The South Asian summer monsoon brings copious rain to agriculture-dependent country India and bulk of the precipitation in central India is attributed to monsoon low pressure systems (LPS). Large ...uncertainty exists in the statistics of LPS during the historical period and in future projections. In this study, we have developed an LPS tracking approach which considers geopotential height anomaly and relative vorticity thresholds. The approach is validated by comparing characteristics of LPS from our tracking scheme with those from previous studies. Our analysis indicates around 14 LPS per year (over 68 LPS-days). 60–70% of monsoon rainfall in north, east and central India is found to be associated with LPS (location is within 1000 km radii of LPS). Over the central Indian region, around 82% of extreme precipitation events occur during LPS days, out of which 47% are on depression and deep depression days and 78% is associated with LPS. 15–25% of monsoon precipitation in central and East Indian states is in the form of extreme precipitation associated with LPS. At many locations in central India, very heavy precipitation (≥ 124.5 mm/day) due to LPS is estimated to have a return period less than 5 years. Further, our analysis shows that the intensity of extreme precipitation is larger by 50% (95th percentile precipitation) to 100% (99th percentile) when the extreme is associated with LPS. Our analysis of extreme precipitation related to LPS has the potential to provide valuable information for flood risk assessment during monsoon season in central India.
Anthropogenic influences and global climate change are expected to alter the land carbon stocks in the future. In this modeling study, using the NCAR Community Earth System Model (CESM), we assess ...the relative importance of CO
2
fertilization, nitrogen deposition, climate change, and land use and land cover changes (LULCC) on the land carbon uptake in three future scenarios used in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our simulations show that CO
2
fertilization is the primary driver of the increase in net primary production (NPP) and total ecosystem carbon (TEC) in the representative concentrations pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios. The effect of nitrogen deposition on NPP and TEC in the future scenarios is small. Climate warming causes increases in NPP in the RCP4.5 and RCP8.5 scenarios, but it leads to loss of TEC in the future scenarios because of increased heterotrophic respiration. LULCC leads to an enhancement of NPP in the future scenarios due to post-harvest regrowth in the RCP2.6 and RCP8.5 scenarios, and due to afforestation in the RCP4.5 scenario. We find that land is a source of carbon in the RC8.5 and RCP2.6 scenarios mainly because of LULCC and climate change, but afforestation and CO
2
fertilization in the RCP4.5 scenario facilitate the land to be a sink. Our findings, albeit from a single model, are in broader agreement with other studies that highlight the need for better land management practices and moderation in climate change for a continued land carbon sink.
Significance Biogeophysical effects such as albedo and evapotranspiration changes due to deforestation were shown by several studies in the past to exert strong influence on local surface ...temperatures. In this study, we assess the remote versus local effects of large-scale deforestation on precipitation in the monsoon regions of the world. In contrast to the dominant role of local effects on temperature changes, we find that the remote effects have a larger influence than local effects on shifting the location of the Intertropical Convergence Zone and hence precipitation in all the monsoon regions. This result has important implications for assessing the net benefits of climate change mitigation strategies such as afforestation/reforestation and for understanding changes in monsoon rainfall in past climates.
In this paper, using idealized climate model simulations, we investigate the biogeophysical effects of large-scale deforestation on monsoon regions. We find that the remote forcing from large-scale deforestation in the northern middle and high latitudes shifts the Intertropical Convergence Zone southward. This results in a significant decrease in precipitation in the Northern Hemisphere monsoon regions (East Asia, North America, North Africa, and South Asia) and moderate precipitation increases in the Southern Hemisphere monsoon regions (South Africa, South America, and Australia). The magnitude of the monsoonal precipitation changes depends on the location of deforestation, with remote effects showing a larger influence than local effects. The South Asian Monsoon region is affected the most, with 18% decline in precipitation over India. Our results indicate that any comprehensive assessment of afforestation/reforestation as climate change mitigation strategies should carefully evaluate the remote effects on monsoonal precipitation alongside the large local impacts on temperatures.
The empirical “amount effect” observed in the distribution of stable water isotope ratios in tropical precipitation is used in several studies to reconstruct past precipitation. Recent observations ...suggest the importance of large‐scale organized convection systems on amount effect. With a series of experiments with Community Atmospheric Model version 3.0 with water isotope tracers, we quantify the sensitivity of amount effect to changes in modeled deep convection. The magnitude of the regression slope between long‐term monthly precipitation amount and isotope ratios in precipitation over tropical ocean reduces by more than 20% with a reduction in mean deep convective precipitation by about 60%, indicating a decline in fractionation efficiency. Reduced condensation in deep convective updrafts results in enrichment of lower level vapor with heavier isotope that causes enrichment in total precipitation. However, consequent increases in stratiform and shallow convective precipitation partially offset the reduction in the slope of amount effect. The net result is a reduced slope of amount effect in tropical regions except the tropical western Pacific, where the effects of enhanced large‐scale ascent and increased stratiform precipitation prevail over the influence of reduced deep convection. We also find that the isotope ratios in precipitation are improved over certain regions in the tropics with reduced deep convection, showing that analyses of isotope ratios in precipitation and water vapor are powerful tools to improve precipitation processes in convective parameterization schemes in climate models. Further, our study suggests that the precipitation types over a region can alter the fractionation efficiency of isotopes with implications for the reconstructions of past precipitation.
Key Points
The slope of amount effect in the tropics reduces with reduction in modeled deep convective precipitation
Increases in large‐scale/shallow convective precipitation partially offset the reduction in the slope of amount effect
The δDprecip values are improved in certain regions in tropics with reduced deep convection
Monsoon low pressure systems (LPS) during the summer monsoon season (June–September) over India are a lifeline for the agriculture‐dependent country. We use the Community Earth System Model ...(CESM1.2.2) to simulate the LPS characteristics (genesis, propagation direction, tracks, intensity, and precipitation) over India and the Bay of Bengal and analyse the influence of Southeast Asian mountains (SAMs) on these characteristics. CESM reproduced most of the LPS characteristics. However, a southward latitudinal shift of about 4° is simulated in the median of the tracks, which is likely due to the weaker simulated upper tropospheric meridional temperature gradient (MTG) in the Indian region. Removal of SAMs is found to have little influence on LPS characteristics. Further, it causes only a slight reduction in mean summer monsoon precipitation (by 5%) and LPS‐related precipitation (from 10.7 ± 1.0 to 10.3 ± 0.8 mm/day) over India, as SAMs have negligible influence on in situ westward propagating LPS which are the major contributors to LPS‐related precipitation. The insensitivity of LPS characteristics and precipitation over India to the removal of SAMs can be attributed to their low height and the insensitivity of MTG to the height of these mountains.
We use the NCAR Community Earth System Model (CESM) to simulate the statistics of summer monsoon low pressure systems (LPS) over India and study the influence of the southeast Asian mountains (SAMs) on these characteristics. CESM could reproduce most of the LPS characteristics but a southward latitudinal shift of 4° is simulated in the median of the tracks. The removal of SAMs is found to have little influence on LPS characteristics and summer monsoon rainfall over India.
Abstract
The heavily industrialised Kanpur region is the most polluted stretch of the Ganga river because of excessive pollutant discharge from the industries. Agricultural runoff along with climate ...change further adds to the pollution risk in this industrialised stretch of Ganga. In this paper, we analyse the potential impacts of climate change and land use change on the water quality in this stretch under hypothetical scenarios using the water quality model, QUAL2K. Water quality indicators of Dissolved Oxygen (DO), Biochemical Oxygen Demand, ammonia, nitrate, total nitrogen, organic-, inorganic- and total phosphorous and faecal coliform are assessed for eight climate change and six land use land cover scenarios. Eutrophic conditions are observed in this stretch of the river for all scenarios, implying severe impacts on aquatic life. DO is identified as the most sensitive indicator to the climate change scenarios considered, while nutrients and faecal coliform are more sensitive to the land use scenarios. Increase in agricultural land area leads to larger nutrient concentration while increase in built-up area causes an increase in faecal coliform concentration. Results from this hypothetical study could provide valuable guidance for improving the water quality of the Ganges in future climate change and land use change scenarios.
Arctic geoengineering wherein sunlight absorption is reduced only in the Arctic has been suggested as a remedial measure to counteract the on-going rapid climate change in the Arctic. Several ...modeling studies have shown that Arctic geoengineering can minimize Arctic warming but will shift the Inter-tropical Convergence Zone (ITCZ) southward, unless offset by comparable geoengineering in the Southern Hemisphere. In this study, we investigate and quantify the implications of this ITCZ shift due to Arctic geoengineering for the global monsoon regions using the Community Atmosphere Model version 4 coupled to a slab ocean model. A doubling of CO
2
from pre-industrial levels leads to a warming of ~6 K in the Arctic region and precipitation in the monsoon regions increases by up to ~15%. In our Arctic geoengineering simulation which illustrates a plausible latitudinal distribution of the reduction in sunlight, an addition of sulfate aerosols (11 Mt) in the Arctic stratosphere nearly offsets the Arctic warming due to CO
2
doubling but this shifts the ITCZ southward by ~1.5° relative to the pre-industrial climate. The combined effect from this shift and the residual CO
2
-induced climate change in the tropics is a decrease/increase in annual mean precipitation in the Northern Hemisphere/Southern Hemisphere monsoon regions by up to −12/+17%. Polar geoengineering where sulfate aerosols are prescribed in both the Arctic (10 Mt) and Antarctic (8 Mt) nearly offsets the ITCZ shift due to Arctic geoengineering, but there is still a residual precipitation increase (up to 7%) in most monsoon regions associated with the residual CO
2
induced warming in the tropics. The ITCZ shift due to our Global geoengineering simulation, where aerosols (20 Mt) are prescribed uniformly around the globe, is much smaller and the precipitation changes in most monsoon regions are within ±2% as the residual CO
2
-induced warming in the tropics is also much less than in Arctic and Polar geoengineering. Further, global geoengineering nearly offsets the Arctic warming. Based on our results we infer that Arctic geoengineering leads to ITCZ shift and leaves residual CO
2
induced warming in the tropics resulting in substantial precipitation decreases (increases) in the Northern (Southern) hemisphere monsoon regions.
Terrestrial and oceanic carbon sinks together sequester >50% of the anthropogenic emissions, and the major uncertainty in the global carbon budget is related to the terrestrial carbon cycle. Hence, ...it is important to understand the major drivers of the land carbon uptake to make informed decisions on climate change mitigation policies. In this paper, we assess the major drivers of the land carbon uptake-CO2 fertilization, nitrogen deposition, climate change, and land use/land cover changes (LULCC)-from existing literature for the historical period and future scenarios, focusing on the results from fifth Coupled Models Intercomparison Project (CMIP5). The existing literature shows that the LULCC fluxes have led to a decline in the terrestrial carbon stocks during the historical period, despite positive contributions from CO2 fertilization and nitrogen deposition. However, several studies find increases in the land carbon sink in recent decades and suggest that CO2 fertilization is the primary driver (up to 85%) of this increase followed by nitrogen deposition (∼10%-20%). For the 21st century, terrestrial carbon stocks are projected to increase in the majority of CMIP5 simulations under the representative concentration pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios, mainly due to CO2 fertilization. These projections indicate that the effects of nitrogen deposition in future scenarios are small (∼2%-10%), and climate warming would lead to a loss of land carbon. The vast majority of the studies consider the effects of only one or two of the drivers, impairing comprehensive assessments of the relative contributions of the drivers. Further, the broad range in magnitudes and scenario/model dependence of the sensitivity factors pose challenges in unambiguous projections of land carbon uptake. Improved representation of processes such as LULCC, fires, nutrient limitation and permafrost thawing in the models are necessary to constrain the present-day carbon cycle and for more accurate future projections.
Abstract
Previous studies have shown that climate sensitivity, defined as the global mean surface temperature change per unit radiative forcing, is smaller for solar radiative forcing compared to an ...equivalent CO
2
radiative forcing. We investigate the causes for this difference using the NCAR CAM4 model. The contributions to the climate feedback parameter, which is inversely related to climate sensitivity, are estimated for water vapor, lapse rate, Planck, albedo, and cloud feedbacks using the radiative kernel technique. The total feedback estimated for CO
2
and solar radiative forcing from our model simulations is −1.23 and −1.45 W m
−2
K
−1
, respectively. We find that the difference in feedback between the two cases is primarily due to differences in lapse rate, water vapor, and cloud feedbacks, which together explain 65% of the difference in total feedback. The rest comes from Planck and albedo feedbacks. The differences in feedbacks arise mainly from differences in the horizontal (meridional) structure of forcing and the consequent warming. Our study provides important insights into the effects of the meridional structure of forcing on climate feedback, which is important for evaluating global climate change from different forcing agents.
Significance Statement
An increase in atmospheric CO
2
concentration or an increase in incoming solar radiation leads to a rise in the radiative budget and consequent climate warming, which is amplified by the presence of multiple climate feedbacks. These feedbacks, from changes in surface albedo, combined effect of water vapor and the vertical lapse rate of temperature, and changes in clouds, differ between solar and CO
2
forcing. Using radiative kernels, this study quantifies these individual feedbacks for an equivalent radiative change caused by an increase in CO
2
or incoming solar radiation, showing how the differences arise from differences in the meridional patterns of warming. In agreement with prior studies, these differences can explain the smaller efficacy of solar forcing compared to CO
2
forcing.
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