Climate change, as an environmental hazard operating at the global scale, poses a unique and ''involuntary exposure'' to many societies, and therefore represents possibly the largest health inequity of our time. According to statistics from the World Health Organization (WHO), regions or populations already experiencing the most increase in diseases attributable to temperature rise in the past 30 years ironically contain those populations least responsible for causing greenhouse gas warming of the planet. Average global carbon emissions approximate one metric ton per year (tC/yr) per person. In 2004, United States per capita emissions neared 6 tC/yr (with Canada and Australia not far behind), and Japan and Western European countries range from 2 to 5 tC/yr per capita. Yet developing countries' per capita emissions approximate 0.6 tC/yr, and more than 50 countries are below 0.2 tC/yr (or 30-fold less than an average American). This imbalance between populations suffering from an increase in climate-sensitive diseases versus those nations producing greenhouse gases that cause global warming can be quantified using a ''natural debt'' index, which is the cumulative depleted CO 2 emissions per capita. This is a better representation of the responsibility for current warming than a single year's emissions. By this measure, for example, the relative responsibilities of the U.S. in relation to those of India or China is nearly double that using an index of current emissions, although it does not greatly change the relationship between India and China. Rich countries like the U.S. have caused much more of today's warming than poor ones, which have not been emitting at significant levels for many years yet, no matter what current emissions indicate. Along with taking necessary measures to reduce the extent of global warming and the associated impacts, society also needs to pursue equitable solutions that first protect the most vulnerable pop-

Significance We develop a transparent climate debt index, termed international natural debt, which combines historical emissions of CO 2 from fossil sources and land use/forestry as well as CH 4 . It covers 205 countries and is a function of emissions, lifetimes, and radiative forcings. This index can be used to assess the implications of choosing between CO 2 and CH 4 control measures and facilitates more accurate international comparisons of a range of climate-change-related phenomena, as illustrated by imposed versus experienced health impacts. Including the two most important greenhouse gases in one index shifts the basic international narrative about differential accountability for climate change.

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