PARIS - Here's a challenge. To fight climate change, get 195 countries around a table to negotiate individual greenhouse gas emission reduction targets.
Add in the historical fact that industrialised nations have emitted the most greenhouse gases.
Mix in demands for an easier deal by poorer nations who feel they should be allowed to keep their economies growing.
Further add in the complication that big developing nations, such as China, India and Brazil are now among the world's top carbon dioxide (CO2) emitters, with China by far the largest, producing about a quarter of all mankind's CO2.
The United States is No. 2, and India is No.3.
The result is gridlock at the negotiating table. So, as the planet keeps getting warmer, climate negotiations needed another approach for a new deal that would start in 2020.
The existing United Nations climate treaty, the Kyoto Protocol, was agreed after nearly 40 rich nations signed up to binding emissions targets.
Poorer nations were not obliged to meet any legally binding caps.
The world has changed a lot since Kyoto was negotiated in the 1990s, with emissions in most developed nations steady or falling and CO2 pollution from poorer nations rising quickly. It quickly became clear that the rich-poor divide on emissions was getting very hazy indeed.
In Paris, negotiators are working on a deal that commits all countries to cutting emissions but through their own individually pledged actions.
The United States' lead negotiator, Mr Todd Stern, on Friday (Dec 4) laid out the logic of the Paris agreement's architecture.
"You cannot engage in the same kind of negotiated back and forth over targets that, for example, happened in Kyoto," he told reporters. "You can't do that when you have 195 countries."
"We saw that from the very beginning and we proposed a bottom-up nationally determined structure where countries were going to make their own determination about what they could do, urged to do the best possible and, with certain processes built in, designed to goad them into doing the best that they could," he said.
And that's what we have on the table in Paris. Ahead of the meeting, most of the 195 nations submitted their climate plans.
"We have 184 targets that have been put forward. This is a way that works, to get countries to come forward with pretty ambitious targets, not perfect, but pretty ambitious."
Mr Stern explained the US position that the targets submitted by nations will not be legally binding.
But he made clear that other parts of a Paris agreement would be.
"The whole structure that involves the accountability for those targets is legally binding, not the targets themselves. I think that's a good compromise and the right way to have brought the largest number of countries into the fold."
The process of measuring, reporting and verifying national actions, called transparency, as well as reporting on national actions and review of those actions would all be legally binding, he said.
This system is designed to accommodate the US. Congress would never agree to a treaty that committed the US to legally binding emissions targets. But the US can support a deal that President Barack Obama can sign by executive order.
The bottom line is that the Paris climate agreement will be a compromise and it won't please everyone.
But to keep pushing for legally binding targets was never going to work for the US, and nor does it suit China and India.
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