Description: Highlights: • Slice and transient simulations of Holocene climate change were performed. • Spatial–temporal patterns of Holocene Asian summer precipitation are investigated. • A tripole pattern of summer precipitation can be seen over monsoonal Asia. • Insolation change is a key factor for Holocene Asian summer monsoon change. • Internal feedbacks are important to Holocene Asian summer precipitation changes. Abstract: Paleoclimate proxy records of precipitation/effective moisture show spatial–temporal inhomogeneous over Asian monsoon and monsoon marginal regions during the Holocene. To investigate the spatial differences and diverging temporal evolution over monsoonal Asia and monsoon marginal regions, we conduct a series of numerical experiments with an atmosphere–ocean–sea ice coupled climate model, the Kiel Climate Model (KCM), for the period of Holocene from 9.5 ka BP to present (0 ka BP). The simulations include two time-slice equilibrium experiments for early Holocene (9.5 ka BP) and present-day (0 ka BP), respectively and one transient simulation (HT) using a scheme for model acceleration regarding to the Earth's orbitally driven insolation forcing for the whole period of Holocene (from 9.5 to 0 ka BP). The simulated summer precipitation in the equilibrium experiments shows a tripole pattern over monsoonal Asia as depicted by the first modes of empirical orthogonal function (EOF1) of H0K and H9K. The transient simulation HT exhibits a wave train pattern in the summer precipitation across the Asian monsoon region associated with a gradually decreased trend in the strength of Asian summer monsoon, as a result of the response of Asian summer monsoon system to the Holocene orbitally-forced insolation change. Both the synthesis of multi-proxy records and model experiments confirm the regional dissimilarity of the Holocene optimum precipitation/effective moisture over the East Asian summer monsoon region, monsoon marginal region, and the westerly-dominated areas, suggesting the complex response of the regional climate systems to Holocene insolation change in association with the internal feedbacks within climate system, such as the air-sea interactions associated with the El Nino/Southern Oscillation (ENSO) and shift of the Intertropical Convergence Zone (ITCZ) in the evolution of Asian summer monsoon during the Holocene.

Description: Highlights: • North Atlantic sea surface temperature exhibits high decadal predictability potential. • Model bias hinders exploiting the decadal predictability potential. • An innovative method was developed to overcome some of the bias problem. • North Atlantic sea surface temperature will stay anomalously warm until about 2030. Abstract: The Atlantic Meridional Overturning Circulation (AMOC), a major current system in the Atlantic Ocean, is thought to be an important driver of climate variability, both regionally and globally and on a large range of time scales from decadal to centennial and even longer. Measurements to monitor the AMOC strength have only started in 2004, which is too short to investigate its link to long-term climate variability. Here the surface heat flux-driven part of the AMOC during 1900–2010 is reconstructed from the history of the North Atlantic Oscillation, the most energetic mode of internal atmospheric variability in the Atlantic sector. The decadal variations of the AMOC obtained in that way are shown to precede the observed decadal variations in basin-wide North Atlantic sea surface temperature (SST), known as the Atlantic Multidecadal Oscillation (AMO) which strongly impacts societally important quantities such as Atlantic hurricane activity and Sahel rainfall. The future evolution of the AMO is forecast using the AMOC reconstructed up to 2010. The present warm phase of the AMO is predicted to continue until the end of the next decade, but with a negative tendency.

Description: The global surface air temperature record of the last 150 years is characterized by a long-term warming trend, with strong multidecadal variability superimposed. Similar multidecadal variability is also seen in other (societal important) parameters such as Sahel rainfall or Atlantic hurricane activity. The existence of the multidecadal variability makes climate change detection a challenge, since Global Warming evolves on a similar timescale. The ongoing discussion about a potential anthropogenic signal in the Atlantic hurricane activity is an example. A lot of work was devoted during the last years to understand the dynamics of the multidecadal variability, and external as well as internal mechanisms were proposed. This review paper focuses on two aspects. First, it describes the mechanisms for internal variability using a stochastic framework. Specific attention is given to variability of the Atlantic Meridional Overturning Circulation (AMOC), which is likely the origin of a considerable part of decadal variability and predictability in the Atlantic Sector. Second, the paper discusses the decadal predictability and the factors limiting its realisation. These include a poor understanding of the mechanisms involved and large biases in state-of-the-art climate models. Enhanced model resolution, improved subgrid scale parameterisations, and the inclusion of additional climate subsystems, such as a resolved stratosphere, may help overcome these limitations.

Description: Twentieth century climate exhibits a strong warming trend. There is a broad scientific consensus that the warming contains a significant contribution from enhanced atmospheric greenhouse gas (GHG) concentrations due to anthropogenic emissions. The climate will continue to warm during the 21st century due to the large inertia of the Earth System and in response to additional GHG emissions, but by how much remains highly uncertain. This is mainly due to three factors: natural variability, model uncertainty, and GHG emission scenario uncertainty. Uncertainty due to natural variability dominates at short time scales of a few years up to a few decades, while at the longer centennial time scales scenario uncertainty provides the largest contribution to the total uncertainty. Model uncertainty is important at all lead times. Furthermore, our understanding of the Earth System dynamics is incomplete. Potentially important feedbacks such as the carbon cycle feedback are not well understood and not even taken into account in many model projections. Yet the scientific evidence is overwhelming that global mean surface temperature will exceed a level toward the end of the 21st century that will be unprecedented during the history of mankind, even if strong measures are taken to reduce global GHG emissions. It is this long-term perspective that demands immediate political action.