For the past four years, Mr de Boer has attempted the impossible: getting all of the nations of the world to agree to a new global treaty on climate change that would avert dangerous global warming.
But the resignation throws the future of the international climate change negotiations – already on life-support after a series of setbacks – into grave doubt.
Copenhagen was not the unmitigated failure that some claimed. At the summit, developed and developing countries did, for the first time, commit together to emissions reductions. And rich countries agreed to pay poor countries to help them cut emissions, though they did not say exactly how.
What overshadowed those successes, however, was the nature of the process itself. By the end of the summit, there were scenes of chaos – world leaders railing against capitalism, halting proceedings, grandstanding and playacting. One delegate slashed her hand on the conference floor to demonstrate that the blood of the poor was being spilled by the rich.
After the conference, the fragile coalition forged in Copenhagen started to show signs of its frailty. President Obama’s administration became mired down in healthcare, its loss of a Senate seat, and renewed fears over jobs and the economy. All of this gave it little attention to turn to climate change.
To make things worse, China started raising doubts in public about the science of climate change. A war of words began as several rich nations accused China of using its allies to prevent a deal at Copenhagen.
Then, just as the UN could have hoped to start picking up the pieces of the process and continuing with its efforts, the sceptics went on the attack. The “climategate” scandal grew in strength and the authority of the IPCC came under question.
Amidst all of these problems, coming on top of the already impossible process of getting 193 countries to agree a new global treaty on one of the thorniest issues facing policymakers today, it is hardly surprising that Mr de Boer decided enough was enough.
Mr de Boer has decided that now is the best time to go, and he is probably right. To resign much later this year would be to write off the prospects of the Mexico climate change conference before it has even begun, and to resign right after Mexico – if it fails to produce a treaty – would be equally invidious. At least this way, he will not formally depart until July, giving the UN time to find a successor and to brief that successor.
Mr de Boer’s past four years have been mired in grinding negotiations. He spent his first months in office grappling with the appalling state of negotiations at that time, when the US was still bent on preventing progress on a possible successor to the Kyoto protocol.
His next year’s task was first to try to persuade the incoming Secretary-General, Ban Ki-moon, of the urgency of the climate problem. In this he was quickly successful, but plans for an unprecedented summit on climate change among heads of state and government, scheduled to take place at the fringes of the UN general assembly, were spoiled by the refusal of George W Bush to attend anything but the closing dinner.
Eventually, Mr Bush was drawn by Angela Merkel of Germany to agree to begin the negotiation process, and at a landmark UN conference in Bali, nations set out a roadmap towards a new treaty. Even that was fraught, however: Mr de Boer appeared to break down in tears on the platform after the final Friday night of negotiations stretched into the Saturday afternoon.
And that left two years of excruciating negotiations leading up to the chaos of the Copenhagen summit.
Mr de Boer’s task was formidable. His successor’s will be even more so.