by 2080, if we assume continued growth rates in consumption of fossil fuels, the numbers of additional people exposed to frequent flooding in river delta areas ... and from coastline cities and villages ... would be counted in hundreds of millions assuming no adaptation measures were implemented."
America, the world's greatest polluter, produces 20 per cent of the global emissions of greenhouse gases but accounts for 4 per cent of the world's population. Sir David said it should not shrink from its responsibilities to the international community. "As the world's only remaining superpower, the United States is accustomed to leading internationally co-ordinated action. But, at present, the US Government is failing to take up the challenge of global warming.
"The Bush administration's current strategy relies largely on market-based incentives and voluntary actions ... But the market cannot decide that mitigation is necessary, nor can it establish the basic international framework in which all actors can take their place."
Sir David's comparison of the dangers of climate change to global terrorism irked Downing Street. When Mr Blair was questioned on 3 February about Sir David's comments before a House of Commons committee, he said that, while terrorism and climate change were both of "critical urgency, I think you can get into a rather cerebral debate about which is more important than the other".
Seven days later, Ivan Rogers, the Prime Minister's principal private secretary, wrote to Sir David telling him to avoid media interviews and to tone down his previous pronunciation that action on climate change was more important than the war on terrorism.