One option that has been discussed is to base a revised treaty on Kyoto’s original rules but to set tougher standards for industrialized nations, and new targets for some developing nations. The European Union has proposed 15 to 30 percent cuts for industrialized nations by 2020. These goals are up to six times higher than the current targets.

Other countries would prefer to start all over again, partly to encourage the US to join in. One alternative, proposed by representatives of Kenya, Mexico, Monaco and Switzerland, would allocate carbon-emission rights according to population. Each person on Earth would have an equal right to emit carbon. For a country to exceed its population-based target, it would have to buy spare entitlements from poor countries or countries that had invested in low-carbon technologies. The problem with this scheme is that it might provide an incentive for countries to increase their populations, leading to an increase, rather than a decrease, in greenhouse gas emissions.

According to another option, countries would adopt specific carbon-reduction technologies, such as maintaining natural carbon sinks, such as rainforests, or generating more electricity from renewables, in exchange for similar pledges from other nations. This option could encourage innovation, but would not necessarily lead to reductions in emissions.

The United States has proposed targets based on Gross Domestic Product. Countries would be required to reduce their economies’ “carbon intensities” by a specific percentage. This plan would create an incentive to move to clean technologies, but not penalize countries that use carbon emissions efficiently to create wealth. It would, however, disadvantage countries in the earliest stages of industrialization, such as India. Such countries typically reach a peak of carbon intensity before carbon emissions decline as concerns about the environment increase.

Developing countries, such as China, South Africa, Indonesia, Argentina, Mexico and Brazil, are beginning to accept that they may have to accept emissions targets. They no longer see climate change as a problem that affects only the rich. Instead, they view it as a barrier to their economic development. Argentina has already suffered a 30 percent decline in electricity production because melting glacier patterns and changing rainfall patterns are reducing flows in rivers used for hydropower.

Many developing countries have already initiated climate-control measures. China is improving its energy efficiency so quickly that in the past decade, it has made only minimal increases in greenhouse gas emissions, despite the exceedingly fast pace at which it has become industrialized. Argentina has the world’s largest fleet of cars powered by natural gas, which produce less carbon per unit of energy than gasoline.

According to British climate negotiator David Warrilow, the world will have to limit carbon dioxide emissions to the atmosphere from human activity to less than 600 billion tons for the entire 21st century. If current trends continue, we will already have emitted 400 billion tons by 2030.