Search Results
Abstract
There are significant discrepancies between observational datasets of Arctic sea ice concentrations covering the last three decades, which result in differences of over 20% in Arctic summer sea ice extent/area and 5%–10% in winter. Previous modeling studies have shown that idealized sea ice anomalies have the potential for making a substantial impact on climate. In this paper, this theory is further developed by performing a set of simulations using the third Hadley Centre Coupled Atmospheric Model (HadAM3). The model was driven with monthly climatologies of sea ice fractions derived from three of these records to investigate potential implications of sea ice inaccuracies for climate simulations. The standard sea ice climatology from the Met Office provided a control. This study focuses on the effects of actual inaccuracies of concentration retrievals, which vary spatially and are larger in summer than winter.
The smaller sea ice discrepancies in winter have a much larger influence on climate than the much greater summer sea ice differences. High sensitivity to sea ice prescription was observed, even though no SST feedbacks were included. Significant effects on surface fields were observed in the Arctic, North Atlantic, and North Pacific. Arctic average surface air temperature anomalies in winter vary by 2.5°C, and locally exceed 12°C. Arctic mean sea level pressure varies by up to 5 mb locally. Anomalies extend to 45°N over North America and Eurasia but not to lower latitudes, and with limited changes in circulation above the boundary layer. No statistically significant impact on climate variability was simulated, in terms of the North Atlantic Oscillation. Results suggest that the uncertainty in summer sea ice prescription is not critical but that winter values require greater accuracy, with the caveats that the influences of ocean–sea ice feedbacks were not included in this study.
Abstract
There are significant discrepancies between observational datasets of Arctic sea ice concentrations covering the last three decades, which result in differences of over 20% in Arctic summer sea ice extent/area and 5%–10% in winter. Previous modeling studies have shown that idealized sea ice anomalies have the potential for making a substantial impact on climate. In this paper, this theory is further developed by performing a set of simulations using the third Hadley Centre Coupled Atmospheric Model (HadAM3). The model was driven with monthly climatologies of sea ice fractions derived from three of these records to investigate potential implications of sea ice inaccuracies for climate simulations. The standard sea ice climatology from the Met Office provided a control. This study focuses on the effects of actual inaccuracies of concentration retrievals, which vary spatially and are larger in summer than winter.
The smaller sea ice discrepancies in winter have a much larger influence on climate than the much greater summer sea ice differences. High sensitivity to sea ice prescription was observed, even though no SST feedbacks were included. Significant effects on surface fields were observed in the Arctic, North Atlantic, and North Pacific. Arctic average surface air temperature anomalies in winter vary by 2.5°C, and locally exceed 12°C. Arctic mean sea level pressure varies by up to 5 mb locally. Anomalies extend to 45°N over North America and Eurasia but not to lower latitudes, and with limited changes in circulation above the boundary layer. No statistically significant impact on climate variability was simulated, in terms of the North Atlantic Oscillation. Results suggest that the uncertainty in summer sea ice prescription is not critical but that winter values require greater accuracy, with the caveats that the influences of ocean–sea ice feedbacks were not included in this study.
Abstract
The impact of land surface representation on GCM simulations of climate change is analyzed using eight climate change experiments, carried out with four GCMs each utilizing two different land surface schemes (LSSs). In the regions studied (Amazonia, the Sahel, and southern Europe) the simulations differ markedly in terms of their predicted changes in evapotranspiration and soil moisture. These differences are only partly as a result of differences in the predicted changes in precipitation and available energy. A simple “bucket model” characterization of each LSS demonstrates that the different hydrological sensitivities are also strongly dependent on properties of the LSS, most notably the runoff, which occurs when evaporation is marginally soil moisture limited. This parameter, “Y c ,” varies significantly among the LSSs, and influences both the soil moisture in the 1 × CO2 control climate, and the sensitivity of both evaporation and soil moisture to climate change. It is concluded that uncertainty in the predicted changes in surface hydrology is more dependent on such gross features of the runoff versus soil moisture curve than on the detailed treatment of evapotranspiration.
Abstract
The impact of land surface representation on GCM simulations of climate change is analyzed using eight climate change experiments, carried out with four GCMs each utilizing two different land surface schemes (LSSs). In the regions studied (Amazonia, the Sahel, and southern Europe) the simulations differ markedly in terms of their predicted changes in evapotranspiration and soil moisture. These differences are only partly as a result of differences in the predicted changes in precipitation and available energy. A simple “bucket model” characterization of each LSS demonstrates that the different hydrological sensitivities are also strongly dependent on properties of the LSS, most notably the runoff, which occurs when evaporation is marginally soil moisture limited. This parameter, “Y c ,” varies significantly among the LSSs, and influences both the soil moisture in the 1 × CO2 control climate, and the sensitivity of both evaporation and soil moisture to climate change. It is concluded that uncertainty in the predicted changes in surface hydrology is more dependent on such gross features of the runoff versus soil moisture curve than on the detailed treatment of evapotranspiration.
Abstract
Future land cover will have a significant impact on climate and is strongly influenced by the extent of agricultural land use. Differing assumptions of crop yield increase and carbon pricing mitigation strategies affect projected expansion of agricultural land in future scenarios. In the representative concentration pathway 4.5 (RCP4.5) from phase 5 of the Coupled Model Intercomparison Project (CMIP5), the carbon effects of these land cover changes are included, although the biogeophysical effects are not. The afforestation in RCP4.5 has important biogeophysical impacts on climate, in addition to the land carbon changes, which are directly related to the assumption of crop yield increase and the universal carbon tax. To investigate the biogeophysical climatic impact of combinations of agricultural crop yield increases and carbon pricing mitigation, five scenarios of land-use change based on RCP4.5 are used as inputs to an earth system model [Hadley Centre Global Environment Model, version 2–Earth System (HadGEM2-ES)]. In the scenario with the greatest increase in agricultural land (as a result of no increase in crop yield and no climate mitigation) there is a significant −0.49 K worldwide cooling by 2100 compared to a control scenario with no land-use change. Regional cooling is up to −2.2 K annually in northeastern Asia. Including carbon feedbacks from the land-use change gives a small global cooling of −0.067 K. This work shows that there are significant impacts from biogeophysical land-use changes caused by assumptions of crop yield and carbon mitigation, which mean that land carbon is not the whole story. It also elucidates the potential conflict between cooling from biogeophysical climate effects of land-use change and wider environmental aims.
Abstract
Future land cover will have a significant impact on climate and is strongly influenced by the extent of agricultural land use. Differing assumptions of crop yield increase and carbon pricing mitigation strategies affect projected expansion of agricultural land in future scenarios. In the representative concentration pathway 4.5 (RCP4.5) from phase 5 of the Coupled Model Intercomparison Project (CMIP5), the carbon effects of these land cover changes are included, although the biogeophysical effects are not. The afforestation in RCP4.5 has important biogeophysical impacts on climate, in addition to the land carbon changes, which are directly related to the assumption of crop yield increase and the universal carbon tax. To investigate the biogeophysical climatic impact of combinations of agricultural crop yield increases and carbon pricing mitigation, five scenarios of land-use change based on RCP4.5 are used as inputs to an earth system model [Hadley Centre Global Environment Model, version 2–Earth System (HadGEM2-ES)]. In the scenario with the greatest increase in agricultural land (as a result of no increase in crop yield and no climate mitigation) there is a significant −0.49 K worldwide cooling by 2100 compared to a control scenario with no land-use change. Regional cooling is up to −2.2 K annually in northeastern Asia. Including carbon feedbacks from the land-use change gives a small global cooling of −0.067 K. This work shows that there are significant impacts from biogeophysical land-use changes caused by assumptions of crop yield and carbon mitigation, which mean that land carbon is not the whole story. It also elucidates the potential conflict between cooling from biogeophysical climate effects of land-use change and wider environmental aims.
Abstract
Extratropical weather systems are an essential feature of the midlatitude climate and global circulation. At the last glacial maximum (LGM), the formation of regions of high transient activity, referred to as “storm tracks,” is strongly affected by the presence of large ice sheets over northern America and Scandinavia and by differences in sea surface temperature (SST) distributions. In the framework of the Palaeoclimate Modelling Intercomparison Project, simulations of the LGM climate have been run with a wide range of atmospheric general circulation models (AGCMs) using the same set of boundary conditions, allowing a valuable comparison between simulations of a climate very different from the present one.
In this study, the authors focus on the storm track representation in the models and its relationship with the surface temperatures, the mean flow, and the precipitation. Storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three-dimensional E vectors, computed from daily output. It is found that the general response to the changes in boundary conditions from present day to LGM is consistent for all models: they nearly all give an eastward shift for both storm tracks, with a larger shift for the Atlantic one. This is intrinsically linked to changes in stationary waves, which is also studied using the E vector diagnostic. Differences between the models reside in the value of the shift of the storm tracks and the change in their amplitude, which the authors analyze in terms of differences in resolution and parameterizations in the models. The sensitivity of the storm tracks to the sea surface temperatures and sea-ice extent are also examined by comparing the differences between prescribed and computed SST simulations. All in all, it is the eastern part of the storm tracks that is found to be most model-dependent, which relates to differences in the simulated climates over America’s west coast and Europe, and has to be taken into account when analyzing GCM climate simulations.
Abstract
Extratropical weather systems are an essential feature of the midlatitude climate and global circulation. At the last glacial maximum (LGM), the formation of regions of high transient activity, referred to as “storm tracks,” is strongly affected by the presence of large ice sheets over northern America and Scandinavia and by differences in sea surface temperature (SST) distributions. In the framework of the Palaeoclimate Modelling Intercomparison Project, simulations of the LGM climate have been run with a wide range of atmospheric general circulation models (AGCMs) using the same set of boundary conditions, allowing a valuable comparison between simulations of a climate very different from the present one.
In this study, the authors focus on the storm track representation in the models and its relationship with the surface temperatures, the mean flow, and the precipitation. Storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three-dimensional E vectors, computed from daily output. It is found that the general response to the changes in boundary conditions from present day to LGM is consistent for all models: they nearly all give an eastward shift for both storm tracks, with a larger shift for the Atlantic one. This is intrinsically linked to changes in stationary waves, which is also studied using the E vector diagnostic. Differences between the models reside in the value of the shift of the storm tracks and the change in their amplitude, which the authors analyze in terms of differences in resolution and parameterizations in the models. The sensitivity of the storm tracks to the sea surface temperatures and sea-ice extent are also examined by comparing the differences between prescribed and computed SST simulations. All in all, it is the eastern part of the storm tracks that is found to be most model-dependent, which relates to differences in the simulated climates over America’s west coast and Europe, and has to be taken into account when analyzing GCM climate simulations.
Abstract
The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated shifts in vegetation belts are too small in both cases. Vegetation changes due to warmer and more arid conditions in the midcontinents of the Northern Hemisphere are consistent with palaeoenvironmental data from North America, but data from Eurasia suggests conditions were wetter at 6 kyr BP than today. The models show quantitatively similar vegetation changes in the intertropical zone, and in the northern and southern extratropics. The small differences among models in the magnitude of the global vegetation response are not related to differences in global or zonal climate averages, but reflect differences in simulated regional features. Regional-scale analyses will therefore be necessary to identify the underlying causes of such differences among models.
Abstract
The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated shifts in vegetation belts are too small in both cases. Vegetation changes due to warmer and more arid conditions in the midcontinents of the Northern Hemisphere are consistent with palaeoenvironmental data from North America, but data from Eurasia suggests conditions were wetter at 6 kyr BP than today. The models show quantitatively similar vegetation changes in the intertropical zone, and in the northern and southern extratropics. The small differences among models in the magnitude of the global vegetation response are not related to differences in global or zonal climate averages, but reflect differences in simulated regional features. Regional-scale analyses will therefore be necessary to identify the underlying causes of such differences among models.
