Opm: Een lezing over dit onderwerp, zowel binnen als buiten Shell, is reeds enkele malen door mij gehouden.
Rem: A lecture on this subject, within and outside Shell, has been given by me a couple of times already

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Climate Change; a summary of some present knowledge and theories

(by Evert Wesker)


In the last decades some notable weather events have occurred. A small but impressive sample of weather incidents:

  • On 15 December 1986 and 10 January 1993 all time pressure lows on the North Atlantic of
    916 (914?) mbar were recorded in the vicinity of Iceland.
  • The summers of 1994 and 1995 broke quite some heat records. A striking example among them:
    an all time high of 24 °C recorded on south-east Greenland!
  • The Typhoon Faxai in December 2001, at the very end of the tropical storm season in the
    West Pacific, had at its moment of maximum intensity sustained winds along its eye wall of
    over 285 km/h, gusts of 350 km/h and a central pressure of 879 mbar.
  • In the year 2005 the US were hit by two strong hurricanes: "Katrina" and "Rita"
    Both made it well into category 5, and inflicted major damage to oil and gas offshore platforms.
    The total damage (direct and indirect) done by "Katrina" was over $100 billion and killed approximately 1500 people, making it the deadliest storm since 1928.
  • In 2010 Pakistan was hit by the largest disaster since its independence. Major floods, due to extreme monsoon rains, in the Indus valley affected more than 20 million people
The years 1995 and 1997 already were very warm years on the meteorological record, but 1998 set a new record of 0.58 °C above the long year average world temperature. In the new millennium this trend continues. 2005 and 2010 were at least as warm as 1998, while in those cases no extreme El Niño was occurring.

It is not surprising that in recent years the issue of human activity induced climate change has drawn more attention than before, although the weather incidents mentioned above might have nothing to do with a possible climate change.
In their report "Climate change 1995 the IPCC (Intergovernmental Penal on Climate Change) came out with the - cautious - conclusion that the present climate change on Earth is (partly) due to CO2 (the most important), CH4, N2O and CFC emissions.

The evidence that the "enhanced greenhouse effect" has actually kicked in has become stronger in recent years. In the third and fourth assessment of the IPCC the conclusions are much firmer.

Another notable example is an article in "Science" by Thomas J. Crowley.

In this little article an overview of some present knowledge and theories is given.

1. The astronomical perspective

1.1 Brightness of the sun

The climate can be influenced greatly by the amount of solar energy caught by the Earth. The first factor to be considered is the evolution of the sun itself. The sun is presently a so called "main sequence" star. In the centre of such a star Hydrogen is converted into Helium in a thermonuclear fusion process. During the "main sequence" stadium a star like the sun slowly increases its energy output by a factor 2. This period is 10 billion years for the sun; then it will become a red giant which will burn the Earth to cinders. During the lifetime of the Earth (the past 4.5 billion years) the sun became 25% brighter! In spite of this the Earth entered a totally frozen state only a couple of times. On the contrary, it has been about as warm or warmer much of the time than in the present ice ages period although the sun was fainter than today.(*) This is maybe due to the high CO2 concentration in the early (-4.5 up to -1 billion yr) atmosphere. Also the albedo of the earth may have been lower than today.

The - in average!! - 11 year sun spot cycle is accompanied by a very modest variation of the energy output (0.05 - 0.1%) of the sun. There appears to be some correlation between weather patterns and the sun spot cycle. However, no explanation giving a clear mechanism why these patterns change is available. (One should be cautious about correlation as such. e.g.: There is a correlation between the number of Storks and the birth number in some parts of Europe ...) To conclude, this matter is still unresolved.(**)

This also holds for the fact that during the climax of the "little ice age" (1645-1715) there were no, or very little, sun spots (the Maunder minimum). No directly measured data are available on the energy output of the sun in those days. In the last IPCC report "Climate Change 2001" the rise in solar irradiance since 1750 is estimated at 0.1 - 0.5 W/m2 (0.04 - 0.2%).

Currently some interesting developments are going on, when considerng the solar actvity. There are indications that the Sun is entering a longer period of low actvity. If there would be a repeat of the "Maunder minimum", the effects of it would become apparent in real time.

Publications from the Potsdam Institute for Climate Impact Research state that the effect to be expected from this will be an average cooling of the Earth of about 0.3°C. This will be easily overwhelmed by the effects of human actvities (burning fossil fuels, etc.), which are currently expected cause a global warming of at least 2°C by 2100.

*) Evidence has been found that during ice age periods about 2.2 billion years ago as well as between 750 and 580 million years ago ice caps were present near the equator. For the Neoproterozoic period between 750 and 580 million years ago a very fascinating theory has been proposed by Hoffman and Schrag involving a very dynamic picture of the Earth oscillating between an extreme greenhouse state and a totally frozen state.
**) Publications quoted in the 1995 IPCC report (see lit. list at the end) indicate that "sun like" stars are 0.2 - 0.6% fainter when no solar spots are present at all.
An interesting hypothesis on the solar spot cycle has been proposed by Lassen and Friis Christensen

1.2 The orbit of the Earth

Milankovitch made a study on the relation between the orbit of the Earth and its climate in the last 2 million of years. The following variations occur in the orbit and orientation:

  • The eccentricity varies by 6% with a 100.000 year period. This gives rise to a 0.3% variation in solar heat flux.
  • The obliquity of the Earth varies between 21.8° and 24.4° with a period of 41.000 year.
  • The precession of the Earth's axis has a period of 25.800 year.

The most "favourable" conditions for the formation of ice caps in the higher latitudes are cold summers. Their occurrence is most likely in a situation where the obliquity is minimal (lowest summer sun) , the eccentricity maximal and the Earth is in its aphelion position in summer. The winters are cold enough anyhow if the summer heat is insufficient to melt away the last winters' snow. More over: Less cold winters in this situation are in general favourable to accumulate more snow.

The opposite is also true. Warm summers can melt away ice caps. Recent publications point in the direction that maxima in summer insulation in the arctic (a high summer sun and the Earth in a perihelium position in summer while its orbit is at a maximum in eccentricity) coincided with the termination of glacial periods.

Milankovitch showed that there is a good correlation between the eccentricity and obliquity cycle and the climate in the northern hemisphere in the last 2 million years. In the early Pleistocene (2.000.000 - 800.000 yr before present) the obliquity cycle (40.000 years) dominates, while in the last 800.000 years the eccentricity cycle (100.000 years) dominates. In the last 800.000 years the "glacials" lasted 5 to 10 times longer than the "interglacials".

An interesting hypothesis is given in this article by W.F. Ruddiman:

Ice-driven CO2 feedback on ice volume

2. The geologic perspective

2.1 Continental drift and oceanic circulation patterns

The Earth has had much warmer climates in the past. In the middle of the Cretaceous the climate in high latitude regions was at least 10 - 15° warmer than nowadays. This is due to two factors:

  • The positions of the continents and the resulting ocean circulation.
  • The higher amount of CO2 in the atmosphere. (maybe to a lesser extent)

In the middle Cretaceous the configuration of the continents was such that the formation of a circum equatorial sea current was possible. From there large gyres brought warm water to the arctic oceans very effectively. Secondly there were no sources of cold abyssal water, like nowadays from Antarctica. The cold abyssal water from Antarctica covers the oceans floors to far over the equator. In the middle Cretaceous the abyssal water of the ocean was warm: 15° - 20 °C! The surface temperature of the arctic oceans was 15 °C. In the figure below this "greenhouse configuration" is depicted:

Greenhouse World

As water vapour is the most powerful greenhouse gas in the atmosphere (its concentration is magnitudes higher than other greenhouse gases) in case of warm ocean conditions it will dominate. The effects of CO2 are felt best in the absence of water, e.g. in winter in the high arctic nowadays. Such winter conditions probably did not occur anywhere during the Cretaceous.

Nowadays the position of the continents, definitively in the southern hemisphere, is in an "icehouse configuration":

  • The formation of a circum equatorial sea current is impossible due to the positions of the America's and Africa.
  • Antarctica is surrounded by a circumpolar current, isolating this continent from the warmer parts of the Earth.
  • Antarctica can play the role of a "cold source", and increases the albedo of the earth (together with the Greenlandic ice cap). Cold abyssal water from Antarctica (0 - 2 °C) can be found on the ocean floors to far beyond the equator.

In the picture below the "icehouse configuration" is shown:

Icehouse World

The present "icehouse configuration" in the southern hemisphere formed approximately 30 million years ago, when the Drake passage (south of South America) opened. The Earth as a whole cooled down from then on, although there were strong fluctuations. In the northern hemisphere the first significant ice caps formed some 3 million years ago. The state of the Gulf stream across the Atlantic plays a major role in the climate of eastern North America and Europe. In those area's the bulk of extra ice, during "glacials", was formed. Around the North Atlantic the variability of the climate during the pleistocene has probably been the greatest on Earth.

To conclude, the position of the continents, and thus the possible positions of the sea currents, is a very important factor influencing the climate on Earth. It is, however, not the only one. Dramatic short term climate variations have occurred over a time span much shorter (extreme cases: a few decades) than those of geological processes like continental drift.

The presented Greenhouse vs. Icehouse hypothesis (source: T.H. van Andel, "New Views on an Old Planet, a history of global change", Cambridge (USA, Mass.) 1994) in this small article has been complemented by another fascinating possibility recently published in Scientific American by Hoffman and Schrag, involving a very dynamic picture of the Earth oscillating between an extreme greenhouse state and a totally frozen state.
It evolves as follows:

  1. Continents are gatherered around the equator. Increased rainfall on the equatoral continents leads to weathering, and a falling CO2 content in the atmosphere. As the sun was 6% fainter some 650 million yr BP a crital threshold is passed and the polar seas start freezing over.
  2. The albedo of the Earth rises drastically, and a run-away icehouse effect occurs. The Earths' Oceans freeze over totally, and the temperature drops to -50 °C.
  3. Volcanic activity goes on, bringing CO2 into the atmosphere. This is not removed and builds up. At some point in time the Earths' temperatures rise enough to start melting the ice in the equatoral regions.
  4. The ice melts quickly after this all over the world. The decreased albedo, combined with the very high CO2 content of the atmosphere, causes the temperature to soar upwards to 50 °C. After this the CO2 is removed and the climate returns to "normal".
During the period 750 - 580 million yr BP the Earth probably went through this cycle four times.

2.2 Volcanism

Volcanism can influence the weather by dust clouds obscuring the sun. Also sulphuric acid aerosols can play such a role.

In the history of the Earth quite spectacular volcanic events have occurred. Two examples are the formation of the "Siberian traps" during the last stage of the formation of the supercontinent Pangea (-245 Myr) and the formation of the "Deccan traps" north-east of Bombay, when India was on its way to Asia (-65 Myr). In both cases 1 to 1.5 million cubic kilometres of basaltic lava was erupted in less than 1 million years.

Both events coincided with mass extinctions, the so called "Permian-Triassic catastrophe" and the "C-T catastrophe". It is now believed that the Dinosaurs perished as a result of a major impact on Earth of a large body in Yucatan, Mexico. But, the aftermath of prolonged volcanic activity may have made the matter still worse for the Dino's. (A "double whammy")

A huge volcanic eruption created the Lake Toba caldera (Indonesia, Sumatra). As far as Ceylon centimetres of ash fell from it. This event, which took place 73500 years ago, might have accelerated the onset of the last glacial period.(*)

For individual volcanic eruptions from the 18th century on their impact on the weather can be estimated. The two most extreme ones are the plinian eruption of the Tambora (Sumbawa, Indonesia) in 1815 and the Laki fissure eruption on Iceland in 1783.

In 1815 the Tambora exploded with a violence unparalleled in recent history; more than 50 km3 of magma was produced, some 200 million tons of sulphuric acid aerosol was ejected into the stratosphere. It probably was one of the causes of "the year without a summer" in 1816. The drop in temperature in the northern hemisphere is estimated at 0.4 - 0.7 °C.

In 1783 there was a prolonged fissure eruption on Iceland, the Laki eruption. It lasted for 8 months. In this period 14 km3 of basaltic lava was erupted and more than 100 million tons of sulphurdioxide was brought into the atmosphere. On Iceland the results were catastrophic: 75% of the livestock died and there was an all out crop failure. The resulting famine took the lives of 25% of the Icelandic population. The aftermath was even greater than in case of the Tambora eruption. In both North America and Europe cold records were broken. The cooling effect of the Laki eruption in the northern hemisphere is estimated at 1 °C.

Other eruptions led to less dramatic effects: Krakatau (Indonesia, 1883, 0.3 °C), Santa Maria (Guatamala, 1902, 0.4 °C), Katmai (Alaska, 1912, 0.2 °C), Agung (Indonesia, 1963, 0.3 °C), El Chichón (Mexico, 1982, 0.5 °C), Pinatubo (Philippines, 1991, 0.5 °C).

Mount St. Helens (USA, 1980) is not on this list. The major blast was more horizontal, and the amount of sulphuric acid aerosols was modest compared to the preceding list of eruptions.

In all cases the effect of individual volcanic eruptions lasts for only about 2 years. In 1993 the dust veil of the Pinatubo had almost disappeared. The matter may be quite different in case of prolonged volcanic activity. E.g.: The Laki fissure eruption was much less violent than the eruption of the Pinatubo, but appeared to have had a much bigger impact on the weather.

Volcanoes also bring CO2 into the atmosphere. However, on the basis of long year averages this amount is modest (20 - 50 million tons of carbon per year) when compared to the CO2 emissions due to human activities (about 8 billion metric tons of carbon per year).

*) Recent publications cast doubt on the possibility that the Toba eruption has accelerated the onset of the last glacial. No evidence pointing to a long range climatic change caused by this "mega eruption" was found in Greenlandic ice from this period.

3. The greenhouse gases' perspective

The atmosphere contains greenhouse gases. Without the presence of these gases the Earth would be as cold as -18 °C in average. The main greenhouse gases are:

  • Water vapour, the most important, except in the high arctic in winter
  • CO2, partly natural (biomass decay, volcanism), partly man made (fossil fuel burning)
  • CH4, partly natural (biomass decay), partly man made (gas field leaks, rice paddy's)
  • N2O, partly man made (decomposition of nitrate fertilisers, fuel burning)
  • CFC's, man made, also cause depletion of the ozone layer in the lower stratosphere, will be phased out in the coming decade(s).
  • Fluorine containing compounds like CF4, SF6 and HCFC's. Their contribution is low, but their stability is (very) high.
   The radiation balance of the atmosphere, average of the entire Earth over 24 h. (Source: IPCC report)

In the history of the Earth the amount of CO2 (and O2!) has varied quite considerably. In the Cambrian, Ordovician, Silurian and Devonian era (570 - 360 million yr ago) and again during the Triassic, Jurassic and Cretaceous era (245 - 65 million yr ago) the CO2 content of the atmosphere was 5 - 15 times higher than today. In the Carboniferous era, on the other hand, it fell 300 million years ago to half of the present quantity. Also during cold periods in the last 2 million years ("glacials") it fell to almost half of the present concentration.

In the Carboniferous era (-300 Myr) and the Pleistocene era (-2 Myr to present) ice age periods coincided with low CO2 concentrations. On the other hand, during an ice age period in the Ordovician/Silurian era (-440 Myr) the CO2 concentration was about 10 times higher than nowadays! Maybe the solar energy output in those days, 2-3% lower, might provide some explanation. It seems (see also: Hoffman and Schrag) that this lower energy output puts the threshold to an ice age "lower" (i.e. easier to pass).

When considering the CO2 content of the atmosphere in the last 150.000 years, the "glacials" match with low CO2 concentrations and "interglacials" with higher concentrations very well. An open question is whether the CO2 preceded the climate changes, coincided with them, or lagged behind with the variations. Is it the cause or the effect? This question is still open, as it is up to now not possible to determine the chronology with sufficient accuracy.

It should be borne in mind that in a cold world the average temperature contrast between polar and (sub)tropical regions is bigger. Such a world is windy and probably dusty. And as has been told in the part on volcanism: Dust makes the world colder.(*)

And so we are once again caught in an intricate net of interrelations. Problems can not be solved by themselves, but all issues must be addressed simultaneously on all fronts.

On the other hand: Whether atmospheric CO2 concentrations, in the natural course of climatic fluctuation, are cause or effect (or both!), there can be no question about it that human activities increase them and "improve" the capability of the Earth's atmosphere to retain its warmth more effectively. The recent series of very warm years since the late eighties now start to provide evidence beyond statistic fluctuations (the "noise level") to prove this point.

In recent modelling the cooling effects of industrial sulphurdioxide and dust emissions ("the human volcano") are included. These emission make the climate 1 - 2 °C cooler in the industrial zones of the northern hemisphere. After inclusion of these effects the fit with the temperature profile over the years and the "predicted" is quite well. If the work of Lassen and Friis Christensen is included as well, the fit is still better as the warming during 1910 - 1940 can be explained as well.

To conclude, the greenhouse gases content of the atmosphere is significantly influencing the climate on Earth. It is still rather uncertain to what extent they will make the climate warmer and where the effects will be felt. Locally reverse (cooling) effects might be possible.

*) Recent publications have addressed an ambiguous aspect on the role of dust. The 1815 Tambora eruption was followed by "the year without a summer" (1816). However, the winter was not cold at all. It appears that dust above low albedo parts of the Earth has a cooling effect but on the other hand it has a warming effect above high albedo parts (snow and ice!).

4. The weather patterns perspective

4.1 The location of ice caps

I start with an amusingly contradictory example. After the climax of the last "glacial" 18.000 years ago, the climate recovered. The ice caps on Scandinavia and Canada were dwindling rapidly. The Scandinavian ice cap by 6500 BC was almost gone, while a significant remainder was still present on the Hudson bay, Labrador and Baffin Island. This caused an asymmetry in the circumpolar vortex, which sent the polar jet stream to a northerly course over Europe. This resulted in warm summers in north-western Europe, thanks to the presence of ice on Labrador! The picture below shows this.

6500 BC

The climate in Europe in those days was about as warm as it is today. Due to the presence of anticyclones over northern Europe the contrasts between summer and winter were bigger than today. This example shows that one should be very cautious to draw conclusions from local observations.

4.2 The role of sea currents

4.2.1 The Atlantic Gulf Stream (and the little ice age)

The Atlantic Gulf Stream has a profound influence on the climate of Europe and, to a lesser extent eastern North America. Eastern sides of oceans in winter are much warmer than western sides of oceans. Lisbon has a January temperature of 11 °C, New York 0 °C. This is easy to understand: In the middle latitudes the dominant winds come from the west. But, if one considers Bergen, Norway (on the north-east coast of the Atlantic Ocean) and compare its January temperature with Anchorage, Alaska (on the north-east coast of the Pacific Ocean at a similar latitude) then the latter is 12 °C colder. This big difference is entirely attributable to the presence of the Atlantic Gulf Stream which puts its marks on the climate in north-western Europe.

The Gulf Stream has varied considerably. In the figure below the shows the limits of the arctic water on two moments in time: Its position during the climax of the "little ice age" (1695) and during a much warmer period in the north Atlantic Ocean (1920-1950).

1695 and 1940

The difference between 1920-1950 and 1695 is dramatic. So were the climatic consequences over north-western Europe. In the years 1690 - 1700 the weather in northern Scotland was so cold that in 8 out of 10 years there was a crop failure. The famine which resulted from this took the lives of one to two thirds of the local population. In this region it was a disaster greater than the Bubonic Plague of the 14th century!

If one regards the oscillations in temperature over the northern Atlantic Ocean since the last glacial period one can observe maxima at 7000 BC, 4500 BC, 2000 BC and 800 AD. The second of this series was the warmest, about 1 - 2 °C warmer then today. An interesting idea has been proposed to explain this: "The salt conveyor".

"The salt conveyor" has two modes: "On" and "Off".

If the salt conveyor is "On" warm and salty water flows into the north Atlantic where it meets cold and dry arctic air. It will cool and sink away. This water will flow southwards at some depth and flow to the Indian and Pacific ocean. In this way salt is transported away into the depth of the oceans. If at a certain point in time the water becomes too warm or too fresh it will not sink away any more and the salt conveyor switches "off". According to model calculations a fresh water flow equal to one quarter of the flow of the Amazon river appears to be enough to stop "the salt conveyor". In the "off" stage a net ice growth in the north Atlantic is possible.

The late Quaternary record suggests that this process oscillates with a period of about 2000 years, which is almost equal to the periods between little ice ages: every 2500 years. Whether this idea is right is still an open matter. Localised events and the impact of ice sheets on the atmospheric circulation have to be taken into account as well. But it shows a way of linking ocean flow patterns with weather patterns. And to close this point: If the "conveyor belt" hypothesis is (partly) right the present warming of the Earth may well switch the conveyor (partly) "off" due to larger amounts of fresh water (melting of ice, increased rainfall from air masses originating from the Pacific), and the north Atlantic would become colder. Another possibility is a warming of the north Atlantic, but less so than areas at similar latitudes in the northern hemisphere.

In the last IPCC report it is stated that the possibility of a cooling of the North Atlantic due to the mechanism described above is probably remote.

(Source: Nature)

In a publication in Nature it was stated that the Atlantic Gulf Stream has recently weakened by 30%. However, severe doubt have been cast on this - it has probably been proven to be wrong - in a later publication.

4.2.2 El Niño and the southern oscillation

In the tropical zone in the Pacific Ocean an oscillation takes place which occasionally has quite some impact on the weather in large parts of the world. Normally the warmest water in the Pacific can be found near Indonesia. It is brought there by trade winds and warmed on its course from the eastern Pacific. Over the equator there is a circulation pattern, a so called Walker cell, in which warm air rises over the west equatorial Pacific and sinks in the eastern Pacific off the coast of Peru. Along the equator an easterly wind blows. This is enhanced by the fact that the already cool water of the Humbolt stream is cooled further by upwelling caused by southerly winds along the coast of Chile. Coriolis forces push the water westwards, and thus enhance upwelling of cold water.

Every few years the air pressure rises in the western Pacific and drops in eastern Pacific. As a result the trade winds are weakened. Sometimes the easterly winds on the equatorial zone are replaced by western winds. The warm water starts to move eastwards, and the water temperature off the coast of Peru rises. The circulation pattern of the Walker cell is relocated. Warm air rises in the central pacific and sinks in the western Pacific. The oscillation is quasi periodic with periods ranging from 2 to 7 years.

The El Niño's of 1982-83 and 1997-1998 were very strong. In 1982-83 the sea water temperature off the coast of Peru rose from 19 °C to 26 °C. It resulted in about 4000 mm of rainfall over a period of a year in a part of Peru which normally receives about 25 mm in the same period. In 1997-1998 this picture was similar.

The effects were great:

  • In 1982-83 there were severe droughts in Indonesia, Australia and southern Africa. The first two can be attributed to the El Niño event; the latter its cause is less certain, but the El Niño event is suspected to have played an important role. In 1997 Indonesia again was struck by droughts.
  • The subtropical jet stream was much stronger in the east Pacific near California, where it even locally merged with the polar jet stream. It resulted in plentiful rainfall and occasionally stormy weather in normally dry and quiet areas like California. Storm systems moved over the Rocky mountains into the region along the Gulf of Mexico.
  • Equador and Peru were raked by floods due to excessive rainfall.

Extreme occurrences of El Niño like in 1982-83 and 1997-1998 may raise the question whether there is a connection between El Niño and global warming. It is not possible to draw this conclusion from the historic records. More over, a recent publication at the KNMI site points in the direction that there is no correlation between the temperature on Earth and the activity of the El Niño cycle.

+ A small article on this subject can be found here.
+ Some notes on the 1997-1998 El Niño are given here.

4.3 Blocking anticyclones

In the general circulation pattern of the Earth the circumpolar vortex, often referred to as the polar jet stream, plays a prominent role. In the arctic the air pressure drops faster with the height than in the lower latitudes due to the temperature. At altitudes of 5 km the weather map of the world is relatively simple. There is a low pressure area at the poles and in the equatorial zone there is a high pressure belt. In general, except south of Asia during the monsoon, the upper winds are westerlies. In the zone where cold air of arctic origin meets warmer air, the so called polar front, the pressure gradient is greatest. In this region the circumpolar vortex is a strong westerly wind: The polar jet stream. At altitudes of 10 - 12 km high it can blow as fast as 300 km/h.

The jet stream meanders and often marks the path of middle latitude storms. If the jet stream is meandering only slightly (a strong west circulation) there are wet and cool summers and wet and mild winters in Europe. However, the meanders can also be large. In this case high pressure areas build up in regions normally frequented by middle latitude storms. The dominant western winds are replaced by easterlies. This weather pattern is called a blockade. A blocking anticyclone bars the way for middle latitude storms. If such a blockade keeps in place for many weeks (months) it often generates extreme weather: Heat and cold waves, droughts and floods depending on the time of year and position relative to the blocking anticyclone.

The summer of 1976 in Europe provides a nice example of a blockade. In June and July western Europe was hot and dry, with many days with maximum temperatures well in the 30-ties. Moscow had one of the coldest (3 - 4 °C below normal) and wettest summers in the century. Mid latitude storms which normally move over the UK and Scandinavia were forced on a track round Scandinavia into Russia by a blockade over western Europe.

Recent hot summers (1994 and 1995) and the cold winter of 1996 in western Europe can all be attributed to blockades. One can argue that because of this they are no proof of a climate change. They are merely extreme weather events which occur every now and then. On the other hand: If blockades occur more often, they do form, in total, a climate change. From reconstructions of weather maps based on historic data it appears that during the little ice age (1450 - 1850) blockades occurred quite often. This point is proven by the fact that there are quite few exceedingly cold winters on the records of this period, but also some extremely hot summers resembling the summers of 1994 and 1995.

4.3.1 The North Atlantic Oscillation

For Europe the weather is mainly "made" on the Atlantic. What happens over there to a large extent determines the weather and the climate of Europe.

And here another oscillition, the so called North Atlantic Oscillation, comes into play. Two extremes in winter can be distinguished:

  1. A strong Iceland low combined with a well developed Azores High.
    The so called North Atlantic Oscillation Index, the difference in air pressure between Portugal and Iceland, is high.
  2. A much weaker Azores High and higher pressures on the North Atlantic.
    Now the North Atlantic Oscillation Index is low.
In case 1. Strong South-Westerly winds bring warm air deep into Europe. It results in mild winters. Sometimes very deep and active mid latitude storms can develop South of Iceland. The winters of the late 80-ties and Early 90-ties were very good examples of this weather pattern. On 10 Januari 1993 an all time low barometric pressure of 912 mbar was recorded South East of Iceland. In the winter of 1990 England was hit by a couple of severe winter storms. The one on 25 Januari 1990 being a notable example.

In case 2. High pressure area's are present over the North Atlantic or Scandinavia. In case of high's over Scandinavia cold waves can engulve Europe. This doesn't mean that no severe weather can arrive from the sea. Northern Scotland in these cases can be hit by 'polar lows', very small hurricane like storm systems which form when very cold air moves over open sea. In Februari 1969 in such a polar low a gust of 118 Kts (218 km/h) was recorded on the airport of Kirkwall, Orkney Islands, during a violent snow shower. Sailors call this type of weather "The Barber". If you venture on deck you get "a nice shave" ...

North Atlantic Oscillation Index
North Atlantic Oscillation Index anomalies.

In The figure above the North Atlantic Oscillation Index anomalies for the winters of 1865 - 2015 are shown. Very striking are the low values in the 60-ties and the very high values in the late 80-ties and 90-ties up to 1995. In the years 1961 - 1965 England experienced 5 years with a "skating Christmas". The 1963 winter was an historic winter. In the late 80-ties and 90-ties the winters, in general, were mild. The winters of 1989 and 1990 were, together with the recent 2007 and 2014 one, "subtropical".

This makes it difficult to prove the point that these winters are a harbinger of the present warming of the climate on Earth. The very warm winters were caused by a "high North Atlantic Oscillation Index" weather pattern. The cold one's (e.g. 1963, and more recently 1979, 1985, 1986, 1987 and 1996) by a low "NAOI" weather pattern.

The present warming of the climate on Earth can not easily be proven by means of local observations and conclusions. Weather tends to slap about from one extreme to the other opposite extreme over the World. While we had balmy weather in Europe during the 1998 New Year, people were dying of hypothermia in Bangla Desh.
In the winter of 1996 NW Europe had a cold winter. The West coast of Greenland had one of the mildest winters of the century.

Recently in a report from the KNMI, The Netherlands, ("De toestand van het klimaat in Nederland 2003"), it was stated that there are some indications of a link between "high NAOI" weather patterns and the present global warming trend.

4.4 Variability of the climate; some concluding remarks

In three occasions, dated at roughly 115.000, 90.000 and 70.000 years ago very rapid climate changes took place in and around the north Atlantic Ocean. The switch from a level comparable with the present prevailing temperatures to ice age climax temperatures appears to have taken place within thousand years, and possibly even within a century. A switch "back to normal" took place 2000 to 5000 years later and the change was equally rapid.

A more recent occurrence of this type took place 12.700 years ago. (The Younger Dryas cold wave) The temperature in western Europe suddenly dropped from a level comparable to the present level by 5 °C within fifty years. Small glaciers reappeared in the Lake District in England. After about 1200 years the climate recovered from this cold spell. This also was a very rapid switch.

In North America some of the climate changes were so quick that they fall within the error margins of Carbon-14 dating.

These events in the past suggest the possible existence of "quantum steps" in (persistent) weather patterns and thus the climate. The once common picture in which climate changes were events which take place very gradually has been replaced in the last 20 years by a picture in which climate changes can take place "almost overnight".

5. Some personal notes; food for thought

Recognition of a new trend or a lasting change of the climate constitutes a difficulty for policy makers. From the four previous chapters one can conclude that the total picture is not a simple black and white one. Climate developments are always to an appreciable extent obscured by short term variations. Weather tends to slap about from one extreme to the opposite extreme from year to year.

On the other hand one may use the metaphor of "the Indian summer": In spite of autumn the weather is like in summer. But still, we all know that the winter inevitably will come.

The evidence is mounting that human activities will make the world (somewhat) warmer. We still have a great uncertainty of to what extent the warming will take place and where its effects will be felt. But one should bear in mind that the great population growth in this century took place under very favourable climatic conditions. Any climate change, warmer or colder, might mean great difficulties to ensure an adequate food production.

The amount of CO2 in the atmosphere inevitable will rise in the coming decades. A modest rise in the standards of living in countries like India and China will imply a big increase in fossil fuel consumption. In a report of the World Energy Council it is expected that, even in case of maximum policy support for development of sustainable energy, the world will still be dependent on fossil (hydro)carbon(s) for over 80% in 2020.

Against this background it may be wise to accelerate developments towards lower energy intensive economies in the developed world. This helps to slow down the increase in greenhouse gases in the atmosphere. It also "buys extra time" for the conversion to an economy based on sustainable energy sources. Besides this many "no regret" actions can be taken; E.g. simply repairing leaky natural gas pipelines and installations might help a great deal!

Besides warming of the climate also extreme weather variations deserve attention. The relentless growth of the world population has made societies much less flexible. A succession of catastrophic cyclones into the Ganges-Bramahputra delta can not be countered by "simply" moving 50 million Bengali's into India. Assuming that it will be "business as usual" anyhow in future, although tempting for politicians with an horizon which does in general not reach beyond the next elections' date, might be courting disaster.

I think that policies should try to anticipate more than today against possible climate changes and extreme weather events. E.g.: policies to ensure sufficient food production all over the world, and not going astray by introducing 'high yield but somewhat vulnerable' crops. Or to put it differently from a Dutch perspective: If you say that dikes are expensive, try a flood!

Evert Wesker, 20 January 1996
Small revisions and additions, 20 August 1996, 6 April 1997, 23 December 1997, 31 January 1998, 22 August 1998, 23 December 1998, 22 October 1999, 14 January 2000, 16 July 2000,
3 February 2001, 8 August 2001, 6 February 2002, 29 May 2003, 27 August 2003,
3 November 2004, 18 August 2005, 1 December 2005, 23 December 2006, July 1st 2007, November 25th 2009, May 28th 2010, January 23rd 2011, December 26th 2012, December 12th 2014 and September 1st 2015


Some literature:

  • C.J. de Jager en E.P.J. van den Heuvel, "Onstaan en Levensloop van Sterren", Zutphen 1972, (in Dutch only)
  • A.C. Phillips, "The Physics of Stars", Chichester 1994
  • J.K. Beatty and A. Chaikin (ed.), "The New Solar System", Cambridge (USA, Mass.) 1990
  • P. Francis, "Volcanoes, A Planetary Perspective", Oxford 1993
  • C.J. van der Ham, C.G. Korevaar, W.D. Moens en P.C. Stijnman, "Meteorologie en Oceanografie voor de zeevaart", Baarn 1990, (in Dutch only)
  • C.D. Ahrens, "Meteorology Today, an introduction to weather, climate and the environment", St. Paul (USA, Minn.) 1991
  • T.H. van Andel, "New Views on an Old Planet, a history of global change", Cambridge (USA, Mass.) 1994
  • H.H. Lamb, "Climate, History and the Modern World", London 1995
  • J.T. Houghton et. al., Climate Change 1995, (published for the IPCC), Cambridge (USA) 1996
  • D.V. Hoyt and K.H. Schatten, "The Role of the Sun in Climate Change", Oxford 1997
  • N. Wells, "The Atmosphere and Ocean", Chichester 1997
  • M.L. Salby, "Fundamentals of Atmospheric Physics", San Diego 1996
  • J.M. Wallace and P.V. Hobbs, "Atmospheric Science, an introductory survey", San Diego 1997
  • Thomas J. Crowley, "Causes of Climate Change Over the Past 1000 Years", Science, Vol. 289 (14 July 2000), pp. 270-277     (go up again)
  • J.T. Houghton et. al., Climate Change 2001: The Scientific Basis, (published for the IPCC), Cambridge (USA) 2001
  • KNMI, "De toestand van het klimaat in Nederland 2003" (in Dutch)

Opm: Een lezing over dit onderwerp, zowel binnen als buiten Shell, is reeds enkele malen door mij gehouden.
Rem: A lecture on this subject, within and outside Shell, has been given by me a couple of times already

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