The United States this week rejected the idea of a global carbon "budget" as a way of figuring out what responsibilities countries must take in addressing climate change.

Speaking in Bonn, Germany, where midyear U.N. climate talks are under way, U.S. Deputy Envoy for Climate Change Jonathan Pershing said the idea of carving up finite atmospheric space will hurt the negotiations and misses the greater objective of helping countries develop sustainably.

"The point is not that everyone is given space to emit. The point is that people be given equitable access to develop," Pershing told a room of negotiators from 192 countries.

"The idea of assigning fixed emissions rights, in our view, misconstrues this core challenge of addressing climate change while promoting development. If you try to reduce this to a formula, you inevitably get driven, in our view, down a blind alley," he said.

The idea that there is a finite amount of global warming-inducing carbon dioxide that countries can release into the atmosphere that should be divided up based on formulas -- like how much of the "carbon space" a nation already has used, for example -- is not a new one. But it is getting a fresh airing as developed and developing countries attempt to square the agreement they made last year in Durban, South Africa, to launch a new global emissions agreement with the need to divide responsibility. The buzzword at issue: equity.

How to define 'equity'?

"It is extremely difficult to define in such a way that the definition would be acceptable to all. And yet equity is at the very heart of the new agreement," said U.N. Framework Convention on Climate Change Executive Secretary Christiana Figueres, opening a workshop for negotiators on the topic.

Under the current and fading Kyoto Protocol system, countries were divided into two categories -- industrialized nations charged with cutting carbon and developing countries with no responsibilities. Under the Durban Platform, all countries have agreed to curb carbon, but developing new "equitable" criteria to understand who does how much poses an enormous challenge.

Figueres called for countries to consider nations' past and future emissions, the capacity of countries to address the consequences of climate change, and countries' economic circumstances. Yet she offered little guidance on how to balance those.

Developing nations this week insisted that a new regime hew close to the old one, which relies almost entirely on ensuring that those with "historical responsibility" for emissions cut CO2.

"Developed countries have emitted too much into the atmosphere, which seriously jeopardizes the opportunity of developing countries to equitable access to sustainable development," said Chinese negotiator He Jiankun.

Do developed nations owe a 'carbon debt'?

"The emissions from developed countries have overexceeded their due share and have created, therefore, a serious inequity in terms of the usage of the atmospheric space," He said. China joined several other countries in calling for developed countries to first drastically reduce their emissions while providing developing nations with finance and technology so that they can also do more to cut carbon.

Bolivia, meanwhile, said Western countries need to pay their "carbon debt" for causing climate change, and India challenged the idea that developing countries want the right to pollute or that focusing on equity will keep ambition low.

"Often our call for equity is understood as a hesitation to act. We would like to emphatically deny this and dispel this notion once and for all," the negotiator said. Rather, he said, poor countries see the right to develop as paramount, and when wealthy countries fail to curb emissions, it hurts already vulnerable nations' chances of moving ahead economically.

The United States and Europe, while acknowledging equity as a key concept, seemed to define it very differently than developing countries did.

E.U. lead negotiator Artur Runge-Metzger agreed that sustainable development and poverty alleviation are key. But he also argued that the old Kyoto ideas of neatly dividing up nations into two categories "will have to be reflected in a changing world." Like Pershing, he argued that no "magic formula" for equity exists and that the real formula lies in decoupling economic growth from emissions.

Said Pershing, "We need to consider equity in as broad a way as possible. If we focus too narrowly or too specifically on one approach, it will inevitably lead to many countries feeling that approach is unfair and that lack of perceived fairness will likely drive us away from an agreement."