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 unless human contributions to historical GHG concentrations are taken into account (Fig. 1.4). Further increases in GHG concentrations are extremely likely to lead to accelerated temperature increases. Depending on these future emissions and concentrations, by the 2081 to 2100 time period, global average temperatures are projected to increase by 2.0°F to 4.7°F (1.1°C to 2.6°C) or as high as 4.7°F to 8.6°F (2.6°C to 4.8°C)1 (IPCC, 2013). The large range is due to uncertainties both in future GHG concentrations and the sensitivity2 of the climate system to GHG concentrations. Warming is projected to be greatest in the high latitudes of the northern hemisphere. Throughout the globe, land areas are generally expected to warm more than ocean regions. High-latitude precipitation is projected to increase in both hemispheres, while many dry regions at subtropical latitudes, such as the Mediterranean region, are projected to become drier. Globally, it is virtually certain that the hottest temperatures will increase in frequency and magnitude, and the coldest temperatures will decrease in frequency and magnitude, although there could be regional exceptions (IPCC, 2012). Both land ice and sea ice volumes are projected to decrease. Ocean acidification is projected to increase as CO2 concentrations rise. This section describes the critical climate hazards related to temperature and precipitation in the New York metropolitan region. For sea level and coastal storms, see Chapters 2 and 4 (NPCC, 2015). Both mean (e.g., annual averages) and extreme (e.g., heavy downpours) quantities are presented. Observations for New York City are placed in a broader context because trends over large spatial scales (regional, national and global) are an important source of predictability with respect to New York City's future climate. Summers in New York City are warm, with cool winters. Annual mean air temperature in New York City (using data from the Central Park weather station) was approximately 54°F from 1971 to 2000. Mean annual temperature has increased at a rate of 0.3°F per decade over the 1900 to 2013 period in Central Park, although the trend has varied substantially over shorter periods (Fig. 1.5). For example, the first and last 30-year periods were characterized by warming (0.38°F per decade and 0.79°F per decade, respectively), whereas the middle segment experienced negligible cooling (−0.04°F per decade). This absence of warming in the middle of the 20th century is evident nationally and globally as well and has been linked to a combination of high sulphate aerosol emissions (a cooling factor) and natural variability. The temperature trend since 1900 for the New York metropolitan region is broadly similar to the trend for the northeast United States (Fig. 1.6).3 Specifically, most of the Northeast has experienced a trend toward higher temperatures, especially in recent decades. This trend is present in both rural and urban weather stations, so it cannot be explained by the urban heat island effect.4 New York City experiences significant precipitation throughout the year, with relatively little variation from month to month in the typical year. Annual average precipitation ranges between approximately 43 and 50 inches, depending on the location within the city. Precipitation has increased at a rate of approximately 0.8 inches per decade from 1900 to 2013 in Central Park (Fig. 1.7). Year-to-year (and multiyear) variability of precipitation has also become more pronounced, especially since the 1970s. The standard deviation, a measure of variability, increased from 6.1 inches from 1900 to 1956 to 10.3 inches from 1957 to 2013. Precipitation in many parts of the larger Northeast region has also increased since the 1900s (Fig. 1.8). However, this long-term trend in the Northeast generally cannot be distinguished from natural variability. Both temperature and precipitation extremes have significant impacts on New York City. When a single climate variable or combinations of variables approach the tails of their distribution, this is referred to as an extreme (see Fig. for an of how an extreme is precipitation are heavy precipitation events generally range from than an to a whereas can range from to location in the New York City experiences heat waves in and periods 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., However, changes in extreme events as maximum and temperatures and extreme at large spatial scales can be to human on global climate (IPCC, 2012). The IPCC Report on the of and to Climate Change concluded that it is very likely that there have been an decrease in the number of cold days and cold and an increase in the number of warm days and warm globally for most land areas with data, including and The also 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 recent events in the United States, such as the drought of or the of (see Box also of the impacts of weather and climate it is not possible to extreme such as to climate sea level rise occurring in the New York metropolitan region, in due to climate increased the and of coastal flooding during the (see also Chapter 2, NPCC, 2015). This is an of how long-term trends in climate variables can the risk of 1971 to New York City days per with maximum temperatures at or per at or and two heat waves per year. The number of extreme events in a given is For example, New York City with at with maximum temperatures at or to the last at or was in and there has been other time on New York City experienced more than two in a with maximum temperatures at or 1971 to Central Park days per with temperatures at or As is the for the number of cold days in a given also from to the In the cool of there were days at or whereas in there were The is the greatest number of cool 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 Central since 1971 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 temperatures, variations in extreme precipitation events are has been a but not significant trend toward more extreme precipitation events in New York City since For example, the with the greatest number of events with 2 inches or more of have occurred since and Because extreme precipitation events to relatively of over large areas are to there is a relatively large of to a significant trend from variability. the larger Northeast region, intense precipitation events as the of have increased by approximately over the period from to et al., 2014). This section presents New York climate projections for the century with the used to the projections. global climate projections are for and extremes of temperature and This section also describes the potential for changes in other variables (e.g., heat and heavy downpours) because projections are or See and (NPCC, 2015) for of the projections and of climate change and impacts has increased in recent there uncertainties that are at scales (Box (IPCC, 2007; 2012). The NPCC2 to present climate uncertainties in to for the use of such as and The is to New York City and the surrounding metropolitan more to mean 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. For climate models not the model results are or there is not a of observations to projections. For 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 both 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 first NPCC (NPCC, the models generally have higher spatial and more model and 2013). The global climate models Earth system models that among aerosols, ice and et al., For example, warming temperatures in an Earth system model lead to changes in and the carbon which can on (a or (a the have also 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., 2012). The of as a that cold can be expected to from time to time as the climate especially at regional and local scales. extended throughout the United States, the reached their ice in the However, over the United States, cold in the were largely by warm in the United States, a states experienced their on Globally, 2013 for the on 2013). The planet has not experienced a month with temperatures since The that global temperatures continue to as GHG concentrations continue to rise not the that regions could cool or that weather could become more extreme in An of and modeling (e.g., et al., is in sea ice could be a characterized by and more weather This is an have been by and and et for However, the potential are given the expected of sea ice et al., 2013) and the high vulnerability to climate projections are based on GCM from the single model the New York metropolitan region. The of the from GCM to GCM because 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 larger for mean changes in temperature and precipitation than for the number of days extreme The mean changes in temperature and precipitation generally across at a land For example, the mean temperature and precipitation change projections for miles from and New miles from from for New York City These are well within the of the climate uncertainty in long-term projections. the projections for changes in extreme events as heat and extreme are expected to be generally across an approximately However, the projections of changes in the frequency of extreme (e.g., days over can be variable within the of a For example, there is large spatial variation in the number of days over across the region as a of such as the urban heat island and the from the The change in the number of days over is variable 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 substantially within the and within the as in the NPCC2 coastal flood NPCC, 2015). This is primarily because coastal throughout the for example, the relatively of and are in to the northern 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 emissions 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 CO2 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., regional models within global but processes not be by changes in and the urban heat island on a warming variability that is largely especially in areas such as the New York metropolitan region. As a as GHG concentrations weather and climate, will especially for extreme events and over time periods (e.g., a cold has that natural variability can be driven by variations that sources of natural variability the and solar weather over periods of time (e.g., can average much of the natural variability, but it not it Observations uncertainties as of uncertainty of weather stations, 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 30-year expressed to the period 1971 to for temperature and The NPCC uses time and a given For example, the time refers to the period from to The NPCC2 has also climate projections for for 2100 a different approach from the 30-year time The is that because the of climate model simulations in it is not possible to a projection for the 30-year time on the for 2100 are an average of two that a trend to the time and that trend to 2100 (see over the of the NPCC projections toward the of the century (Box 1.3). For example, the RCPs not the possible carbon and other associated with climate The Earth system models in used by the NPCC2 could the potential for increased methane and carbon from the extreme warming More the potential for such as that could carbon from the atmosphere, increases the into the future The combination of 35 GCMs and two RCPs a of for temperature and For 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 periods are to the climate model results for the period to Mean temperature change projections are the a of the between future and is than model The is a for local projections et al., et al., Mean precipitation change is based on the of a given future precipitation to that of precipitation as a The greatest impacts 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 combinations are in the of of temperature change and in the of change in to the observed 1971 to temperature and precipitation data from Central Park to of This approach to projections of extreme events not account for possible changes in variability over which are not well This section presents climate projections for the and 2100 for precipitation, and extreme events. temperatures are extremely likely for the New York metropolitan region in the coming decades. simulations project increases the of this GCM simulations increases in precipitation, but precipitation variability is precipitation projections are certain than temperature projections. The projected future temperature changes in 1.1 and Figure that by the New York City's mean temperatures throughout a to of a The middle range of projections temperatures by 2.0°F to by the to by the and to by the temperatures increase by to increases are projected to be for of the year. 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 large in climate due to the of GHGs in the atmosphere and the or of the climate system and the oceans 1.1 that regional precipitation is projected in the middle 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 variability. Figure that precipitation is characterized by large historical variability, with is the New York metropolitan region's drought of in the Precipitation increases are expected to be during the of precipitation changes in are with approximately the models precipitation increases and decreases (see for projections). their extreme events can have large impacts on New York City's infrastructure, natural and This section describes how the of heat cold and intense precipitation in the New York metropolitan region are projected to change in the coming decades. The extreme projections in 1.2 are based on observed data for Central The total number of as days with a maximum temperature at or or is expected to increase as the century the the frequency of days at or increase by more than to the 1971 to by the the frequency more than by the the frequency more than days are expected to relatively the increase in their frequency of is projected to the change in days at or The frequency and of heat as or more days with maximum temperatures at or are very likely to increase. In the frequency of extreme cold as the number of days per with temperatures at or is projected to decrease approximately by the more than by the and approximately by the the increase in annual precipitation is expected to be relatively larger increases are expected in the and of extreme precipitation in this as at 2, or 4 at Because parts of New York including parts of coastal and extreme precipitation days than Central Park, extreme precipitation days than in the for Central Park in the future as For of the extreme climate future changes are at local scales to projections. For example, the between extreme precipitation events and different of storms, and between and are For the NPCC projections based on and expert 1.3). the of the heat are very likely to both due to higher temperatures and because warmer air can more The combination of high temperatures and high can effects by the human to cool and heat (see Chapter NPCC, 2015). as intense precipitation at and are very likely to increase in frequency 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 frequency and of ice and for the New York metropolitan region from the of global climate models large climate changes and the potential for large 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 critical 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 emissions in New York City of New 2014). GHG emissions are a global New York City's on emissions in the United States 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, air and other variables will be critical in the of regional climate modeling will also how projected changes throughout the due to including coastal and different urban 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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