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Abstract

Given limited scientific agreement on approaches and methodologies, estimates of climate-change adaptation costs vary widely. Here, we present a meta-analysis of aggregate adaptation costs in developing countries, across three roughly homogeneous groups of estimates, i.e. national plan-based, bottom-up science-based, and global top-down estimates. We show that the level of global warming, a country's economic status, and methodology applied, are the main determinants for the estimated costs of adaptation. Not surprisingly, adaptation costs are much higher at high levels of global warming by 2050 and 2100, diverging from low levels of warming from the 2030s. Consequently, strong global mitigation action could reduce the adaptation costs by three quarters by 2100. Next, adaptation costs are higher for high-income countries in absolute dollar value, but costs are higher relative to gross domestic product for low-income countries. The integrated assessment model based estimates are at the higher end of the range at the global scale, but the estimates based on the sectoral impacts aggregation approach are higher in case of bottom-up estimates. Regardless of the methodology applied, current climate finance pledges of USD100 billion by 2020 - for both mitigation and adaptation - would fall far short of estimated global adaptation costs.