Holocene land use change in impacts on

climate

Joy Singarayer

Modern land use change

Modern land use change

Global agriculture

Land use change impacts

• Changes in atmospheric

composition (CO2, CH4 etc.)

• Biogeophysical changes at

the surface

Why look at our ancestors’ impacts?

• Defining ‘the Anthropocene’ and its start

• Long term human disturbance of carbon cycle/climate

• Important for model (climate, vegetation) evaluation

• Defining baselines for climate or ecosystem conservation

The Anthropocene

• Human impacts on environment outweigh natural

• Nuclear testing era? Industrialization? Or before?

• Ice cores and other records show marked deviation from

the last 800,000 years

The Early Anthropogenic Hypothesis

• Ruddiman’s (2003) grand

idea

• Humans have had a

measurable impact on

GHGs and climate since

~8000 years ago

Ruddiman, 2013

The Neolithic revolution

• From foraging to farming

• Proto-farming: Natufians, 13 kyr – 9kyr BP

The Neolithic revolution

• Independent centres of origin

• Rapid spread (people, technologies, culture)

• Sedentism: population aggregation, social hierarchies, technological advances etc.

Pre-industrial perspectives

Rice irrigation Chuodon (6280BP)

Çatalhöyük, Turkey (9500-770BP)

Minoans, Crete (5000-3500BP)

Egypt Old Kingdom (4700-4200BP) Great Wall (started 2700BP)

Roman Empire (2100 – 1600BP)

10kyr 8kyr 6kyr 4kyr 2kyr 0kyr

Hypothesis testing

Testing with models

• E.g. Joos et al (2004) using veg/carbon models

• Mechanisms: SST change, carbonate compensation, terrestrial uptake, coral buildup

Evidence for Methane

Ruddiman, 2003

Testing with models

Testing with models

• Model captures last glacial cycle variation

• Late Holocene methane rise :southern tropics source

• Suggests smaller role for humans Singarayer et al., Nature, 2011

Caveats

1. Carbon/methane models often driven by

climate models forced with observed GHGs

2. Potentially large anthropogenic carbon

source could be masked by natural sink

(e.g. peatland accumulation)

3. Large uncertainty in Holocene land use

reconstruction

Global land use reconstructions

• Based on population density, land suitability,

and per capita use - assumptions

Kaplan et al., 2010

Global land use reconstructions

• Archaeological constraints and pollen etc.

REVEALS (Sugita et al)

Including land use estimates

• High resolution record of interpolar gradient

• Constrains latitudinal distribution of emissions

• Suggests natural and human emissions necessary to explain last 2.5kyr

Mitchell et al., Science, 2013

Constraining carbon budgets

• Using CO2 and carbon isotopic data

• Suggest humans become significant driver in last

3000 years

• Highest estimates of early land use not supported

Stocker et al, 2017

What about the biogeophysical impacts?

• Use global climate model, HadCM3

Climatic impact of Holocene land use

5000BC

4000BC

3000BC 2000BC 1000BC

0AD 1000AD 1850AD

• Run model with/without agricultural development 500yr mean

• Annual temperature anomalies up to 2 degrees C

• Summer temperature anomalies up to 5 degrees C

Smith, Singarayer et al, 2016

JJA temperature impacts

Precipitation anomalies

DJF JJA

ITCZ southward shift

in all seasons

Monsoons decrease

in intensity

Temporal evolution

-10 -2 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 2 10

Temperature anomaly, 6kyr – PI (°C)

A B C

D E

-10 -2 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 2 10

Temperature anomaly, 6kyr – PI (°C)

A B C

D E

Surface Air Temperature 6ka BP – 0ka BP (Preindustrial) No land use With land use

Bartlein et al., 2013, data

How far back is our influence significant?

• Global impact of agriculture on climate

potentially extends back several millennia

• Biogeochemical and biogeophysical factors

possibly opposing impacts

• Uncertainties in land use estimates

• Prior to the Holocene…?

Pleistocene impacts

• Out of Africa migration

10-30 k years

~150 k years

~60-70 k years

~40-60 k years

~40 k years

Pleistocene impacts

10-30 k years

~150 k years

~60-70 k years

~40-60 k years

~40 k years

Pleistocene megafauna extinctions

• 97 of 150 genera extinct between 50-10kyr

• Natural or human induced

Koch and Barnosky, 2006

Potential impacts of megafauna loss

• Tree expansion (CO2 sequestration, albedo,

evapotranspiration)

• Altered fire regimes

• Seed dispersal (community composition)

• Ecosystem structure and function

• Nutrient dispersal

• Reduced methane emissions

Prehistory of tropical forests

Roberts et al., 2017

• Human impacts stretch into prehistory

• Local to global scale

• New records and models are enabling more

nuanced pictures to emerge

• What are the combined impacts?

• Implications for future land use policy?

Acknowledgements

• Paul Valdes, Jed Kaplan, David Richards, T

Davies-Barnard, Clare Smith

• Previous work funded by BBC, NERC,

University of Reading, Leverhulme Trust