Modeling of Climate Change Consequences for Phase III Watershed Implementation Plans

June 14, 2018

The consequences of global climate change are already evident in the Chesapeake Bay and watershed, at least in terms of warmer water temperatures and sea-level rise, and will influence virtually every aspect of the structure and function of the ecosystem in the coming decades. The 2014 Chesapeake Watershed Agreement set a new goal for climate resiliency to withstand adverse impacts from changing climate conditions. Since then, an important focus has been to estimate, using the Chesapeake Bay Program’s (CBP) management models of the watershed and the estuary, the effects of climate change on achieving water quality standards in the estuary.  Based on these estimates, the task was to determine any adjustments that should be considered in the nutrient load reductions included in the Phase III Watershed Implementation Plans (WIPs) to achieve the Agreement’s 2025 TMDL commitments.  Results were presented at the December 2017 Scientific and Technical Advisory Committee (STAC) meeting.

The modeling assumed specific projections of increased temperature, changes in precipitation, and a rise in mean sea level based on guidance from scientific community, including a 2016 STAC workshop report. Projections made for 2025 from a 1991-2000 reference period were: relative sea-level rise of 17 cm (extrapolated from tide gauge trends at Sewells Point), temperature increase of 1.1°C (based on downscaled climate model projections of annual air temperature), and an overall 3.1% increase in precipitation (based on extrapolation of trends observed in the 88-year annual PRISM data for major basins of the watershed). A single set of assumptions was used rather than scenarios or ranges.

The focus in model outcomes was the effect on hypoxia in the deep channel of the mainstem Bay, because that is the most recalcitrant impairment of water quality. The basic question was, given the assumed climate changes, what would be the variance in hypoxia from the TMDL 2025 objective for achieving dissolved oxygen standards (i.e., if nutrient loads were reduced consistent with the present WIPs by 2025)? Directionally, hypoxia would be expected to increase with increased temperature because of the decrease in oxygen saturation of warmer waters and the increase in rates of metabolic consumption of dissolved oxygen. Hypoxia would be expected to worsen with increased precipitation that would yield more freshwater runoff and deliver more nutrients. On the other hand, sea-level rise would likely result in some alleviation of hypoxia as the estuary would deepen and have greater inflows of cooler and more oxygen-rich ocean water.

Using the assumptions listed above, the models estimated that, on balance, the total nitrogen and total phosphorus loads would change minimally, but nitrate and dissolved phosphate loads would have increased by 2.4% and 1.6%, respectively. This, plus the warmer temperatures, would be sufficient to decrease dissolved oxygen criteria attainment by 7%. Additional analyses indicated that "raising the level of effort of all jurisdictions using the approved planning target method"resulted in an estimate that basin-wide load reductions would require basin-wide load reductions of 9.1 million pounds (4.1 million kg) of nitrogen and 0.49 million pounds (0.22 million kg) of phosphorus per year would be required compensate for the effects of climate change by 2025. The additional climate-change offsets for nitrogen that would have to be reduced are not insignificant with respect to the reductions from 2013 levels yet to be reduced over the next seven years for several of the jurisdictional WIPs.

In considering this advice, the CBP’s Principals’ Staff Committee (PSC) was struck by how rather small changes in assumptions on climate changes affected these estimates. Preliminary model estimates that had been provided earlier in 2017 had suggested that the extent of hypoxia would decrease (improve) with climate change. The rather substantial change in the assessment then provided in December 2017 resulted principally from a reduction in the assumed sea-level rise from 30 cm to 17 cm, and also consideration of the form of the additional nutrient loads, disproportionately more soluble nitrate and orthophosphate.

The PSC indicated that it would also like to consider information on how modifying “best management practices” could affect nutrient loads under the changing climate. As explained in this briefing paper, the PSC directed the CBP’s Climate Resiliency and Modeling Workgroups to address the uncertainties in current scientific understanding so that in 2021, the Partnership can consider the results of updated methods, techniques and studies and revisit estimated loads due to climate change. Toward this end, a STAC workshop has been approved and is being planned for later in 2018. It will inform the development and implementation of a complete and fully operational climate change modeling and assessment system in 2019. After refinement, findings on climate change will be implemented into the jurisdictions' 2022-2023 WIP milestones.

This unconventional CRMS session seeks to increase community awareness of the consequences of climate change and solicit its input toward addressing the particular challenge of adjusting WIPs.  Moderated by Don Boesch, President Emeritus of the University of Maryland Center for Environmental Science, the session will include an overview of the modeling efforts to date by Lewis Linker, CBP Modeling Coordinator, and the perspectives of a panel of four scientists familiar with both the existing climate modeling framework and the decision-making process.

• Maria Herrmann, Penn State University, chair of STAC Review of Climate Change Assessment Framework
• Zack Easton, Virginia Tech University, co-chair of STAC Review of Watershed Model Phase 6
• Lee Currey, Maryland Department of the Environment, principal technical advisor to Secretary Grumbles, chair of the Principals Staff Committee
• Ray Najjar, Penn State University, lead author of a 2010 paper potential climate-change impacts on the Chesapeake Bay

The session will include active, facilitated audience participation in the discussion. Outcomes of the session will help shape a STAC workshop on Chesapeake Bay Program Climate Change Modeling 2.0 in September 2018, and, subsequently, the refinement of the WIP analyses and PSC decisions.