Text A How Much Can We Really Trust Climate Models...
Michael D.Lemonick
Yale Environment 360:Better scientific research and powerful computers mean climate models are getting even more reliable.
NASA not only measures sea surface temperature from space using powerful scientific instruments,but it also studies temperature processes in advanced computer models.
A chart appears on page 45 of the 2007 Synthesis Report of the Intergovernmental Panel on Climate Change(IPCC),laying out projections for what global temperature and sea level should look like by the end of this century.Both are projected to rise,which will come as no surprise to anyone who's been paying even the slightest attention to the headlines over the past decade or so.In both cases,however,the projections span a wide range of possibilities.The temperature,for example,is likely to rise anywhere from 1.8°C to 6.4°C(3.2°F to 11.5°F),while sea level could increase by as little as 7 inches or by as much as 23—or anywhere in between.
It all sounds appallingly vague,and the fact that it's all based on computer models probably doesn't reassure the general public all that much.For many people,“model”is just another way of saying“not the real world.”In fairness,the wide range of possibilities in part reflects uncertainty about human behavior:The chart lays out different possible scenarios based on how much CO2 and other greenhouse gases humans might emit over the coming century.Whether the world adopts strict emission controls or decides to ignore the climate problem entirely will make a huge difference to how much warming is likely to happen.
But even when you factor out the vagaries of politics and economics,and assume future emissions are known perfectly,the projections from climate models still cover a range of temperatures,sea levels,and other manifestations of climate change.And while there's just one climate,there's more than one way to simulate it.The IPCC's numbers come from averaging nearly two dozen individual models produced by institutions including the National Center for Atmospheric Research(NCAR),the Geophysical Fluid Dynamics Laboratory(GFDL),the U.K.'s Met Office,and more.All of these models have features in common,but they're constructed differently—and all of them leave some potentially important climate processes out entirely.So the question remains:How much can we really trust climate models to tell us about the future?
The answer,says Keith Dixon,a modeler at GFDL,is that it all depends on questions you're asking.“If you want to know‘is climate change something that should be on my radar screen’”he says,“then you end up with some very solid results.The climate is warming,and we can say why.Looking to the 21st century,all reasonable projections of what humans will be doing suggest that not only will the climate continue to warm,you have a good chance of it accelerating.Those are global-scale issues,and they're very solid.”
The reason they're solid is that,right from the emergence of the first crude versions back in the 1960s,models have been at their heart a series of equations that describe airflow,radiation and energy balance as the Sun warms the Earth and the Earth sends some of that warmth back out into space.“It literally comes down to mathematics,”says Peter Gleckler,a research scientist with the Program for Climate Model Diagnosis and Intercomparison at Livermore National Laboratory,and the basic equations are identical from one model to another.“Global climate models,”he says,echoing Dixon,“are designed to deal with large-scale flow of the atmosphere,and they do very well with that.”
The problem is that warming causes all sorts of changes—in the amount of ice in the Arctic,in the kind of vegetation on land,in ocean currents,in permafrost and cloud cover and more—that in turn can either cause more warming,or cool things off.To model the climate accurately,you have to account for all of these factors.Unfortunately,says James Hurrell,who led the NCAR's most recent effort to upgrade its own climate model,you can't.“Sometimes you don't include processes simply because you don't understand them well enough,”he says.“Sometimes it's because they haven't even been discovered yet.”
A good example of the former,says Dixon,is the global carbon cycle—the complex interchange of carbon between oceans,atmosphere,and biosphere.Since atmospheric carbon dioxide is driving climate change,it's obviously important,but until about 15 years ago,it was too poorly understood to be included in the models.“Now,”says Dixon,“we're including it—we're simulating life,not just physics.”Equations representing ocean dynamics and sea ice also have been added to climate models as scientists have understood these crucial processes better.
Other important phenomena,such as changes in clouds,are still too complex to model accurately.“We can't simulate individual cumulus clouds,”says Dixon,because they're much smaller than the 200-kilometer grid boxes that make up climate models'representation of the world.The same applies to aerosols—tiny particles,including natural dust and manmade soot—that float around in the atmosphere and can cool or warm the planet,depending on their size and composition.
But there's no one right way to model these small-scale phenomena.“We don't have the observations and don't have the theory,”says Gleckler.The best they can do on this point is to simulate the net effect of all the clouds or aerosols in a grid box,a process known as“parameterization.”Different modeling centers go about it in different ways,which,unsurprisingly,leads to varying results.“It's not a science for which everything is known,by definition,”says Gleckler.“Many groups around the world are pursuing their own research pathways to develop improved models.”If the past is any guide,modelers will be able to abandon parameterizations one by one,replacing them with mathematical representations of real physical processes.
Sometimes,modelers don't understand a process well enough to include it at all,even if they know it could be important.One example is a caveat that appears on that 2007 IPCC chart.The projected range of sea-level rise,it warns,explicitly excludes“future rapid dynamical changes in ice flow.”In other words,if land-based ice in Greenland and Antarctica starts moving more quickly toward the sea than it has in the past—something glaciologists knew was possible,but hadn't yet been documented—these estimates would be incorrect.And sure enough,satellites have now detected such movements.“The last generation of NCAR models,”says Hurrell,“had no ice sheet dynamics at all.The model we just released last summer does,but the representation is relatively crude.In a year or two,we'll have a more sophisticated update.”
Sophistication only counts,however,if the models end up doing a reasonable job of representing the real world.It's not especially useful to wait until 2100 to find out,so modelers do the next best thing:They perform“hindcasts,”which are the inverse of forecasts.“We start the models from the middle of the 1800s,”says Dixon,“and let them run through the present.”If a model reproduces the overall characteristics of the real-world climate record reasonably well,that's a good sign.
What the models don't try to do is to match the timing of short-term climate variations we've experienced.A model might produce a Dust Bowl like that of the 1930s,but in the model it might happen in the 1950s.It should produce the ups and downs of El Niño and La Niña currents in the Pacific with about the right frequency and intensity,but not necessarily at the same times as they happen in the real Pacific.Models should show slowdowns and accelerations in the overall warming trend,the result of natural fluctuations,at about the rate they happen in the real climate.But they won't necessarily show the specific flattening of global warming we've observed during the past decade—a temporary slowdown that had skeptics declaring the end of climate change.
It's also important to realize that climate represents what modelers call a boundary condition.Blizzards in the Sahara are outside the boundaries of our current climate,and so are stands of palm trees in Greenland next year.But within those boundaries,things can bounce around a great deal from year to year or decade to decade.What modelers aim to produce is a virtual climate that resembles the real one in a statistical sense,with El Niños,say,appearing about as often as they do in reality,or hundred-year storms coming once every hundred years or so.
This is one essential difference between weather forecasting and climate projection.Both use computer models,and in some cases,even the very same models.But weather forecasts start out with the observed state of the atmosphere and oceans at this very moment,then project it forward.It's not useful for our day-to-day lives to know that September has this average high or that average low;we want to know what the actual temperature will be tomorrow,and the day after,and next week.Because the atmosphere is chaotic,anything less than perfect knowledge of today's conditions(which is impossible,given that observations are always imperfect)will make the forecast useless after about two weeks.
Since climate projections go out not days or weeks,but decades,modelers don't even try to make specific forecasts.Instead,they look for changes in averages—in boundary conditions.They want to know if Septembers in 2050 will be generally warmer than Septembers in 2010,or whether extreme weather events—droughts,torrential rains,floods—will become more or less frequent.Indeed,that's the definition of climate:the average conditions in a particular place.
“Because models are put together by different scientists using different codes,each one has its strengths and weaknesses,”says Dixon.“Sometimes one(modeling)group ends up with too much or too little sea ice but does very well with El Niño and precipitation in the continental U.S.,for example,”while another nails the ice but falls down on sea-level rise.When you average many models together,however,the errors tend to cancel.
Even when models reproduce the past reasonably well,however,it doesn't guarantee that they're equally reliable at projecting the future.That's in part because some changes in climate are non-linear,which is to say that a small nudge can produce an unexpectedly large result.Again,ice sheets are a good example:If you look at melting alone,it's pretty straightforward to calculate how much extra water will enter the sea for every degree of temperature rise.But because meltwater can percolate down to lubricate the undersides of glaciers,and because warmer oceans can lift the ends of glaciers up off the sea floor and remove a natural brake,the ice itself can end up getting dumped into the sea,unmelted.A relatively small temperature rise can thus lead to an unexpectedly large increase in sea level.That particular non-linearity was already suspected,if not fully understood,but there could be others lurking in the climate system.
Beyond that,says Dixon,if three-fourths of the models project that the Sahel(the area just south of the Sahara)will get wetter,for example,and a fourth says it will dry out,“there's a tendency to go with the majority.But we can't rule out without a whole lot of investigation whether the minority is doing something right.Maybe they have a better representation of rainfall patterns.”Even so,he says,if you have the vast majority coming up with similar results,and you go back to the underlying theory,and it makes physical sense,that tends to give you more confidence they're right.The best confidence-builder of all,of course,is when a trend projected by models shows up in observations—warmer springs and earlier snowmelt in the Western U.S.,for example,which not only makes physical sense in a warming world,but which is clearly happening.
(1,953 words)
New Words＆Expressions
1.accelerate［ək΄seləreIt］v.to increase the speed of
2.aerosol［΄eərəsɒl］n.a metal container in which liquids are kept under pressure and forced out in a spray气雾器；喷雾器
3.blizzard［΄blIzəd］n.a violent and heavy snowstorm
4.biosphere［΄baIəʊsfIə］n.the part of the earth's surface and atmosphere where there are living things生物圈
5.caveat［΄kæviæt］n.process to suspend proceedings；a warning
6.cumulus［kjuːmjələs］n.a type of thick white cloud formed when hot air rises very quickly
7.diagnosis［ˌdaIəg΄nəʊsIs］n.determining the nature of a disease from observation of symptoms
8.fluctuation［ˌflʌktʃu΄eIʃən］n.a wave motion；moving up and down；being irregular
9.geophysics［ˌdʒiːəʊ΄fIzIks］n.study of the earth's magnetism,meteorology,etc.
10.glaciologist［gleIsI΄ɒlədʒIst］n.a person who studies glaciers,or more generally ice and natural phenomena that involve ice
11.grid［grId］n.system of overhead cables carrying on pylons,for distributing electric current over a large area；network of squares on maps,numbered for reference
12.inverse［ˌIn΄vзːs］adj.inverted；in the opposite order,position or arrangement
13.linear［΄lIniə(r)］adj.of or in lines；of length
14.lubricate［΄luːbrIkeIt］vt.put oil or grease into machine parts to make them work easily
15.lurk［lзːk］vi.to keep out of view lying in wait or ready to attack
16.manifestation［ˌmænIfe΄steIʃn］n.making clear
17.parameterization［pəræmItəˌraI΄zeIʃən］n.参数化
18.percolate［΄pзːkəleIt］v.(of liquid)to pass slowly；to filter
19.permafrost［΄pзːməfrɒst］n.permanently frozen subsoil(in the polar region)
20.precipitation［prIˌsIpI΄teIʃn］n.fall of rain,sleet,snow or hail
21.projection［prə΄dʒekʃn］n.the act of projecting；prominence
22.reassure［ˌriːə΄ʃʊə］vt.remove the fears or doubts of
23.scenario［sə΄nɑːriəʊ］n.(pl-s)written outline of a play,an opera,a film with details of the scenes
24.soot［sʊt］n.black powder in smoke,or left by smoke on surface
25.sophistication［səˌfIstI΄keIʃn］n.being expert or having knowledge of some technical subject
26.simulate［΄sImjuleIt］vt.to pretend to be；to pretend to have or feel
27.simulation［ˌsImju΄leIʃn］n.imitation；pretence
28.vagaries［΄veIgəriz］n.(pl)unexpected events or changes that cannot be controlled and can influence a situation
29.vegetation［vedʒi΄teiʃ(ə)n］n.plants in general
Notes
1.This text is taken from climate change,The Guardian on 18 January,2011.
2.NASA:National Aeronautics and Space Administration(U.S.)
3.IPCC(联合国政府间气候变化专门委员会)：The Intergovernmental Panel on Climate Change(IPCC)was set up jointly by the World Meteorological Organization and the United Nations Environment Programme to provide an authoritative international statement of scientific understanding of climate change.The IPCC's periodic assessments of the causes,impacts and possible response strategies to climate change are the most comprehensive and up-to-date reports available on the subject,and form the standard reference for all concerned with climate change in academic,government and industry worldwide.This Synthesis Report is the fourth element of the IPCC Fourth Assessment Report“Climate Change 2007”.Through three working groups,many hundreds of international experts assess climate change in this Report.
4.PCMDI:Program for Climate Model Diagnosis and Intercomparison.PCMDI was established in 1989 at the Lawrence Livermore National Laboratory(LLNL),located in the San Francisco Bay area,in California.Their staff includes research scientists,computer scientists,and diverse support personnel.They are primarily funded by the Regional and Global Climate Modeling(RGCM)Program and the Atmospheric System Research(ASR)Program of the Climate and Environmental Sciences Division of the U.S.Department of Energy's Office of Science,Biological and Environmental Research(BER)Program.
The PCMDI's mission is to develop improved methods and tools for the diagnosis and intercomparison of general circulation models(GCMs)that simulate the global climate.The need for innovative analysis of GCM climate simulations is apparent,as increasingly more complex models are developed,while the disagreements among these simulations and relative to climate observations remain significant and poorly understood.The nature and causes of these disagreements must be accounted for in a systematic fashion in order to confidently use GCMs for simulation of putative global climate change.
5.hindcast:In oceanography and meteorology,backtesting is also known as hindcasting.A hindcast is a way of testing a mathematical model;known or closely estimated inputs for past events are entered into the model to see how well the output matches the known results.Hindcasting usually refers to a numerical model integration of a historical period where no observations have been assimilated.This distinguishes a hindcast run from a reanalysis.Oceanographic observations of salinity and temperature as well as observations of surface wave parameters such as the significant wave height are much scarcer than meteorological observations,making hindcasting more common in oceanography than in meteorology.Also,since surface waves represent a forced system where the wind is the only generating force,wave hindcasting is often considered adequate for generating a reasonable representation of the wave climate with little need for a full reanalysis.Hindcasting is also used in hydrology for model stream flows.
6.Glaciology is the scientific study of glaciers,or more generally ice and natural phenomena that involve ice.Glaciology is an interdisciplinary earth science that integrates geophysics,geology,physical geography,geomorphology,climatology,meteorology,hydrology,biology,and ecology.The impact of glaciers on people includes the fields of human geography and anthropology.The discoveries of water ice on the Moon,Mars and Europa add an extraterrestrial component to the field,as in“astroglaciology”.
7.El Niño is the warm phase of the El Niño Southern Oscillation(commonly called ENSO)and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific(between approximately the International Date Line and 120°W),including off the Pacific coast of South America.El Niño Southern Oscillation refers to the cycle of warm and cold temperatures,as measured by sea surface temperature,SST,of the tropical central and eastern Pacific Ocean.El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific.The cool phase of ENSO is called“La Niña”with SST in the eastern Pacific below average and air pressures high in the eastern and low in the western Pacific.The ENSO cycle,both El Niño and La Niña,causes global changes of both temperatures and rainfall.Mechanisms that cause the oscillation remain under study.
Reading Comprehension
Ⅰ.Questions for discussion
1.What is the essential difference between weather forecasting and climate projection analyzed in the text?
2.What are the limitations of climate models?
3.Why are scientists still working on climate models,with all their limitations?
Ⅱ.Judge,according to the text,whether the following statements are true(T)or false(F).
1.Global temperature and sea level are projected to rise,which will surprise everyone who has been paying more attention to the headlines over the past decade or so.
2.For many people,“model”is just another way of saying“not the real world”.
3.Although there's just one climate,there are a few ways to simulate it.
4.All reasonable projections of what humans will be doing suggest that the climate should not continue to warm though you have a chance of accelerating.
5.Anything less than perfect knowledge of today's conditions will make the forecast useless after about two weeks because the atmosphere is chaotic.
Vocabulary
Ⅰ.Fill in the blanks with words that best complete the sentences.
( )1.The popularity of the film shows that the revi ewers'fears were completely________.
A.unjustified B.unjust C.misguided D.unaccepted
( )2.The multinational corporation was making a t ake-over_______for a properly company.
A.application B.bid C.proposal D.suggestion
( )3.The party's reduced vote was________of lack of support for its policies.
A.indicative B.positive C.revealing D.evident
( )4.The________of beer and alcohol in New Zealand is very high.
A.consumption B.use C.drink D.absorption
( )5.As for Anne,I am not sure about her________in Italian.
A.fluency B.clarity C.coherence D.excellence
( )6.If the fire alarm is sounded,all residents are requested to________in the courtyard.
A.assemble B.converge C.crowd D.accumulate
( )7.Last year,the crime rate in Chicago has sharply________.
A.declined B.lessened C.descended D.slipped
( )8.The most successful way to solve the language problem while a foreign play is being performed is________translation.
A.instantaneous B.spontaneous C.simultaneous D.homogeneous
( )9.My cousin likes eating very much,but he isn't very________about the food he eats.
A.special B.peculiar C.particular D.specific
( )10.Undergraduate students have no________to the books in the school library.
A.access B.entrance C.way D.path
Ⅱ.Fill in each blank with a word chosen from the box in its appropriate form.
reassurance percolate caveat simulation acceleration lurk
sophisticate manifestation lubrication project fluctuate scenario
1.The____________of the industrial structure also needs the support of elements of knowledge.
2.But if the hospital would____________me that I would not be at risk,then I would happily donate.
3.This____________should not cloud the central issue:a coup in a region which has shed authoritarianism should not be allowed to stand.
4.The method of TWS radar simulation is analyzed in this paper,and corresponding metrics are computed for a____________designed in this paper.
5.This seems like a neat way to eradicate killer bacteria that____________on surfaces in hospital rooms.
6.The municipal finance must give a full play to financial policy if the underdeveloped area wants to____________its economy.
7.The water gradually____________down through the sands.
8.That leaves only one other suspect—the oil used to____________the engine.
9.And its outstanding virtue is the minimal________of the rolling force produced by the rolling mill structure.
10.____________of the extreme climate events under the global warming is always of great importance in the global change research field.
11.Self-identity crisis is a concentrated____________of contemporary cultural crisis.
12.It is an important content to____________natural scenery in Computer Graphics.
Cloze
Of the four choices given below for each blank,choose the one that best fits into the passage.
One of the most difficult aspects of deciding whether 1 climatic events reveal evidence of the impact of human activities is that it is hard to get a(n) 2 of what 3 the natural variability of the climate.We know that over the past millennia the climate 4 major changes without any significant human 5 .We also know that the global climate system is immensely 6 and that everything is in some way connected,and so the system 7 of fluctuating in unexpected ways.We need therefore to know 8 the climate can vary of its own accord in order to interpret with confidence the extent to 9 recent changes are natural as opposed to being the result of human activities.
Instrumental 10 do not go back far enough to provide us with 11 measurements of global climatic variability on timescales longer than a century.What we do know is that as we include longer time 12 ,the record shows increasing evidence of slow swings in climate between different regimes.To build up a better picture of fluctuations appreciably further back in time require us to use proxy records.
What the proxy records make abundantly clear i s that there have been significant 13 changes in the climate over timescales longer than a few thousand years.Equally 14 ,however,is the relative 15 of the climate in the past 10,000 years.
To the extent that the 16 of the global climate from these records can provide a measure of its true variability,it should at least indicate 17 all the natural causes of climate change have combined.These include the chaotic fluctuations of the atmosphere,the slower but equally erratic behavior of the oceans, 18 in the land surfaces,and the extent of ice and snow.Also included will be any variations that have 19 from volcanic activity,and possibly 20 activities.
( )1.A.current B.past C.nowadays D.future
( )2.A.guess B.measure C.standard D.estimate
( )3.A.builds B.establishes C.constitutes D.founds
( )4.A.has undergone B.underwent
C.undergoes D.has been undergone
( )5.A.interruption B.invading C.residence D.intervention
( )6.A.complicated B.difficult C.terrible D.strange
( )7.A.is capable B.has ability C.is responsible D.has been able
( )8.A.how long B.how much C.which D.what
( )9.A.that B.what C.how D.which
( )10.A.records B.estimates C.activities D.shows
( )11.A.true B.reliable C.real D.correct
( )12.A.gaps B.periods C.intervals D.stops
( )13.A.natural B.human C.global D.national
( )14.A.striking B.confusing C.surprising D.unlikely
( )15.A.fluctuation B.change C.balance D.stability
( )16.A.coverage B.change C.date D.study
( )17.A.that B.how C.which D.what
( )18.A.changes B.forms C.stability D.possibility
( )19.A.increased B.raised C.arisen D.grown
( )20.A.human B.animal C.beast D.land