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The Royal Society of Edinburghin association with the International Centre for Mathematical Sciences

Climate Change During the Last 10,000 Years: Reconstructions and Uncertainties

Professor Heinz Wanner, University of BernProfessor John Haslett, Trinity College, Dublin

Professor Gabriele Hegerl, University of Edinburgh

Wednesday 14 July 2010

Report by Andrew Schurer

As part of the 11th International Meeting on Statistical Climatology (11 IMSC), in partnership with theRoyal Society of Edinburgh, ProfessorHeinz Wanner and Professor John Haslett explained to a

packed audience details about climate change during the last 10,000 years. The speakers addressedboth the factors affecting the climate and the problem of uncertainties in our knowledge, as well as thedifficulties facing scientists in communicating the nature of this uncertainty to others.

The first speaker was Professor Heinz Wanner, an expert in palaeoclimate reconstructions, whodiscussed the key factors affecting past climate. He showed how the climate has changed over thepast 20,000 years, warming from an initially cold ice age period until the present interglacial period,called the Holocene, by showing a graph of temperature reconstructed from observations made fromGreenland ice cores. The warming trend shown was not smooth however, and after an initial strongwarming, the temperature fell rapidly again back to ice age conditions for a short period of 600700years, known as the Younger Dryas, caused by melt-water fluxes from the Laurentide ice sheetinfluencing the Gulf Stream. After another period of warming during the Holocene, the climate becamecomparatively stable, albeit with observable temperature fluctuations in the mid-latitudes and polarareas, and precipitation fluctuations in the equatorial areas. Professor Wanner mentioned that it hasbeen proposed that this relative stability could have played a crucial role in the development ofhumankind.

A strong factor in the Holocene climate has been the slow melting of the ice sheets, in particular thegradual melting of the Laurentide ice sheet over northern North America. This melting resulted in astrongly rising sea level until about 7000 years ago. Also important is the melting of sea ice. Sea icereflects light, as can be clearly seen from satellites in space, as opposed to the sea which absorbsand therefore appears black; consequently the quantity of sea ice is hugely influential for our climate.Since we have no precise information about its quantity, this represents one of the greatestuncertainties in our understanding of the Holocene climate.

One of the key drivers of climate during the Holocene is the change in solar radiation due to variationsin the Earth's orbit round the Sun. Over the last 10,000 years, the summer solar insolation in theNorthern Hemisphere has declined by approximately 40Wm-2, a significantly large amount.Conversely, the summer insolation in the Southern Hemisphere has been gradually increasing. Thesechanges have had a large influence on the climate of the Holocene, which can be roughly divided intofour periods. The first, temperate period is characterised by the Northern Hemisphere ice sheet melt,and a successive warming due to the stronger solar insolation. The middle, warm, period, alsoreferred to as the optimal period, has the warmest temperature, also attributable to warming from theocean, which integrated the high Northern Hemisphere summer insolation. The third cool periodshows a gradual cooling due to a drop in Northern Hemisphere solar insolation. Finally, over the last100150 years, this temperature trend has been reversed, with the temperature showing a warming.This change in the energy input has other effects, most notably a change in the position of theIntertropical Convergence Zone (ITCZ), which is an area of high convection near the Equator. The

solar insolation change has caused the ITCZ to move southward which, in turn, has caused aweakening of the monsoons over the last 6,000 years. Also important is the effect of the change inenergy input on glacier dynamics. Observations of tree-rings, moraines and sediments show that over

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the last 6,000 years glaciers have been advancing in the Northern Hemisphere while they possiblyretreated in the Southern Hemisphere.

As well as these long-term trends the observational record also shows periods of rapid climate changewhere the temperature rapidly cools, referred to as Bond cycles, identified by looking at sediments inthe Atlantic Ocean. It is thought that some of the early events may be due to changes in thethermohaline circulation; however the cause of the later events is still highly uncertain.

Looking at the last 1,000 years, temperature reconstructions show that on average temperature washigher during the Medieval Warm Period (MWP) and lower during the Little Ice Age (LIA). However, ifwe look at maps showing temperature anomalies for these periods, the situation is more complex. Itcan be clearly seen that while many areas were warmer during the MWP, there were also areas whichwere actually colder; equally during the LIA, although many areas were colder, there were also areasshowing increased temperature.

Studies show that over the last 1,000 years, volcanoes have been strongly influential on the climate,having a cooling effect; in addition, changes in solar activity have also had some influence. Over thelast couple of centuries, the human (anthropogenic) influence has been rapidly growing due to theburning of fossil fuels. Models including these forcings have been developed and the effect of theseforcings, both in the model results and in observations, can be seen. These models show us that theanthropogenic forcings outweigh the natural forcings over the most recent period, with a high degreeof certainty. It is a still an open question as to what the climate would have been like now if theanthropogenic influence had been removed, although it is thought that it would have been similar toeither the LIA or to conditions experienced around 1900.

Professor Wanner concluded his talk by discussing the impact of recent climate change on mankind.People in the richer developed countries in the north such as Scotland and Switzerland (his owncountry), might be able to afford to adapt to the changes, but people in the developing world,particularly in the areas predicted to get drier around Asia, Africa and Latin America, will face realproblems.

The second speaker was Professor John Haslett, a statistician from Trinity College, Dublin. Heprimarily addressed the issue of uncertainty and how this can be communicated. How much do weknow these things? How much of them do we know?

Professor Haslett showed how it is possible for scientists to communicate uncertainty to otherscientists by the use of error bars. But how does the media do uncertainty? He demonstrated this byuse of an example. Data from Glendalough, a glacial valley in Ireland, indicated a period of rapidclimate change in the past. About 10,000 years ago, there was a period of rapid warming, when theclimate went from an extremely severe cold climate to one not unlike todays climate. No one knowsfor sure over how short a period this warming took place; however the scientists are reasonablycertain that it probably happened over a period of less than 30 years, and maybe even as short asseven. A national newspaper took up this story and illustrated it with a large picture showing, on oneside, a current photo of Glendalough and, on the other, a photo of the same location with glaciers and

ice superimposed, with a title How Wicklow went from Arctic to mild in 7 years ....

Scientists have investigated this episode by collecting many different types of data, called proxies,from around Glendalough; for example pollen found in mud cores. It is then possible for scientists tocalculate many different scenarios that are consistent with the data. This then allows for plots whichshow statistical uncertainty, and make possible statements such as ninety five percent of modelswhich fit the data lie within a certain range. Rapid climate change such as this has been found tooccur frequently in the past, and is an important, significant and interesting phenomenon. When theaudience were asked whether the photos from the newspaper were more memorable than thescientific plots, the majority picked the photos, thus illustrating the problems in communicatingscientific ideas and uncertainty to the public.

Professor Haslett explained that although mathematicians can do proofs, statisticians, scientists andeven the law struggle with them, (although in many cases it is possible to disprove an idea byproposing a single counter example). This then means uncertainty is involved. It is possible to study

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our uncertainty statisticians make a living out of it, but it is much harder to communicate it. Thewebsite Understanding Uncertainty, set up by David Spiegelhalter, the Winton Professor of the PublicUnderstanding of Risk, does a good job of this.

The talk concluded by discussing the large discrepancy between the scientific view on climate changeand the public. The scientific community, almost to a man, is united in believing one thing, but publicopinion instead is divided. Communicating the science to the public is an important and difficult taskand one in which what the press say is probably more important than what the scientists say.

QUESTIONS

ProfessorGabriele Hegerl, the Chair of the 11 IMSC Scientific Programme Committee, joined the twospeakers to take questions from the audience.

One audience member asked a question about pollen analysis, and whether the spatial movement ofplants is taken into account. Professor Haslett answered that it depends on the nature of the analysis,and in particular on the timescale of interest and the plant in question; but agreed that there are somepeople studying this in great detail.

Another person said that he was heartened to hear so much talk about uncertainty and felt that this isnot normally communicated enough. Professor Hegerl said that the IPCC reports have a carefulexplanation of uncertainty and the problem is with the interface with the media. Professor Haslettpointed out that it is not entirely fair to blame the press, since it is their job to get people to buy andread their articles and that it is a huge challenge to communicate uncertainty. Professor Wanner saidthat it is important to explain uncertainties and to be modest and open minded.

One person asked about the difference between statistical inference models and physical models andhow important they are, and in particular how physical the solar models are. Professor Wanneranswered that we need both, and explained that reconstructions of solar forcing are based onisotopes, and that there are only direct measurements for calibration from 1974, leading touncertainties. Professor Hegerl agreed that both kinds of models are important and said that bringingstatisticians and climatologists together is the one of the aims of the ongoing conferencein Edinburghon Statistical Climatology. Professor Haslett added that it is not a case of theory ordata statisticiansuse them both and that statistical models are about more than just spotting patterns; they are alsopredictive.

An audience member asked about the relationship between tree rings and temperature, and whetherthe statistical linear regressions performed are correct. Professor Haslett answered that this is not ayes/no question, and that it is not something to believe in or not believe in, rather it is more importantto look at usefulness and what we can learn from tree rings and we can quantify the uncertainty.Professor Hegerl added that the temperature reconstructions are based on many different kinds ofproxies, and although tree rings are important, they are not the only evidence; for example, bore holesform a completely separate line of evidence for past climate, and the failure of scientists to

communicate this to the public has been a problem.

One person questioned the link between carbon dioxide and temperature. Professor Wanneranswered that there are still uncertainties concerning past climate, and much work is being done, butthere is a growing body of evidence to support the link. Professor Hegerl added that the directradiative response to CO2 is well known; however the feedbacks are less certain. Scientists are tryingto constrain this using many lines of evidence and, although there is still uncertainty, there is strongevidence showing that the feedbacks are positive.

A Vote of Thanks was proposed by John Toland, FRS FRSE Professor of Mathematics at theUniversity of Bath.

Opinions expressed here do not necessarily represent the views of the RSE, nor of its Fellows

The Royal Society of Edinburgh, Scotlands National Academy, is Scottish Charity No.SC000470