Estimating the Global Impacts of Climate Variability and Change During the 20th Century

Estimates of the impacts of observed climate change during the 20th century obtained by different integrated assessment models (IAMs) are separated into their main natural and anthropogenic components. The estimates of the costs that can be attributed to natural variability factors and to the anthropogenic intervention with the climate system in general tend to show that: 1) during the first half of the century, the amplitude of the impacts associated to natural variability is considerably larger than that produced by anthropogenic factors and according to most models the effects of natural variability were mainly negative. These non-monotonic impacts are mostly determined by the low-frequency variability and the persistence of the climate system; 2) IAMs do not agree on the sign (nor on the magnitude) of the impacts of anthropogenic forcing but indicate that they steadily grew over the first part of the century, rapidly accelerated since the mid 1970's, and decelerated during the first decade of the 21st century. The economic impacts of anthropogenic forcing range in the tenths of percentage of the world GDP by the end of the 20th century; 3) the impacts of natural forcing are about one order of magnitude lower than those associated to anthropogenic forcing and are dominated by the solar forcing. Human activities became dominant drivers of the infrapolated economic impacts at the end of the 20th century, rivaling in magnitude with those of natural variability. FUNDn3.6 allows to further decompose the natural and anthropogenic contributions into different sectors. The benefits of anthropogenic contribution in agriculture and energy are shown to outweigh the losses in health and water resources.