Achim Steiner says reaching an agreement in 2020 instead of at next month's Durban conference would be 'very high risk'

The United Nations' environment chief has slammed plans by the world's richest nations to put off a global treaty on climate change to 2020, saying the proposals were "very high risk".

Achim Steiner, executive director of the UN environment programme, said postponing an agreement – which was meant to be signed in 2013 – to the end of this decade was a "political choice" rather than one based on science.

The Guardian revealed this week that most of the world's rich economies have quietly decided to shelve plans for a global agreement on climate change to take effect within the next few years, instead pushing for an agreement by the end of 2015 or 2016, and coming into effect until 2020. Scientists and economists have said this plan risks leading to catastrophic and irreversible climate change.

Steiner is the first senior UN official to speak out against the plans, which will be aired next week at the latest round of UN climate negotiations in Durban, South Africa.

He said: "Those countries that are currently talking about deferring an agreement [to come into force] in 2020 are essentially saying we are taking you from high risk to very high risk in terms of the effects of global warming. This is a choice – a political choice. Our role, working with the scientific community, is to bring to the attention of the global public that this is the risk that policymakers and governments will expose us to."

UNEP's chief scientist, Joseph Alcamo, highlighted the potential dangers of putting off an operational agreement until the end of this decade. "Every year it becomes more difficult to keep within 2C [of warming above pre-industrial levels – the limit of safety, according to scientists, beyond which climate change becomes catastrophic and irreversible]."

He said: "Every year, we build more power plants. Every year, we build more buildings that are not efficient. Every year, our options [to avoid climate change] get less and less."

Niklas Hoehne, of the consultancy Ecofys, said: "What's important is that the effect [of increasing emissions] is cumulative. The more we wait the more we have to do, and the less we do before 2020, the more reductions [in emissions] we have to do beyond 2020 – and the longer we wait, the more expensive that becomes."

Steiner was also scathing about the proposal, which is gaining currency in some quarters, that a global climate agreement is not necessary, because companies and governments will reduce emissions by themselves without the pressure of an international commitment. This would not happen, he said. "The world has no option but to reach a binding agreement. If we don't have a global agreement, we become captive to the narrow self-interest of countries who only see the competitive advantage rationale in whether to act [on emissions] or not."

Steiner was speaking in London at the launch of a new UNEP report that shows the current pledges made by governments around the world to cut emissions will be inadequate to keep warming below 2C. He called on governments to increase their ambition, for instance by agreeing at Durban to move to the upper end of their range of emissions-reduction targets. Several countries, including China and the European Union, have committed to a range of targets, some of them depending on the actions of other countries.

Alcumo said: "The gap [between emissions pledges and the targets needed] can be bridged, but over the past few years the gap has increased rather than decreased."

Lord Stern of Brentford, chair of the Grantham Research Institute on Climate Change at the London School of Economics, reinforced Steiners call for more ambitious targets. He said: "This report shows that national pledges for reducing emissions by 2020, if fully delivered, would take the world about half of the way between 'business as usual' and a path that would give us a 50-50 chance of avoiding global warming of more than 2C.

"That path would require global annual emissions of greenhouse gases to peak within the next ten years, and then reduce sharply to, at most, half current levels by 2050."

But he added that emissions had risen in the past year in the absence of an international agreement binding the world's major economies. He warned that action must be urgent: "There is a serious risk that countries will lock in high-carbon infrastructure now, which will make it more difficult and expensive to reduce emissions over the next few decades."