Human-caused long-term changes in global aridity

AbstractWidespread aridification of the land surface causes substantial environmental challenges and is generally well documented. However, the mechanisms underlying increased aridity remain relatively underexplored. Here, we investigated the anthropogenic and natural factors affecting long-term global aridity changes using multisource observation-based aridity index, factorial simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6), and rigorous detection and attribution (D&A) methods. Our study found that anthropogenic forcings, mainly rising greenhouse gas emissions (GHGE) and aerosols, caused the increased aridification of the globe and each hemisphere with high statistical confidence for 1965–2014; the GHGE contributed to drying trends, whereas the aerosol emissions led to wetting tendencies; moreover, the bias-corrected CMIP6 future aridity index based on the scaling factors from optimal D&A demonstrated greater aridification than the original simulations. These findings highlight the dominant role of human effects on increasing aridification at broad spatial scales, implying future reductions in aridity will rely primarily on the GHGE mitigation.

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Spatial Relationship between Precipitation and Runoff in Africa

10.5194/hess-2018-424 ◽

2018 ◽

Cited By ~ 1

Author(s):

Fidele Karamage ◽

Yuanbo Liu ◽

Xingwang Fan ◽

Meta Francis Justine ◽

Guiping Wu ◽

...

Keyword(s):

Land Surface ◽

Spatial Scales ◽

Spatial Relationship ◽

Resource Planning ◽

Climatic Conditions ◽

Runoff Coefficient ◽

Global Precipitation Climatology Centre ◽

High Runoff ◽

Monthly Runoff

Abstract. Lack of sufficient and reliable hydrological information is a key hindrance to water resource planning and management in Africa. Hence, the objective of this research is to examine the relationship between precipitation and runoff at three spatial scales, including the whole continent, 25 major basins and 55 countries. For this purpose, the long-term monthly runoff coefficient (Rc) was estimated using the long-term monthly runoff data (R) calculated from the Global Runoff Data Centre (GRDC) streamflow records and Global Precipitation Climatology Centre (GPCC) precipitation datasets for the period of time spanning from 1901 to 2017. Subsequently, the observed Rc data were interpolated in order to estimate Rc over the ungauged basins under guidance of key runoff controlling factors, including the land-surface temperature (T), precipitation (P) and potential runoff coefficient (Co) inferred from the land use and land cover, slope and soil texture information. The results show that 16 % of the annual mean precipitation (672.52 mm) becomes runoff (105.72 mm), with a runoff coefficient of 0.16, and the remaining 84 % (566.80 mm) evapotranspirates over the continent during 1901–2017. Spatial analysis reveals that the precipitation–runoff relationship varies significantly among different basins and countries, mainly dependent on climatic conditions and its inter-annual variability. Generally, high runoff depths and runoff coefficients are observed over humid tropical basins and countries with high precipitation intensity compared to those located in subtropical and temperate drylands.

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Influence of radiative forcing factors on ground–air temperature coupling during the last millennium: implications for borehole climatology

Climate of the Past ◽

10.5194/cp-14-1583-2018 ◽

2018 ◽

Vol 14 (11) ◽

pp. 1583-1606 ◽

Cited By ~ 7

Author(s):

Camilo Melo-Aguilar ◽

J. Fidel González-Rouco ◽

Elena García-Bustamante ◽

Jorge Navarro-Montesinos ◽

Norman Steinert

Keyword(s):

Air Temperature ◽

Land Surface ◽

Radiative Forcing ◽

Spatial Scales ◽

Ground Surface ◽

Surface Air Temperature ◽

Small Variation ◽

Last Millennium ◽

Reconstruction Methods

Abstract. Past climate variations may be uncovered via reconstruction methods that use proxy data as predictors. Among them, borehole reconstruction is a well-established technique to recover the long-term past surface air temperature (SAT) evolution. It is based on the assumption that SAT changes are strongly coupled to ground surface temperature (GST) changes and transferred to the subsurface by thermal conduction. We evaluate the SAT–GST coupling during the last millennium (LM) using simulations from the Community Earth System Model LM Ensemble (CESM-LME). The validity of such a premise is explored by analyzing the structure of the SAT–GST covariance during the LM and also by investigating the evolution of the long-term SAT–GST relationship. The multiple and single-forcing simulations in the CESM-LME are used to analyze the SAT–GST relationship within different regions and spatial scales and to derive the influence of the different forcing factors on producing feedback mechanisms that alter the energy balance at the surface. The results indicate that SAT–GST coupling is strong at global and above multi-decadal timescales in CESM-LME, although a relatively small variation in the long-term SAT–GST relationship is also represented. However, at a global scale such variation does not significantly impact the SAT–GST coupling, at local to regional scales this relationship experiences considerable long-term changes mostly after the end of the 19th century. Land use land cover changes are the main driver for locally and regionally decoupling SAT and GST, as they modify the land surface properties such as albedo, surface roughness and hydrology, which in turn modifies the energy fluxes at the surface. Snow cover feedbacks due to the influence of other external forcing are also important for corrupting the long-term SAT–GST coupling. Our findings suggest that such local and regional SAT–GST decoupling processes may represent a source of bias for SAT reconstructions from borehole measurement, since the thermal signature imprinted in the subsurface over the affected regions is not fully representative of the long-term SAT variations.

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An Integrated Variational Framework for Mapping Evapotranspiration by Assimilating GOES LST and SMAP data

10.5194/egusphere-egu2020-2026 ◽

2020 ◽

Author(s):

Leila farhadi ◽

Abedeh Abdolghafoorian

Keyword(s):

Soil Moisture ◽

Great Plains ◽

Land Surface ◽

Water Cycle ◽

Spatial Scales ◽

Coupled Model ◽

Balance Model ◽

Parameter Estimates ◽

Dynamic Feedback ◽

Unknown Parameters

Evapotranspiration (ET) is a key component of terrestrial water cycle that plays an important role in the Earth system. Aaccurate estimation of ET is crucial in various hydrological, meteorological, and agricultural applications. In situ measurements of ET are costly and cannot be readily scaled to regional scales relevant to weather and climate studies. Therefore, there is a need for techniques to make quantitative estimates of ET using land surface state observations that are widely available from remote sensing across a range of spatial scales.In this work, A variational data (VDA) assimilation framework is developed to estimate ET by assimilating Soil Moisture Active Passive (SMAP) soil moisture and Geostationary Operational Environmental Satellite (GOES) land surface temperature data into a coupled dual-source energy and water balance model.The VDA framework estimates the key parameters of the coupled model, which regulate the partitioning of available energy (i.e., neutral bulk heat transfer coefficient (CHN) and evaporative fraction from soil (EFS) and canopy (EFC)). The uncertainties of the retrieved unknown parameters are estimated through the inverse of Hessian of cost function, obtained using the Lagrangian methodology. Analysis of the second-order information provides a tool to identify the optimum parameter estimates and guides towards a well-posed estimation problem.The VDA framework is implemented&#160;over an area of 21780 km2&#160;in the U.S. Southern Great Plains (with computational grid size of 0.05 degree) during a nine-month period. The maps of retrieved evaporation and transpiration are used to study a number of dynamic feedback mechanisms between the land and atmosphere, such as the dependence of evapotranspiration on vegetation and soil moisture.

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Anthropogenic and Natural Contributions to the Lengthening of the Southern Hemisphere Summer Season

Journal of Climate ◽

10.1175/jcli-d-20-0084.1 ◽

2020 ◽

Vol 33 (24) ◽

pp. 10539-10553

Author(s):

Evan Weller ◽

Bo-Joung Park ◽

Seung-Ki Min

Keyword(s):

Greenhouse Gas ◽

Southern Hemisphere ◽

Dominant Role ◽

Coupled Model ◽

Internal Variability ◽

Southern Annular Mode ◽

Summer Season ◽

External Forcings ◽

Regional Trends

AbstractThis study provides the first quantitative assessment of observed long-term changes in summer-season timing and length in the Southern Hemisphere (SH) and its subregions over the past 60 years, enabling a global completeness by complementing such characteristics previously reported for the Northern Hemisphere (NH). Using an objective algorithm that is based on temperature indices, relative measures of summer onset, withdrawal, and duration are determined at each land location over the period 1953–2012. Significant widespread summer-season lengthening, due to earlier onset and delayed withdrawal, has occurred across the SH, a longer period for extreme heat-wave events and wildfires to potentially occur. The asymmetric magnitude (onset vs withdrawal) in summer-season lengthening is slightly less over the SH than over the NH. Contributions of anthropogenic and natural factors to the observed trends in summer-season characteristics were investigated using phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel simulations integrated with observed external forcings [anthropogenic plus natural (ALL)], greenhouse gas forcing only (GHG), and natural forcing only [solar and volcanic activities (NAT)]. Overall, consistent with the NH, increased greenhouse gases were the main cause of observed changes in the SH, with negligible contribution from other external forcings. ALL and GHG simulations also reproduced a slight tendency for earlier summer onset to contribute more to summer lengthening. Proportions of observed regional trends in summer-season indices attributable to trends in long-term internal variability in the SH, namely, the interdecadal Pacific oscillation (IPO) and southern annular mode (SAM), suggests such variability can only explain up to ~12%, supporting the dominant role of greenhouse gas forcing.

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On the roles of circulation and aerosols in the decline of mist and dense fog in Europe over the last 30 years

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-23987-2009 ◽

2009 ◽

Vol 9 (6) ◽

pp. 23987-24027

Author(s):

G. J. van Oldenborgh ◽

P. Yiou ◽

R. Vautard

Keyword(s):

Land Surface ◽

Atmospheric Dynamics ◽

Spatially Correlated ◽

Transportation Risk ◽

Climate Simulations ◽

Low Visibility ◽

Dense Fog ◽

Aerosol Emissions ◽

Sulphur Emission

Abstract. Fog and mist are meteorological phenomena that have significant contributions to temperature variations. Understanding and predicting them is also crucial for transportation risk management. It has been shown that low visibility phenomena over Europe have been declining over the past three decades. The trends in mist and haze have been correlated to atmospheric aerosol trends. However, dense fog has not received yet such focus. The goal of this paper is to examine the roles of synoptic atmospheric circulation and aerosol content on the trends of dense fog. We show that sulphur emission trends are spatially correlated with visibility trends, with a maximum correlation when visibility is between 1 km and 10 km. We find that atmospheric dynamics overall contributes up to 40% of the variability of the frequency of fog occurrences. This contribution is spatially variable and highly depends on the topography and the season, with higher values in the winter. The observed long-term circulation changes do not contribute much to the trends in low visibility found in the data. This process is illustrated on three stations (De Bilt, Zürich Airport and Potsdam) for which a long-term visibility data and a thorough meteorological description are available. We conclude that to properly represent fog in future climate simulations, it is necessary to include realistic representations of aerosol emissions and chemistry, land surface properties and atmospheric dynamics.

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Disentangling the impacts of anthropogenic aerosols on terrestrial carbon cycle during 1850-2014

10.5194/egusphere-egu21-14841 ◽

2021 ◽

Author(s):

Yuan Zhang ◽

Philippe Ciais ◽

Olivier Boucher ◽

Fabienne Maignan ◽

Ana Bastos ◽

...

Keyword(s):

Land Surface ◽

Dominant Role ◽

Diffuse Radiation ◽

Carbon Fluxes ◽

Surface Model ◽

Anthropogenic Aerosols ◽

Radiation Quality ◽

Anthropogenic Aerosol ◽

C Cycle ◽

Aerosol Emissions

Aerosols have a dimming and cooling effect and change hydrological regimes, thus affecting carbon fluxes, which are sensitive to climate. Aerosols also scatter sunlight, which increases the fraction of diffuse radiation, increasing photosynthesis. Although previous studies have quantified the impacts of some of these factors separately, there remains no clear conclusion whether the physical impacts of aerosols on land carbon fluxes is larger through diffuse radiation change than through changes in other climate variables. In this study, we quantified the overall physical impacts of anthropogenic aerosols on land C fluxes and explored the contribution from each factor using a set of factorial simulations driven by climate and aerosol data from the IPSL-CM6A-LR experiments from 1850 to 2014. A newly-developed land surface model which distinguishes diffuse and direct radiation in canopy radiation transmission, ORCHIDEE_DF, was used. Specifically, a sub-grid scheme was developed to distinguish the cloudy and clear sky conditions. We found that anthropogenic aerosol emissions since 1850 cumulatively enhanced the land C sink by 22.6 PgC. 78% of this C sink enhancement is contributed by aerosol-induced increase in the diffuse radiation fraction, which is much larger than the effect of the aerosol-induced dimming. The cooling of anthropogenic aerosols increases the C sink in low latitudes but decreases the C sink in high latitudes and overall slightly increases the global land C sink. Compared with radiation and temperature changes, aerosol-induced precipitation changes have limited impacts. The dominant role of diffuse radiation changes in affecting historical land C fluxes found in this study implies that future aerosol emissions may have a much stronger impacts on the C cycle through changing radiation quality than through changing climate alone. Earth system models need to take into account the diffuse radiation fertilization effect, in order to better evaluate the impacts of climate change mitigation scenarios.

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On the roles of circulation and aerosols in the decline of mist and dense fog in Europe over the last 30 years

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-4597-2010 ◽

2010 ◽

Vol 10 (10) ◽

pp. 4597-4609 ◽

Cited By ~ 26

Author(s):

G. J. van Oldenborgh ◽

P. Yiou ◽

R. Vautard

Keyword(s):

Land Surface ◽

Atmospheric Dynamics ◽

Spatially Correlated ◽

Transportation Risk ◽

Climate Simulations ◽

Low Visibility ◽

Dense Fog ◽

Aerosol Emissions ◽

Sulphur Emission

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Spatial Distribution of Global Landscape Evaporation in the Early Twenty-First Century by Means of a Generalized Complementary Approach

Journal of Hydrometeorology ◽

10.1175/jhm-d-19-0208.1 ◽

2020 ◽

Vol 21 (2) ◽

pp. 287-298 ◽

Cited By ~ 8

Author(s):

Wilfried Brutsaert ◽

Lei Cheng ◽

Lu Zhang

Keyword(s):

Spatial Distribution ◽

Wind Speed ◽

Land Surface ◽

Spatial Scales ◽

Surface Air Temperature ◽

Aridity Index ◽

Routine Data ◽

Net Radiation ◽

Atmospheric Conditions ◽

Near Surface

AbstractA generalized implementation of the complementary principle was applied to estimate global land surface evaporation and its spatial distribution. The single parameter in the method was calibrated as a function of aridity index, mainly on the basis of runoff and precipitation data for 524 catchments in different parts of the world. The spatial distribution of annual evaporation from Earth’s land surfaces for 2001–13 was then calculated at a spatial resolution of 0.5°, by means of an available global net radiation dataset (commonly referred to as CERES SYN1deg-Day) and a global forcing dataset (referred to as CRU-NCEP v7) for near-surface temperature, humidity, wind speed, and air pressure. The results are shown to agree with reliable previous estimates by more elaborate methods. The global average evaporation for 2001–13 was found to be 472.65 mm a−1 or 36.96 W m−2. The present method should allow not only future updates but also retroactive historical analyses with routine data of net radiation, near-surface air temperature, humidity, wind speed, and precipitation; its main advantage is that the environmental aridity is deduced from atmospheric conditions and requires no knowledge of surface characteristics, such as soil moisture, vegetation, and terrain, which are highly variable and often difficult to quantify at larger spatial scales. Because they are strictly measurement based, the results can serve also as a reality check for different aspects of climate and related models.

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Critical impact of vegetation physiology on the continental hydrologic cycle in response to increasing CO2

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1720712115 ◽

2018 ◽

Vol 115 (16) ◽

pp. 4093-4098 ◽

Cited By ~ 62

Author(s):

Léo Lemordant ◽

Pierre Gentine ◽

Abigail S. Swann ◽

Benjamin I. Cook ◽

Jacob Scheff

Keyword(s):

Land Surface ◽

Dominant Role ◽

Human Life ◽

Major Effect ◽

Hydrologic Cycle ◽

Hydrologic Change ◽

Hydrological Stress ◽

Precipitation Changes

Predicting how increasing atmospheric CO2 will affect the hydrologic cycle is of utmost importance for a range of applications ranging from ecological services to human life and activities. A typical perspective is that hydrologic change is driven by precipitation and radiation changes due to climate change, and that the land surface will adjust. Using Earth system models with decoupled surface (vegetation physiology) and atmospheric (radiative) CO2 responses, we here show that the CO2 physiological response has a dominant role in evapotranspiration and evaporative fraction changes and has a major effect on long-term runoff compared with radiative or precipitation changes due to increased atmospheric CO2. This major effect is true for most hydrological stress variables over the largest fraction of the globe, except for soil moisture, which exhibits a more nonlinear response. This highlights the key role of vegetation in controlling future terrestrial hydrologic response and emphasizes that the carbon and water cycles are intimately coupled over land.

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Future trends in global water vs. energy-controlled evaporative regimes

10.5194/egusphere-egu21-3166 ◽

2021 ◽

Author(s):

Jasper Denissen ◽

Adriaan Teuling ◽

Wantong Li ◽

Markus Reichstein ◽

Andy Pitman ◽

...

Keyword(s):

Soil Moisture ◽

Land Surface ◽

Climate Model ◽

Global Climate ◽

Coupled Model ◽

Energy Availability ◽

Water Control ◽

Near Surface ◽

Global Patterns

Water and energy availability govern the exchange of carbon, energy and water between the land surface and the atmosphere and therefore exert influence on near-surface weather. Roughly one can distinguish between two evaporative regimes: One limited by available energy (under wet conditions) and one limited by available soil moisture (under dry conditions). The transition between these evaporative regimes has been studied on local to global scales using observational and modelled datasets. This revealed the complexity of defining this transition, as it varies both in space and time and is sensitive to climate, soil and vegetation characteristics.In this study, we characterized this transition by comparing the correlations of evaporation anomalies with (i) soil moisture anomalies (proxy for strength of water control) and (ii) temperature anomalies (proxy for strength of energy control). In the first step, we use observation-based data to derive global patterns of evaporative regimes and establish that the regime transition is sensitive to not only long-term average soil moisture, but also long-term average temperature. Analyzing historical and future climate model simulations from the Coupled Model Intercomparison Project (CMIP6), we found that the ensemble mean of the CMIP6 models produces similar global patterns and sensitivities to energy and water availability. However, there is ample disagreement between results of individual models, with the largest spread around the transition zones. Further, the disagreement between individual models on the total area of water-limited regions increases gradually in time from historical to future experiments. In the next step, we attribute trends in evaporative regimes to trends in water and energy availability, CO2 and vapor pressure deficit. This research reveals how global climate change translates into regional-global scale trends in water- vs. energy-controlled evaporative regimes. Our observational results can constrain modelled global evaporative regimes and inform future model development to decrease the substantial spread across the present model ensemble.

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