Guest post by Paul Homewood (reposted from his blog Not a lot of people know that please visit and bookmark) Part 1 of a three part series.
Hubert Lamb was one of the leading climatologists of his time, indeed described in one obituary as the greatest. He spent most of his career at the UK Met Office before founding and becoming the first director of the Climatic Research Unit. He wrote many books, but perhaps “Climate: Present , Past & Future” was the most significant. Here we review Volume 2, amounting to 836 pages, which particularly looks at climatic trends over the centuries.
Originally published in 1977, the volume offers great insights into the thinking not only of Lamb himself, but also of many of his peers. Not only does Lamb give us the benefit of his own work and experience, but much of his research is into work carried out by a host of other scientists of his time and earlier.
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.
Climate during the Holocene
The Holocene begins around 10000 BC, at the end of the last Ice Age, and continues to the present. In this section, we will look at the period leading up to the Medieval Warming Period.
How did temperatures in this first part of Holocene compare with today’s and what confidence can we have in their accuracy and extent? Lamb presents a good deal of evidence to suggest that, for much of the period, temperatures were warmer than now. For instance he presents much evidence from glaciers.
It was after 2000-1500 BC that most of the present glaciers in the Rocky Mountains south of 57 o N were formed and that major re-advance of those in the Alaskan Rockies first took place.
And at their subsequent advanced positions – probably around 500 BC as well as between 1650 and 1850 AD – the glaciers in the Alps regained an extent, estimated in the Glockner region, at about 5 times their Bronze Age Minimum, when all the smaller ones had disappeared.
Treeline studies, including Southern Hemisphere sites, paint a similar picture. Quoting a study by Markgraf in 1974, which encompassed the Alps, Carpathians, Rockies, Japan, New Guinea, Australia, New Zealand, East Africa and the Andes, Lamb writes :-
Summer temperatures in these regions were 2 C higher than now in the warmest postglacial times (around 5000 BC).
He then quotes a similar study by Lamarche in 1973:-
Study of the Upper Tree Line on the White Mountains in California, similarly indicates warm season temperatures about 2C higher than today all through the warmest millenia, from before 5500 BC until about 2200 BC.
[Many recent studies in Baffin Bay, Greenland and Iceland come to similar conclusions, i.e. that for much of the Holocene, temperatures were higher than now and also that the Little Ice Age was probably the coldest period in the last 10000 years.]
What about the cooler periods Lamb mentions?
He describes this as the “Sub Atlantic Period” from about 1000 BC.
Glacier advances, changes in the composition of the forests, and the retreat of the forest from its previous northern and upper limits, indicate significant cooling of world climates, its start being detectable in some places (e.g. Alaska, Chile, China) from as early as 1500 BC.
In Europe, the most marked change seems to have been from 1200-700 BC. By 700-500 BC, prevailing temperatures must have been about 2.0C lower than they had been half a millenium earlier, and there was a great increase of wetness everywhere north of the Alps.
Another aspect of the centuries of colder climate around 500 BC in NW Europe was evidently their storminess. There was perhaps a final climax of the first of these epochs of marked storminess in the great North Sea storm, or storms, about 120-114 BC, which altered the coasts of Jutland and NW Germany in a great sea flood, “The Cymbrian Flood”, which set off the migration of the Celtic (Cymbrian) and Teutonic peoples who had been living in these areas.
The probable course of prevailing temperatures in Europe and the Far East has been presented in Fig 16.22. [Not shown]. In both regions, the last few centuries BC register some general rise in temperature, representing a recovery from the coldest conditions of the onset of the Sub Atlantic climatic period, which had culminated in great glacier advances in the Alps (HEUBERGER 1968), at various times between about 900 and 300 BC, and apparently a lower snowline in the high mountains of Lebanon and elsewhere in the Near East and Equatorial Africa.
Lamb goes on to describe how temperatures recovered in the period leading up to the MWP.
There was a gradual fluctuating recovery of warmth in Europe over the 1000 years after 600 BC, particularly after 100 BC, leading to a period of warmth and apparently high sea level around 400 AD. [We would recognise this as the Roman Warming Period].
The Roman agricultural writer, Saserna, wrote that in the last century BC, cultivation of the olive and vine were spreading further north in Italy, where in the previous century, winters had been too cold for transplants to survive (WARNER ALLEN 1961).
After some reversion to colder and wetter climates in the next 300-400 years, sharply renewed warming from about 800 AD led to an important warm epoch.
Medieval Warming Period
Lamb had no doubt that the MWP was real and global.
Evidence already cited at various places in this volume suggests that, for a few centuries in the Middle Ages, the climate in most parts of the world regained something approaching the warmth of the warmest postglacial times.
He cites many examples in Europe and North America which indicate warmer temperatures than now.
- The northern limit of vineyards with a long history of cultivation lay some 300-500 km north of the limit of commercial vineyards in the 20thC.
- In many parts of England there are traces of medieval tillage far above anything attempted in the present century, even in wartime: up to 350 m above sea level on Dartmoor and 320 m in Northumberland.
- The tree line and upper limits of various crops on the hills of Central Europe were higher than today.
- Mining operations at high levels in the Alps which had long been abandoned were reopened, and water supply ducts were built to take water from points which were subsequently overrun by glaciers and are in some cases still under ice.
- In Central Norway the area of farming spread 100-200m up valleys and hillsides from 800 – 1000 AD, only to retreat just as decisively after 1300 AD.
- The Viking colonies in W and SW Greenland were able to bury their dead sheep in soil that has since been permanently frozen.
- It was also a warm period generally from N Mexico to N Canada, where forest remnants between 25 and 100 km north of the present limit have been found, radio carbon dated between 880 and 1140 AD.
[Recent studies, that have found evidence that Alaskan glaciers were smaller in the MWP than now, tie in with this North American conclusion.]
But as Lamb makes clear, the warming was not limited to the Northern Hemisphere.
- Holloway (1954) has reported evidence from the forest composition of a warmer climate in South Island, New Zealand, between about 700 AD and 1400 AD, than in the centuries before and after.
- On the coast of East Antarctica, at Cape Hallett, a great modern penguin rookery seems, from radiocarbon dating tests, to have been first colonised between about 400 and 700 AD, presumably during a phase of improving climate, and to have been occupied ever since.
Little Ice Age
Lamb has this to say about the extent of the LIA.
The period we are discussing has been dubbed “The Little Ice Age” because, not only in Europe but in most parts of the world, the extent of snow and ice on land and sea seems to have attained a maximum as great as, or in most cases greater than, at any time since the last major ice age.
Lamb also recognises that there were timing differences between the two hemispheres when he points out
On the whole the culmination seems to have come earlier in the NH, particularly in N America, the Arctic and China/Japan, and later in the SH, where the maximum advance of the glaciers in Chile seems to have been in the 18thC and the greatest extent of ice on the Antarctic Ocean may have been as late as around 1900.
There was, however, an important late climax of the Arctic sea ice around Iceland between 1780 and 1830, and many glaciers in the Alps reached their greatest extent towards 1850.
He sums this period up very well.
The course of the climatic deterioration over 500 years from 1200 AD can quite well be traced by its effects under the following headings.
- Increasing spread of the Arctic sea ice into all the northernmost Atlantic and around Greenland, forcing the abandonment of the old sailing routes to Greenland, which had been used from 1000-1300 AD.
- Advances of the inland ice and permafrost in Greenland and of glaciers in Iceland, Norway and the Alps.
- Lowering of the treeline on the heights in Central Europe and the Rockies.
- Increasing wetness of the ground and spread of lakes and marshes in many places in North, West and Central Europe, and all over Northern Russia and Siberia.
- Increasing frequency of the freezing of rivers and lakes.
- Evidence of increasing severity of the windstorms and resulting sea floods and disasters by shifting sand.
- In the records of harvest failure.
- In the records and archaeology of abandoning crop growing, tillage and vineyards, abandoned farms and villages.
- In the incidence of disease and death among human and animal populations.
As to the causes, Lamb explains
It is reasonable to consider the whole sequence, from about the time of Christ, through the early medieval warm centuries and the cold climate that followed, to our own times, as an oscillation on the same time scale, and possibly of basically the same nature, as the Bolling & Allerod oscillations in Late Glacial Times, the Piora oscillation [around 3000 BC], and the Bronze Age and early Iron Age changes in the last 4000 years.
[For further information on the LIA, I would recommend Brian Fagan’s excellent book “The Little Ice Age].
In Parts II and III, we will look at how climate changed during the 20th Century, what the future had in store, and the impact of man on the climate.
I believe that the book is currently out of print, but a new publication is due out in December from Routledge.