Abstract
The global mean 1900–2015 warming simulated by 42 Coupled Models Inter-comparison Project, phase 5 (CMIP5) climate models varies between 0.58 and 1.70 °C. The observed warming according to the NASA GISS temperature analysis is 0.95 °C with a 1200 km smoothing radius, or 0.86 °C with a 250 km smoothing radius. The projection of the future 2015–2100 global warming under a moderate increase of anthropogenic radiative forcing (RCP4.5 scenario) by individual models is between 0.7 and 2.3 °C. The CMIP5 climate models agree that the future climate will be warmer; however, there is little consensus as to how large the warming will be (reflected by an uncertainty of over a factor of three). A parsimonious statistical regression model with just three explanatory variables [anthropogenic radiative forcing due to greenhouse gases and aerosols (GHGA), solar variability, and the Atlantic Multi-decadal Oscillation (AMO) index] accounts for over 95 % of the observed 1900–2015 temperature variance. This statistical regression model reproduces very accurately the past warming (0.96 °C compared to the observed 0.95 °C) and projects the future 2015–2100 warming to be around 0.95 °C (with the IPCC 2013 suggested RCP4.5 radiative forcing and an assumed cyclic AMO behavior). The AMO contribution to the 1970–2005 warming was between 0.13 and 0.20 °C (depending on which AMO index is used) compared to the GHGA contribution of 0.49–0.58 °C. During the twenty-first century AMO cycle the AMO contribution is projected to remain the same (0.13–0.20 °C), while the GHGA contribution is expected to decrease to 0.21–0.25 °C due to the levelling off of the GHGA radiative forcing that is assumed according to the RCP4.5 scenario. Thus the anthropogenic contribution and natural variability are expected to contribute about equally to the anticipated global warming during the second half of the twenty-first century for the RCP4.5 trajectory.
Highlights
During the past century the Earth has experienced a considerable warming
We show that the relative contribution of the natural climate variability to the anticipated global warming will likely increase during the second half of the current century due to the expected levelling off of the anthropogenic forcing under the RCP4.5 scenario
A compromise between complexity and accuracy leads to a parsimonious model with just three statistically significant explanatory variables, namely greenhouse gases and aerosol (GHGA), solar variability (SOL), and Atlantic Multi-decadal oscillation (AMO), which still accounts for 95.51 % of the observed (1900–2015) temperature variance (Table 2)
Summary
During the past century the Earth has experienced a considerable warming. most of the warming has been attributed to the burning of fossil fuels, the division between natural and anthropogenic components remains uncertain. For future climate change projections up to the year 2100 we use the total radiative forcing (CO2, CH4, O3, N2O, halocarbons, and anthropogenic aerosols) prescribed by the IPCC (2013) for the RCP4.5 scenario. This number represents an approximate increase of 4.5 W/m2 between the year 2100 and its pre-industrial value. In the following regression analysis we consider aerosol radiative forcing including an indirect effect as prescribed by the IPCC (2013), and we classify the CMIP5 models according to their aerosol treatment following the description in Table 9.1 of Flato et al (2013). The collinearity affects the interpretability of the regression model, it does not affect its predictability
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