Transcript PPT - Atmospheric Chemistry Modeling Group

The Human Influence on Climate:
How much is known, What’s in store for us?
Loretta Mickley
Harvard University
CO2 concentrations, Mauna Loa
Some background: What do I do?
• Develop chemical models
of the atmosphere
• Analyze measurements of
chemical species
• Examine the link between
chemistry and climate
Calculated ozone concentrations in ppb at
the surface, annually averaged.
The earth’s energy budget: Incoming
+ sulfate
aerosol
+ soot
aerosol
Soot aerosol absorbs
incoming sunlight.
Sulfate aerosol reflects
sunlight back to space.
The earth’s energy budget: Outgoing
Infrared radiation = heat
Without greenhouse gases,
earth would be cold (0oF)
+ soot aerosol
What could change the energy budget (and climate)?
+ aerosol
+ aerosol
+ aerosol
(ice)
ocean
Concentrations of carbon dioxide and methane have risen
sharply since pre-industrial times.
Carbon dioxide: 33% rise
BW 5
The MetOffice. Hadley Center for Climate Prediction and Research.
Methane: 100% rise
What is radiative forcing?
Radiative forcing = calculated heating/cooling of atmosphere,
convenient yardstick. Units = W/m2
Radiative
forcing since
1760
IPCC 2001
How have global surface temperatures changed over the last
140 years?
SPM 1a
Jones et al 2001, Folland et al. 2001
Variations in surface temperature over the Northern Hemisphere
for the past 1,000 years
Large uncertainty
Mann et al, 1998
How has climate varied over the last 25,000 years?
Proxy for temperature changes
Back in time
Last 10,000 years =
relatively warm period,
era of agriculture, rise
of civilization
Data from ice cores.
Last ice age
IPCC 2001
Temperature changes going 400,000 years back
Back in time
Series of ice ages.
Reasons for
temperature swings
= changes in earth’s
orbit or tilt, amplified
by changes in
greenhouse gases??
Sowers and Bender, 1995
How has temperature changed by region?
Most rapid
temperature increase
over high northern
latitudes.
(Possible reason=
positive ice feedback)
Jones et al, 2001
How do you diagnose reasons for observed changes?
No way to do control experiment (n=1)
Models = tools to understanding past climate
change, predicting future.
Two kinds of models:
•
Energy balance models (simpler,
quicker to run)
•
General circulation models
(calculate circulation of winds and ocean
currents, more complicated)
Could the observed warming be due to random variability?
Temperature anomoly (oC)
Model output
Observed
temperature
3 attempts to
simulate climate
without any forcings
(no volcanos or
solar changes).
Also, observed
temperature trend,
1850-2000.
Observed signal
larger than model
variability.
Stouffer et al, 1999
What caused the warming over the last 150 years?
Volcanos, solar changes
3 attempts to fit
model changes
to observations.
Greenhouse gases, manmade aerosol
Best match
All forcings
Stott et al, 2000, Tett et al 2000
Can we simulate the last 1000 years in Northern Hemisphere?
reconstructed
temperatures
thermometer
temperatures
energy balance
model
Model includes volcanic
aerosol, solar variations,
greenhouse gases and
manmade aerosol.
Much “tuning” in this model!
Crowley et al, 2000
Caveats regarding models
•
•
Models usually calibrated to get present-day climate right.
Models unable to reproduce abrupt climate change, have difficulty with
patterns of changes.
Back in time
Series of ice ages.
Models have
trouble capturing
large temperature
swings.
How are temperatures projected to change over the next
100 years?
Model result
depends on which
scenario is chosen.
Estimated
temperature
increase =
1.4 - 5.8 oC
(large uncertainty)
IPCC, 2001
What other possible changes could we see?
•
•
•
More heat waves?
More stagnation events (and more high ozone days)?
More droughts in some regions, more intense rainfall elsewhere?
(changes in hydrological cycle)
Many uncertainties!
Possible surprises?
Extra slides
IPCC 2001
IPCC 2001