Patterns in Global, Regional, and Local Groundwater Depth

Groundwater.water.usgs.gov

water.usgs.gov

Up until this point, there has been no unifying effort to create a global map of groundwater tables.  However, a global, comprehensive map of the location and depth of water tables throughout the world can help with finding global patterns of groundwater movement.  Fan et al. compiled all existing government records of groundwater tables from over a million well sites around the globe.  Where government records were not available, they used data from published literature.  The compiled map was not complete though;  the water tables in many places around the globe remain unrecorded.  In order to look more closely and completely at global, regional, and local trends in groundwater distribution and depth the researchers also used a pre-existing groundwater table model.  They found that groundwater tends to be shallowest in the most humid climates, in wetland regions, in arid valleys and along the edges of continents, especially in areas with long, flat, plains of wetlands leading up to the coast.  Additionally, the model looked for the influence of three forces—climate, terrain, and sea level—on the water table depth (WTD).  While sea level has the strongest influence on WTD globally, regionally climate and topographic gradient are most important, and locally they found that terrain can override climate boundaries and lead to climatic anomalies like oases. —Alison Marks

 
Fan, Y., Li, H., Miguez-Macho, G. Global Patterns of Groundwater Table Depth.  Science 339, 940–943.

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Natural Solar-Volcanic and Anthropogenic Greenhouse Gas Climate Changes Cause Different Variations in Precipitation and Sea Surface Temperatures

Liu et al. (2013) investigated two aspects of precipitation change under climate change conditions:  changes in precipitation levels and sea surface temperatures (SSTs).  These two aspects, the change in global mean precipitation and the spatial distribution of precipitation changes, are found by Liu et al. to have different responses to increases in greenhouse gases from anthropogenic sources than to combined solar radiation and volcanic forcings.  This supports the incongruity between the predictions of climate change reconstructions based on palaeoproxy evidence, naturally occurring records of climate conditions and time, and the current best climate models.  While increased atmospheric and sea temperatures, as a result of a warming event, have historically caused an increase in precipitation due to an increase of energy in the atmosphere and therefore a more energetic system, the aerosols introduced during the production of greenhouse gases through the burning of fossil fuels suppress precipitation, leading to less overall precipitation in a greenhouse gas influenced environment.   Similarly, the spatial distribution of precipitation is affected by the forms of warming, causing different gradients of SSTs.  Several climate models predict that these gradients change differently under the solar-volcanic warming than greenhouse gas associated warming, again probably due to the presence of aerosols.  When the findings from both the change in global mean precipitation and the change in spatial distribution of precipitation are considered, it becomes clear that the method of climate change is important in predicting the way that precipitation patterns will change.—Alison Marks

Liu, J., Wang, B., Cane, M., Yim, S., Lee. J., 2013.  Divergent Global Precipitation Changes Induced by Natural Versus Anthropogenic Forcing.  Nature 493, 656–659.

Jian Liu and his colleagues used the ECHO-G model to run millennium climate simulations.  The ECHO-G is an atmosphere-ocean model that is able to reproduce accurate short-term and present-day climates by coupling an oceanic circulation model and an atmospheric model.  Comparisons between this and other more complicated climate models found them to be relatively similar, adding a sense of validity to the present model. 
Liu et al. found that the difference in global mean precipitation change between the two forms of climate change can be best explained through the tropospheric energy budget.  The troposphere, the lowest layers of earth’s atmosphere, has a certain amount of energy available to do work.  This energy is responsible for the warming and evaporation of water from the oceans, the formation of clouds, and other atmospheric activities.  When more energy is put into this system, i.e. during global warming, more water should evaporate and more precipitation should fall.  However, the release of anthropogenic greenhouse gases is accompanied by the release of aerosols, fine particles suspended in the air, which discourage cloud formation and precipitation.  Therefore, climate change due to greenhouse gas release can have a net negative relationship with annual precipitation.
More involved modeling was used by Liu et al.to consider the possible differences between solar-volcanic and greenhouse gas climate change with regard to the distribution of precipitation.  They began by identifying a solar-volcanic mode during the last millennium, the Medieval Warm Period (1100-1200 CE).  This period was then compared, using both proxy evidence and climate modeling, to the Little Ice Age (1630-1730 CE) to determine the changes in precipitation and SST during the warm period.  The Medieval Warm Period is characterized by a stronger SST gradient across the Pacific Ocean, stronger winds across the Pacific, stronger atmospheric circulation, and, as a result, higher precipitation.  Liu et al. compared these results to climate modeling that only considered greenhouse gases as a climate change forcing, both for the periods 1860-2000 CE and 1990-2100 CE.  The period between 1990-2100 CE was modeled using the A1B scheme proposed by the IPCC.  In the A1B climate model, all energy forms are improved, but all forms, both carbon-neutral and not, continue to be used in equal amounts.  Both time periods had similar SST and precipitation trends predicted by the model, however the results during the later time period were exacerbated to a greater degree.  The later time period had a slightly smaller increase in SST gradient change than the solar-volcanic period and a comparison of precipitation changes shows that the two climate change forces caused precipitation change in different regions.  While the solar-volcanic model was drier than the greenhouse gas model in the central Pacific, the greenhouse gas model was drier along the tropics.  Overall, the greenhouse gas model creates about 40% less precipitation change per degree Celsius of temperature change than the solar-volcanic model. 

The research done by Liu et al. helps to explain the complicated nature of climate change.  The source causing the climate change can greatly impact the oceanic SSTs and precipitation patterns.  Their work also makes it clear that while palaeoproxy data can be useful in helping to ascertain a general idea of the future through comparison, some of the aspects of the climate change will not follow the same pattern as past warming events. 

Decadal Rain Patterns Along East African Coast Controlled by Eastern Indian Ocean Sea Surface Temperatures

The eastern African Coast has been experiencing a long, steady decrease in annual rainfall over the past few decades, including an especially dry period between 2010–2011.  After a failure of the seasonal rainy season in 2011, Tierney et al. (2013) investigated the link between regional precipitation and sea surface temperatures (SSTs) in the Indian and Pacific Oceans, hoping to determine if natural climate variance or anthropogenic climate change were responsible for the drought.  The authors collected water level data from lakes in eastern Africa and used a series of simulations to compare the water levels to both atmosphere-ocean climate models and approximated SSTs in the Indian Ocean over the last 700 years to determine if oceanic climate in the Indian or equatorial Pacific Oceans affected precipitation along the eastern coast of Africa.  A significant relationship was found between the eastern Indian Ocean SSTs and rainfall in the Cape Horn region.  The authors speculated that the SSTs in the eastern Indian Ocean were able to affect a large atmospheric circulation pattern that has a localized anomaly over the Indian Ocean, the Walker circulation.  Under this model, the current homogenous SSTs across the Indian Ocean could be suppressing the Walker circulation anomaly over the Indian Ocean and causing the current drought conditions in Africa.  Definite conclusions as to the cause of current SSTs in the Indian Ocean could not be reached;  the resulting drought conditions could be a part of the natural cycle of the region or a result of anthropogenic climate change.—Alison Marks

Tierney, Jessica E., Jason E. Smerdon, Kevin J. Anchukaitis, and Richard Seager. Multidecadal Variability in East African Hydroclimate Controlled by the Indian Ocean. Nature 493, 389–392.

Tierney et al. averaged water level records from seven lakes in Eastern Africa and calculated water level variance over the last 700 years to determine spatial and temporal trends of water availability in the area.  Because the lake records aren’t uniform by year in the same way as tree rings might be, they used a Monte Carlo empirical orthogonal function (MCEOF).  The MCEOF compared the datasets from the lakes against one another to align the data sets by year.  From that data, Tierney et al. were able to separate the region into two areas with opposing trends:  the eastern coast/Horn of Africa and the Rift Valley area, located south and inland from Cape Horn. The eastern coast/Horn of Africa area was identified as the target region and all following analysis was done on the data collected from that region.  The data from the lakes in the eastern coast region were then analyzed with a simulation using a 1,300 and 3000 year atmosphere-ocean climate circulation control models to determine if the changes in water level overtime could be attributed to changes in climate.  The data were analyzed in increments of 50 years.  The authors found that the region was wetter when the models predicted warm sea surface temperatures in the west Indian Ocean and cooler sea surface temperatures in the east Indian Ocean.  No significance was found between the El Niño–Southern Oscillation (ENSO), which is mainly impacted by Pacific Ocean SSTs, and the water levels along the African coast.  From this point on in the study, the possibility of the conditions of the Pacific Ocean being a contributing factor to the precipitation in eastern Africa was disregarded.  Finally, the scientists used a recent sediment reconstruction of the Makassar Strait, a set of consistent SSTs in the western Pacific Ocean over the last millennium, to approximate the SSTs of the Indian Ocean over the same period of time.  This approximation was done by using the known influence of the Indonesian Throughflow from the Pacific Ocean to the Indian Ocean to approximate the temperatures present in both the eastern and western Indian Ocean at the time. 
The relationship between the SSTs in the Indian Ocean and the rainfall levels in the Cape Horn region can be viewed through an understanding of the general trends found in the datasets.  The MCEOF test found that in the target region there was a period of drought between 1300 and 1400 CE, followed by a transition to a wetter climate until 1700 to 1750 CE, when peak rainfall levels were reached.  After 1750 CE, there was a gradual decrease in precipitation that has continued into current day.  When these SSTs were compared to the water level trends, Tierney et al. found that the highest SSTs in the eastern Indian Ocean during this time correlated with the wettest periods along the eastern African coast.  This is probably due to the Walker circulation anomaly above the Indian Ocean.  The Walker circulation is an atmospheric climate system that interacts mostly over the equatorial Pacific Ocean, but can be affected by stratified temperatures between the eastern and western Indian Oceans.  When the temperature in the western Indian Ocean becomes warmer than the eastern side, the warmer air along the western side moves up, and then cycles down along into the east.  This creates a front along the eastern coast of Africa and leads to cloud formation and eventually precipitation. 

Despite these correlations, the researchers were not able to make any definite conclusions about their findings.  The Indian Ocean SST data set was created through inference and approximation.  Although the trends suggest that the more stratified the temperatures of the Indian Ocean are, the stronger the Walker circulation affects the African coast, there is not enough certainty that the SSTs determined over the last one thousand years for the Indian Ocean are accurate.  Tierney et al. were also not able to form any definite conclusions on the cause of the current drought along the eastern coast of Africa.  Although the results suggest that the current drought could just be a continuation of the current climate regime, the last major climate peak in the area, between 1770 and 1750 CE, occurred during the Little Ice Age.  Because that wet period can be correlated with another major climate event that was caused by radiative forcing, the current climate situation cannot be disregarded as a possible cause for the droughts being experienced along Cape Horn.  More certain data on historical SSTs needs to be investigated before a significant conclusion can be made.