No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid-to-late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment

Vegetation and soil feedbacks at the last glacial maximum

Vegetation feedback at the Last Glacial Maximum (LGM, about 21,000 calendar years ago) remains an unresolved question. A global atmospheric general circulation model (AGCM) is asynchronously coupled with an equilibrium terrestrial biosphere model in the present study. The coupled model is then used to investigate the influences of vegetation and soil feedbacks on the LGM climate. It is found that the simulated geographical distribution of vegetation at the LGM differs from the present pattern dramatically, and glacial vegetation cover tends to be reduced on average. Vegetation feedback alone leads to an annual Surface temperature decrease of 0.31 degrees C over the LGM ice-free continental areas. Additional soil feedback reinforced vegetation-induced cooling over high latitude Eurasia and from the eastern Middle East eastward to the Indian Peninsula significantly. In the tropics, a terrestrial annual surface cooling of 0.45 degrees C is produced by vegetation and soil feedbacks. it is shown that vegetation and soil feedbacks partly reduce data-model discrepancy as produced by the AGCM alone in some regions such as Central Africa, the Indian Peninsula, South China, and North Australia. (C) 2008 Elsevier B.V. All rights reserved. [References: 51

Last Glacial Maximum over China: Sensitivities of climate to paleovegetation and Tibetan ice sheet

With the boundary conditions appropriate for the Last Glacial Maximum (LGM), including ice sheets, sea surface temperatures, sea-ice distribution, atmospheric CO2 concentration, the Earth’s orbital parameters, topography, and coastline, the atmospheric general circulation model of the Institute of Atmospheric Physics (IAP-AGCM) computes colder and drier conditions than for present day. Global annual-average surface temperature decreased by 5.3°C, and terrestrial precipitation was down by 29%. It is shown that IAP-AGCM LGM simulation compares favorably to results from other AGCMs, and/but generally shows a weak terrestrial cooling when compared to paleoclimatic reconstructions in tropics. The 21 ka (ka: thousands of years ago) vegetation reconstruction is introduced into the model to study the regional climate response to the changes in vegetation and associated soil characteristics over China. The additional cooling due to these two changes reduces, to a certain degree, the model-data discrepancies. In addition, under the precondition of continental ice existing over part of the Tibetan Plateau at the LGM, the authors examine the regional climate response to the continental ice. It follows that the glacial-age environment over the Tibetan Plateau is a very important factor for 21 ka climate simulation in East Asia

Dramatic increase in the probability of 2006-like compound dry and hot events over Southwest China under future global warming

Compound dry and hot events, the combination of high temperature and scarce rainfall, are receiving attention in recent times due to their devastating stress on ecosystems, agricultural production, and public health worldwide. Knowing the risk of compound dry and hot events, particularly for the severe ones on record, is essential for developing effective measures to mitigate the negative impacts. However, most of the existing investigations only focus on the long-term trend of compound dry and hot events during the past decades, and future changes in those record-breaking events remain sparsely reported, especially in China. With a focus on the typical severe compound dry and hot event over Southwest China in summer 2006, this study quantifies the future probability of such compound extreme under various emission and societal development scenarios. Results show that the compound dry and hot event in 2006 is the most severe during 1901–2020, with widespread spatial extent in Southwest China. The observed change in temperature plays a dominant role in the variation of compound severity. Based on Coupled Model Intercomparison Project phase 6 (CMIP6) multi-model simulations, we further show that the maximum ratio of the increase in the area affected by compound dry and hot events to that of 2006 is projected to probably be about twofold under the Shared Socioeconomic Pathway (SSP) 1–2.6 scenario in about 2040, while it would likely climb to nearly triple for the other two scenarios SSP2-4.5 and SSP5-8.5 in about 2060 and 2050, respectively. These findings could provide a support for urgent adaptation and mitigation efforts against compound dry and hot events to mitigate the impact