Can Ocean Models Help to Understand the Role of the Ocean in

Can Ocean Models Help to Understand
the Role of the Ocean in Absorbing
Human-Produced Carbon Dioxide?
Keith Rodgers, Princeton University
Intergovernmental Panel on Climate Change (IPCC):
[the 4th report (AR4) in 2007 won the Nobel Peace Prize]
Climate change over last 50 years as “unequivocal”
Changes are 95% likely to be the result of human activity
The report predicted serious negative consequences if we
continue to increase CO2 concentrations in the atmosphere
It is important to understand the types of tools that are currently
being used by the climate research community to study this
Generally speaking, there are two groups of climate researchers:
Observational community: collect data at sea, etc.
Modeling Community (construction and use of climate models)
In fact both groups use models and data, but in distinct ways.
Other speakers at this symposium have focused on the work of
the first group
I would like to convey the way in which the second group
contributes to climate research
Earth System Models: used to understand climate change
Best thought of as consisting as a number of sub-models that represent:
Physical processes (ocean and atmosphere each as a separate
model); These models have at their core “physical laws”, such
as Newton’s Law (F=ma)
Ecosystem Models/Carbon cycle models (ocean/land treated
Building models of biological systems is very difficult!
As a result of our lack of understanding, these models have
less “skill” than dynamical models
In contrast to “statistical models” used by observational
community, Earth System Models try to capture the real
physical cause of relationships between component systems and
Why do we need models?
Can’t we understand future climate change based on work of observational community?
Consider changes in
atmospheric CO2 recorded
at Mauna Loa (Hawaii) over
the time interval since 1959:
Can ask question: given that temperatures have changed 0.5-1.0°C
over a time interval where atmospheric CO2 concentrations have
increased by 70 ppm over 50 years, doesn’t this give us a way to predict
how climate will change if atmospheric CO2 increases by (for example)
140 ppm over the next 50 years?
The Earth System should be assumed to be “nonlinear”
For “linear” systems, multiplying the perturbation by a factor of two
results in the amplitude of the response being twice as large
For example, by Newton’s Law (F=ma), a train will accelerate
twice as fast if the locomotive exerts twice the force in pulling
the train (ignoring friction etc.)
It is known that there are a great number of “nonlinear”
processes in the different components of the Earth System, and
thus the climate system needs to be treated in general as a
nonlinear system
For example, increasing atmospheric CO2 concentrations by
70ppm from current values could lead to changes larger than 1°C
Thus “Earth System Models” are recognized as being critical part of
climate research, and Earth System Models are being developed at a
number of climate centers in response to the understanding gained through
the IPCC process
Historically numerical models first focused on “weather prediction” (1950s-1960s)
Demonstrated clearly the key role
of “waves” in determining the
behavior of atmosphere for
Subsequent work has
demonstrated conclusively that
related wave behavior in both the
ocean and the atmosphere is
critical to understanding El Nino
(climate variability)
Three dimensional models: physical
Models of the physics of the
atmosphere and the ocean are
typically defined on a three
dimensional “lattice” structure
This may consist for each medium
(ocean and atmosphere) of
40 levels (boxes)
180 latitudinal boxes
360 longitudinal boxes
and thereby
40x180x360=2.5 million boxes !!!
Using such models requires
computers significantly larger than
consumer laptops/desktops
Evolution of typical atmospheric model (horizontal)
resolution during history of IPCC project
First assessment report (1980s)
Second assessment report (1995)
Third assessment report (early 2000s)
Fourth assessment report (2007)
Why is CO2 important for climate?
First described quantitatively by Svante Arrhenius in 1896
Carbon has been a key component of life and carbon cycling
throughout geological history, and it carbon cycle research
focuses on a wide range of “timescales”
Very Long Timescales (tectonics and volcanism)
Human timescales (decades to centuries): main focus of IPCC
What is the role of the ocean in the global carbon cycle
over timescales of decades to centuries?
Where is carbon going in the ocean?
Data-based estimate of Takahashi et al. (2002)
Blue regions: net transfer of CO2 into the ocean
Red regions: net transfer of CO2 to the atmosphere
Where is carbon accumulating in the ocean?
Data-based estimate of Sabine et al. [2004] for 1990s
A variety of methods indicate that for the recent past 1/3 of the CO2
released through human activity has been absorbed by the ocean
How much CO2 will be taken up by the ocean in the future?
First step is to evaluate ocean model components of Earth
System Models (evaluation of skill for modern climate); this is
most effectively done with ocean-only models experiments where
the ocean currents are intended to follow historical changes
Second step is to evaluate Earth System Models over prediction
for 21st century changes
First step: compare ocean carbon uptake estimated by Sabine et al.
[2004] with simulated uptake from available ocean carbon models
[Sabine et al., 2004]
Japanese ocean
German ocean
ocean model
French ocean
Ocean model
Air-sea fluxes of CO2 for French
(IPSL) Earth System model
Air-sea fluxes of CO2 based on
Takahashi et al. [2002] data
Preliminary result from Earth System models: there are significant changes in
carbon exchanges (between the ocean and atmosphere) over 21st century
Evidence that the ocean carbon cycle is changing
North Atlantic Ocean
Observational studies suggest that ocean uptake of CO2 in the
North Atlantic may be decreasing ;
Ocean model studies are divided on this point
Southern Ocean
Recent modeling studies have raised the question of whether
the Southern Ocean uptake of CO2 may also be decreasing
Needs for future research
Earth System Models
Needed to predict future changes
Observational Networks
Needed to understand processes and constrain models