(Editor’s Note: Earthjustice attorneys Erika Rosenthal and Martin Wagner are blogging live from the Copenhagen climate conference. Here is today’s post by Erika.)

In the opening days of the Copenhagen climate negotiations, France and South Africa are looking like rock stars for the commitments they’ve made to reduce carbon emissions.

France announced that it would reduce its carbon emissions 30 percent below 1990 levels by 2020. And South Africa said it would cut the growth of its carbon emissions by 34 percent (below expected levels) by 2020 and 42 percent by 2025—if it gets aid from developed countries.

Why does leadership for South Africa only commit to reduce its growth in emissions when France’s leadership actually results in a reduction in emissions? The answer lies in principle of fairness across nations and generations which is the cornerstone of the UN Framework Convention on Climate Change.

The Convention calls on all countries to "protect the climate system for the benefit of present and future generations of humankind, on the basis of equity and in accordance with their common but differentiated responsibilities and respective capabilities." The short hand for this principle is the oft- repeated (at these negotiations, at least) acronym "CBDR": common but differentiated responsibilities.

CBDR grows out of one of the most basic rules of environmental protection: the polluter, not society as a whole, should pay for the pollution. In other words, if you broke it, you fix it.

In the climate context, CBDR acknowledges that all nations bear a "common" responsibility to solve the problem. We are all in this together, and it’s going to require all our best efforts to get ourselves out.

But we are not all equally responsible for getting us into this climate crisis. Far from it. That’s where "differentiated" comes in. Some nations have contributed vastly more to the problem and, importantly, have reaped enormous economic benefits in the process.

Developed nations like the United States became rich by using power from burning fossil fuels like coal and oil—and emitting enormous amounts of greenhouse gases. Whether or not we meant to contribute to global warming, the comfort in which we live today is a result of those emissions. In fact, according to the UN Human Development Report, rich countries are responsible for "about 7 out of every 10 tonnes of CO2 that have been emitted since the start of the industrial era."

Bringing it down to a personal level, an average American is responsible for 20 times the emissions of an average Indian. Even in rapidly industrializing China, the long-term climate impact of a child born today is less than one fifth the impact of a child born in the U.S.

The developing world in contrast has contributed only a small fraction of historical emissions. Even today about one sixth of the world’s population is so low-income that it produces no significant emissions at all. Moreover these are the people who are most vulnerable to the effects of global warming—and are already suffering from the droughts and floods and water shortages and surging seas that we see reported on the front pages with ever greater frequency.

It is these stark realities that give rise to the CBDR principle—the recognition that the greatest responsibility for stopping climate change lies with those who have benefited from it the most and have more resources to tackle the problem.

So "leadership" is rightly defined differently depending on a country’s historical responsibility and its current circumstances. There is little doubt what is the appropriate measure of leadership for most countries. The question is which ones will measure up to the bar set by France and South Africa when Heads of State arrive in Copenhagen next week.