New York City Panel on Climate Change 2015 ReportChapter 1: Climate Observations and Projections

1.1 The global climate system 1.2 Observed climate 1.3 Climate projections 1.4 Conclusions and recommendations During 2013 and 2014, numerous international (IPCC, 2013) and national (Melillo et al., 2014; Gordon, 2014) reports have concluded that human activities are changing the climate, leading to increased vulnerability and risk. Since the industrial revolution, fossil fuel burning, industrial activity, and land use changes have led to a 40% increase in heat-trapping carbon dioxide (CO2), and an approximately 150% increase in methane (CH4), another powerful greenhouse gas (GHG), has been observed. Global temperatures have increased by close to 1°C since 1880 as the upper oceans have warmed and polar ice has retreated. These and other climate changes are projected to accelerate as greenhouse gas concentrations continue to rise. In the coming decades, climate change is extremely likely to bring warmer temperatures in the New York metropolitan region (see Box. 1.1 and Fig.1.1 for key definitions and terms). Heat waves are very likely to increase; total annual precipitation will likely increase and brief, intense rainstorms are very likely to increase. Because of incomplete knowledge about exactly how much climate change will occur, choosing among policies for reducing future damages requires prudent risk management (Yohe and Leichenko, 2010; Kunreuther et al., 2013). Given differing risk tolerances among stakeholders, a risk management approach allows for a range of possible climate change outcomes to be examined with associated uncertainties surrounding their likelihoods. Climate change refers to a significant change in the state of the climate that can be identified from changes in the average state or the variability of weather and that persists for an extended time period, typically decades to centuries or longer. Climate change can refer to the effects of (1) persistent anthropogenic or human-caused changes in the composition of the atmosphere and/or land use, or (2) natural processes such as volcanic eruptions and Earth's orbital variations (IPCC, 2013). A GCM is a mathematical representation of the behavior of the Earth's climate system over time that can be used to estimate the sensitivity of the climate system to changes in atmospheric concentrations of greenhouse gases (GHGs) and aerosols. Each model simulates physical exchanges among the ocean, atmosphere, land, and ice. The NPCC2 uses 35 GCMs for temperature and precipitation projections. RCPs are sets of trajectories of concentrations of GHGs, aerosols, and land use changes developed for climate models as a basis for long-term and near-term climate-modeling experiments (Figure 1.2; Moss et al., 2010). RCPs describe different climate futures based on different amounts of climate forcingsb. These data are used as inputs to global climate models to project the effects of these drivers on future climate. The NPCC2 uses a set of global climate model simulations driven by two RCPs, known as 4.5 and 8.5, which had the maximum number of GCM simulations available from World Climate Research Programme/Program for Climate Model Diagnosis and Intercomparison (WCRP/PCMDI). RCP 4.5 and RCP 8.5 were selected to bound the range of anticipated GHG forcings at the global scale. On the basis of the selection of the 2 RCPs and 35 GCM simulations, local climate change information is developed for key climate variables—temperature, precipitation, and associated extreme events. These results and projections reflect a range of potential outcomes for the New York metropolitan region (for a full description of projection methods, see Section 1.3). A climate hazard is a weather or climate state such as a heat wave, flood, high wind, heavy rain, ice, snow, and drought that can cause harm and damage to people, property, infrastructure, land, and ecosystems. Climate hazards can be expressed in quantified measures, such as flood height in feet, wind speed in miles per hour, and inches of rain, ice, or snowfall that are reached or exceeded in a given period of time. Uncertainty denotes a state of incomplete knowledge that results from lack of information, natural variability in the measured phenomenon, instrumental and modeling errors, and/or from disagreement about what is known or knowable (IPCC, 2013). See Box 1.3 for information on sources of uncertainty in climate projections. The New York City Panel on Climate Change 2 (NPCC2) projections can be used to inform planning across multiple governmental scales (e.g., city, county, state) in the New York metropolitan region. Such coordinated efforts can serve as test cases for successful local, state, and federal coordination for integrated climate adaptation initiatives. This chapter describes the global climate system, and presents observed temperature and precipitation trends and projections for the region. Chapter 2 (NPCC, 2015) focuses on sea level rise and possible changes in coastal storms. Chapter 3 and Chapter 4 (NPCC, 2015) describe efforts to better understand the region's vulnerability to coastal flooding during coastal storms. The treatment of likelihood related to the NPCC projections is similar to that developed by the Intergovernmental Panel on Climate Change Fourth and Fifth Assessment Reports (IPCC, 2007; 2013), with six likelihood categories (Box 1.1 and Fig. 1.1). The assignment of climate hazards to these categories is based on observed data, global climate model simulations, published literature, and expert judgment. The global climate system is comprised of the atmosphere, biosphere, hydrosphere, cryosphere, and lithosphere. The components of the climate system interact over a wide range of spatial and temporal scales. The Earth's climate is largely driven by the energy it receives from the sun. This incoming solar radiation (shortwave radiation) is partly absorbed, partly scattered, and partly reflected by gases in the atmosphere, by aerosols, by the Earth's surface, and by clouds. The Earth reemits the energy it receives from the sun in the form of longwave, or infrared, radiation. Under equilibrium conditions, there is an energy balance between the outgoing terrestrial longwave radiation and the incoming solar radiation. Without the presence of naturally occurring GHGs in the atmosphere, this balance would be achieved at temperatures of approximately −33°F (−18°C). An atmosphere containing GHGs is relatively opaque to terrestrial radiation. Such a planet achieves radiative balance at a higher surface temperature than it would without GHGs. On Earth, the increase in GHG concentrations due to human activities such as fossil fuel combustion, cement making, deforestation, and land use changes has led to a surface warming of almost 1.8°F (1°C) and a range of climate changes including upper ocean warming, and loss of land and sea ice. Key components of Earth's radiative balance are illustrated in Figure 1.3. In the 2013 Fifth Assessment Report (IPCC AR5), the IPCC documented a range of observed climate trends. Global surface temperature has increased about 1.5°F (0.85°C) since 1880. Both hemispheres have experienced decreases in net snow and ice cover, and global sea level has risen by approximately 0.5 to 0.7 inches (1.3 to 1.7 cm) per decade over the past century (Hay et al., 2015). More recently, since the 1990s, the global sea level rise rate has accelerated to approximately 1.3 inches (3.2 cm) per decade (see Chapter 2, NPCC, 2015, for New York metropolitan region sea level rise observations and projections). Droughts (in regions such as but not limited to the Mediterranean and West Africa) have grown more frequent and longer in duration. In the United States, Canada, and Mexico (as well as other regions), intense precipitation events have become more common. Hot days and heat waves have become more frequent and intense, and cold events have decreased in frequency. The upper oceans have warmed and become more acidic (IPCC, 2013). As temperatures have warmed in the atmosphere and ocean, biological systems have responded as well; for example, spring has been arriving earlier, and fall has been extending later into the year, in many mid- and high-latitude regions (IPCC, 2014). The IPCC AR5 states that there is a greater than 95% chance that warming temperatures since the mid-20th century are primarily due to human activities. Atmospheric concentrations of the major GHG carbon dioxide (CO2) are now approximately 40% higher than in preindustrial times. Concentrations of other important GHGs, including methane (CH4) and nitrous oxide (N2O), have increased by close to 150% and close to 20%, respectively, since preindustrial times. The warming that occurred globally over the 20th century cannot be reproduced by GCMs human to GHG concentrations are into in GHG concentrations are extremely likely to to accelerated temperature on these future and by the to time period, global average temperatures are projected to increase by to to or as high as to to (IPCC, 2013). The range is due to uncertainties in future GHG concentrations and the of the climate system to GHG is projected to be in the high of the the land are to more than ocean precipitation is projected to increase in many regions at such as the Mediterranean are projected to become it is that the temperatures will increase in and and the temperatures will in and there be (IPCC, Both land ice and sea ice are projected to is projected to increase as concentrations rise. This describes the climate hazards related to temperature and precipitation in the New York metropolitan region. sea level and coastal see 2 and 4 (NPCC, 2015). Both (e.g., annual and extreme (e.g., heavy are for New York City are in a trends over spatial scales national and are an important of with to New York future climate. in New York City are with temperature in New York City data from the weather approximately from to annual temperature has increased at a rate of per decade over the to 2013 period in the has over example, the and were by warming per decade and per the experienced per This of warming in the of the 20th century is and globally as well and has been to a of high and natural The temperature since for the New York metropolitan region is similar to the for the United of the has experienced a higher in This is in and weather it cannot be by the heat New York City significant precipitation the year, with relatively from to in the average precipitation between approximately and on the the has increased at a rate of approximately inches per decade from to 2013 in variability of precipitation has become more since the The a of increased from inches from to to inches from to in many of the region has increased since the this long-term in the cannot be from natural Both temperature and precipitation have significant on New York a climate or of approach the of their this is to as an extreme (see Fig. for an of how an extreme is precipitation are heavy precipitation events range from than an to a can range from to in the New York City heat waves in and of cold weather in in extreme events at local scales such as the New York metropolitan region are not significant due to high natural variability and limited et al., changes in extreme events as maximum and temperatures and extreme at spatial scales can be to human on global climate (IPCC, The IPCC Report on the of and to Climate Change concluded that it is very likely that there have been an in the number of cold days and cold and an increase in the number of days and globally for land with data, including and The that there have been significant trends in the number of heavy precipitation events in regions the (e.g., and has on the significant effects that extreme climate events have on New York City (see Chapter 2, Box events in the United States, such as the drought of or the of (see Box of the of weather and climate it is not possible to extreme such as to climate sea level rise occurring in the New York metropolitan in due to climate increased the and of coastal flooding during the (see Chapter 2, NPCC, 2015). This is an of how long-term trends in climate can the risk of to New York City days per with maximum temperatures at or per at or and two heat waves per The number of extreme events in a given is example, New York City with at with maximum temperatures at or to the at or in and there has been other time on New York City experienced more than two in a with maximum temperatures at or to days per with temperatures at or As is the for the number of cold days in a given from to the In the of there were days at or in there were The is the number of days at or since precipitation events are as the number of per of precipitation at or 2, and 4 inches per for New York City the weather in since and New York City days per with or more of rain, 3 days per with 2 inches or more of rain, and days per with 4 inches or more of As with extreme variations in extreme precipitation events are has been a but not significant more extreme precipitation events in New York City since example, the with the number of events with 2 inches or more of have occurred since and Because extreme precipitation events to relatively of over are to there is a relatively of to a significant from the intense precipitation events as the of have increased by approximately over the period from to et al., 2014). This presents New York climate projections for the century with the used to the projections. global climate projections are for and of temperature and This describes the potential for changes in other (e.g., heat and heavy projections are or See and (NPCC, 2015) for of the projections and of climate change and has increased in there uncertainties that are at scales (Box (IPCC, 2007; The NPCC2 to climate uncertainties in to for the use of such as and The is to New York City and the surrounding metropolitan more to changes in climate and to future extreme events (e.g., et al., Kunreuther et al., 2013). The NPCC2 a range of climate outcomes for temperature and precipitation from GCM simulations based on two et al., 2010). The RCPs a range of possible future global concentrations of GHGs, other important such as aerosols, and land use changes over the results from 35 GCMs are used to temperature and precipitation projections for the New York metropolitan region. climate models not the model results are or there is not a of observations to projections. these a projection of the likely of change is on the basis of expert judgment. Both the and used in the IPCC AR5 (IPCC, 2013). GCMs are mathematical of the behavior of the Earth's climate system over time that can be used to estimate the sensitivity of the climate system to changes in atmospheric concentrations of GHGs and aerosols. Each model simulates physical exchanges among the ocean, atmosphere, land, and ice. the past decades, climate models have increased in and as physical of the climate system has The GCM simulations used by the NPCC2 are from the Model Intercomparison et al., and were developed for the IPCC to the climate model simulations from used in the NPCC (NPCC, the models have higher spatial and more model and 2013). The global climate models Earth system models that among aerosols, ice and et al., example, warming temperatures in an Earth system model to changes in and the carbon which can on or the have been a number of in and models better of and that can at spatial scales. These and other have led to better of many climate such as sea ice et al., The of as a that cold can be to from time to time as the climate at and local scales. extended the United States, the reached their ice in the over the United States, cold in the were largely by in the United States, a states experienced their on 2013 for the on 2013). The planet has not experienced a with temperatures since The that global temperatures continue to as GHG concentrations continue to rise not the that regions or that weather become more extreme in An of and modeling (e.g., et al., is in sea ice be a by and more weather This is an have been by and and et for the potential are given the of sea ice et al., 2013) and the high vulnerability to climate projections are based on GCM from the model the New York metropolitan region. The of the from GCM to GCM GCMs in spatial the over which are These spatial range from as as miles by miles by to as as miles by miles by with an average of approximately miles by miles by The changes by the NPCC2 in temperature and precipitation time (e.g., 3 of warming by a given future time are to the New York metropolitan region. The spatial of of the NPCC2 projections is for changes in temperature and precipitation than for the number of days extreme The changes in temperature and precipitation across at a land example, the temperature and precipitation change projections for miles from and New miles from from for New York City These are well the of the climate uncertainty in long-term projections. the projections for changes in extreme events as heat and extreme are to be across an approximately the projections of changes in the of extreme (e.g., days over can be the of a example, there is spatial in the number of days over across the region as a of such as the heat and the from the The change in the number of days over is as well et al., 2013). the NPCC2 projections for total sea level change are for the New York metropolitan region (see Chapter 2, NPCC, projected changes in flood will the and the as in the NPCC2 coastal flood NPCC, 2015). This is primarily coastal the for example, the relatively of and are in to the and the the of uncertainty in climate projections concentrations of GHGs, aerosols, and land use GHG concentrations will on and and (e.g., methane from in a warming and/or RCPs are used to possible of the climate system to changes in GHGs and other Climate models are used to how much warming and other changes for a given change in important The temperature effects of are well but models in their as changes in and ice with that how much warming will A set of climate models is used to the range of such and local changes that from global and Climate model results can be or (e.g., models global but processes not be by changes in and the heat on a warming variability that is largely in such as the New York metropolitan region. As a as GHG concentrations weather and climate, will for extreme events and over time (e.g., a cold has that natural variability can be driven by variations that sources of natural variability the and solar weather over of time (e.g., can average much of the natural but it not it uncertainties as of uncertainty of weather errors, and in the of data it is not possible to future temperature or precipitation for a or year, GCMs are for the likely range of changes over time The NPCC2 projections use time of expressed to the period to for temperature and The NPCC uses time and a given example, the time refers to the period from to The NPCC2 has climate projections for for a different approach from the time The is that the of climate model simulations in it is not possible to a projection for the time on the for are an average of two that a to the time and that to (see over the of the NPCC projections the of the century (Box 1.3). example, the RCPs not the possible carbon and other associated with climate The Earth system models in used by the NPCC2 the potential for increased methane and carbon from the extreme warming More the potential for such as that carbon from the atmosphere, the into the future The of 35 GCMs and two RCPs a of for temperature and time period, the results a climate range of which can be used in were to GCM and to of the two selected The results for future time are to the climate model results for the period to temperature change projections are the a of the between future and is than model The is a for local projections et al., et al., precipitation change is based on the of a given future precipitation to that of precipitation as a The of extreme temperature and precipitation the of on than Because from climate models is more than and the NPCC2 uses a projection for extreme events. changes in temperature and precipitation are based on the for the annual changes time in of the are in the of of temperature change and in the of change in to the observed to temperature and precipitation data from to of This approach to projections of extreme events not for possible changes in variability over which are not well This presents climate projections for the and for precipitation, and extreme events. temperatures are extremely likely for the New York metropolitan region in the coming simulations project the of this GCM simulations in precipitation, but precipitation variability is precipitation projections are than temperature projections. The projected future temperature changes in 1.1 and Figure that by the New York temperatures a to of a The range of projections temperatures by to by the to by the and to by the temperatures increase by to are projected to be for of the The two RCPs project similar temperature changes to the the temperature changes by RCP 8.5 are higher than by RCP decades for the different RCPs to in climate due to the of GHGs in the atmosphere and the or of the climate system and the oceans 1.1 that precipitation is projected in the range to increase by approximately by the by the and by the projected changes in precipitation range from to In the projected changes in precipitation associated with GHGs in the global climate models are to Figure that precipitation is by with is the New York metropolitan region's drought of in the are to be during the of precipitation changes in are with approximately the models precipitation and decreases (see for projections). their extreme events can have on New York infrastructure, natural and This describes how the of heat cold and intense precipitation in the New York metropolitan region are projected to change in the coming The extreme projections in 1.2 are based on observed data for The total number of as days with a maximum temperature at or or is to increase as the century the the of days at or increase by more than to the to by the the more than by the the more than days are to relatively the increase in their of is projected to the change in days at or The and of heat as or more days with maximum temperatures at or are very likely to increase. In the of extreme cold as the number of days per with temperatures at or is projected to approximately by the more than by the and approximately by the the increase in annual precipitation is to be relatively are in the and of extreme precipitation in this as at 2, or 4 at Because of New York including of coastal and extreme precipitation days than extreme precipitation days than in the for in the future as of the extreme climate future changes are at local scales to projections. example, the between extreme precipitation events and different of and between and are the NPCC projections based on and expert 1.3). the of the heat are very likely to due to higher temperatures and warmer can more The of high temperatures and high can effects by the human to and heat (see Chapter NPCC, 2015). as intense precipitation at and are very likely to increase in and in are to the of the it is more likely than not that will increase in the New York metropolitan region et al., is how drought risk in the New York metropolitan region change in the As the century snowfall is likely to become with the snow in (IPCC, changes in the of snowfall per are is how the and of ice and for the New York metropolitan region from the of global climate models climate changes and the potential for In the coming decades, the NPCC that climate change is extremely likely to bring warmer temperatures to New York City and the surrounding region. Heat waves are very likely to increase. annual precipitation is likely to and brief, intense rainstorms are very likely to increase. is more likely than not that will become more there significant uncertainties long-term climate these projections would the climate what has been experienced This chapter information that can be used to but a is that the of extreme warming a New The to extreme warming are to the in GHG in New York City of New 2014). GHG are a global New York on in the United and is the NPCC has a of how the as a be by climate more is on and and (see Chapter NPCC, 2015) of precipitation, and other will be in the of climate modeling will how projected changes the due to including coastal and different land The NPCC approach a range of possible outcomes and to projections as information and climate model results become Such are as the of climate change

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