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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
• 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
Testing with models
• E.g. Joos et al (2004) using veg/carbon models
• Mechanisms: SST change, carbonate compensation, terrestrial uptake, coral buildup
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
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
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
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 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
• 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?