finding 20.1 : key-message-20-1

Freshwater is critical to life throughout the Caribbean. Increasing global carbon emissions are projected to reduce average rainfall in this region by the end of the century (likely, high confidence), constraining freshwater availability, while extreme rainfall events, which can increase freshwater flooding impacts, are expected to increase in intensity (likely, medium confidence). Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater in coastal aquifers (very likely, high confidence). Increasing variability in rainfall events and increasing temperatures will likely alter the distribution of ecological life zones and exacerbate existing problems in water management, planning, and infrastructure capacity (likely, medium confidence).

This finding is from chapter 20 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

The majority of our Key Messages were developed over the course of two separate author meetings. The first occurred March 9–10, 2017, and the second on May 3, 2017. Both meetings were held in San Juan, Puerto Rico; however, people were also able to join remotely from Washington, DC, Raleigh, North Carolina, and the U.S. Virgin Islands (USVI). In addition, the author team held weekly conference calls and organized separate Key Message calls and meetings to review and draft information that was integral to our chapter. To develop the Key Messages, the team also deliberated with outside experts who are acknowledged as our technical contributors.

Description of evidence base:

The average global atmospheric carbon dioxide (CO2) concentration has increased from 378 parts per million (ppm) in 2005 to over 406 ppm during April of 2017. The rate of increase over this period appears to be constant, and there is no indication that the rate will decrease in the future.0b94246c-91be-4f95-ae61-d36fdf775ff3 Several climate change studies have concluded that owing to increased atmospheric CO2 and the consequent global climate change, rainfall will likely decrease in the region between now and the end of the century (e.g., Meehl et al. 2007, Biasutti et al. 2012, Campbell et al. 2011, Cashman et al. 201003abb6ea-0525-4fac-a321-121ca0727673,89123dd9-0946-403e-b223-9745e46800ad,812fc7b5-a2f0-4e62-a767-6b8dc494ea60,e73b29bb-f4e5-47a7-9b18-45697e8e7bdf). Neelin et al. (2006)680629ff-ef00-462b-9c16-d98dc1d3c163 and Scatena (1998)17117749-f561-47d3-96fe-a62683b61369 have predicted increasingly severe droughts in the region in the future. Several downscaling studies, which specifically considered Puerto Rico, predict a reduction in rainfall by the end of the century72d1011e-bdff-49c0-b00f-8222c2a350ea,650b2907-85b1-4b76-a339-a9ec1703c5bd,744497bd-974c-497e-bf74-34ff514c0f83 and constraints on freshwater availability. Furthermore, Taylor et al. (2018)9de0b9c6-ce59-4940-8a6c-a244f1fa7a9c used the most recent generation of global climate models and demonstrated that when global warming increases from 1.5°C to 2°C above the preindustrial values (1861–1900), the Caribbean experiences a shift to predominantly drier conditions. Small watersheds that feed reservoirs are typical of the Caribbean region, and they are less able to serve as a buffer for rainfall variability. Small watersheds exhibit variable drainage patterns, which in turn affect evapotranspiration, groundwater infiltration, and surface water runoff. Drainage patterns in watersheds are also affected by the specific geometry, configuration, and orientation in relation to the average direction of wind over the region, as well as the morphology of rivers. With a projected reduction in rainfall up to 30% on average for the island by the end of the century,650b2907-85b1-4b76-a339-a9ec1703c5bd certain watersheds will likely be less able to buffer rainfall variability and will likely see water deficits in the near future. Increasing variability in rainfall events and increasing temperatures will likely exacerbate existing problems in water management, planning, and infrastructure capacity.

Streamflow is estimated using hydrologic models that are calibrated to networks of stream gauges and precipitation measurements. Reservoirs are considered in a permanent supply deficit if the annual streamflow leaving these reservoirs falls below zero after estimating withdrawals for human consumption, evapotranspiration, and rainfall. Projections of when deficit conditions could occur (circa 2025) are estimated using climate models.a045f06c-0964-4286-9b5a-9b625da4eb2d

Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater in coastal aquifers. In Puerto Rico, groundwater quality can change when the water table is below sea level in coastal areas or when the intensity of pumping induces local upconing of deeper, poor-quality water.553e2d0a-c0ad-4540-9c5f-1f47374129ec Upconing is the process by which saline water underlying freshwater in an aquifer rises upward into the freshwater zone due to pumping.4375edd4-4f85-4a9f-bf62-37c2985ade2b When the water table is below sea level, the natural discharge of groundwater along the coast is reversed and can result in the inland movement of seawater or the upconing of low-quality water.1b555f67-0af6-4f16-882b-0c253117b9c8,9b520861-46d3-45dc-92a0-5c2af79d9429 Diminished aquifer recharge and, to a lesser extent, increased groundwater withdrawals during 2012–2015 resulted in a reduction in the freshwater saturated thickness of the South Coast Aquifer. With sea level rise, groundwater quality will likely deteriorate even further in coastal aquifers in Puerto Rico.

New information and remaining uncertainties:

As global changes continue to alter the hydrological cycle across the region, water resources are expected to be affected in both quantity and quality. There is still uncertainty as to the extent and severity of these global changes on small island nations such as Puerto Rico and the USVI, despite notable advancements in downscaled modeling exercises. Current climatological observations have presented an overall increase in mean annual precipitation across Puerto Rico.0049e302-7751-4977-91ff-0df54d0ab326 However, climate model projections point toward an overall decrease in annual mean precipitation toward 2050 and an increase in rainfall intensity for extreme rainfall,72d1011e-bdff-49c0-b00f-8222c2a350ea,650b2907-85b1-4b76-a339-a9ec1703c5bd,56d77153-c8fc-4fcf-a7f0-fa0e843936f1,66a435ae-179c-49f4-981b-248d647b9579,744497bd-974c-497e-bf74-34ff514c0f83,e031a298-836a-4dbb-b5c8-f4f0ddb047a3 including rainfall associated with hurricanes. There is more uncertainty regarding the frequency and duration to changes in extreme rainfall within the region.650b2907-85b1-4b76-a339-a9ec1703c5bd,56d77153-c8fc-4fcf-a7f0-fa0e843936f1,744497bd-974c-497e-bf74-34ff514c0f83

Selected CMIP3 (Coupled Model Intercomparison Project, phase 3) and CMIP5 global climate models (GCMs) capture the general large-scale atmospheric circulation that controls seasonal rainfall patterns within the Caribbean5d493a0a-db95-418d-ad99-148d753db96a and provide justification that these GCM projections can be further downscaled to capture important rainfall characteristics associated with the islands.e16c77ed-0eaf-4fa6-8c98-256a28794b3b Systemic dry biases exist, however, in the GCMs.5d493a0a-db95-418d-ad99-148d753db96a And many GCMs fail to capture the bimodal precipitation pattern in the region.56d77153-c8fc-4fcf-a7f0-fa0e843936f1 The CMIP3 generation of GCMs that do capture the bimodal rainfall pattern predict extreme drying at the middle and end of this century.650b2907-85b1-4b76-a339-a9ec1703c5bd,56d77153-c8fc-4fcf-a7f0-fa0e843936f1 The CMIP5 generation of GCMs also projects drying by the middle and end of the century, but the magnitude of drying is not as large. Local and island-scale processes could affect these projected changes, since the land surface interacts with and affects both precipitation and evaporation rates.f0dee221-fc70-498e-a618-4b272642bab2

Assessment of confidence based on evidence:

There is high confidence that freshwater availability will likely be constrained by the end of the century and medium confidence that extreme rainfall events will likely increase in intensity. There is high confidence that sea level rise will very likely cause saltwater intrusion impacts on coastal freshwater aquifers. There is medium confidence about likely changes to ecological life zones but low confidence about the distributional effects on the existing terrestrial ecosystems in the region.

References :

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