Coupled climate-society modeling of a realistic scenario to achieve a sustainable Earth

A conceptual model was developed to project the global warming for this century. This model incorporated several important factors associated with the climate and society. Under the forcing of anthropogenic carbon dioxide, the climate system is represented by a global mean surface air temperature (SAT) and carbon storage, which is separated into the atmosphere, land and oceans. The SAT rises due to the atmospheric carbon, which is partially absorbed by the terrestrial ecosystem and the ocean. These absorption rates are reduced by the rising SAT. The anthropogenic carbon dioxide is emitted by society, which is described by global energy production (P) and energy efficiency/carbon intensity (E), yielding a rate of P/E. P consists of the energy production per capita (H) and the population (M) in developed countries and regions, P = H × M. These society components were set to grow, based on the historical record from the last 50 years, while societal incentives to reduce the growth rate H and to increase E in proportion to the increase in SAT were introduced. It is shown that, among the basic scenarios in the Special Report on Emissions Scenarios (SRES) for this century, medium-level carbon emission—where the growth rate of H is reduced by 30% and E is doubled, with 1°C of warming—could be achieved. Until the end of this century, both the terrestrial ecosystem and the oceans act as sinks. If societal incentives are eliminated, carbon emission approaches the upper limit considered in the SRES scenarios, and the terrestrial ecosystem changes into a source of carbon dioxide. Since H and E are closely related to lifestyle and technology, respectively, individuals in the developed countries are urged to change their lifestyles, and institutions need to develop low-carbon technologies and spread them to developing countries. When society achieves medium-level carbon emission for a couple of centuries, oceanic absorption was found to become more crucial than terrestrial absorption, so oceanic behavior has to be estimated more accurately.