Land-cover and Ecological Changes in Tibet During the Last 15,000 Years John Birks University of...

Land-cover and Ecological Changes in Tibet During the

Last 15,000 Years

John BirksUniversity of Bergen

University College LondonUniversity of Oxford

Bjerknes Centre 10 Year Anniversary ConferenceClimate Change at High Latitudes

September 3-6 2012

Land-cover, environmental, and ecological changes have recurred over temporal scales from annual/decadal to multimillennial.

Changes organised into a nested hierarchy of time scales (and also spatial scales).

Such a hierarchy puts climatic and ecological dynamics of past 15,000 years into a broader temporal context.

Many factors have driven ecological dynamics in last 15 kyr. All derived, we assume, ultimately from climate dynamics or interactions with prevailing climate.

Fire determined by fuel, ignition, and fire weather. Fuel influenced by hydroclimate (via primary production, vegetation structure, decomposition, dryness of surface and crown fuels), as are ignition (lightning, human activity), and fire weather (temperature, humidity, wind velocity, turbulence).

Hydroclimate represents a climate-system response to various forcings

external – insolation, CO2, dust

internal – circulation patterns influencing transport of heat and water vapour.

Linkages between climate forcing, hydroclimatic response, and ecosystem response all comprise a nested hierarchy.

Jackson et al. (2009)

Multiple time scales of climate forcing, hydroclimatic response, and ecosystem response for Yellowstone National ark, Wyoming. See major changes in hydroclimate and ecological responses.

Donald Rumsfeld, US Secretary of Defense 12 February 2002

“There are known knowns. There are things we know we know. We also know there known unknowns; that is to say, we know there are some things we do not know. But there are also unknown unknowns. There are things we do not know we don’t know.”

Rumsfeld’s epistemological analysis reveals three classes. Addition of a fourth – unknown knowns – gives us a useful scheme for identifying and addressing sources of uncertainty in palaeoclimate and palaeoecological research in terms of congnisance, ignorance, knowledge, and uncertainty.

Cognisance (or awareness)

Ignorance

Knowledge Known knowns

Unknown knowns

Uncertainty Known unknowns

Unknown unknowns

Jackson (2012)

Known knowns – more or less solid facts, observations, or inferences based on best available evidence (e.g. Younger Dryas)

Known unknowns – sources of error and uncertainty. Try to minimise and estimate. Things we know but that we don’t know adequately (e.g. prediction errors)

Unknown unknowns – represent ignorance, often lead to major scientific surprises (e.g. Dansgaard-Oescheger and Heinrich events). Constantly becoming known.

Unknown knowns – things we know so well that we are no longer explicitly aware we know them (e.g. hidden and unquestioned assumptions, old literatures now ignored or not available in electronic format – “ignorance creep”).

Consider some aspects of land-cover and ecological change on the Tibetan Plateau in last 15 kyr based on our work done in last 10 yrs mainly with Ulirke Herzschuh (Potsdam)

What about cognisance, ignorance, knowledge, and uncertainty for Tibet in 2002?

Known knowns +

Known unknowns +

Unknown unknowns (+)

Unknown knowns -

15,000 Years

COHMAP (1993, Kutzbach (1981), Kutzbach & Otto-Bliesner (1982)

Kutzbach & Ruddiman (1993)

Boundary conditions for COHMAP simulation with the Community Climate Model (CCM) for the last 18,000 yrs

Values at 9000 yrs for precession and obliquity resulted in 7% more global average solar radiation. CCM simulation gives intensified continent-scale Northern Hemisphere monsoon circulation. Driven by northern summer insolation variations at the 23,000 yr precession period. Support for this comes from

CH4 signal in ice-cores where there is a dominant 23,000 yr signal, and

δ18Oair variations after ice-volume is allowed for have strong 23,000 yr variations, resulting from changes in global biomass (‘Dole’ effect) driven by monsoons

Known known

What were the hydroclimatic responses?

Lake-level synthesis (75 lakes) from central Asia for last 50,000 yrs (Herzschuh 2006).

Known knowns but becoming unknown knowns

High effective moisture from most areas dominated by the Indian monsoon (e.g. Tibetan Plateau) 11,000-7000 (5000) yr BP

What were the ecological and land-cover responses?

Annual precipitation 250-300 mm yr-1

75% falls June-September as part of the Indian summer monsoon

Vegetation around Qinghai Lake - 2006

Vegetation moisture limited – dry alpine-steppe

Qilian Shan Mountains – up to 5803 m

Vegetational history

How to interpret it? Unknown unknown

Regional-scale pollen is a function of regional vegetation

Regional vegetation is a function of regional climate

Regional-scale pollen is an indirect function of climate and can be used to reconstruct past climate

Stages in deriving and applying modern pollen-climate transfer function (based on unpublished diagram by Steve Juggins)

Vegetation types on the Tibetan Plateau

Strong modern pollen-modern precipitation signal (RMSEP = 62 mm, range 104-7673 mm). Provides quantitative basis for fine-resolution climate reconstruction

Qinghai Lake pollen stratigraphy

Climate reconstructions

Model simulations

CLIMBER run with vegetation changing with changing climate. Strongest reduction between 4000 and 6000 yr BP. Tree cover decreased from 25 to 8.5%. Known unknowns because of uncertainties

Major ecological and land-cover response, namely forest decline over much of Tibetan Plateau, to decreasing Indian summer monsoon activity at about 6000 yr BP. Clear hydroclimatic, ecological, and land-cover responses to regional-scale climate change.

Biomass changes since mid Holocene based on Earth System Models (Dallmeyer et al. 2011)

6 kyr BP Today

Forest 41.4% 28.3%

Shrub 5.3% 12.3%

Grassland 38.1% 42.3%

Other 15.2% 17.1%

Decrease of living biomass by 0.62 kgC m-2

Total biomass decreased by 1.9 kgC m-2

About 6.64 PgC released due to natural land-cover change

Superimposed on this broad regional picture of changes in Indian summer monsoon, increasing evidence for last 15,000 yrs of

Major drought episodes – 5.6-4.9, 4.4-3.9, and 2.8-2.4 cal kyr BP (Shen et al. 2008)

Cold events – 10.3-10.0, 7.9-7.4, 5.9-5.4, 4.8-4.2, 1.7-1.3, and 0.6-0.1 cal kyr BP (Mischke & Zhang 2010)

Developed modern pollen–leaf-area index (LAI) calibration function to reconstruct extent of past land-cover using remote sensing data (normalised difference vegetation index: NDVI) based on Advanced Very High Resolution Radiometry (AVHRR) (Herzschuh et al. 2010)

Unknown unknowns becoming known knowns and known unknowns

Highest cover changes at Qinghai Lake (forest steppe). Alpine steppe and high-alpine vegetation at Hurleq, Zigetang, and Koucha all very stable over last 9 kyr.

Evidence for abrupt change in last 15 kyr: ~11.5 and ~5.0-4.5 cal kyr BP and AD 1300. Little evidence for change in monsoon at 8.2 cal kyr BP. Possibly triggered by variations in other components of the climate system (Morill et al. 2003)

Unknown unknowns

Asynchronous evolution of Indian and East Asian summer monsoons. Indian Monsoon has maximum wet conditions in early Holocene. East Asian areas have maximum moisture in mid Holocene. Possibly due to strengthened Hadley Circulation over Tibetan Plateau in early Holocene resulting in subsidence in East Asian monsoon region (Wang et al. 2010)

Unknown unknowns

Atmospheric CH4 and monsoon activity (Zhou 2011)

Opposite trends in CH4 and monsoon index in late Holocene. CH4 increase a result of expansion of rice paddy fields Anthropogenic CH4

emissionsCH4 emissions from paddiesArea of rice cultivationEstimated CH4 emissions from paddies

Unknown unknown

What is the role of human activity in determining modern vegetation of

Tibetan Plateau?

Today contains world’s largest alpine ecosystem – Kobresia pygmaea grassland

Miehe et al. (2008)

Origins (Herzschuh et al. 2011)

1.Grazing by large herbivores (yaks, etc.) – almost no archaeological evidence. Known unknowns

2.CO2 changes – model Tibetan vegetation using BIOME4 model at 375 ppm CO2 (today), 260 ppm (early Holocene), and 650 ppm (future)

Propose that replacement by drought-resistant alpine steppe (adapted to low CO2) by mesic Kobresia vegetation since 7 kyr BP is response to CO2 increase in mid-late Holocene

Unknown unknowns

15,000 Years

Very much in area of unknown unknowns

Various problems

1. Poor chronologies

2. Few high-resolution studies3. Significant variability over last 1000 yrs,

possibly linked to onset of Little Ice Age, but poor chronological control limits detailed interpretation (e.g. isotopes, grain size, C/N ratios, alkenones) (Henderson & Holmes 2009)

4.Aquatic biological systems suggest no climatically induce ecological threshold have been crossed, with modest diatom responses to warming in last 2 centuries (Wischnewski et al. 2011)

5. Some hints of variations in winter and summer monsoon activity (Liu et al. 2009)

Intensified winter monsoon in grey correlate with Bond events O-2Monsoon changes at this scale may be forced by solar output and oceanic-atmospheric circulation patterns

6. Monsoon Asia Drought Atlas (MADA) contains reconstructions of dryness and wetness since AD 1300 (Cook et al. 2010) based on tree-rings.

Times in last millennium when monsoon failed and megadroughts resulted.

Asian and N American megadroughts in late 19th and early 20th centuries are linked to Pacific SST of El Niño-Southern Oscillation (ENSO). MADA shows that drought and wetness in monsoon region are spatially heterogeneous even though area is under the influence of one broad-scale circulation pattern.

Are they a response to anthropogenically climatic change or part of inherent monsoon spatial variability?

150-15 Years1. Glacial retreat in Tibetan Himalaya and Bhutan

Bhutan, 2009

extent of glacier 1921 2007

2. Increase in alpine grassland at high altitudes (e.g. >5000 m on Mt Everest) (Liu et al. in press)

3. Increased steppe vegetation cover and higher pollen production in last 160 yrs, possibly due to increased monsoonal activity (Herzschuh et al. 2006)

4.Elevational shifts of shrubs (e.g. Hippophae tibetana) on Everest (Wang et al. in press)

Many unknown unknowns

What about cognisance, ignorance, knowledge and uncertainty in 2012?

2002 2012

Known knowns + ++

Known unknowns + ++

Unknown unknowns (+) +

Unknown knowns - (+)

Given present state of knowledge, scenario planning as used in business and finance may provide an effective basis for using diverse scientific findings from the Tibetan studies (and many others) in decision-making.

Scenario-planning uses a combination of scientific input, expert opinion, and forecast data to develop alternative scenarios for future.

Contracts with attempts at developing ‘precise’ quantitative assessments of future conditions that may be hindered or biased by many compounding uncertainties and qualifications.

Science data

Expertise

Forecast data

Scenario A

Scenario B

Scenario C

So much to do and to discover. Tragic that access to Tibet is now so limited unless you have ‘friends in ‘right’ places’ and none of those are native Tibetans!

Acknowledgements

Ulrike Herzschuh, Alfred Wegener Institute, Potsdam (Marie Curie Fellow in Bergen 2005)

La Duo, Lhasa University (First Tibetan PhD in Biology 2008)

Hilary Birks

Geritt Lohmann

Cathy Jenks

Stefan Mischke