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by Holli Riebeek • design by Robert Simmon • May 11, 2007

The original version of this fact sheet, published in 2002 andwritten by John Weier, is archived as a PDF.

Over the last five years, 600 scientists from theIntergovernmental Panel on Climate Change siftedthrough thousands of studies about global warmingpublished in forums ranging from scientific journalsto industry publications and distilled the world’saccumulated knowledge into this conclusion:“Warming of the climate system is unequivocal.”

Far from being some future fear, global warming ishappening now, and scientists have evidence thathumans are to blame. For decades, cars and factorieshave spewed billions of tons of greenhouse gases intothe atmosphere, and these gases caused temperaturesto rise between 0.6°C and 0.9°C (1.08°F to 1.62°F)over the past century. The rate of warming in the last50 years was double the rate observed over the last100 years. Temperatures are certain to go up further.

The effects of global warmingare already being feltworldwide. The Larsen-B IceShelf on the Antarctic Peninsulacollapsed over 35 days in early2002, prompted by 3°C ofwarming since the 1940s. (NASAimage by Jesse Allen, based onMODIS data.)

This article may contain outdated information. The most recent version of the Earth Observatory's Global Warming Factsheet is available at the following URL: http://earthobservatory.nasa.gov/Features/GlobalWarming/

But why should we worry about a seemingly smallincrease in temperature? It turns out that the globalaverage temperature is quite stable over long periodsof time, and small changes in that temperaturecorrespond to enormous changes in the environment.For example, during the last ice age, when ice sheetsa mile thick covered North America all the way downto the northern states, the world was only 9 to 15degrees Fahrenheit colder than today. Much ofmodern human civilization owes its existence to thestability in the average global temperature since theend of the last ice age—a stability that allowedhuman cultures to transition from roaming, hunter-gatherer societies into more permanent, agriculture-supported communities. Even the temperature changeof a degree or two that has occurred over the lastcentury is capable of producing significant changes inour environment and way of life.

Cars, factories, and powerplants pump billions of tons ofcarbon dioxide into theatmosphere every year. Since1750, carbon dioxide levelshave increased 35 percent,while temperatures have goneup between 0.6°C and 0.9°C.Scientists have very highconfidence that increasedconcentrations of greenhousegases are causing the planet towarm. (Photograph ©2002Travel Geographer. NASAgraphs by Robert Simmon,based on carbon dioxide datafrom Dr. Pieter Tans, NOAA/ESRLand temperature data fromNASA Goddard Institute forSpace Studies.)

In the future, it is very likely that rising temperatureswill lead to more frequent heat waves, and virtuallycertain that the seas will rise, which could leave low-lying nations awash in seawater. Warmertemperatures will alter weather patterns, making itlikely that there will be more intense droughts andmore intense rain events. Moreover, global warmingwill last thousands of years. To gain an understandingof how global warming might impact humanity, it is

One inevitable consequence ofglobal warming is sea-level rise.In the face of higher sea levelsand more intense storms,coastal communities facegreater risk of rapid beacherosion and damage fromdestructive storms like theintense nor’easter of April 2007.(Photograph ©2007metimbers2000.)

of how global warming might impact humanity, it isnecessary to understand what global warming is, howscientists measure it, and how forecasts for the futureare made.

What is Global Warming? Global warmth begins with sunlight. When light from

the Sun reaches the Earth, roughly 30 percent of it isreflected back into space by clouds, atmosphericparticles, reflective ground surfaces, and even oceansurf. The remaining 70 percent of the light isabsorbed by the land, air, and oceans, heating ourplanet’s surface and atmosphere and making life onEarth possible. Solar energy does not stay bound upin Earth’s environment forever. Instead, as the rocks,the air, and the sea warm, they emit thermal radiation,or infrared heat. Much of this thermal radiationtravels directly out to space, allowing Earth to cool.

Some of this outgoing radiation, however, is re-absorbed by water vapor, carbon dioxide, and othergases in the atmosphere (called greenhouse gasesbecause of their heat-trapping capacity) and is thenre-radiated back toward the Earth’s surface. On thewhole, this re-absorption process is good. If therewere no greenhouse gases or clouds in theatmosphere, the Earth’s average surface temperaturewould be a very chilly -18°C (0°F) instead of thecomfortable 15°C (59°F) that it is today.

What has scientists concerned now is that over thepast 250 years humans have been artificially raisingthe concentration of greenhouse gases in theatmosphere at an ever-increasing rate. By 2004,humans were pumping out over 8 billion tons ofcarbon dioxide per year. Some of it was absorbed by“sinks” like forests or the ocean, and the restaccumulated in the atmosphere. We produce millionsof pounds of methane by allowing our trash todecompose in landfills and by breeding large herds ofmethane-belching cattle. Nitrogen-based fertilizersand other soil management practices lead to therelease of nitrous oxide into the atmosphere.

Once these greenhouse gases get into the atmosphere,they stay there for decades or longer. According tothe Intergovernmental Panel on Climate Change(IPCC), since the industrial revolution began in about1750, carbon dioxide levels have increased 35 percent

Land, air, and oceans absorbmost of the energy that comesfrom the Sun (yellow arrows,left), and the rest is reflectedback into space. The land andoceans re-emit the energy theyabsorb (red arrows, right) in theform of heat, which is thenabsorbed by greenhouse gasesin the atmosphere. In turn,greenhouse gases re-emit theheat, sending it back to thesurface. This heat-trappingeffect keeps the Earthcomfortably warm, but thebuild-up of greenhouse gasesintensifies the effect, drivingtemperatures up. (Imageadapted from Kiel andTrenberth, 1997, by DebbiMcLean.)

1750, carbon dioxide levels have increased 35 percentand methane levels have increased 148 percent.Paleoclimate readings taken from ice cores and fossilrecords show that these gases, two of the mostabundant greenhouse gases, are at their highest levelsin at least the past 650,000 years. Scientists have veryhigh confidence (a phrase the IPCC translates to“greater than 90 percent certainty”) that the increasedconcentrations of greenhouse gases have made itmore difficult for thermal radiation to leave the Earth,and as a result, Earth has warmed.

Evidence for Global Warming Recent observations of warming support the theory

that greenhouse gases are warming the world. Overthe last century, the planet has experienced the largestincrease in surface temperature in 1,300 years. Theaverage surface temperature of the Earth rose 0.6 to0.9 degrees Celsius (1.08°F to 1.62°F) between 1906and 2006, and the rate of temperature increase nearlydoubled in the last 50 years. Worldwidemeasurements of sea level show a rise of about 0.17meters (0.56 feet) during the twentieth century. Theworld’s glaciers have steadily receded, and Arctic seaice extent has steadily shrunk by 2.7 percent perdecade since 1978.

Even if greenhouse gas concentrations stabilizedtoday, the planet would continue to warm by about0.6°C over the next century because it takes years forEarth to fully react to increases in greenhouse gases.As Earth has warmed, much of the excess energy has

Spilling from the ColumbiaIcefield in the Rocky Mountainsof western Canada, theAthabasca Glacier has beenshrinking by about 15 metersper year. In the past 125 years,the glacier has lost half itsvolume and has retreated morethan 1.5 kilometers. Athabascais just one of Earth’s manyglaciers that are dwindling asglobal temperatures climb.Since 1960, glaciers around theworld have lost an estimated8,000 cubic kilometers of ice.(Photograph ©2005 HughSaxby. Glacier graph adaptedfrom Dyurgerov and Meier,2005.)

As Earth has warmed, much of the excess energy hasgone into heating the upper layers of the ocean.Scientists suspect that currents have transported someof this excess heat from surface waters down deep,removing it from the surface of our planet. Once thelower layers of the ocean have warmed, the excessheat in the upper layers will no longer be drawndown, and Earth will warm about 0.6°C (1° F).

But how do scientists know global warming is causedby humans and that the observed warming isn’t anatural variation in Earth’s climate? Scientists usethree closely connected methods to understandchanges in Earth’s climate. They look at records ofEarth’s past climates to see how and why climatechanged in the past, they build computer models thatallow them to see how the climate works, and theyclosely monitor Earth’s current vital signs with anarray of instruments ranging from space-basedsatellites to deep sea thermometers. Records of pastclimate change reveal the natural events—such asvolcanic eruptions and solar activity—that influencedclimate throughout Earth’s history. Today, scientistsmonitor those same natural events as well as human-released greenhouse gases and use computer modelsto determine how each influences Earth’s climate.

Reconstructing Past Climate ChangeLike detectives at a crime scene, scientists reconstructpast climate changes by looking for evidence left inthings like glacial ice, ocean sediments, rocks, andtrees. For example, glacial ice traps tiny samples ofEarth’s atmosphere, giving scientists a record ofgreenhouse gases that stretches back more than650,000 years, and the chemical make-up of the iceprovides clues to the average global temperature.From these and other records, scientists have built arecord of Earth’s past climates, or “paleoclimates.”Paleoclimatology allowed scientists to show thatclimate changes in the past have been triggered byvariations in Earth’s orbit, solar variation, volcaniceruptions, and greenhouse gases.

The annual layers of snowpacked in glacial ice preservea record of climate stretchingback hundreds of thousands ofyears. To reconstruct pasttemperatures and atmosphericconditions, scientists use icecores, tree rings, oceansediments, cave rocks, andother natural records thatpreserve a signature of theclimate. By understanding howEarth’s climate has changed inthe past, scientists gain insightinto how and why it mightchange in the future.(Photograph ©2005 RetoStöckli.)

Building a Climate Model Next, to understand how sunlight, air, water, and land

come together to create Earth’s climate, scientistsbuild climate models—computer simulations of theclimate system. Climate models include the

climate system. Climate models include thefundamental laws of physics—conservation ofenergy, mass, and momentum—as well as dozens offactors that influence Earth’s climate. Though themodels are complicated, rigorous tests with real-world data hone them into robust tools that allowscientists to experiment with the climate in a way nototherwise possible. For example, when scientists atNASA’s Goddard Institute for Space Studies (GISS),NASA’s division spearheading climate modelingefforts, put measurements of volcanic particles fromMount Pinatubo’s 1991 eruption into their climatemodels well after the event, the models reported thatEarth would have cooled by around 0.5°C a year orso later. The prediction matched cooling that hadbeen observed around the globe after the eruption.

As the models reconstruct events that match theclimate record, researchers gain confidence that themodels are accurately duplicating the complexinteractions that drive Earth’s climate. Scientists thenexperiment with the models to gain insight into whatis driving climate change. By experimenting with themodels—removing greenhouse gases emitted by theburning of fossil fuels or changing the intensity of theSun to see how each influences the climate—scientists can use the models to explain Earth’scurrent climate and predict its future climate. So far,the only way scientists can get the models to matchthe rise in temperature seen over the past century is toinclude the greenhouse gases that humans have putinto the atmosphere. This means that, according to themodels, humans are responsible for most of thewarming observed during the second half of thetwentieth century.

Mount Pinatubo’s 1991 eruptionpumped volcanic gases highinto the atmosphere. The gasesinteracted with water vapor toform a reflective shade ofaerosol particles (top graph)that stretched far beyond thePhilippines, where the volcanois located. The global averagetemperature dipped half adegree Celsius until theparticles (sulfates) cleared afew years later. Scientists testand refine global climatemodels by comparing modelpredictions of temperaturechange after events like theeruption to actual observations(bottom graph). When modelsreliably match observations,scientists gain confidence thatthe models accuratelyrepresent Earth’s climatesystem. (NASA graphs by RobertSimmon, based on data fromNASA Goddard Institute forSpace Studies.)

Major Climate Influences Effect on SurfaceTemperature

Explanation Source Related EOArticles

Anthropogenic Greenhouse gases Warming Gases absorb energy emitted bythe Earth, re-radiating heattoward the surface.

Burning fossil fuelsin factories andvehicles, fertilizers,decomposing trash,livestock

CarbonCycle

Ozone Warming Ozone absorbs sunlight, so adepletion of ozone cools theatmosphere, while a build-up ofozone warms the surface.

Chlorofluorocarbonsdestroystratospheric ozone,while the interactionof sunlight withcertain pollutantscreates surfaceozone

Tango in theAtmosphere;

Ozone

Surface Albedo Cooling Albedo is the percent of sunlightthe surface reflects.Deforestation increases albedo.Soot that falls on snow and icereduces albedo. The effect offorest loss outweighs the effectof the soot, resulting in netcooling.

Land use and sooton snow

ArcticReflection;

Earth’sAlbedo inDecline

Aerosols (particle pollution) Cooling Particles in the atmosphereshade the Earth’s surface,offsetting global warming asmuch as 40 percent. Sootabsorbs heat and warms theatmosphere.

Volcanoes, burningfossil fuel, otherhuman sources,fires, dust

Aerosols &ClimateChange

Aerosols (Cloud albedo effect) Cooling Because human-producedaerosols are smaller and morenumerous than natural aerosols,they increase the amount andbrightness of clouds.

Clouds formedaround pollutionreleased by cars,factories, powerplants, etc.

Clouds areCooler than

Smoke;Changing

Our WeatherOne

Smokestackat a Time

Solar Variability Warming andCooling

An increase in sunlight bringsmore energy to the Earth andvice-versa.

Natural solar cycles ACRIMSAT;Solar Max;

SORCE;Under a

Variable Sun

But why do scientists trust results from climatemodels when models seem to have so much troubleforecasting the weather? It turns out that trends areeasier to predict than specific events. Weather is ashort-term, small-scale set of measurements ofenvironmental conditions, while climate is theaverage of those conditions over a large area for along time. The difference between predicting weatherand climate is similar to the difference betweenpredicting when a particular person will die versuscalculating the average life span of an entirepopulation. Given the large number of variables thatinfluence conditions in Earth’s lower atmosphere, andgiven that chaos also plays a larger role on shorterand smaller scales of time and space, weather ismuch harder to predict than the averages that make upclimate.

However, the longer the time scale, the harder itbecomes to predict climate. Scientists understandhow certain processes that drive Earth’s climate worknow, and so they can accurately predict how eventslike Pinatubo’s eruption will cool the globe’s averagetemperature. But they don’t understand how everyaspect of the climate system will change as the planetwarms. Feedback loops—in which change in one partof the climate system produces change in another part—make climate harder to forecast as scientists lookfarther into the future. For example, what will happento clouds as Earth warms? Will high-flying, heat-absorbing clouds that would cause additional heatingbecome more frequent than dense, sunlight-blockingclouds? Will changes be regional or global, and howwill they affect global climate? As of now, scientistscan’t answer these questions, and the uncertaintiesmean that global climate models provide a range ofpredictions instead of a highly detailed forecast.

Models integrate the manyfactors that influence Earth’sclimate to determine how theywork together to createtoday’s climate and how theymight influence climatechange in the future.

Observing Global WarmingClimate models and paleoclimate information tellscientists what kinds of symptoms to look for when

Climate is what you expect;weather is what you get. Thisgraph compares long-term

scientists what kinds of symptoms to look for whendiagnosing global warming. Ocean temperatures andacidity should rise as the oceans soak up more heatand carbon dioxide. Global temperatures arepredicted to increase, with the largest temperatureincreases over land and at the poles. Glaciers and seaice will melt and sea levels will rise. Like a patient ina hospital, Earth is closely monitored for thesesymptoms by a fleet of satellites and surfaceinstruments. NASA satellites record a host of vitalsigns including atmospheric aerosols (particles fromthings like factories, fires, or erupting volcanoes),atmospheric gases, energy from Earth’s surface andthe Sun, ocean surface temperatures, global sealevels, the extent of ice sheets, glaciers and sea ice,plant growth, rainfall, cloud structure, and more. Onthe ground, networks of weather stations maintaintemperature and rainfall records, and buoys measuredeep ocean temperatures.

graph compares long-termaverage high and lowtemperatures (dark lines) to theactual daily high and lowtemperatures in New York City’sCentral Park during 2006.Although the averagetemperatures vary gradually asthe seasons change,temperatures fluctuate wildlyfrom day-to-day. (NASA graphby Robert Simmon, based ondata from the NationalWeather Service ForecastOffice.)

Along with paleoclimate data, these sources revealthat the planet has been warming for at least the last400 years, and possibly the last 1000 years. As ofnow, warming after 1950 cannot be explained withoutaccounting for greenhouse gases; natural influencessuch as volcanic eruptions or changes in the Sun’soutput cannot account for the observed temperatureschanges.

Occasional violent volcanic eruptions, such as Mt.Pinatubo, pump gases like sulfur dioxide and aerosolshigh into the atmosphere where they can linger formore than a year, reflecting sunlight and shadingEarth’s surface. The cooling influence of this aerosol“shade” is greater than the warming influence of thevolcanoes’ greenhouse gas emissions, and thereforesuch eruptions cannot account for the recent warmingtrend.

An increase in solar output also falls short ofexplaining recent warming. NASA satellites havebeen measuring the Sun’s output since 1978, andwhile the Sun’s activity has varied a little, theobserved changes were not large enough to accountfor the warming recorded during the same period.Climate simulations of global temperature changesbased only on solar variability and volcanic aerosolssince 1750—omitting greenhouse gases— are able tofit the record of global temperatures only up untilabout 1950.

About half the carbon dioxideemitted into the air fromburning fossil fuels dissolves inthe ocean. While this processreduces the amount ofgreenhouse gases in theatmosphere, it raises the acidityof ocean water (just likecarbonated water, which isacidic). This map shows thetotal amount of human-madecarbon dioxide in ocean waterfrom the surface to the seafloor. Blue areas have lowamounts, while yellow regionsare rich in anthropogeniccarbon dioxide. High amountsoccur where currents carry thecarbon-dioxide-rich surfacewater into the ocean depths.(Map adapted from Sabine etal., 2004.)

about 1950.

The only viable explanation for warming after 1950 isan increase in greenhouse gases. It is well establishedtheoretically why carbon dioxide, methane, and othergreenhouse gases should heat the planet, andobservations show that they have.

Satellite measurements of theSun’s activity since 1978 revealthe Sun’s eleven-year sunspotcycle. When the Sun is moreactive, it sends slightly moreenergy to the Earth, andenergy levels dip when the Sunis quieter. Though the Sun’sactivity has varied over thepast three decades, thevariation is too small to explainthe rapid warming seen onEarth during the same period.(Graph adapted from thePMOD World RadiationCenter.)

Predicting Future Warming As the world consumes ever more fossil fuel energy,

greenhouse gas concentrations will continue to rise,and Earth’s average surface temperature will risewith them. Based on plausible emission scenarios, theIPCC estimates that average surface temperaturescould rise between 2°C and 6°C by the end of the21st century.

At first glance, these numbers probably do not seemthreatening. After all, temperatures typically change afew tens of degrees whenever a storm front movesthrough. Such temperature changes, however,represent day-to-day regional fluctuations. Whensurface temperatures are averaged over the entireglobe for extended periods of time, it turns out thatthe average is remarkably stable. Not since the end ofthe last ice age 20,000 years ago, when Earth warmed

Global warming will not affectall places on Earth the sameway. Climate models predictthat warming will be greatest inthe Arctic and over land.Models also give a range oftemperature predictions basedon different emission scenarios.If humans limit greenhouse gasemissions (low growth), then thetemperature change over thenext century will be smaller thanthe change predicted if

the last ice age 20,000 years ago, when Earth warmedabout 5°C, has the average surface temperaturechanged as dramatically as the 2°C to 6°C changethat scientists are predicting for the next century.

Scientists predict the range of temperature increaseby running different scenarios through climatemodels. Because scientists can’t say how humansociety may change over the next century, or howcertain aspects of the climate system (such as clouds)will respond to global warming, they give a range oftemperature estimates. The higher estimates are madeon the assumption that the entire world will continueto use more and more fossil fuel per capita. Thelower estimates come from best-case scenarios inwhich environmentally friendly technologies such asfuel cells and solar panels replace much of today’sfossil fuel combustion. After inputting estimates forfuture greenhouse gas emissions, scientists run themodels forward into many possible futures to arriveat the range of estimates provided in the IPCC report.The estimates are being used to predict how risingtemperatures will affect both people and naturalecosystems. The severity of environmental changewill depend on how much the Earth’s surface warmsover the next century.

the change predicted ifhumans do not limit emissions(high growth). (©2007 IPCCWG1 AR-4.)

The Intergovernmental Panel onClimate Change estimates thatEarth will warm between twoand six degrees Celsius over thenext century. The range inestimate comes from runningdifferent emission scenariosthrough several different globalclimate models. Scenarios thatassume that people will burnmore and more fossil fuelprovide the estimates in the topend of the temperature range,while scenarios that assumethat greenhouse gas emissionswill grow slowly give lowertemperature predictions. Theorange line provides anestimate of what globaltemperatures would have beenif greenhouse gases had stayedat year 2000 levels. (©2007IPCC WG1 AR-4.)

Potential Effects of Global Warming The most obvious impact of global warming will be

changes in both average and extreme temperature andprecipitation, but warming will also enhance coastalerosion, lengthen the growing season, melt ice capsand glaciers, and alter the range of some infectiousdiseases, among other things.

For most places, global warming will result in morehot days and fewer cool days, with the greatestwarming happening over land. Longer, more intenseheat waves will become more frequent. High latitudesand generally wet places will tend to receive morerainfall, while tropical regions and generally dryplaces will probably receive less rain. Increases inrainfall will come in the form of bigger, wetterstorms, rather than in the form of more rainy days. Inbetween those larger storms will be longer periods oflight or no rain, so the frequency of drought willincrease. Hurricanes will likely increase in intensitydue to warmer ocean surface temperatures.

It is impossible to pin any one unusual weather eventon global warming, but evidence is emerging thatsuggests that global warming is already influencingthe weather. The IPCC reports that both heat wavesand intense rain events have increased in frequencyduring the last 50 years, and human-induced globalwarming more likely than not contributed to thetrend. Satellite-based rainfall measurements showtropical areas got more rain in the form of largestorms or light rainfall instead of moderate stormsbetween 1979 and 2003. Since the 1970s, the areaaffected by drought and the number of intensetropical cyclones also have increased, trends thatIPCC scientists say were more likely than notinfluenced by human activities, though in the case ofcyclones, the record is too sparse to draw any certainconclusions.

Global warming will shift majorclimate patterns, possiblyprolonging and intensifying thecurrent drought in the U.S.Southwest. The white ring ofbleached rock on the once-redcliffs that surround Lake Powellindicate the drop in water levelover the past decade, theresult of repeated winters withlow snowfall. (Photograph©2006 Tigresblanco.)

Phenomena and direction of trend Likelihood that trend occurredin late 20th century

Likelihood of a humancontribution to observed trend

Likelihood offuture trends

Warmer and fewer cold days andnights over most land areas

Very likely Likely Virtually certain

Warmer and more frequent hot daysand nights over most land areas

Very likely Likely (nights) Virtually certain

Warm spells/heat waves. Frequencyincreases over most land areas

Likely More likely than not Very likely

Heavy precipitation events. Frequency(or proportion of total rainfall fromheavy falls) increases over most areas

Likely More likely than not Very likely

Area affected by droughts increases Likely in many regions since1970s

More likely than not Likely

Intense tropical cyclone activityincreases

Likely in some regions since1970

More likely than not Likely

Increased incidence of extreme highsea level (excludes tsunamis)

Likely More likely than not Likely

Potential Effects of Global Warming

The weather isn’t the only thing global warming willimpact: rising sea levels will erode coasts and causemore frequent coastal flooding. The problem isserious because as much as 10 percent of the world’spopulation lives in coastal areas less than 10 meters(about 30 feet) above sea level. The IPCC estimatesthat sea levels will rise between 0.18 and 0.59 meters(0.59 to 1.9 feet) by 2099 because of expanding seawater and melting mountain glaciers.

Apart from drivingtemperatures up, globalwarming is likely to causebigger, more destructive storms,more widespread drought, andcoastal damage from high sealevels. With some exceptions,the tropics will likely receive lessrain (orange) as the planetwarms, while the polar regionswill receive more precipitation(green). White areas indicatethat fewer than two-thirds ofthe climate models agreed onhow precipitation will change.Stippled areas reveal wheremore than 90 percent of themodels agreed. (©2007 IPCCWG1 AR-4.)

These estimates of sea level rise may be low,however, because they do not account for changes inthe rate of melt from the world’s major ice sheets. Astemperatures rise, ice will melt more quickly. Newsatellite measurements reveal that the Greenland andWest Antarctic ice sheets are shedding about 125billion tons of ice per year—enough to raise sealevels by 0.35 millimeters (0.01 inches) per year. Ifthe melting were to accelerate, the rise in sea levelcould be significantly higher. For instance, the lasttime global temperatures were a degree or so warmerthan today, sea levels were about 6 meters (20 feet)higher, with the water mainly coming from themelting of the Greenland and the West Antarctic icesheets. Neither ice sheet is likely to disappear before2100, but there is the danger that global warmingcould initiate massive losses from the Greenland andAntarctic ice sheets that will continue or evenaccelerate over future centuries.

Global warming is also putting pressure onecosystems, the plants and animals that co-exist in aparticular climate. Warmer temperatures have alreadyshifted the growing season in many parts of the

Sea levels crept up about 20centimeters during thetwentieth century. Most of therise happened because waterexpands as it warms, thoughmelting mountain glaciers alsocontributed to the change. Sealevels are predicted to go upbetween 0.18 and 0.59 metersover the next century, thoughthe increase could be greater ifice sheets in Greenland andAntarctica melt more quicklythan predicted. Higher sealevels will erode coastlines andcause more frequent flooding.(Graph ©2007 Robert Rohde.)

shifted the growing season in many parts of theglobe. Spring is coming earlier, and that means thatmigrating animals have to start earlier to follow foodsources. And since the growing season is longer,plants need more water to keep growing or they willdry out, increasing the risk of fires. Shorter, milderwinters fail to kill insects, increasing the risk that aninfestation will destroy an ecosystem. As the growingseason progresses, maximum daily temperaturesincrease, sometimes beyond the tolerance of the plantor animal. To survive the climbing temperatures, bothmarine and land-based plants and animals havestarted to migrate towards the poles. Those speciesthat cannot migrate or adapt face extinction. TheIPCC estimates that 20-30 percent of plant andanimal species will be at risk of extinction iftemperatures climb more than 1.5° to 2.5°C.

The people who will be hardest hit will be residentsof poorer countries who do not have the resources tofend off changes in climate. As tropical temperaturezones expand, the reach of some infectious diseaseslike malaria will change. More intense rains andhurricanes, rising sea levels, and fast-meltingmountain glaciers will lead to more severe flooding.Hotter summers and more frequent fires will lead tomore cases of heat stroke and deaths, and to higherlevels of near-surface ozone and smoke, which wouldcause more ‘code red’ air quality days. Intensedroughts could lead to an increase in malnutrition. Ona longer time scale, fresh water will become scarcerduring the summer as mountain glaciers disappear,particularly in Asia and parts of North America. Onthe flip side, warmer winters will lead to fewer cold-related deaths, and the longer growing season couldincrease food production in some temperate areas.

Ultimately, global warming will impact life on Earthin many ways, but the extent of the change is up tous. Scientists have shown that human emissions ofgreenhouse gases are pushing global temperatures up,and many aspects of climate are responding to thewarming in the way that scientists predicted theywould. Ecosystems across the globe are already

As much as 10 percent of theworld’s population lives incoastal regions where theelevation is less than 10 metersabove sea level. Thesecommunities will becomeincreasingly prone to stormdamage and flooding as sealevels rise. Among the mostvulnerable countries isBangladesh, which has lowelevation, a high populationdensity, and is one of theworld’s poorest nations. Redareas indicate populations thatlive less than 10 meters abovesea level, while green areasshow the population density inareas with an elevation greaterthan 10 meters. (Imagecourtesy Socioeconomic Dataand Applications Center.)

would. Ecosystems across the globe are alreadyaffected and surprising changes have already takenplace. Polar ice caps are melting, plants and animalsare migrating, tropical rain is shifting, and droughtsare becoming more widespread and frequent. Sincegreenhouse gases are long-lived, the planet willcontinue to warm and changes will continue tohappen, but the degree to which global warmingchanges life on Earth depends on our decisions.

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