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Climatic Change:
What Can We Hope To Know?
Starley L. Thompson
Climate and Carbon Cycle Modeling Group
Atmospheric Science Division
Energy and Environment Directorate
Lawrence Livermore National Laboratory
Climatic Change: What Can We Hope To Know?
Contents
• Global Warming:
The observational evidence
• Causes of Climate Change:
Natural and anthropogenic causes
• The Future:
Predictions and The Big Unknowns
Climatic Change: What Can We Hope To Know?
The Earth’s Climate is Changing
• Global average increase of 0.6 ºC
over the 20th century
• 1990s were the warmest decade in
the instrumental record
Observed Change in Global Mean Temperature Since 1860
Climatic Change: What Can We Hope To Know?
The Earth’s Climate is Changing
• The increase in temperature during the 20th century is likely to have
been the largest of any century during the past 1000 years.
Estimated Change in Northern Hemisphere Temperature Since 1000 AD
Climatic Change: What Can We Hope To Know?
A Collective Picture of a Warming World
• Length of freeze-free
• Global sea level rise of 0.1 to
season has lengthened in
mid and high latitudes
0.2 meters in the 20th century
• 10% decrease in land
extreme low temperatures
since 1950
snow cover since 1960s
• Retreat of mountain
glaciers in the 20th century
• 10-15% decrease in
summer sea ice extent
since 1950s
• Reduction in the frequency of
• 2-4% increase in frequency
of heavy precipitation events in
mid and high latitudes of the
northern hemisphere
• A few areas have not warmed,
mainly some parts of the southern
hemisphere oceans and Antarctica
Climatic Change: What Can We Hope To Know?
Carbon Dioxide
• Atmospheric CO2 has
increased by 31% since
1750.
• The present amount of
CO2 has likely not been
exceeded during the
past 20 million years
Hawaii
Ice Cores
• The current rate of
increase of CO2 is
unprecedented in the
past 20,000 years.
A Cause of Global Warming?
Atmospheric CO2 for the past millenium.
Climatic Change: What Can We Hope To Know?
Human Influence on the Atmosphere
During the Industrial Era
Concentrations of three wellmixed greenhouse gases
Carbon Dioxide
• Atmospheric methane has
increased by 151% since
1750. About half of current
emissions are anthropogenic.
Methane
• Atmospheric nitrous oxide
has increased by 17% since
1750. About a third of current
emissions are anthropogenic.
Nitrous Oxide
Climatic Change: What Can We Hope To Know?
Human Influence on the Atmosphere
During the Industrial Era
Sulphate Aerosols
Simulated global sulphate aerosol
distribution for October
Measured sulphate in
Greenland ice and SO2
emissions since 1900
Climatic Change: What Can We Hope To Know?
CO2 currently accounts for 60% of
the radiative forcing from well-mixed
greenhouse gases
Climatic Change: What Can We Hope To Know?
Simulating Global Warming
A standard
procedure is to
increase the
carbon dioxide
(CO2) in a
climate model
simulation and
then compare
the results to a
control run
without a CO2
increase.
NOTE: Atmospheric
CO2 is expected to
double well before
the end of the 21st
century.
Change in annual mean temperature (ºC) resulting
from a doubling of CO2 in the atmosphere.
Source: Govindasamy and Caldeira (2000)
Climatic Change: What Can We Hope To Know?
Climate as a Computational Problem
The atmosphere and
oceans are fluids on a
rotating sphere.
Building a Climate Model?
You will need
• Equations of fluid motion
and thermodynamics
Applied on
• A four dimensional global
grid (3 space + 1 time)
Including
• “Physics” (e.g., clouds,
sunlight, etc.)
Image Copyright  1987 by Fulcrum, Inc.
Climatic Change: What Can We Hope To Know?
Then, combine together and run
on a rather large computer.
The Limits to Climate Computation
Latitude
Climate is Derived From Weather
• Climate modeling is really
weather modeling
Height
Longitude
• We start from an initial state and
compute forward in time by small
steps.
Each halving of grid spacing
requires eight times more
computer power.
Unresolved Processes
An example of how the Earth
is spatially gridded in current
operational climate models
Climatic Change: What Can We Hope To Know?
• Clouds and precipitation occur
on spatial scales much finer than
the explicit computational grid.
How Do We Know The Models Are Right?
We cannot know with certainty until we perform the experiment,
but we can do proxy validation to increase our confidence.
Detailed records
of the
post WW II era
Historical climate
of the
19 & 20th centuries
Paleoclimates
of the
geologic past
Climatic Change: What Can We Hope To Know?
Model
Intercomparison
Confidenc
e
In Model
Results
Testing Individual
Model Components
Q: How can we predict climate if
we can’t predict the weather?
A: The chaotic details of
weather are not predictable
beyond several days, but the
statistics of climate are, in
principle, predictable.
Natural Forcing Only
• Natural (solar + volcanic) forcing
alone does not account for
warming in the past 50 years.
• Adding human
Anthropogenic Forcing Only
influences (greenhouse
gases and sulphate
aerosols) brings the
models and
observations into pretty
good agreement.
Natural and Anthropogenic Forcing
Testing by
Simulating the
20th Century
Climatic Change: What Can We Hope To Know?
Projecting Into The Future:
CO2 Increase Scenarios
• Emissions of CO2 due to
fossil fuel burning will be the
dominant influence on
atmospheric CO2 in the 21st
century
• Stabilization of CO2 at twice
the pre-industrial level will
require emissions to drop to
below 1990 levels in less
than 50 years.
• Emissions will need to
continue to decrease steadily
thereafter to a very small
fraction of current emissions.
Climatic Change: What Can We Hope To Know?
Projecting Into The Future:
Temperature Change
• The rate of warming is very
likely to be without precedent
in at least the last 10,000
years
• The range of uncertainty
due to climate models is
comparable to the range of
uncertainty due to CO2
scenarios.
• It is very likely that nearly
all land areas will warm more
rapidly than the global
average.
Climatic Change: What Can We Hope To Know?
• Global Increase Ranging From 1.4 to
5.8 ºC from 1990 to 2100.
Extreme Weather and Climate Events
Climatic Change: What Can We Hope To Know?
A Grand Challenge:
Climate and Carbon Cycle Interaction
• Human activities perturb the
natural carbon cycle.
Atmosphere: 730
Yearly Increase: 4
Fossil fuel use: 6
Deforestation: 2
• We put about 8 gigatons of
carbon (as CO2) into the
atmosphere per year:
Manmade Source
-------------------------Fossil Fuel Burning:
Cement Production:
Land Use Change:
Gt C/yr
--------6
0.1
1.7
• About half the emitted CO2
accumulates in the atmosphere.
The remainder is taken up by the
land and oceans.
Climatic Change: What Can We Hope To Know?
Net
120
2
Vegetation: 500
122
Net
102
104
Soil: 1500
2
Recoverable
Fossil Fuel:
10,000
Ocean: 38,000
The Global Carbon Cycle.
Numbers indicate annual flows and
reservoirs in gigatons of carbon.
The Global Carbon Cycle
Is Sensitive to Climate Change
Climate change will affect the uptake of anthropogenic
carbon dioxide by the oceans and terrestrial biosphere.
Major
Carbon
Sink
Regions in
Oceans
-8.0
0
Present carbon uptake by the
oceans (Moles CO2 /square
meter/year)
Climatic Change: What Can We Hope To Know?
3.5
Simulated interannual variability of carbon
uptake by terrestrial vegetation arising from
interannual variability of sea surface
temperatures
Moving from Specified to Predicted CO2
• Currently, projections of climate change do this:
Specified
Atmospheric CO2
Concentration
Climate
Model
Future Climate
• More credible projections will need to do this:
Specified
CO2
Emissions
Climatic Change: What Can We Hope To Know?
Combined
Climate and
Carbon Cycle
Model
CO2 Concentration
Future Climate
Beyond Coupling: Making the Models Useful
• The current coarse resolution of climate models hampers their utility
for climate change impacts studies.
• The models need to provide information relevant to planners and
policymakers.
Achieving a Congressional Resolution:
Current Global Climate
Model Grid Area
300 km
Climatic Change: What Can We Hope To Know?
Average Area of a
Congressional District
150 km
A Big Unknown: Clouds and Water Vapor
Changes in Climate Models
Coarse Spatial Grids
Used in Typical Climate
Models Result in the Use
of Approximations for
Clouds and Rain
Topography at 300 km grid resolution
Climatic Change: What Can We Hope To Know?
The global mean
temperature
response of climate
models to a doubling
of CO2 varies from
about 1.5 to 4.5 ºC.
This range of
uncertainty is mostly
due to climate
feedbacks from water
vapor and clouds
Response of Ocean Circulation
to Global Warming
The overturning circulation is
driven by the sinking of
dense water at high latitudes
Dots mark sites
of deep water
formation
Density is determined by
temperature and salinity,
thus the term thermohaline
circulation (THC)
The overturning THC
component in the Atlantic
Ocean is important for
northern hemisphere heat
balance
Source: Rahmstorf, Nature (1999)
Climatic Change: What Can We Hope To Know?
Two Stable Climate States?

Climate models can produce two stable states for the
THC strength

No THC (thermohaline collapse) is one possible
state.

Similar behavior is seen in the geologic record.
Climatic Change: What Can We Hope To Know?
Global Warming and THC Collapse

Anything that acts to decrease water density
at Atlantic high latitudes can promote THC
collapse.

Global warming can do this in two ways:

Increase water temperature at high latitudes

Increase freshwater input to high latitudes
Climatic Change: What Can We Hope To Know?
Summary
• Global Warming:
Happening now. 0.6 ºC so far. 1.4 to 5.8 ºC by 2100.
• Climate Predictions:
Harder to predict regional details and extreme events
• Big Unknowns in Climate Processes:
Clouds and water vapor
Coupled carbon cycle and climate feedbacks
Ocean thermohaline circulation response
Climatic Change: What Can We Hope To Know?