New York City Panel on Climate Change 2019 Report Chapter 2: New Methods for Assessing Extreme Temperatures, Heavy Downpours, and Drought

Abstract

The focus of NPCC3 is on high-risk events involving extreme temperatures, extreme precipitation, and drought. Current trends are presented using historical climate records of high temperature, cold snaps, humidity, and extreme precipitation for the New York metropolitan region. The geographical span of the New York metropolitan region considered here includes, in addition to New York City, adjacent sections of New Jersey such as Newark, Jersey City and Elizabeth, as well as other nearby locations in New York such as Yonkers and Long Island. Historical records of droughts in the Delaware watershed region are also examined. Each climate extreme is analyzed for detection of current trends, and future projections are updated for high-temperature extremes as a test of new methods that could be utilized by NPCC4. These represent finer temporal and spatial resolutions that may be of practical use to key stakeholders in New York City for planning purposes and/or emergency responses. They include local projections of extreme heat and demonstrate the role of the heterogeneous landscape of the city in each process (e.g., how the urban heat island (UHI) affects city neighborhoods differently). Each section of the chapter presents definitions, baselines, methods, and projections, along with uncertainties and recommendations for future work. As in NPCC2, NPCC3 makes use of definitions, measurements, baselines, and scenarios to represent how the probabilities of climate events may change in the future. Here, the focus is on extreme events. For most climate hazards, the definitions of extremes are consistent with the NPCC2, specifically for extreme heat, cold spells, and precipitation. NPCC3 confirms the temperature and precipitation projections of NPCC2 as those of record for use in planning. Based on emerging science, NPCC3 introduces a new methodology for analyzing heat and precipitation extremes that could be used for developing future projections of record in NPCC4. In NPCC2, temperature analyses included projections of average temperature changes and changes in heat waves and hot days. NPCC3 explores new methodologies for downscaling heat extremes and introduces new metrics to analyze historical and projected humidity. For precipitation, NPCC2 developed quantitative projections for average rainfall and daily maximum rainfall events, and NPCC3 introduces a methodology for quantifying projections for sub-daily heavy downpour rain events. In addition, NPCC3 examined how current observations of temperature and precipitation changes compare to projected changes from NPCC2 into the 2020s the from to the of and that observations from to for and NPCC2 projections are to consistent with projected changes in average for temperature and precipitation be with of the role that in the As NPCC3 from a focus on average to the in to the of the for the extreme and the for are the with NPCC2 is For the for heat waves is is the as NPCC3 for extreme heat events and for NPCC2 used the with NPCC3 downscaling and future projections for extreme heat on and downscaling for is included in the projections as a new The section of extreme with a of cold and The section on urban makes use of records of and to demonstrate the spatial of extreme events For a precipitation record on is used to in the New York City watershed and of to are used to how extreme droughts in the the new extreme projections, downscaling is used In the of a that the role of the urban in heat events and events. from the of the are used for projections of extreme for future projections are consistent with NPCC2 are updated to for climate in the of temperature are in on the and as by The of as the of and are to in New York City the to and extreme heat events as heat Here, the of heat waves to the that of with of waves a of These include and and and with a the (UHI) the of extreme heat events and and New York City, the most urban in the with a and to and the of events and in the future. heat projections developed on and with on local urban projections that for and that may the and of high-temperature events. of is the may heat waves by for may by the for the and and include heat from and of and heat in and and of the role in how to high heat As with the to heat of to of high-temperature such as and heat section presents extreme heat and projections for New York City using new methods, for urban of climate is in spatial and are to and The downscaling used by NPCC3 is to the of climate by and to a of observations in the from the of NPCC2 that with records to the projections to the New York metropolitan region using the projected changes are to daily In as in NPCC2, the climate projections are on climate by to as and to as high The of is to the uncertainties emerging from the of as well as those to the of future and on and use For NPCC3 the as the NPCC2 and methods are in change to a change in the of the climate that be from changes in the average the of and that for to change to the of changes in the of the and/or such as and is a of the of the climate that be used to to of and use Each the and are of of of and changes developed for climate as a for and climate on of climate These are used as to to the of on future The of by as and The of by is here as a and that by as a the of the of the and local climate change is developed for key climate precipitation, and extreme events. 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