Essay on Greenhouse Effect | Atmosphere | Geography

Here is a compilation of essays on the ‘Greenhouse Effect’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on the ‘Greenhouse Effect’ especially written for school and college students.

Essay on Greenhouse Effect

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Essay Contents:

1. Essay on the Introduction to Greenhouse Effect
2. Essay on the Tropospheric Lapse Rate of Temperature
3. Essay on the Enhanced Greenhouse Effect
4. Essay on Climate Sensitivity
5. Essay on Human-Made Tropospheric Aerosols
6. Essay on the Ocean and Response Time for Climate Change
7. Essay on Feedbacks: Water Vapour, Ice and Snow, Clouds
8. Essay on the Global Carbon Cycle
9. Essay on the Natural Climatic Variability

Essay # 1. Introduction to 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. On cooling, it emits less and at longer wavelengths. 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 long-wave 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 radiation. 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° C. At -18°C, about 240 Watts per sq. mt. (W/m²) 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°C and sends an average of roughly 390 W/m² of infrared upwards. After the absorption and emission processes just outlined, 240 W/m² 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/m² of outgoing terrestrial infrared trapped by earth’s pre-industrial atmosphere. It warms earth’s surface by about 33°C. As an aside, note that garden glasshouses retain heat mainly by lack of convection and advection. 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. Clouds are mother big player in the game. 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.

This can be summarized as:

i. Outgoing terrestrial infrared trapped (warming) about 30 W/m².

ii. Solar radiation reflected back to space (cooling) nearly 50 W/m².

iii. Net cloud affects (cooling) roughly 20 W/m².

Earth’s present reflectivity or albedo (whiteness) is near 0.3. This means that about 30% or slightly over 100 W/m² of the sun’s incoming radiation is reflected back to space, while roughly 240 W/m² or about 70% is absorbed. Clouds cause almost half of earth’s current albedo and perhaps 20% of the natural greenhouse effect.

Globally averaged, the surface constantly gains radiative energy, whereas the atmosphere scores a loss. Sending up about 390 W/m², the surface absorbs roughly 170 W/m² solar radiations and over 300W/m² infrared back radiation from greenhouse gases and clouds. The atmosphere, clouds included, radiates both up- and downward, altogether over 500 W/m².

It absorbs roughly 70 W/m² solar radiation and 350 W/m² 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.

Essay # 2. Tropospheric Lapse Rate of Temperature:

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°C cooling/km of altitude.

This is the so-called global mean tropospheric lapse rate. It indicates the average rate of cooling with height. For illustration, if the amount of the mean tropospheric lapse rate should increase by 1°C/km, then the mean air temperature at 5 km altitude would drop by 5°C.

Basically, earth’s surface temperature and the greenhouse effect tend to go up and down with the amount of the troposphere lapse rate. Earth’s effective radiating temperature of -18°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. Now imagine that the amount of the global mean tropospheric lapse rate goes up, while anything else remains equal, 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 enhanced lapse rate. The difference between surface emission and emission to space, that is, the greenhouse effect, increases. Vice versa, if the magnitudes 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. 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 mid-latitudes 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.

Essay # 3. The Enhanced Greenhouse Effect:

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. Methane (CH₄) increased from roughly 0.8 ppmv in 1800 to more than 1.7 ppmv in 1992.

Nitrous oxide (N₂O) rose from a pre-industrial level of about 0.275 ppmv to 0.310 or so ppmv in 1992. 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 transparent, 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 radiation, in W/m², 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. Rising greenhouse gas levels cause positive radiative forcing.

Aerosols can cause negative radiative forcing. Radiative forcing due to human-made greenhouse gases is currently estimated at about 2.5 W/m². CO₂ causes roughly 1.6 W/m² of this, while methane contributes about 0.5 W/m². Doubling the CO₂ level from its, pre-industrial 280 to 560 ppmv amounts to a radiative forcing of a bit over 4 W/m². 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.

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.

Earth’s surface will most probably warm, although it is uncertain by how much mill 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.

Essay # 4. 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°C for a CO₂ doubling, with a best estimate of 2.5°C. Since 1890, average global surface temperature went up by about 0.5°C with an uncertainty of roughly 0.15°C both ways- the true warming is likely to lie somewhere between 0.3 and 0.6°C.

This estimate takes into account any known error sources, including urban heat island bias, relocation of stations, changes in measuring practices and varying coverage of the globe. About 0.3°C warming until 1940 and 0.1 °C cooling until 1975 were followed by renewed warming. 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.

On average, both the surface and lower-to-middle troposphere warmed by about 0.1 °C per decade since 1960. From 1979 to 1995, however, the surface warmed by 0.13°C per decade, while the lower-to-middle troposphere cooled by 0.05°C per decade. Gaps in the southern ocean’s 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. Since 1960, the lower stratosphere cooled markedly by roughly -0.35°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. It is currently hopeless to draw conclusions from the observed temperature record about the present or future amount of greenhouse gas induced warming.

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:

i. Human-made greenhouse gases warming,

ii. Human-made tropospheric aerosols cooling and

iii. 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. This is a far cry from quantifying the human influence, 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 the estimated range of climate sensitivity to greenhouse gases.

Essay # 5. 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.

Even if the global averages of aerosol and greenhouse gas forcing cancel, their different distributions 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 long lived greenhouse gases. 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. ‘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 double 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 return may affect atmospheric circulation? Aerosols are hard to measure.

Size, shape, composition and regional distribution of the particles vary. So do their effects on climate. Aerosols can cause not just local but also distant responses, because heat or 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.

Essay # 6. Ocean and Response Time for Climate Change:

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 vapour.

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. 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. For illustration, imagine a CO₂ rise to 560 ppmv (twice the pre-industrial 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°C surface warming by the year 2300, with widely varying time responses. The sixteenth expert estimated 0.3°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.

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%. 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.

Essay # 7. Feedbacks: Water Vapour, 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°C. However, there are feedbacks, including though not confined to water vapor feedback probably positive, ice-snow-albedo feedback presumably positive, cloud feedback poorly understood and biological feedbacks. 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, 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 suggest that some part of the feedback formerly ascribed to water vapor may instead stem from lapse rate changes. 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. 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. 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 space-wards.

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°C for a CO₂, doubling. Depending on whether and how cloud cover changes, the cloud feedback could almost halve or almost double the warming. A recent inter-comparison of 15 climate models showed mostly small to modest negative or positive cloud feedbacks.

Essay # 8. The Global Carbon Cycle:

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. 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 draw down 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 either 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 invegetation and soils may eventually win out.

Temporarily, however, carbon may be released, especially if large and rapid changes should cause forests to die back. Turning to the ocean, a sea surface warming of 1°C may increase atmospheric CO₂ by up to 10 ppmv through degassing. 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.

The export of organic carbon from the surface to deeper ocean layers, known as the biological pump, is not or little affected by CO₂ availability, 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, pre-industrial 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. 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. 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 de-glaciation, 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. 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.

Essay # 9. Natural Climatic Variability:

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, variations 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°C. Unraveling climate’s natural vagaries may take a long time, because sufficiently long and detailed climatic records are scarce. The Little Ice Age, from about 1450 to the 19th century, and the Medieval Warm Period, from perhaps the 9th to the 14th century, is cases in point.

The data, including historical, tree ring, coral and ice core records are scanty, 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 cooling and warming’s appearing to have been common. For illustration, summers in northwest Sweden were, by and large, warmer than their 1860-1959 mean between A.D. 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 A.D. 1000 and warm summers between 1200 and 1400. 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.

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. The warming since around 1900 appears to be one of the globally most uniform temperature shifts during, at least, the past several centuries.

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/m². 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/m².

Without knowing natural climatic variations reasonably well, elucidating their causes is difficult. Even the causes of well-known events can be hard to identify. In 1976-77 the behavior of El Nino-Southern Oscillation appears to have changed.

El Nino episodes got more frequent, 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.

Essay # 10. 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 significantly to the cooling and warming of the last two glacial cycles. Ice cores from Greenland and Antarctica indicate that there was a close link between greenhouse gases and temperature.

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.

The level of nitrous oxide (N₂O) probably also varied more or less in phase with temperature. The variations of these trace gases may account for up to about 50% of the estimated temperature changes. 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 de-glaciations, CO₂ may have risen in phase with temperature or with an, in geologic terms, modest lag of up to about 1000 years. 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, 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. In spite of this, the effect is hard to quantify.

During the last de-glaciation, 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/m². The meltdown of the huge glacial ice shields reduced earth’s albedo, accounting for another perhaps 3 to 3.5 W/m². These figures are compatible with the IPCC estimate of about 1.5 to 4.5°C surface warming for a CO₂ doubling.

How cold was the last ice age? This is not yet clear. Tropical oceans, for instance, may have been between 1 and 5°C cooler than they are now, and Greenland may have been several degrees colder than previously thought. 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/m² need not be equal.

Glaciations and de-glaciations are triggered by variations in earth’s orbit. Tilt of earth’s axis, season of the perihelion (closest approach 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 de-glaciation. The direct effect of the “orbital trigger” was too small to cause glaciation or de-glaciation. Instead, a cascade of feedbacks and interacting processes with widely varying time scales led to the final result. Shifts in atmospheric or oceanic circulation may occur within decades. Southward spread of tundra or pole-ward 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, and the gist of the eventual outcome are known. In today’s climate change its aerosol component is poorly known. For the next 25,000 years, high northern latitude summer insolation will not drop anywhere near its minimum of 115,000 years ago.

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. 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.

Sudden climatic shifts during the last ice age 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. 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 is 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, evaporation, and 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.

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.

Human-made greenhouse gases and aerosols will change our climate. It is uncertain by how much, how swiftly and with what twists the climate will change. The present best estimates may well overstate the risk, but they may as well understate it.

Climate change resembles a gamble with high slakes. Current knowledge of the carbon cycle suggests that atmospheric CO₂ will respond sluggishly to CO₂ emissions changes. The response of the climate system to a given CO₂ level takes decades in longer. Barring surprises, the lag time between changes in CO₂ emissions and their eventual effects on climate is very long.