School of Earth and Environment
INSTITUTE FOR CLIMATE & ATMOSPHERIC SCIENCE
Importance of carbon dioxide physiological forcing
on projected Amazonian precipitation change
Tom Richardson
[email protected]
Piers Forster, Tim Andrews + PDRMIP Team
Introduction
• Amazon rainforest is a key component of the climate system
• The vegetation and carbon balance are sensitive to changes in precipitation
• CMIP5 projections indicate reductions in precipitation over eastern Amazon
but there is considerable uncertainty and model spread
• Atmospheric forcing agents can affect precipitation directly through
adjustments as well as temperature dependent feedbacks
• Carbon dioxide can drive adjustments in precipitation due to reduced stomatal
opening of plants, which reduces evapotranspiration
• Evapotranspiration is a key source of atmospheric moisture over Amazon
region
• This study investigates how atmospheric forcing agents affect Amazonian
precipitation and by what mechanisms, to help understand projected
precipitation changes.
PDRMIP Data and Methods
PDRMIP data:
• Five forcing scenarios – 2xCO2, 3xCH4, 5xSO4, 10xBC, 2%SOL
• Adjustment and feedback components separated using fSST method
• Total precip response taken as mean change for years 51-100 in coupled runs
Local energy and moisture budgets:
Use local energy budget and moisture budget changes to understand precip
response.
𝐿𝛿𝑃 = 𝛿𝐿𝑊𝐶 − 𝛿𝑆𝑊𝐴 − 𝛿𝑆𝐻 + 𝛿𝐻 = 𝛿𝐿𝐻 + 𝐿𝛿𝑀
P - local precipitation
LWC - longwave radiative cooling
SWA - shortwave absorption
SH - sensible heat flux from the surface
H - the dry static energy flux divergence
LH - latent heat flux from the surface
M - moisture convergence
δ - represents a perturbation between climates
PDRMIP Total Precip Response to Forcing
• SO4 and Sol have less
effect on precip due to
counteracting
adjustment and
feedback responses
• CO2 causes large
reduction in precip over
central and eastern
Amazon mainly due to
adjustment
• BC causes large
reduction in precip over
all of Amazon due
mainly due to feedback
ECA region precip and EB response
• Negative adjustment
due to CO2 mainly due
to change in
partitioning of LH and
SH fluxes
• Negative feedback due
to BC mainly due to
circulation changes (H)
• Large spread in
feedback response due
to uncertain circulation
feedbacks
Isolating physiological effects of CO2
To isolate physiological effects we difference Amip and sstClim simulations:
Amip – do not include effects of CO2 on plant stomata
sstClim – do include effects of CO2 on plant stomata
Data:
Output from 12 CMIP5 models
Isolating physiological effects of CO2
• Physiological effects
account for almost all
drying over Amazon
• Physiological effects
also doubles the
inter-model standard
deviation
Simple Model
We estimate impact of CO2 on future Amazonian rainfall change for 2081-2100
using a simple model based on PDRMIP results:
𝛿𝑃 = 𝑅 × 𝐹𝐶𝑂2 + (𝛿𝑇 × 𝐻𝑆)
δP = change in precipitation
R = precipitation adjustment per unit TOA forcing
FCO2 = CO2 TOA forcing
δT = global mean surface temperature change
HS = hydrological sensitivity for Amazon
Projected Amazonian Precip Change for 2081-2100
relative to pre-industrial
• CO2
adjustment
dominates
projected
change in
simple model
• CO2 accounts
for projected
drying over
eastern
Amazon in
CMIP5
models
Conclusions
• Increased BC produces robust drying over much of Amazon region associated
with northward shift of ITCZ
• Increased CO2 drives reduced precipitation over Amazon region , particularly in
the east.
• Model-mean response to CO2 dominated by negative adjustment component
driven predominantly by physiological effects reducing evapotranspiration
• Simple model suggests projected drying over ECA region predominantly driven
by CO2 physiological effect – thus projected change independent of rising
temperatures
• However in individual models temperature driven circulation feedbacks can be
large, but due to little agreement, cancel out in mean
• Highlights importance of reducing uncertainties associated with vegetation
schemes