1,721,031 research outputs found
Changing the Climate Sensitivity of an Atmospheric General Circulation Model through Cloud Radiative Adjustment
No full textConducting probabilistic climate projections with a particular climate model requires the ability to vary the model’s characteristics, such as its climate sensitivity. In this study, the authors implement and validate a method to change the climate sensitivity of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3), through cloud radiative adjustment. Results show that the cloud radiative adjustment method does not lead to physically unrealistic changes in the model’s response to an external forcing, such as doubling CO2 concentrations or increasing sulfate aerosol concentrations. Furthermore, this method has some advantages compared to the traditional perturbed physics approach. In particular, the cloud radiative adjustment method can produce any value of climate sensitivity within the wide range of uncertainty based on the observed twentieth century climate change. As a consequence, this method allows Monte Carlo–type probabilistic climate forecasts to be conducted where values of uncertain parameters not only cover the whole uncertainty range, but cover it homogeneously. Unlike the perturbed physics approach that can produce several versions of a model with the same climate sensitivity but with very different regional patterns of change, the cloud radiative adjustment method can only produce one version of the model with a specific climate sensitivity. As such, a limitation of this method is that it cannot cover the full uncertainty in regional patterns of climate change
The Madden–Julian oscillation wind-convection coupling and the role of moisture processes in the MM5 model
Full text linkThe Madden–Julian oscillation (MJO) produced by a mesoscale model is investigated using standardized statistical diagnostics. Results show that upper- and lower-level zonal winds display the correct MJO structure, phase speed (8 m s[superscript −1]) and space–time power spectrum. However, the simulated free atmosphere moisture, outgoing longwave radiation and precipitation do not exhibit any clear MJO signal. Yet, the boundary layer moisture, moist static energy and atmospheric instability, measured using a moist static energy instability index, have clear MJO signals. A significant finding is the ability of the model to simulate a realistic MJO phase speed in the winds without reproducing the MJO wind-convection coupling or a realistic propagation in the free atmosphere water vapor. This study suggests that the convergence of boundary layer moisture and the discharge and recharge of the moist static energy and atmospheric instability may be responsible for controlling the speed of propagation of the MJO circulation.National Science Foundation (U.S.) (Grant ATM0733698
Climate Change Impacts on Extreme Events in the United States: An Uncertainty Analysis
Full text linkExtreme weather and climate events, such as heat waves, droughts and severe precipitation events, have substantial impacts on ecosystems and the economy. However, future climate simulations display large uncertainty in mean changes. As a result, the uncertainty in future changes of extreme events, especially at the local and national level, is large. In this study, we analyze changes in extreme events over the US in a 60-member ensemble simulation of the 21st century with the Massachusetts Institute of Technology (MIT) Integrated Global System Model–Community Atmosphere Model (IGSM-CAM). Four values of climate sensitivity, three emissions scenarios and five initial conditions are considered. The results show a general intensification of extreme daily maximum temperatures and extreme precipitation events over most of the US. The number of rain days per year increases over the Great Plains but decreases in the northern Pacific Coast and along the Gulf Coast. Extreme daily minimum temperatures increase, especially over the northern parts of the US. As a result, the number of frost days per year decreases over the entire US and the frost-free zone expands northward. This study displays a wide range of future changes in extreme events in the US, even simulated by a single climate model. Nonetheless, it clearly shows that under a reference emissions scenario with no climate policy, changes in extreme events reach dangerous levels, especially for large values of climate sensitivity. On the other hand, the implementation of a stabilization scenario drastically reduces the changes in extremes, even for the highest climate sensitivity considered.This work was partially funded by the US Environmental Protection Agency under Cooperative
Agreement #XA-83600001. The Joint Program on the Science and Policy of Global Change is
funded by a number of federal agencies and a consortium of 40 industrial and foundation
sponsors. For a complete list of sponsors, see: http://globalchange.mit.edu. This research used the
Evergreen computing cluster at the Pacific Northwest National Laboratory. Evergreen is
supported by the Office of Science of the US Department of Energy under Contract No.
DE-AC05-76RL01830
Ocean colour signature of climate change
Full text linkMonitoring changes in marine phytoplankton is important as they form the foundation of the marine food web and are crucial in the carbon cycle. Often Chlorophyll-a (Chl-a) is used to track changes in phytoplankton, since there are global, regular satellite-derived estimates. However, satellite sensors do not measure Chl-a directly. Instead, Chl-a is estimated from remote sensing reflectance (RRS): the ratio of upwelling radiance to the downwelling irradiance at the ocean’s surface. Using a model, we show that RRS in the blue-green spectrum is likely to have a stronger and earlier climate-change-driven signal than Chl-a. This is because RRS has lower natural variability and integrates not only changes to in-water Chl-a, but also alterations in other optically important constituents. Phytoplankton community structure, which strongly affects ocean optics, is likely to show one of the clearest and most rapid signatures of changes to the base of the marine ecosystem
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Advancing the Modeling of Climate Impacts on Annual and Perennial Crops to Inform Adaptation Pathways: Lessons from California Almonds and U.S. Maize
Full text linkAgriculture faces intensifying challenges from climate change, threatening both perennial and annual crops that are crucial to global food and nutrient security. This dissertation advances the modeling of climate impacts on agriculture by developing and applying a novel statistical framework for perennial crops, and by scaling and calibrating a process-based model for annual crops, thereby improving yield projections under climate change and informing adaptation planning.The first study develops a novel modeling framework that integrates climate modeling, horticultural science, and statistical yield modeling, and applies it to California almonds. Results show that increasing minimum temperatures and humidity during the bloom and pollination stage, along with heat stress during the growing stage, are primary drivers of yield losses. Climate change is projected to reduce almond yields by up to 49% by 2100 under the high warming scenario (SSP585). However, sustained innovation gains could more than offset climate damages, highlighting the joint role of technological progress and climate adaptation in perennial systems.The second study expands AquaCrop, a process-based crop model developed by the Food and Agriculture Organization, for regional-scale applications. A flexible Python-based gridded implementation is developed and calibrated using two decades of county-level U.S. maize yield data, which substantially improves model performance compared to the default field-scale calibrated AquaCrop. This county-level calibration approach enables AquaCrop to capture both spatial and temporal dynamics of observation data more effectively than existing approaches, demonstrating the value of regional calibration for large-scale applications and establishing a scalable framework for future continental and global assessments.The third study applies the county-level calibrated AquaCrop to examine U.S. maize production under climate change. Driven by CMIP6 climate projections, simulations indicate that U.S. maize production could decline by 17% by the end of the century under the high warming scenario (SSP585), with particularly severe impacts in the Corn Belt region. Analyses of adaptation strategies show that irrigation, fertilization, and spatial relocation of maize systems can potentially offset climate damages, but feasibility is constrained by water availability, land competition, and resource efficiency. County-level projections provide actionable insights for agricultural policy, crop insurance, and farm-level planning
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TEMulation: A Forest Productivity Emulator of the Terrestrial Ecosystem Model with Extreme Gradient Boosting
Full text linkForests play a critical role in the global carbon cycle, and the age of the forest is a key driver of a forest's carbon productivity. However, the environmental conditions the forest experiences growing also drive global variation in global carbon stocks and fluxes. Integrated Assessment Models would benefit from a representation of forest productivity that is impacted by local climate conditions. The Terrestrial Ecosystem Model (TEM) represents the forest carbon cycle response to age and environmental conditions for different forest types, however it is a complex, process-based model and cannot run fast enough to reasonably be part of a coupled, human-Earth model system. Therefore we have developed a statistical emulator of TEM using XGBoost, which can emulate TEM with high skill in a fraction of the run time
Climatology and Trends in the Forcing of the Stratospheric Ozone Transport
Full text linkAbstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/)A thorough analysis of the ozone transport was carried out using the Transformed-Mean Eulerian (TEM) tracer transport equation and the European Centre for Medium-Range Weather Forecasts (ECMWF) Re- Analysis (ERA-40). In this budget analysis, the chemical net production term, which is calculated as the residual of the other terms, displays the correct features of a chemical sink and source term, including location and seasonality, and shows a good agreement in magnitude compared to other methods of calculating ozone loss rates. This study provides further insight into the role of the eddy ozone transport and underlines its fundamental role in the recovery of the ozone hole during spring. The trend analysis reveals that the ozone hole intensification over 1980-2001 time period is not directly related to the trend in chemical losses, but more specifically to the balance in the trends in chemical losses and transport. That is because, in the SH from October to December, the large increase in the chemical destruction of ozone is balanced by an equally large trend in the eddy transport, associated with a small increase of the mean transport. This study shows that the increase in the eddy transport is characterized by more poleward ozone eddy flux by transient waves in the midlatitudes and by stationary waves in the polar region. This is primarily due to the presence of storm tracks in the midlatitudes and of the asymmetric Antarctic topography and ice-sea heating contrasts near the pole. Overall, this study makes clear of the fact that without an increase in the eddy ozone transport over the 1980-2001 time period, the ozone hole over Antarctica would be drastically more severe. This underlines the need for careful diagnostics of the eddy ozone transport in modeling studies of long-term changes in stratospheric ozone.This study received support from the MIT Joint Program on the Science and Policy of Global Change, which is funded by a consortium of government, industry and foundation sponsors and the National Science Foundation grant ATM073369
Climatology and Trends in the Forcing of the Stratospheric Zonal-Mean Flow
Full text linkAbstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/)The momentum budget of the Transformed Eulerian-Mean (TEM) equation is calculated using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). This study outlines the considerable contribution of the dissipative forcing, identified as a gravity wave drag, to the forcing of the zonal-mean flow. A trend analysis shows that, in recent times, the onset and break down of the Northern Hemisphere (NH) stratospheric polar night jet occur later. This temporal shift is associated with long-term changes in the planetary wave activity that are mainly due to synoptic waves. In the Southern Hemisphere (SH), the polar vortex shows a tendency to persist further into the SH summertime. This is explained by a statistically significant decrease in the intensity of the stationary EP flux divergence over the 1980-2001 period. The prevailing theory explaining the long-term changes in the stratospheric polar vortex postulates that ozone depletion leads to a strengthening of westerly winds which in turn causes the reduction in wave activity in high latitudes. We show that the strongest component in the dynamical response to stratospheric ozone changes is in fact the feedback of planetary wave activity on the zonal wind. Finally, we identify long-term changes in the Brewer-Dobson circulation that are mainly caused by trends in the planetary wave activity during winter and by trends in the gravity wave body force otherwise.This study received support from the MIT Joint Program on the Science and Policy of Global Change, which is funded by a consortium of government, industry and foundation sponsors and the
National Science Foundation grant ATM0733698
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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