Global Climate Change: Anthropogenic Warming versus Multidecadal Natural Oscillations

During the past five decades, global air temperatures have been warming at a rather high rate (IPCC-2013) resulting in scientific and social concern. This warming trend is observed in field and model data and affects both air temperatures over land and over the ocean. IPCC further attributes this temperature increase to the total increase in radiative forcing and that this is primarily caused by the increase in the atmospheric concentration of CO2 during the last 200 years. However, the warming rate changes with time and this has led to question the causes underlying the observed trends. Here, we analyze recent measured and modeled data on global mean surface air temperature anomalies (GMTA) covering the last 160 years using spectral techniques. The spectral analysis of the measured data shows two major signals, a strong secular trend (ST) and a clear multidecadal sinusoidal oscillation (MDV) that resembles the Atlantic Multidecadal Oscillation (AMO). The observed acceleration of the warming during the period from 1970 to 2000 therefore appears to be caused by a superimposition of anthropogenic-induced warming (~60%) with the positive phase of a multidecadal oscillation (~40%). The recent slowdown (hiatus) of this tendency is likely due to a shift in the MDV phase. This change in the MDV phase could mask the effect of global warming also in the forthcoming decades and the current hiatus could be already a manifestation of this phenomenon. The same spectral analysis applied to the simulations and ensemble means from global circulation models participating in the CMIP5 forecasting project does provide significant different results. Most current generation global circulation models (CMIP5) do not reproduce the MDV and are missing the actual temperature hiatus. Therefore, it is less likely that these models could correctly forecast the temperature evolution during the coming decades. We propose here to use the climate dynamics that is inherent in the GMTA data for statistical forecasting of the temperature until 2100. These forecasts, based on the analyzed secular trend and the multidecadal oscillations are indeed capable of reproducing the actual hiatus. We consider this as a successful validation of the method. The statistical forecasts generally result, in comparison to CMIP5 forecasts, in much lower temperature increases for 2100 of only about 1oC, assuming a business as usual scenario. Global mean air temperatures could be even decreasing for the next 2-3 decades. Henceforth, for a correct assessment of the anthropogenic-induced warming of the global air temperatures in the future natural multidecadal temperature oscillations should be taken into account. The origin of the MDV does remain unclear and future research must show the real causes for this climate signal. Investments in basic model development and in better model validation should be increased to achieve more realistic hindcasts, especially capturing the hiatus before focusing again on forecasting. Especially the “black hole” in Earth’s energy balance must be discovered to understand where the additional energy from the increasing radiative forcing is ending up (ice melting or the deep ocean being suitable candidates).

JRC.H.1 - Water Resources