Query: The warmer the planet gets, the more ozone forms, and by mid-century, Americans will likely suffer a 70 percent increase in unhealthy ozone smog, the National Center for Atmospheric Research has projected.

Relevant passages:
Philadelphia <=SEP=> Philadelphia ( -LSB- ˌfɪləˈdɛlfi.ə -RSB- ) is the largest city in the Commonwealth of Pennsylvania and the sixth-most populous city in the United States , with an estimated population of 1,567,872 and more than 6 million in the seventh-largest metropolitan statistical area , . Philadelphia is the economic and cultural anchor of the Delaware Valleya region located in the Northeastern United States at the confluence of the Delaware and Schuylkill Rivers with 7.2 million people residing in the eighth-largest combined statistical area in the United States .   In 1682 , William Penn , an English Quaker , founded the city to serve as capital of the Pennsylvania Colony . Philadelphia played an instrumental role in the American Revolution as a meeting place for the Founding Fathers of the United States , who signed the Declaration of Independence in 1776 and the Constitution in 1787 . Several other key Philadelphia events during the Revolution include the First and Second Continental Congress , the preservation of the Liberty Bell , the Battle of Germantown , the Siege of Fort Mifflin , and the Philadelphia Convention . Philadelphia was one of the nation 's capitals in the Revolutionary War , and served as temporary U.S. capital while Washington , D.C. , was under construction . In the 19th century , Philadelphia became a major industrial center and railroad hub that grew from an influx of European immigrants . The majority of European immigrants have come from Ireland , Italy and Germany -- the three largest reported ancestry groups in the city . The city became a prime destination for African Americans during the Great Migration of the 20th century , as well as Puerto Ricans , surpassing two million occupants by 1950 .   The area 's many universities and colleges make Philadelphia a top international study destination , as the city has evolved into an educational and economic hub . With a gross domestic product of $ 388 billion , Philadelphia ranks ninth among world cities and fourth in the nation . Philadelphia is the center of economic activity in Pennsylvania and is home to seven Fortune 1000 companies . The Philadelphia skyline is growing , with a market of almost 81,900 commercial properties in 2016 including several nationally prominent skyscrapers . The city is known for its arts , culture , and rich history , attracting over 41 million domestic tourists alone in 2015 . Philadelphia has more outdoor sculptures and murals than any other American city . Fairmount Park , when combined with the adjacent Wissahickon Valley Park in the same watershed , is one of the largest contiguous urban park areas in the United States . The 67 National Historic Landmarks in the city helped account for the $ 10 billion generated by tourism . Philadelphia is the birthplace of the United States Marine Corps , and is also the home of many U.S. firsts , including the first library ( 1731 ) , first hospital ( 1751 ) and medical school ( 1765 ) , first Capital ( 1777 ) , first stock exchange ( 1790 ) , first zoo ( 1874 ) , and first business school ( 1881 ) . Philadelphia is the only World Heritage City in the United States .

Ozone depletion <=SEP=> Ozone depletion describes two distinct but related phenomena observed since the late 1970s : a steady decline of about four percent in the total amount of ozone in Earth 's stratosphere ( the ozone layer ) , and a much larger springtime decrease in stratospheric ozone around Earth 's polar regions . The latter phenomenon is referred to as the ozone hole . In addition to these well-known stratospheric phenomena , there are also springtime polar tropospheric ozone depletion events .   The details of polar ozone hole formation differ from that of mid-latitude thinning but the most important process in both is catalytic destruction of ozone by atomic halogens . The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants , solvents , propellants , and foam-blowing agents ( chlorofluorocarbon ( CFCs ) , HCFCs , freons , halons ) . These compounds are transported into the stratosphere by winds after being emitted at the surface . Both types of ozone depletion were observed to increase as emissions of halocarbons increased .   CFCs and other contributory substances are referred to as ozone-depleting substances ( ODS ) . Since the ozone layer prevents most harmful UVB wavelengths ( 280 -- 315 nm ) of ultraviolet light ( UV light ) from passing through the Earth 's atmosphere , observed and projected decreases in ozone generated worldwide concern , leading to adoption of the Montreal Protocol that bans the production of CFCs , halons , and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane . It is suspected that a variety of biological consequences such as increases in sunburn , skin cancer , cataracts , damage to plants , and reduction of plankton populations in the ocean 's photic zone may result from the increased UV exposure due to ozone depletion .

Global warming <=SEP=> Global warming , also referred to as climate change , is the observed century-scale rise in the average temperature of the Earth 's climate system and its related effects . Multiple lines of scientific evidence show that the climate system is warming . Many of the observed changes since the 1950s are unprecedented in the instrumental temperature record which extends back to the mid 19th century , and in paleoclimate proxy records over thousands of years .   In 2013 , the Intergovernmental Panel on Climate Change ( IPCC ) Fifth Assessment Report concluded that `` It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century . '' The largest human influence has been emission of greenhouse gases such as carbon dioxide , methane and nitrous oxide . Climate model projections summarized in the report indicated that during the 21st century the global surface temperature is likely to rise a further 0.3 to for their lowest emissions scenario and 2.6 to for the highest emissions scenario . These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing .   Future climate change and associated impacts will differ from region to region around the globe . Anticipated effects include warming global temperature , rising sea levels , changing precipitation , and expansion of deserts in the subtropics . Warming is expected to be greater over land than over the oceans and greatest in the Arctic , with the continuing retreat of glaciers , permafrost and sea ice . Other likely changes include more frequent extreme weather events including heat waves , droughts , heavy rainfall with floods and heavy snowfall ; ocean acidification ; and species extinctions due to shifting temperature regimes . Effects significant to humans include the threat to food security from decreasing crop yields and the abandonment of populated areas due to rising sea levels . Because the climate system has a large `` inertia '' and greenhouse gases will stay in the atmosphere for a long time , many of these effects will not only exist for decades or centuries , but will persist for tens of thousands of years .   Possible societal responses to global warming include mitigation by emissions reduction , adaptation to its effects , building systems resilient to its effects , and possible future climate engineering . Most countries are parties to the United Nations Framework Convention on Climate Change ( UNFCCC ) ,  whose ultimate objective is to prevent dangerous anthropogenic climate change . Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 C-change relative to pre-industrial levels , with efforts made to limit warming to 1.5 C-change .   Public reactions to global warming and concern about its effects are also increasing . A global 2015 Pew Research Center report showed a median of 54 % consider it `` a very serious problem '' . There are significant regional differences , with Americans and Chinese ( whose economies are responsible for the greatest annual CO2 emissions ) among the least concerned .