HH Lamb–“Climate: Present, Past & Future–Vol 2”–In Review–Part II
By Paul Homewood
In Part I of the review of HH Lamb’s highly influential work, “Climate: Present, Past & Future”, we looked at climate history from the early Holocene up to 1900 AD.
This second part will review what Lamb had to say about 20thC climate, up to 1977, when the book was published, and what he believed the future had in store.
Again, I would remind readers that everything that follows is based on Lamb’s writings in this volume; any comments of mine will be within [ brackets ]. I would also point out that sections in italics are direct quotations from the book.
20th Century temperatures
According to Lamb
Air temperatures prevailing over most of the globe reached their maximum about the early 1940’s, and their subsequent fall to the levels maintained from 1965-75, or after, seems likely to be their longest continued downward trend since 1700.
He goes into more detail, using the Central England Temperature record.
(Figure 18.1) shows the history of temperatures prevailing in Central England from the mid 17thC to 1973. From about 1700 to around 1945, it is a one way story, a trend towards greater warmth, that was interrupted only by various shorter term fluctuations.
In consequence, the temperatures prevailing between about 1920 and 1960 represented a recovery to something near the level, which seems to have prevailed for a longer time in the early Middle Ages.
Lamb believed the CET was indicative of global temperatures.
From the parallelism with the curves representing widely separated regions of the earth, and the near agreement between the annual curve and the sequence of 5 year mean temperatures averaged over the whole world from 1870-1969, it seems that the changes in England of temperature have probably been a fair approximation to the average of the whole earth.
Effects of a cooler climate
Concerning the post 1940 cooling, Lamb comments.
Kukla & Kukla (1974) report that the area of snow and ice, integrated over the year across the Northern Hemisphere, was 12% more in 1973 than in 1967, when the first satellite surveys were made.
He then goes into a lot of detail about the effects of the changing climate of the 1960’s and 1970’s, summarising the changing precipitation patterns which resulted from a cooler climate.
- Greater concentration of the equatorial rains near the equator.
- Reduced rainfall and failures of the monsoons over the zones near 10-20o N and 12-20o S (and farther South).
- Much smaller changes over middle latitudes, where the most significant feature has been the very awkward type of variability from year to year, associated with the behaviour of blocking systems and meridional circulation patterns.
- Substantially increased downput (mainly snow) in the highest Northern Latitudes.
Lamb offers many examples of such changes.
Examples of the consequences of these features include a number of serious items besides the extremes of cold and warmth, drought and flood associated with the occurrences of blocking in middle latitudes.
The greater yield of equatorial rains since 1961 over the equator led to abrupt rises of the levels of the great lakes there, drowning harbours and much land.
But , far more serious were the droughts in the zones to the North and South. In the Sahel between 200,000 and 400,000 died in the drought of 1972-73.
In those parts of N and NW India, near the limit reached by the summer monsoon, Bryson (1973) has noted a corresponding effect, scarcely less threatening to the inhabitants than the 6 year drought from 1968-73 in the West African Sahel.
In the first quarter of the century, there was a severe drought in N and NW India every 3rd or 4th year. Then, as the Earth warmed up and the circumpolar vortex contracted, the monsoon rains penetrated regularly into Northern India, and drought frequency declined to 2 in 36 years, from 1925-60. But since 1960, with the cooling of the Earth and the southern movement of the subtropical high pressure areas, drought frequency has been increasing again and the probability may be now more than once a decade.
Bryson adds that if a drought frequency like that which prevailed at the start of the century were to occur now, with India’s population having increased by a factor of 4, the human and political consequences would be enormous.
[While the droughts referred to are well known, it is interesting to learn about the increase in blocking patterns, which Hansen and co would like to blame nowadays on warming].
Possible causes of climatic change
Intriguingly Lamb refers to a study by Schneider & Mass in 1975 and comments
As regards causation of the climatic change from 1600 – 1970, Schneider & Mass, while admitting evidence for effects of great dust producing volcanic eruptions and man’s output of CO2, were able to simulate most of the supposed course of global mean surface air temperature over the past 370 years, by use of a formula expressing variations of the solar energy available in terms of sunset numbers, using Kondratiev & Nikolsky’s relationship which gives maximum solar energy at sunspot number 80.
What the future held
How did Lamb and his fellow climate scientists see climate developing in the years after the book was published?
Because of the pressing need for climatic foresight, a number of scientifically based forecasts of the natural climate have been attempted already. Those known to the WMO Working Group Climatic Fluctuations, which reported in 1972, and a few independently derived ones collected since are listed in Appendix VI.
…….Nevertheless, despite the wide variety of physical and statistical approaches involved, there is a remarkable degree of agreement between the forecasts for the next 30 years collected in Appendix VI.
Appendix VI goes on to describe each of these forecasts, which Lamb then summarises as follows.
These forecasts have all been attempts at prognosis of the natural climate. They should all be subject to the provisos that the tendency of the climate is not affected by :-
1) Any great outburst of volcanic activity.
2) Any new impact of human activity.
[ This is important to understand. Existing impacts from human activity were already built into the assumptions; only if such impacts increased or changed, would the forecasts be affected.]
Lamb then goes on.
There is a considerable measure of agreement between the 24 forecasts listed. Expectation of a trend towards colder climates with weakened general atmospheric circulation from 1950 or 1960 onwards seems to have been well verified by the actual weather to date.
Most forecasts expect this regime to continue into the 21st C , possibly into the second half of that century, in some cases with a further sharp cooling about 1980, and somewhat easier conditions for a time in the first half of that century.
The Japanese global survey and forecast usefully stresses the increased variability and incidence of extremes of temperature and rainfall in recent years.
The reference to the Japanese study, (Japanese Meteorological Agency – 1973), intrigued me. It was set up “In response to public demand for information about the apparent increase in incidence of exceptional weather in recent years”. It was based on a study of 150 stations across the world and found, that in terms of departure from the previous 30 year average:-
1) In the case of monthly mean temperatures, anomalies [i.e. warmer and/or colder] had increased with each successive decade since 1940, with a predominance of departure towards the low temperature side. From 1961 on, the frequency of significantly cold months was more than double that of significantly warm months.
2) There was no noteworthy variation in the occurrence of precipitation anomalies until 1960, but in the later years their frequency was very significantly increased with cases of deficient rainfall about 1.6 times as frequent as cases of excessive precipitation.
In Part III, we look at the impact of man on climate change.