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Path: senator-bedfellow.mit.edu!faqserv From: jan.schloerer@medizin.uni-ulm.de (Jan Schloerer) Newsgroups: sci.environment,sci.answers,news.answers Subject: Climate change: some basics Supersedes: <sci/climate-change/basics_866621822@rtfm.mit.edu> Followup-To: sci.environment Date: 16 Aug 1997 10:40:57 GMT Organization: University of Ulm Lines: 1084 Approved: news-answers-request@MIT.EDU Expires: 29 Oct 1997 10:38:26 GMT Message-ID: <sci/climate-change/basics_871727906@rtfm.mit.edu> Reply-To: jan.schloerer@medizin.uni-ulm.de (Jan Schloerer) NNTP-Posting-Host: penguin-lust.mit.edu Summary: Nontechnical outline of some basic climate change topics, including natural and enhanced greenhouse effect, feedbacks, aerosols, natural climatic variability, and ice cores. Keywords: climate change, greenhouse effect, radiation balance, aerosols, carbon cycle, natural variability, past climates X-Last-Updated: 1997/04/05 Originator: faqserv@penguin-lust.MIT.EDU Xref: senator-bedfellow.mit.edu sci.environment:143771 sci.answers:6917 news.answers:109925 Archive-name: sci/climate-change/basics Version: 2.02 Last-modified: 05 April 1997 Posting-Frequency: about every two months Changes April 1997: Minor patches in sections 6 and 7, amendment in section 11, some references and web sites added. Changes Oct 1996: Many modifications and amendments, some references replaced by pointers to [IPCC 95]. Climate change: some basics Subject: 1. Introduction By outpouring greenhouse gases humankind has launched an experiment of geologic proportions. Will this experiment, if countermeasures worth mentioning are delayed for some more decades, cause serious consequences during the next century ? Alas, there is no simple yes-no-answer to this question. Climate, its natural vagaries, and the long-term effects of rising greenhouse gas levels are only partially understood. The shortest defensible answer I can think of, a first approximation so to speak: it is roughly an even bet, fifty-fifty. The longer greenhouse gas emissions go on uncurbed, the worse the odds. A nontechnical, by no means comprehensive outline of some of the basic science behind this answer follows. Potential impacts and responses are not addressed. Please note that this is not my field. I have a fair idea of the broad picture, but I don't understand all the technical niceties. I have attempted to sketch some basics in a way which most readers with some interest in our planet's workings might be able to understand. Jan Schloerer jan.schloerer@medizin.uni-ulm.de Subject: 2. Contents 1. Introduction 2. Contents 3. The natural greenhouse effect 4. Tropospheric lapse rate 5. The enhanced greenhouse effect. Radiative forcing 6. Climate sensitivity. The modern temperature record 7. Human-made tropospheric aerosols 8. Ocean and response time 9. Feedbacks: water vapor, ice and snow, clouds 10. The global carbon cycle. Biological feedbacks 11. Natural climatic variability 12. Ice record of greenhouse gases and last glaciation 13. Conclusion 14. Further reading. References 15. Some web sites 16. Acknowledgements. Administrivia. How to get this file Subject: 3. The natural greenhouse effect The sun's radiation, much of it in the visible region of the spectrum, warms our planet. On average, earth must radiate back to space the same amount of energy which it gets from the sun. Being cooler than the sun, earth radiates in the infrared. (An object, when getting warmer, radiates more energy and at shorter wavelengths. On cooling, it emits less and at longer wavelenghts. Lava or heated iron are examples.) The wavelengths at which the sun and the earth emit are, for energetic purposes, almost completely distinct. Often, solar radiation is called shortwave, whereas terrestrial infrared is called longwave radiation. Greenhouse gases in earth's atmosphere, while largely transparent to incoming solar radiation, absorb most of the infrared emitted by earth's surface. The air is cooler than the surface, emission declines with temperature, so the air or, rather, its greenhouse gases emit less infrared upwards than the surface. Moreover, while the surface emits upwards only, the air's greenhouse gases radiate both up- and downwards, so some infrared comes back down. Clouds also absorb infrared well. Again, cloud tops are usually cooler and emit less infrared upwards than the surface, while cloud bottoms radiate some infrared back down. All in all, part of the infrared emitted by the surface gets trapped. Satellites, viewing earth from space, tell us that the amount of infrared going out to space corresponds to an `effective radiating temperature' of about -18 o C. At -18 o C, about 240 watts per square metre (W/m**2) of infrared are emitted. This is just enough to balance the absorbed solar radiation. Yet earth's surface currently has a mean temperature near 15 o C and sends an average of roughly 390 W/m**2 of infrared upwards. After the absorption and emission processes just outlined, 240 W/m**2 eventually escape to space; the rest is captured by greenhouse gases and clouds. The `natural greenhouse effect' can be defined as the 150 or so W/m**2 of outgoing terrestrial infrared trapped by earth's preindustrial atmosphere. It warms earth's surface by about 33 o C. As an aside, note that garden glasshouses retain heat mainly by lack of convection and advection [Jones]. The atmospheric `greenhouse' effect, being caused by absorption and re-emission of infrared radiation, is a misnomer. We won't get rid of it, though ;-) Under clear sky, roughly 60-70 % of the natural greenhouse effect is due to water vapor, which is the dominant greenhouse gas in earth's atmosphere. Next important is carbon dioxide, followed by methane, ozone, and nitrous oxide [IPCC 90, p 47-48]. Clouds are another big player in the game. Beginners please don't confuse clouds with water vapor: clouds consist of water droplets or ice particles or both. Under cloudy sky the greenhouse effect is stronger than under clear sky. At the same time, cloud tops in the sunshine look brilliantly white: they reflect sunlight. Globally and seasonally averaged, clouds currently exert the following effects: Outgoing terrestrial infrared trapped (warming) about 30 W/m**2 Solar radiation reflected back to space (cooling) nearly 50 W/m**2 Net cloud effect (cooling) roughly 20 W/m**2 Earth's present reflectivity or albedo (whiteness) is near 0.3. This means that about 30 % or slightly over 100 W/m**2 of the sun's incoming radiation is reflected back to space, while roughly 240 W/m**2 or about 70 % is absorbed. Almost half of earth's current albedo and perhaps 20 % of the natural greenhouse effect is caused by clouds. Quantities involving clouds are hard to measure and may vary by a few W/m**2, depending on whom you listen to. Globally averaged, the surface constantly gains radiative energy, whereas the atmosphere scores a loss. Sending up about 390 W/m**2, the surface absorbs roughly 170 W/m**2 solar radiation and over 300 W/m**2 infrared back radiation from greenhouse gases and clouds. The atmosphere, clouds included, radiates both up- and downwards, altogether over 500 W/m**2. It absorbs roughly 70 W/m**2 solar radiation and 350 W/m**2 terrestrial infrared. The surface's radiative heating and the atmosphere's radiative cooling are balanced by convection and by evaporation followed by condensation. When evaporating, water takes up latent heat; when water vapor condenses, as happens in cloud formation, latent heat is released to the atmosphere. Information in this section comes from [Berger] and [Hartmann, chapters 2-4], unless indicated otherwise. Subject: 4. Tropospheric lapse rate At any given location, the temperature profile of the air column varies between day and night, from winter to summer. At times and places the air may get warmer higher up (an inversion). Globally averaged, the troposphere, the lower about 10 to 15 km of our atmosphere, gets cooler with height. A typical value cited is 6.5 o C cooling / km of altitude. This is the so-called global mean tropospheric lapse rate. Some people attach a plus, others attach a minus sign to this rate [Hartmann, p 3, 69] [Sinha]. In any case, it indicates the average rate of cooling with height. For illustration, if the amount of the mean tropospheric lapse rate should increase by 1 o C / km, then the mean air temperature at 5 km altitude would drop by 5 o C. Basically, earth's surface temperature and the greenhouse effect tend to go up and down with the amount of the tropospheric lapse rate. To see why, recall that infrared emitted from the surface rarely reaches space directly: greenhouse gases and clouds absorb most of it. Earth's effective radiating temperature of -18 o C corresponds to an apparent radiating altitude of 5 or so km. The bulk of the infrared escaping to space comes from the middle and upper troposphere. On its way up, little of this radiation gets caught: still higher up the air is thin, there are few greenhouse gases and clouds [Hartmann, p 28, 59-60]. Now imagine that the amount of the global mean tropospheric lapse rate goes up, while anything else remains equal (a wild simplification, but never mind). Then the middle and upper troposphere get cooler and emit less infrared to space. The sun keeps shining, so earth's radiation budget gets out of balance. The surface (and troposphere) must warm until they emit enough infrared to restore the balance under the enhan- ced lapse rate. The difference between surface emission and emission to space, that is: the greenhouse effect, increases. Vice versa, if the magnitude of the global mean tropospheric lapse rate drops, then the middle and upper troposphere warm and emit more infrared to space. To regain the balance, the greenhouse effect must decline. Once again, this is simplified in order to convey the basic idea. The mean tropospheric lapse rate is a balance between many processes of energy transfer, like radiation, convection, evaporation, cloud formation, and large scale air motions. Data from the midlatitudes and tropics suggest that local lapse rate changes currently tend to amplify local variations of surface temperature and of the greenhouse effect. It is unclear whether and how the global mean tropospheric lapse may change with a changing global climate [Sinha] [Soden]. Finally, note that if the surface warms, while the lapse rate remains unchanged, then the troposphere will warm by the same amount as the surface. Infrared emission to space will rise accordingly. Subject: 5. The enhanced greenhouse effect. Radiative forcing Since around 1800 and especially during the past few decades, human activities have increased the atmospheric levels of several greenhouse gases. To name a few: Carbon dioxide (CO2) went up from about 280 ppmv (parts per million by volume) in the year 1800 via 315 ppmv in 1958 to about 358 ppmv in 1994 [IPCC 95, p 16, 78] [Keeling]. Methane (CH4) increased from roughly 0.8 ppmv in 1800 to more than 1.7 ppmv in 1992. Nitrous oxide (N2O) rose from a preindustrial level of about 0.275 ppmv to 0.310 or so ppmv in 1992 [IPCC 94, p 87-8, 91-2]. The resulting enhanced greenhouse effect is often expressed in terms of `radiative forcing'. To get a feeling for this notion, suppose that greenhouse gas levels go up, while anything else, including temperature, is kept fixed. Adding greenhouse gases renders the atmosphere more opaque to outgoing infrared radiation. Thus the mean altitude from which infrared emitted upwards makes it to space (5 or so km) rises. As mentioned, the troposphere gets cooler with height. With rising emission altitude, both earth's effective radiating temperature and, consequently, the amount of infrared emitted to space decline. The influx of solar radiation, to which greenhouse gases are almost trans- parent, changes little. So the net influx (the difference between what goes in and out) is now positive instead of being zero. Radiative forcing means a _change_ in the net downward flux of radia- tion, in W/m**2, at the tropopause, the borderline between troposphere and stratosphere. Eventually the climate system must respond and re- adjust the net flux to zero, but temporarily this flux may get positive or negative. Given some perturbation like a change in greenhouse gas or aerosol levels, radiative forcing is estimated with tropospheric and surface temperatures (the response of which takes decades) _kept fixed_ at their unperturbed values [IPCC 94, p 169-71]. Rising greenhouse gas levels cause positive radiative forcing. Aerosols, to be described later, can cause negative radiative forcing. Radiative forcing due to human-made greenhouse gases is currently estimated at about 2.5 W/m**2. CO2 causes roughly 1.6 W/m**2 of this, while methane contributes about 0.5 W/m**2. Doubling the CO2 level from its preindustrial 280 to 560 ppmv amounts to a radiative forcing of a bit over 4 W/m**2. If business goes on as usual, the combined effect of the rising greenhouse gas levels is likely to reach the equivalent of a CO2 doubling around the year 2050 and will hardly stop there [IPCC 90, p 52] [IPCC 95, p 108-18, 321]. An enhanced greenhouse effect disturbs earth's radiation balance: less infrared gets out, while the sun keeps shining. This cannot last, the balance must be restored. At least one of the following things must happen: earth's surface and troposphere may warm (lapse rate remaining unchanged), earth's albedo may go up, the amount of the mean tropospheric lapse rate may drop (the latter, though, might also rise and thus enhance surface warming), or other changes in earth's climate system may curb the enhanced greenhouse effect. In short, something has to give. Monkeying with earth's radiation balance will change the climate in some way. Earth's surface will most probably warm, although it is uncertain by how much and how swiftly. In addition, there will probably be a gamut of other changes, some of which, like changes in the water cycle, are even harder to predict and may become more troublesome than warming [IPCC 95] [Morgan]. Subject: 6. Climate sensitivity. The modern temperature record To the best of present knowledge, the so-called equilibrium surface warming, also known as the `climate sensitivity', is likely to sit somewhere between 1.5 and 4.5 o C for a CO2 doubling, with a best estimate of 2.5 o C [IPCC 95, p 34, 48]. Since 1890, average global surface temperature went up by about 0.5 o C with an uncertainty of roughly 0.15 o C both ways: the true warming is likely to lie somewhere between 0.3 and 0.6 o C. This estimate takes into account any known error sources, including urban heat island bias, relocation of stations, changes in measuring prac- tices and varying coverage of the globe. About 0.3 o C warming until 1940 and 0.1 o C cooling until 1975 were followed by renewed warming. [IPCC 90, chapter 7.4] [IPCC 95, p 26-8, 141-6] Surface and low to mid-tropospheric temperature are often confused, but they are not interchangeable. For tropospheric temperatures, the radiosonde and satellite record go back to 1958 and 1979, respectively. Both records are similar since 1979. On average, both the surface and lower-to-middle troposphere warmed by about 0.1 o C per decade since 1960. From 1979 to 1995, however, the surface warmed by 0.13 o C per decade, while the lower-to-middle troposphere cooled by 0.05 o C per decade. Gaps in the southern oceans surface data and errors in the tropical satellite record may contribute to the difference, but there are physical reasons as well. Surface and tropospheric temperatures responded differently to El Nino-Southern Oscillation, to volcanic eruptions, and probably also to deep Aleutian (1976-88) and Iceland (~1980-95) winter lows. [Hurrell 96/97] [IPCC 95, p 146-8, 165-6] Since 1960, the lower stratosphere cooled markedly by roughly -0.35 o C per decade. Both rising CO2 levels and stratospheric ozone depletion tend to cool the stratosphere. Initial model results suggest that, at the moment, stratospheric ozone loss may play the lead. It may also have a hand in the slight cooling of the upper troposphere over the past decades. [IPCC 95, p 109-11, 148-9] [Ramaswamy] [Santer] It is currently hopeless to draw conclusions from the observed tempe- rature record about the present or future amount of greenhouse gas induced warming. (Nonetheless, this is attempted time and again ;-) Apart from the amount of the eventual warming, its speed is uncertain as well. A given rate of warming does not by itself reveal when and at what level the warming is eventually going to stop. Moreover, the effects of several factors cannot yet be disentangled. Among these, the presumably most important three are: human-made greenhouse gases warming human-made tropospheric aerosols cooling natural climatic variability cooling or warming The geographic and vertical pattern of the temperature changes suggests an influence from human-made greenhouse gases and aerosols as well as from stratospheric ozone depletion [IPCC 95, chapter 8] [Ramaswamy] [Santer] [Tett]. This is a far cry from quantifying the human influen- ce, let alone the extent of future climate change. Taking into account numerous factors that can affect climate, climatologists can only say that the observed changes are consistent with (though no proof for) the estimated range of climate sensitivity to greenhouse gases. Subject: 7. Human-made tropospheric aerosols Aerosols are tiny (0.001 to 10 micrometres) airborne particles. In the troposphere, the lower about 10 to 15 km of our atmosphere, human-made aerosols have greatly increased since about 1850. They present a large source of uncertainty in assessing human influences on climate. `Fine' aerosol particles with sizes between about 0.1 and 1 micrometre can influence climate in two ways. Under clear sky they scatter and absorb solar radiation; some of the scattered sunlight goes back to space (the direct effect). Acting as cloud condensation nuclei, they may enhance reflectivity and life-time of clouds (indirect effect). Sulfur dioxide from fossil fuel burning, yielding sulfate particles after oxidation, is presently the largest source of fine human-made aerosols. Another large source is organic and elemental carbon from burning of tropical forests and savannahs. Globally averaged, fine human-made tropospheric aerosols may currently cancel about 50 % of the warming effect of human-made greenhouse gases. So far, though, the uncertainty range is large, stretching from roughly 10 to 100 %. [IPCC 94, sections 3, 4.4, 4.7] [IPCC 95, sections 2.3, 2.4.2] [Schwartz] Moreover, global averages are misleading. Even if the global averages of aerosol and greenhouse gas forcing cancel, their different distri- butions may cause climatic changes. With life-spans of up to over 100 years, human-made greenhouse gases are fairly evenly distributed. Most tropospheric aerosols are washed out after about a week, they are unevenly distributed. Human-made sulfate aerosols occur mainly down- wind of northern industrialized areas. Most biomass smoke rises from tropical land areas during the dry season. Cutting back sulfur dioxide emissions or biomass burning reduces the aerosol load quickly, leaving over the more longlived greenhouse gases. [Andreae] [IPCC 94 and 95] By the way, roughly one third of the tropospheric sulfate load has natural precursors, mainly oceanic dimethyl sulfide (DMS) and volcanic sulfur dioxide. Violent volcanic eruptions, like Pinatubo 1991, give rise to stratospheric sulfate aerosols which, being more long-lived than their tropospheric cousins, tend to warm the stratosphere and to cool the troposphere and surface for a few years. [IPCC 94, p 135-7, 141-4, 186-9] [IPCC 95, p 115-6, 148, 504-6] 'Coarse' aerosols with particle sizes between 1 and 10 micrometres include mineral dust raised by wind blowing over dry soils. Human influences like over-cultivation and soil erosion may have up to doubled the flux of mineral dust. Mineral dust is most abundant over North Africa, the Arabian Sea, and South Asia. It scatters sunlight and absorbs outgoing terrestrial infrared. One study suggests that these two effects largely cancel at the top of the atmosphere. If so, mineral dust has little effect on earth's overall radiation balance, although it regionally cools the surface and warms the air, which in turn may affect atmospheric circulation. However, as with sulfate aerosols and biomass smoke, there are large uncertainties. [Andreae] [Sokolik] [Tegen] Pinning down aerosol effects more precisely will be tough. Aerosols are hard to measure. Size, shape, composition and regional distribu- tion of the particles vary. So do their effects on climate. Aerosols can cause not just local but also distant responses, because heat or rather, in the case of many aerosols, coolness is transported by the atmosphere and ocean. Assessing the climatic effects of aerosols involves modeling of regional climates and of clouds, both of which are not yet very reliable. [Andreae] [IPCC 94/95] [Peter] [Schwartz] [Sokolik] [Wielicki, p 2127-29, 2146] Subject: 8. Ocean and response time It is not known whether it will take decades or centuries until equilibrium is approached for a given enhanced level of greenhouse gases. Much of this uncertainty stems from poorly known behavior of the ocean. The ocean covers about 70 % of the globe, it transports large amounts of heat, and it is the major source of atmospheric water vapor. The atmosphere and land are affected by variations of the ocean surface only, which in turn depend on the coupling between the ocean surface and the deeper ocean. With its huge heat capacity, the ocean slows down climate change. On the other hand, due to the deep ocean's slow response, temperature may continue to rise for centuries after stabilization of greenhouse gas levels. The topmost so-called `mixed layer', being warmer and less dense than the deeper layers, tends to stay on top. Cool, particularly salty (thus dense) surface water sinks and deep water forms in the northern North Atlantic and near Antarctica. Subsurface water wells up near eastern margins of oceans. For other regions of the ocean, the extent to which surface and deeper waters are exchanged is less clear. The replacement time for the deep ocean is many centuries. In heat capacity, a water column of about 2.5 m depth matches the atmosphere lying above it. Less than 2 m of water match an average land surface. [Hartmann, p 84-5, chapter 7] [IPCC 95, p 210-4, 290]. How much heat will the ocean's deeper layers store before things get really going ? Already in a "stable" climate, ocean circulation is likely to vary a good deal. A changing climate may entail major changes in ocean currents. For instance, North Atlantic deep water formation may decline or become more variable, which, in this region, may inhibit warming or even produce cooling. Unfortunately, not even the ocean's present state is fully known. This should improve over the next decade, but tracking down natural variations lasting decades to centuries may be not so easy. Exchange processes between surface and deeper layers of the ocean are among the ocean models' weaknesses. Improving the models is difficult, as the dearth of observational data hinders judging whether a given model behavior is reasonable [IPCC 95, p 166-7, 210-4, 266-7, 302-4, 317, 346, 526, 530]. At this point, the above question is unanswerable. For illustration, imagine a CO2 rise to 560 ppmv (twice the preindus- trial level) until about 2050, with CO2 remaining constant thereafter. Assume that other greenhouse gases and human-made aerosols remain at their 1990 levels. For this scenario, 15 out of 16 leading US climate scientists offered a best guess of between 2 and 4 o C surface warming by the year 2300, with widely varying time responses. The sixteenth expert estimated 0.3 o C and didn't provide a time response. By 2050, 9 of the 15 respondents expected roughly 50 to 70 % of the eventual warming, in line with recent estimates from climate models [IPCC 95, p 297-300]. The remaining 6 divided equally between swifter and slower warming. By 2100 most participants expected 80 % or more of the eventual warming, two suspected a sluggish response of below 25 % [Morgan, p 469A, 472A, figure 5]. These numbers shouldn't be taken too seriously, yet they highlight the pickle. By the way, all 16 researchers estimated some chance, between 8 and 40 %, that uncertainty about climate sensitivity could grow by a quarter or more after a 15-year research program [Morgan, table 1]. Subject: 9. Feedbacks: water vapor, ice and snow, clouds If nothing except surface and air temperature changed (and if human- made aerosols vanished), then a CO2 doubling would eventually warm earth's surface by 1 to 1.2 o C [Hartmann, p 231-2] [IPCC 95, p 30]. However, there are feedbacks, including though not confined to: water vapor feedback probably positive ice-snow-albedo feedback presumably positive cloud feedback poorly understood biological feedbacks see next section It is widely assumed that warming, which tends to enhance evaporation, will increase the water vapor content of the troposphere. This should amplify the warming, as water vapor is the dominant greenhouse gas [Hartmann, p 232-4] [IPCC 95, p 197, 200-1, 210]. [Lindzen] proposed that, in warmer tropics, deep convective clouds might rain out more thoroughly. This might dry the tropical upper troposphere and curb the tropical water vapor feedback. The available data on spatial patterns and short-term changes of upper-tropospheric humidity do not support Lindzen's notion. However, spatial and short-term variations need not be reliable surrogates for global climate change. The same data sug- gest that some part of the feedback formerly ascribed to water vapor may instead stem from lapse rate changes, the effects of which were outlined in section 4 [Sinha] [Soden]. Snow and ice reflect much of the incident sunlight back to space, thus a reduction of snow and ice cover is likely to enhance warming. Details remain hazy. Feedbacks between cloud cover and changes in sea ice and snow cover are poorly understood. Another hurdle is the interplay between atmosphere, surface ocean, and sea ice, in particular at the always present ice-free patches and near sea ice margins [IPCC 95, p 156-7, 204, 214, 216, 267, 347]. The cloud feedback may be large, yet not even its sign is known. Low clouds tend to cool, high clouds tend to warm. High clouds tend to have lower albedo and reflect less sunlight back to space than low clouds. Clouds are generally good absorbers of infrared, but high clouds have colder tops than low clouds, so they emit less infrared spacewards. To further complicate matters, cloud properties may change with a changing climate, and human-made aerosols may confound the effect of greenhouse gas forcing on clouds. With fixed clouds and sea ice, models would all report climate sensitivities between 2 and 3 o C for a CO2 doubling. Depending on whether and how cloud cover changes, the cloud feedback could almost halve or almost double the warming [Hartmann, p 68, 71-5, 249] [IPCC 94, p 150-4, 183-5] [IPCC 95, p 34-5, 201-10, 345-6] [Wielicki]. A recent intercomparison of 15 climate models showed mostly small to modest negative or positive cloud feedbacks. Sadly, the validity of this result is doubtful [IPCC 95, p 205-6]. Subject: 10. The global carbon cycle. Biological feedbacks Here, it's tempting to list some numbers :-) Gt = gigatonne = 10**9 metric tonnes, the mass of one cubic kilometre of water 1 GtC corresponds to ~3.67 Gt CO2 2.12 GtC or ~7.8 Gt CO2 correspond to 1 ppmv CO2 in the atmosphere. ppmv = parts per million by volume Carbon reservoirs in GtC Atmosphere (1990) 750 Surface ocean 1000 Terrestrial vegetation 600 Marine biota 3 Soils & detritus 1600 Dissolved organic carbon 700 Deep ocean 38000 Natural carbon fluxes in GtC/year, <--> denotes a two-way flux Atmosphere --> terrestrial vegetation 120 Photosynthesis Terrestrial vegetation --> atmosphere 60 Respiration Terrestrial vegetation --> soils & detritus 60 Soils & detritus --> atmosphere 60 Respiration Atmosphere <--> surface ocean 90 Surface ocean <--> deep ocean 100 Human-made CO2 in GtC/year, average fluxes 1980-1989, estimated 90 % confidence intervals in parentheses [IPCC 95, p 79] Carbon dioxide sources: Fossil fuel burning, cement production 5.5 (5.0-6.0) Changes in tropical land use 1.6 (0.6-2.6) Total emissions 7.1 (6.0-8.2) Partitioning among reservoirs: Storage in the atmosphere 3.3 (3.1-3.5) Oceanic uptake 2.0 (1.2-2.8) Northern Hemisphere forest regrowth 0.5 (0.0-1.0) Other terrestrial sinks: CO2 fertilization, N fertilization, climatic variations 1.3 (-0.2-2.8) Except for atmospheric CO2, carbon reservoirs and natural fluxes are hard to measure. Their estimates vary somewhat across the literature. Carbon enters and leaves the atmosphere largely as CO2. Other fluxes involve various carbon compounds. The above irreverently lumps land animals with soils and detritus, and it omits many other details as well. For instance, both volcanic CO2 and CO2 removal via silicate weathering are in the order of 0.1 GtC/year and play a role on geologic time scales only. [IPCC 95, chapters 2.1, 9, 10] [Butcher, chapter 11] [Siegenthaler] CO2 uptake by land plants through photosynthesis is roughly balanced by plant and soil respiration. Depending on whether photosynthesis exceeds or falls below respiration, the net result is CO2 drawdown or CO2 release. Today, photosynthesis is probably slightly ahead. In future, climatic changes or rising CO2 level may trigger feedbacks that curb or speed up the rise of atmospheric CO2. To name a few: CO2 fertilization should promote photosynthesis and draw down some CO2, as long as respiration doesn't catch up. Warming may stimulate or slow down both photosynthesis or respiration, depending, among others, on soil moisture. The mix of species in ecosystems is likely to shift, which in turn may affect atmospheric CO2. Dieback of vegetation can release CO2. The overall effect of these and other feedbacks is hard to tell. Ecosystem models tentatively suggest that carbon storage in vegetation and soils may eventually win out. Temporarily, however, carbon may be released, especially if large and rapid changes should cause forests to die back. [IPCC 95, chapters 2.1 and 9] Turning to the ocean, a sea surface warming of 1 o C may increase atmospheric CO2 by up to 10 ppmv through degassing [IPCC 94, p 57]. More importantly, marine life, in spite of its low biomass, takes up and releases about 50 Gt of carbon annually. Marine biological production occurs largely in the sunlit surface and is thought to be limited mostly by nitrogen. Surface nutrient supplies are replenished primarily through transport from deeper ocean layers. (In the open ocean, iron can be limiting; it enters the ocean mainly in airborne dust and via rivers.) The export of organic carbon from the surface to deeper ocean layers, known as the biological pump, is not or little affected by CO2 availabilility, but it may be affected by changes in temperature, cloud cover, ocean currents, nutrients availability, or ultraviolet radiation. These and other marine biological processes are complex. Researchers cannot yet say how they will respond to disturbances. It has been estimated that, with no biological pump, preindustrial atmospheric CO2 would have been 450 instead of 280 ppmv, whereas a marine life seizing all available surface nutrients could have lowered this to 160 ppmv. On the other hand, preliminary results suggest that changes in the biological pump may affect atmospheric CO2 only by 10s rather than 100s of ppmv. [IPCC 94, p 57-8] [IPCC 95, p 79-80, chapter 10] Biological feedbacks on climate are not limited to the carbon cycle. For instance, dimethyl sulfide (DMS) from the ocean is a major natural source of tropospheric sulfate aerosols. Shifts in DMS production may affect marine cloud cover and surface temperature. DMS production is hard to predict, because it depends, among many others, on the local biomass and mix of species. [IPCC 95, p 488, 504-6] Back to the land, spreading of boreal forest into tundra may lead to warmer winters. Trees protrude above the snow-covered ground, they reflect less sunlight back to space than snow-covered tundra. During and after deglaciation, the expansion of boreal forests amplified the warming of northern land areas. The reverse process, displacement of boreal forest by tundra, probably played a role in the onset of the last glaciation. For another example, rising CO2 tends to improve the water-use efficiency of vegetation. Plants may then release less water vapor to the ambient air. Regionally, this may warm the surface and affect precipitation and soil moisture. [Gallimore] [IPCC 95, p 217-21, 450, 469-71] These few illustrations should do to show that, for better or for worse, human land-use changes like de- or reforestation can make a difference. Subject: 11. Natural climatic variability What course would earth's temperature have taken without human influences ? We don't know. [Burroughs] opens his intriguing book on weather cycles: "The history of meteorology is littered with whitened bones of claims to have demonstrated the existence of reliable cycles in the weather." Too little is known about natural climatic fluctuations lasting decades to centuries. Some players that may cause climatic variations on this time scale: atmospheric variability including shifts of the polar front, varia- tions in the circulation of the North Atlantic and Pacific Ocean, solar variability, volcanism. During the Holocene, the past about 10,000 years, these factors, taken together, probably did not cause global mean surface temperature changes exceeding 1 o C [Rind]. Unraveling climate's natural vagaries may take a long time, because sufficiently long and detailed climatic records are scarce [IPCC 95, p 173-4, 180-1, 411, 418-21]. The Little Ice Age, from about 1450 to the 19th century, and the Medieval Warm Period, from perhaps the 9th to the 14th century, are cases in point. The data, including historical, tree ring, coral and ice core records, are gappy, in particular for the tropics and southern oceans. The global patterns of the climatic changes and the mechanisms behind these changes are not yet known. Formerly it was presumed that both the Medieval Warm Period and the Little Ice Age were globally more or less uniform. Now the available data begin to suggest that no major globally synchronous cool or warm period occurred during the past millennium. Instead, asynchronous regional coolings and warmings appear to have been common. [Bradley] [Crowley & North, chapter 5] [Hughes] [IPCC 95, p 174-7] [Overpeck] [Rind] For illustration, summers in northwest Sweden were, by and large, warmer than their 1860-1959 mean between AD 1000 and 1200 and, again, between 1400 and 1550. From 1200 to 1400, summers tended to be cooler. Year-round sea surface temperatures in the Sargasso Sea appear to have taken a similar course. On the other hand, summer temperatures over the northern Urals show more or less the opposite pattern with cool summers around AD 1000 and warm summers between 1200 and 1400 [Briffa] [Keigwin 96]. Over Northern Hemisphere land areas, summers tended to be cool during the 16th, 17th and 19th century, though with strong regional differences. Chinese summers, for instance, were unusually cool around 1650. This spell was weaker over the northern Urals and at other Arctic sites, it is absent or barely noticeable in a central European and in some North American records [Bradley] [Briffa]. There are not yet enough data to tell whether the so-called Medieval Warm Period, globally averaged, was warmer than the Little Ice Age, let alone the present century. The Little Ice Age, though not a globally synchronous cooling spell, was probably, on average, cooler than the last hundred years [Bradley] [Hughes] [IPCC 95, p 174]. The warming since around 1900 appears to be one of the globally most uniform temperature shifts during, at least, the past several centuries [Crowley & North, p 99] [Overpeck]. Several clues suggest a decline of solar activity during the Maunder Minimum (about 1645-1715), amounting to a radiative forcing of somewhere between -0.5 and -1.5 W/m**2. Decline and subsequent rise of solar activity to its present level may have contributed to the Little Ice Age and to the warming thereafter. Solar forcing since 1850 has been tentatively estimated at between +0.1 and +0.5 W/m**2. [IPCC 94, p 189-92, 194] [IPCC 95, p 115-8] Without knowing natural climatic variations reasonably well, elucida- ting their causes is difficult. Even the causes of wellknown events can be hard to identify. 1976-77 the behavior of El Nino-Southern Oscillation appears to have changed. El Nino episodes got more fre- quent, sea surface temperatures in the tropical Pacific tended to be high, precipitation over the tropics and subtropics from Africa to Indonesia declined. While some model results suggest that greenhouse gas induced climate change may look similar, it is still open whether this was incipient climate change or a natural fluctuation [IPCC 95, p 153-55, 165] [Meehl]. Subject: 12. Ice record of greenhouse gases and last glaciation During the past millennium, until about the 19th century, atmospheric greenhouse gas levels varied little and hence, during that time, probably contributed little to climatic variations. On a longer time scale, changes of greenhouse gas levels probably contributed signifi- cantly to the coolings and warmings of the last two glacial cycles. Ice cores from Greenland and Antarctica indicate that there was a close link between greenhouse gases and temperature [Raynaud]. For instance, the Vostok ice core from Antarctica exhibits a striking correlation between temperature and the concentrations of carbon dioxide (CO2) and methane (CH4) over the past 220,000 years [Jouzel]. The level of nitrous oxide (N2O) probably also varied more or less in phase with temperature [Raynaud, p 928]. The variations of these trace gases may account for up to about 50 % of the estimated temperature changes [Crowley, p 2364] [Raynaud, p 932]. CO2 was most important, while methane and nitrous oxide contributed less. During the onset of the last glaciation, the CO2 decrease markedly lagged the onset of the cooling. During the past two deglaciations, CO2 may have risen in phase with temperature or with an, in geologic terms, modest lag of up to about 1000 years [Raynaud, p 931]. Whether greenhouse gases led or lagged the climatic change, that is, whether they were a primary cause for the change or whether they acted as a positive feedback (which amplified a climatic shift already under way), is important for finding out just exactly what happened, but it is not by itself relevant for estimating the effect of the trace gases on surface temperature [Raynaud, p 932]. In spite of this, the effect is hard to quantify. During the last deglaciation, roughly 18,000 to 10,000 years ago, the rise of trace gas levels amounted to a radiative forcing of about 2.5 to 3 W/m**2. The meltdown of the huge glacial ice shields reduced earth's albedo, accounting for another perhaps 3 to 3.5 W/m**2. These figures are compatible with the IPCC estimate of about 1.5 to 4.5 o C surface warming for a CO2 doubling. They do not permit to narrow down the uncertainty, there remain many unknowns [Crowley, p 2366]. Perhaps most important: How cold was the last ice age ? This is not yet clear. Tropical oceans, for instance, may have been between 1 and 5 o C cooler than they are now [IPCC 95, p 173-4], and Greenland may have been several degrees colder than previously thought [Cuffey]. Another point to keep in mind: The sensitivity of earth's present climate and the sensitivity of the last glacial maximum's climate to a radiative forcing of so or so many W/m**2 need not be equal. The starting positions differ. Glaciations and deglaciations are triggered by variations in earth's orbit. Tilt of earth's axis, season of the perihelion (closest ap- proach of earth to sun, now in January), and eccentricity of earth's elliptical orbit vary. These variations cause, among others, changes in high northern latitude summer insolation, which are critical for the waxing and waning of ice sheets. Northern summer insolation was unusually low at the onset of the last glaciation around 115,000 years ago, it was high during deglaciation. The direct effect of the "orbital trigger" was too small to cause glaciation or deglaciation. Instead, a cascade of feedbacks and interacting processes with widely varying timescales led to the final result. Shifts in atmospheric or oceanic circulation may occur within decades. Southward spread of tundra or poleward expansion of boreal forests can take centuries to millennia. Over 10,000s of years, the weight of an ice sheet depresses the underlying bedrock, which eases melting. Many twists of the story, like the frequent partial breakdowns of ice sheets, remain enigmatic, even though the trigger and the gist of the eventual outcome are known [Crowley & North, chapters 6-7] [Eddy, chapters 17 & 21] [Gallimore] [IPCC 95, p 177-9] [Keigwin 95]. In today's climate change gamble even the trigger or, rather, its aerosol component is poorly known. As we are at it: For the next 25,000 years, high northern latitude summer insolation will not drop anywhere near its minimum of 115,000 years ago [Eddy, p 40-41]. Subject: 13. Conclusion We need to know just about everything. ... Is climate system modelling the ultimate example of hubris, or, by chopping away at areas of ignorance, will we truly improve our predictive capability ? David Rind, Nature 363 (1993), 312 Current climate models tend to predict gradual climate change. This is no guarantee against unpleasant surprises. Climate models as well as the knowledge fed into the models are far from perfect [IPCC 95, p 416-8, chapters 2, 4-6, 9-11] [Morgan] [Wielicki]. Rapid changes in atmospheric circulation, of ocean currents, in ecosystem functioning, or in the West Antarctic ice sheet's behavior may not be likely, yet such risks can, at present, neither be excluded nor quantified. [IPCC 95, p 45-6, 213, 304, 389, 525, 527-8] [Morgan] Vice versa, sudden climatic shifts during the last ice age [IPCC 95, p 177-9] [Keigwin 95] do not imply that similar shifts must necessarily happen in the near future: during glaciation the ice sheets were much larger and less stable than they have been for the past 10,000 years. Past climates help to understand the climate system's workings, but they do not readily reveal what to expect. Our climate seems to be headed for a "warm atmosphere-cold pole combination" which may be unique in earth history. No completely satisfactory geologic analog is known [Crowley & North, chapter 14] [Eddy, p 17-27, 39-71] [Overpeck]. Much of the public debate focuses on warming, an admittedly likely reaction of the climate system. Disturbing earth's radiation balance, however, may change the climate in a host of other potentially serious ways. Warming need not even be the practically most relevant part of the response. This is why many climatologists prefer the term `climate change' over `global warming'. For example, spatial and seasonal patterns of precipitation, evapora- tion, soil moisture and river runoff may shift. These in turn may affect agriculture and freshwater availability, which are critical for many poor countries and a potential source of migrations and conflicts. Cloud patterns, ocean currents, atmospheric circulation or the distribution of extreme weather events may change. Terrestrial and marine life will be affected and may in turn affect the climate via changes, for instance, of carbon storage, evaporation, or albedo [IPCC 95, chapters 9-10]. The risk of rapid climate change is linked to many other problems of concern, like population growth, poverty, loss of biodiversity, or stratospheric ozone depletion. Building a balanced public perception of the risks posed by climate change is difficult. There is an almost irresistible temptation to view extreme weather events, like droughts or storms, as signs of climate change, even if they are well within the limits of natural variability. At the same time, gradual change tends to go unnoticed. Natural climatic variability can lead to temporary coolings; these would be perceived as all-clears by many. We are up against a long- distance race and tend towards a sprinter's outlook. [Maunder, p 75] Human-made greenhouse gases and aerosols will change our climate. There is no free lunch, we cannot alter earth's radiation balance for nothing. It is uncertain by how much, how swiftly and with what twists the climate will change. This is dubious comfort, since uncertainty cuts two ways. The present best estimates may well overstate the risk, but they may as well understate it. Climate change resembles a gamble with high stakes. Current knowledge of the carbon cycle suggests that atmospheric CO2 will respond sluggishly to CO2 emissions changes [IPCC 95, p 82-5, 323]. The response of the climate system to a given CO2 level takes decades or longer. Barring surprises, the lag time between changes in CO2 emissions and their eventual effects on climate is very long. It is an open question how soon the uncertainties can be narrowed down, and whether climatologists will be able to predict details reliably before they start to happen in the real world [IPCC 95] [Morgan]. There is a natural inclination to wait and see until we know what we shall have to face. By then it may be too late. Subject: 14. Further reading. References Introductory articles, mainly on questions not addressed here: [Schelling] pensive, allround: science, impacts, responses [Ausubel] potential impacts: critical, though not complacent [Morgan] presents the scientific pickle in a nutshell [Trenberth] skills and limits of climate models [White] history, some basics, climate debate up to 1990 For details, you might try [Houghton, chapters 1-7] or, if you want to dig deeply, the reports [IPCC 90/94/95] by Working Group I of the Intergovernmental Panel on Climate Change (IPCC). Working Groups II and III address impacts and responses [IPCC 95 II/III]. For the physics of climate, [Hartmann] is a moderately technical starter, while the professionals often turn to the more rigorous [Peixoto]. [Andreae] Meinrat O. Andreae, Raising dust in the greenhouse. Nature 380 (1996), 389-390 [Ausubel] Jesse H. Ausubel, A second look at the impacts of climate change. American Scientist 79 (1991), 210-221 [Berger] A. Berger and Ch. Tricot, The greenhouse effect. Surveys in Geophysics 13 (1992), 523-549 [Bradley] Raymond S. Bradley and Philip D. Jones, `Little Ice Age' summer temperature variations: their nature and relevance to recent global warming trends. The Holocene 3 (1993), 367-376 [Briffa] Keith R. Briffa, Philip D. Jones, Fritz H. Schweingruber, Stepan G. Shiyatov & Edward R. Cook, Unusual twentieth-century summer warmth in a 1,000-year temperature record from Siberia. Nature 376 (1995), 156-159 [Burroughs] William James Burroughs, Weather Cycles: Real or Imaginary? Cambridge University Press 1992 [Butcher] Samuel S. Butcher, Robert J. Charlson, Gordon H. Orians & Gordon V. Wolfe, eds, Global Biogeochemical Cycles. San Diego, CA, Academic Press 1992 [Crowley] Thomas J. Crowley, Geological assessment of the greenhouse effect. Bulletin of the American Meteorological Society 74 (1993), 2363-2373 [Crowley & North] Thomas J. Crowley, Gerald R. North, Paleoclimatology. Oxford University Press 1991 [Cuffey] Kurt M. Cuffey, Gary D. Clow, Richard B. Alley, Minze Stuiver, Edwin D. Waddington, Richard W. Saltus, Large Arctic temperature change at the Wisconsin-Holocene glacial transition. Science 270 (1995), 455-458. Also: Doug MacAyeal, ibid. 444-445. Richard Kerr, Science 272 (1996), 1584-1585 [Eddy] J.A. Eddy and H. Oeschger (eds), Global Changes in the Perspective of the Past. Chichester, UK, John Wiley & Sons 1993 [Gallimore] R.G. Gallimore & J.E. Kutzbach, Role of orbitally induced changes in tundra area in the onset of glaciation. Nature 381 (1996), 503-505. Also: Mark Chandler, Trees retreat and ice advances, ibid. 477-478 [Hartmann] Dennis L. Hartmann, Global Physical Climatology. San Diego, CA, Academic Press 1994 [Houghton] John Houghton, Global Warming: The Complete Briefing. Lion Publishing, Oxford, UK / Elgin, Illinois, US 1994. Albatross Books, Sutherland, Australia 1994 [Hughes] Malcolm K. Hughes and Henry F. Diaz, Was there a `Medieval Warm Period', and if so, where and when ? Climatic Change 26 (1994), 109-142 [Hurrell 96] James W. Hurrell and Kevin E. Trenberth, Satellite versus surface estimates of air temperature since 1979. Journal of Climate 9 (1996), 2222-2232. Also at: http://www.cgd.ucar.edu:80/cas/papers/jclim96/ [Hurrell 97] James W. Hurrell & Kevin E. Trenberth, Spurious trends in satellite MSU temperatures from merging different satellite records. Nature 386 (13 March 1997), 164-167 [IPCC 90] Climate Change - The IPCC Scientific Assessment J.T. Houghton et al., eds, Cambridge University Press 1990 [IPCC 94] Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IPCC IS92 Emission Scenarios. J.T. Houghton et al., eds, Cambridge University Press 1995 [IPCC 95] Climate Change 1995: The Science of Climate Change. J.T. Houghton et al., eds, Cambridge University Press 1996 [IPCC 95 II] Climate Change 1995: Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Robert T. Watson et al., eds, Cambridge University Press 1996 [IPCC 95 III] Climate Change 1995: Economic and Social Dimensions of Climate Change. James P. Bruce et al., eds, Cambridge University Press 1996. [Jones] M.D.H. Jones and A. Henderson-Sellers, History of the greenhouse effect. Progress in Physical Geography 14, 1 (1990), 1-18 [Jouzel] J. Jouzel, N.I. Barkov, J.M. Barnola, M. Bender, 13 more authors, Extending the Vostok ice-core record of paleoclimate to the penultimate glacial period. Nature 364 (1993), 407-412 [Keeling] C.D. Keeling, T.P. Whorf, M. Wahlen & J. van der Plicht, Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980. Nature 375 (1995), 666-670 [Keigwin 95] Lloyd D. Keigwin, The North Pacific through the millennia. Nature 377 (1995), 485-486 [Keigwin 96] Lloyd D. Keigwin, The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274 (29 Nov 1996), 1504-1508 [Lindzen] R.S. Lindzen, Climate dynamics and global change. Annual Reviews of Fluid Mechanics 26 (1994), 353-378 [Maunder] W. John Maunder, Dictionary of Global Climate Change. London, UCL Press / New York, Chapman and Hall 1992 [Meehl] Gerald A. Meehl and Warren M. Washington, El Nino-like climate change in a model with increased atmospheric CO2 concen- trations. Nature 382 (1996), 56-60 [Morgan] M. Granger Morgan, David W. Keith, Climate change: Subjective judgments by climate experts. Environmental Science & Technology 29, 10 (1995), 468A-476A [Overpeck] Jonathan T. Overpeck, Paleoclimatology and climate system dynamics. Reviews of Geophysics 33, Supplement (July 1995), 863-871 [Peixoto] Jose Peixoto and Abraham H. Oort, Physics of Climate. Institute of Physics Publishing, 1992 [Peter] Thomas Peter, Airborne particle analysis for climate studies. Science 273 (1996), 1352-1353 [Ramaswamy] V. Ramaswamy, M.D. Schwarzkopf & W.J. Randel, Fingerprint of ozone depletion in the spatial and temporal pattern of recent lower-stratospheric cooling. Nature 382 (1996), 616-618 [Raynaud] D. Raynaud, J. Jouzel, J.M. Barnola, J. Chappellaz, R.J. Delmas, C. Lorius, The ice record of greenhouse gases. Science 259 (1993), 926-934 [Rind] David Rind & Jonathan Overpeck, Hypothesized causes of decade- to-century-scale climate variability: climate model results. Quaternary Science Reviews 12 (1993), 357-374 [Santer] B.D. Santer, K.E. Taylor, T.M.L. Wigley, T.C. Johns, P.D. Jones, 8 more authors, A search for human influences on the thermal structure of the atmosphere. Nature 382 (1996), 39-46 [Schelling] Thomas C. Schelling, Some economics of global warming. The American Economic Review 82 (March 1992), 1-14 [Schwartz] Stephen E. Schwartz and Meinrat O. Andreae, Uncertainty in climate change caused by aerosols. Science 272 (1996), 1121-22 [Siegenthaler] U. Siegenthaler & J.L. Sarmiento, Atmospheric carbon dioxide and the ocean. Nature 365 (1993), 119-125 [Sinha] Ashok Sinha, Relative influence of lapse rate and water vapor on the greenhouse effect. Journal of Geophysical Research 100 (1995), 5095-5103 [Soden] Brian J. Soden and Rong Fu, A satellite analysis of deep convection, upper-tropospheric humidity, and the greenhouse effect. Journal of Climate 8 (1995), 2333-2351 [Sokolik] Irina N. Sokolik & Owen B. Toon, Direct radiative forcing by anthropogenic airborne mineral aerosols. Nature 381 (1996), 681-683 [Tegen] Ina Tegen, Andrew A. Lacis & Inez Fung, The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380 (1996), 419-422 [Tett] Simon F.B. Tett, John F.B. Mitchell, David E. Parker, Myles R. Allen, Human influence on the atmospheric vertical structure: detection and observations. Science 274 (15 Nov 1996), 1170-1173 [Trenberth] Kevin E. Trenberth, The use and abuse of climate models. Nature 386 (13 March 1997), 131-133 [White] Robert M. White, The great climate debate. Scientific American 263, 1 (July 1990), 18-25 [Wielicki] Bruce A. Wielicki, Robert D. Cess, Michael D. King, David A. Randall, and Edwin F. Harrison, Mission to Planet Earth: Role of clouds and radiation in climate. Bulletin of the American Meteorological Society 76 (1995), 2125-2153 Subject: 15. Some web sites If this article is too technical for your taste, you might try the introduction by Granger Morgan, Tom Smuts, and others at http://www.gcrio.org/gwcc/toc.html The quarterly _Consequences_, edited by John A. Eddy, has readable articles by first rate scientists on past climates, climate models, and more. Published by Saginaw Valley State University, Michigan. http://www.gcrio.org/CONSEQUENCES/introCON.html For summaries of the 1995 IPCC reports see http://www.unep.ch/ipcc/ipcc95.html UNEP's Information Unit on Climate Change (IUCC) at Geneva offers concise fact sheets covering science, impacts and responses: http://www.unep.ch/iucc/fs-index.html Some entry points to the myriad of research and other web sites: http://www.epa.gov/globalwarming/ http://climate.gsfc.nasa.gov/ http://gcmd.gsfc.nasa.gov/ (Global Change) http://www.ncdc.noaa.gov/ http://www.nerc-bas.ac.uk/public/icd/wmc/met.links.html http://www-eosdis.ornl.gov/ http://www.ucar.edu/dss/faq/ (Meteorology FAQ) Some sites linked to sci.environment http://www.access.digex.net/~rmg3/ ftp://ftp.access.digex.net/pub/access/rmg3/sci.faqs/ http://www.mnsinc.com/richp/sci_env.html/ Introduction to sea level change and ice sheets by Robert Grumbine, Robert Parson's Ozone Depletion FAQ, Torsten Brinch's FAQ on ground level ozone, an article on atmospheric CO2, and more. Subject: 16. Acknowledgements. Administrivia. How to get this file Acknowledgements: My wife Rosemarie and Dave Halliwell patiently and friendly endured an inordinate amount of murky drafts. Michael Tobis, Robert Grumbine, Paul Farrar, and many others helped with explanations, comments, and suggestions. Caveat: This is not my field. Those climatologists who told me their opinion so far found the article reasonable. Sole responsibility for errors and misconceptions is mine, though. Corrections are welcomed, the more so as time for maintaining this article is scarce. However, please note the motto: "Not overly detailed" ;-) Students should not use this article as a reference for school projects. They should instead use it as a pointer to some of the published literature. Copyright (c) 1997 by Jan Schloerer, all rights reserved. This article may be posted to any USENET newsgroup, on-line service and BBS, as long as it is posted in its entirety and includes this caveat and copyright statement. However, please inform me, so I know where the article goes. This article may not be distributed for financial gain, it may not be included in commercial collections or compilations without the express written permission of the author. How to get this file: Among others, this article is archived at ftp://rtfm.mit.edu/pub/usenet/news.answers/sci/climate-change/basics http://www.lib.ox.ac.uk/internet/news/faq/sci.environment.html http://www.cs.ruu.nl/wais/html/na-bng/sci.environment.html Further archives are listed in "Introduction to the *.answers newsgroups" which is regularly posted to the *.answers newsgroups. If you do not have access to anonymous ftp or to the world-wide web, send the following email message to mail-server@rtfm.mit.edu send usenet/news.answers/sci/climate-change/basics If you want to find out more about the mail server, send a message to it containing the word "help" (without the quotation marks). Jan Schloerer jan.schloerer@medizin.uni-ulm.de Uni Ulm Biometrie & Med.Dokumentation D-89070 Ulm, Germany