Summary
| Subject keyword(s) | Astronomy, Climate, Climatology, Earth and space science, Earth science, Education (General), Geoscience, Global climate change, History/Policy/Law, Meteorology, Physical sciences, Policy issues, Science, Space Science, Space sciences, Weather |
|---|---|
| Grade level | Elementary School, Middle School, High School, Informal Education |
| Intended audience | Learner |
| Resource type | Reference Material, Text |
| Resource format | text, text/html |
| Rights | This site is owned and operated by WGBH Online. No material from wgbh.org or any Web site owned, operated, licensed or controlled by WGBH may be copied, reproduced, republished, uploaded, posted, transmitted, or distributed in any way, except that you may download one copy of the materials on any single computer for your personal, non-commercial home use only, provided you keep intact all copyright and other proprietary notices. Modification of the materials or use of the materials for any other purpose is a violation of WGBH's copyright and other proprietary rights. For purposes of this Agreement, the use of any such material on any other Web site or networked computer environment is prohibited. All trademarks, servicemarks, and trade names are proprietary to WGBH Educational Foundation or Public Broadcasting Service. |
Found in collection(s)
Click on the logo to get more information about the collection.
Content contained within the resource
The suggestion that humans are changing theclimate tends to evoke two main responses. One reaction is deeply skeptical:"How could humans be a significant player in an epic cosmic scheme in which theEarth's climate varies enormously between frozen ice ages, much cooler thantoday, and periods like the Cretaceous when the average temperature was perhaps20 degrees hotter than today?" The other common response is sympathetic: "Ofcourse the climate is changing: just look at how warm last winter was. Anyonecan see it's changing!"Climate change is a highly complex subject, spanningseveral technical disciplines--from meteorology to climate modelling, fromeconomics to paleoclimatology. It's also highly politicized and contentious.To help the average citizen navigate this debate, here are some frequentlyasked questions. > Is weather the same thingas climate? No. We experience weather locally, at one place and at onetime. As everyone knows, weather can vary enormously over the course of oneyear (100 degrees F or more) and between one region and another. Climatologystudies the average weather of the entire globe over centuries. > How muchnatural variation has there been in climate in the past? Climate's naturalvariation has been large. Paleoclimatologists, who attempt to reconstruct theclimate's of past eras, have evidence that at times the Earth's averagetemperature has been 10 degrees cooler than today and at other times 20 degreeswarmer than today. Many natural factors (operating on very different timescales) are known to change climate: subtle variations in the Earth's orbitround the sun; the positions of the continents as the tectonic plates move; theoutput of the sun's light, the eruption of volcanoes, ocean patterns etc. > How difficult has it been to discriminate human influence on climate fromnatural influences? It's been very difficult. Climatologists only havecomprehensive global temperature and rain fall measurements going back acentury. Using so-called "proxys" such as corals, tree rings, ice cores andlake sediments, scientists have been able to reconstruct longerrecords--back about 1,000 years. Over this period, the last 100 years appearsto stand out. (see graph) Since 1900, the Earth's average globaltemperature has warmed by about 1 degree F--hardly noticeable to most of us,but a rapid increase compared to most natural changes. Interestingly the shapeof this curve is not a straight line. Much of the warming occurred early in thecentury, then it leveled out for three decades around mid century. This, it'sargued, shows that human greenhouse gas build-up is only one factor influencingthe climate. Other factors--for example, the changing output of the sun, otherpollutants like sulphate aerosols which have a cooling effect and,volcanoes--also contribute to the aggregate effect.Why are many scientists andpolicy experts worried about global warming, even though there is no "smokinggun" demonstrating that serious human-caused global warming is underway? Themain reason people are concerned is the existence of a phenomenon called theGreenhouse Effect. Trace gases in the atmosphere like carbon dioxide, methaneand water vapor can trap infrared radiation escaping into space. Without suchnaturally occurring gases in our atmosphere, the Earth's average temperaturewould be 0 degrees F instead of the comfortable 59 degrees F it is today.However, too much CO2, can produce an inferno. (Venus is a good example of aplanet with an atmosphere made almost entirely of CO2.) Since carbon dioxide isgiven off when fossil fuels like oil, coal and gas are burned, it follows thatmore heat energy should be trapped in the climate system causing the globaltemperature to increase. > How much has the amount of CO2 in the atmosphereincreased? Charles David Keeling's work on the mountain of Mauna Loa inHawaii established (see graph) that since 1957 the CO2 level has risen exponentiallyand that the industrial CO2 diffuses throughout the entireatmosphere in about 18 months. Samples from the Vostock Ice in the Antarctichave extended the Keeling curve backwards in time, showing that CO2 has risenone third since the industrial revolution, and it is higher than at any timefor the past 450,000 years. > What part of this increased CO2 isman-made? Most of it. Each year about 7 billion tons of carbon (which turnsinto over 20 billion tons of CO2) are released by cars, factories and energyproduction (see graph). About one half of this is absorbed by the oceans and the biosphere.The rest stays in the atmosphere, joining other post-industrial revolution CO2and remaining there for an average residence of 100 years. If things continueas they are--with the developing world rapidly industrializing--then by 2100,the world's annual emission of carbon will exceed 20 billion tons (70 billiontons of CO2) and the amount (concentration) built up in the atmosphere will beat least triple the pre-industrial level. > Do we know just how this extra CO2will change the climate? Not really. Greenhouse gases like CO2 and methane"force" the climate by trapping energy in the climate system. But it's justpart of a very complex system involving other forcings. For example, asmentioned above, there are forcings like the sun's illumination which variesover cycles of a decade or more. Or, volcanoes which spread ash into the airand create a cooling effect. Other human pollutants like sulfate aerosols(also produced by coal burning) actually cool the climate system by reflectingincoming sunlight. But the biggest uncertainty is due to feedbacks which eitheramplify (positive) or dampen (negative) the effects of a forcing. The principalpositive feedback is water vapor. Increased CO2 causes warming...leading tomore water vapor which is itself a greenhouse gas...leading to more warming.Warmer ocean water bubbles out the dissolved CO2 which then leads to morewarming, more water vapor and so on. But there are negative feedbacks also.Water vapor can form clouds which reflect incoming sunlight and thus have acooling effect. Finally, there are delays or inertias in the system, notablythe oceans which can absorb large amounts of heat for a time. The oceans,therefore, act as a break on the climate system retarding change, delaying anywarming and cooling. To make things worse, the regional effect of a climatechange is not intuitive. In high latitudes, the increased water vapor mightfall as snow leading to major surprises as increased fresh water run off haltsor slows the so-called thermohaline circulation (driving the Gulf Stream) whichplunges far northern surface waters to the abyssal depths returning them to thesouthern hemisphere. Interrupting this current might plunge the North Atlanticinto a deep freeze. > To control greenhouse gas emissions, do we have to phaseout fossil fuels? Sooner or later, yes. Because CO2 stays in the atmospherefor about a century, simply freezing the annual emissions won't stop theatmospheric concentrations rising. To stabilize the atmospheric levels of CO2,be it at double, triple, quadruple preindustrial levels (or more), willeventually involve phasing out fossil fuels entirely. This is extraordinarilydifficult.Fossil fuels are the basis of modern urban civilization. This highlyconcentrated form of energy which can be used for electricity, heating andtransportation has enabled the evolution of large modern cities and a moderninfrastructure to ship people and goods all over the planet. Today, fossilfuels are the primary source of 87% of the world's energy; the remainder comingfrom biomass (firewood and charcoal), nuclear and hydoelectric. Fossil fuelsare not only concentrated and convenient, they are remarkably cheap. > What arethe challenges in cutting our dependence on fossil energy? Perhaps thebiggest challenge is to cut our dependence at a time when the world'spopulation and energy demands are both rising rapidly. Over the next century,world population will almost double to over 10 billion at least. And thestandard of living of billions of people will rise as they seek many of thematerial benefits of the developed nations. This will require lots of energy;rising from today's energy consumption rate of 10 terawatts (trillion watts) toa rate of over 20 TW by the middle of the century and 40 TW or more by 2100. > Will improvements in energy efficiency help? Just a little. Switchingfrom high carbon sources like coal to lower carbon sources like natural gaswill also certainly buy the world time. The good news is that nations like theU.S. are using energy more efficiently. In a typical American house, theappliances are much more efficient than they were two decades ago: thetoasters, TVs, cookers and boilers. The bad news is there are many moreappliances like VCRs, computers, video games (many of which are left on); thehomes are larger; and the number of automobiles we drive has increased. > Ifthe only long-term solution is to replace fossil fuels, where will our energycome from? There are forms of energy which produce little or no carbon.However, currently, there is no known source (with the possible exception of nuclear energy capableof producing the massive amounts of energy the world will need. Hydroelectricpower is essentially carbon free, but the available rivers have all beenexploited and hydro will never contribute more a few percent of the total.Nuclear energy is well established--440 plants produce 17% of the world'selectricity with no carbon dioxide or other air pollutants. But public concernsabout safety issues currently prevent a major expansion in thissource.So-called "new renewables" are continually mentioned as possiblesolutions, but it's difficult to see how sources like solar, wind and biomasscan ever play a major role. Solar and wind are intermittent sources (the windonly blows 25% of the time) which gather very small amounts of energy perhectare. Biomass, essentially the construction of energy plantations, whichgrow cellulose for burning or conversion into ethanol, is even lessconcentrated. To produce 10 trillion watts (the current world powerconsumption) using biomass would mean using 10% of the world's land surfacearea (the area currently given over to agriculture.) To sum up--there's no smoking gun to prove beyond doubt we are already changing the climate. But, ifcurrent trends continue, the amount of CO2 entering and accumulating in theatmosphere will go on rising. Given the vast reserves of fossil fuels, sooneror later we will put enough CO2 into the atmosphere to force a climate change.Just what this climate change will be, nobody knows. But once we have changedthe atmosphere, it will take hundreds of years to return to the way it wasbefore the industrial revolution. > home > the debate > carbon diet > faqs > stories in ice > discussion > beyond fossil fuels > water world > program excerpt > graphs > resources/links > synopsis > NOVA > FRONTLINE > wgbh New Content Copyright ©2000 PBS Online and WGBH/NOVA/FRONTLINE Support provided by For new content visit the redesigned NOVA site