The legacy of our CO2 emissions: a clash of scientific facts, politics and ethics

Of the carbon dioxide that we emit, a substantial fraction remains in the atmosphere for thousands of years. Combined with the slow response of the climate system, this results in the global temperature increase resulting from CO2 being nearly proportional to the total emitted amount of CO2 since preindustrial times. This has a number of simple but far-reaching consequences that raise important questions for climate change mitigation, policy and ethics. Even if anthropogenic emissions of CO2 were stopped, most of the realized climate change would persist for centuries and thus be irreversible on human timescales, yet standard economic thinking largely discounts these longterm intergenerational effects. Countries and generations to first order contribute to both past and future climate change in proportion to their total emissions. A global temperature target implies a CO2 "budget" or "quota", a finite amount of CO2 that society is allowed to emit to stay below the target. Distributing that budget over time and between countries is an ethical challenge that our world has so far failed to address. Despite the simple relationship between CO2 emissions and temperature, the consequences for climate policy and for sharing the responsibility of reducing global CO2 emissions can only be drawn in combination with judgments about equity, fairness, the value of future generations and our attitude towards risk