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Global Climate Change:
What Controversies?
Bryan C. Weare
Atmospheric Science Program University of California, Davis
IPCC Fourth
Assessment
Reports
Is the Global
Temperature Really
Increasing or is it Just
“Cyclical”?
Global Temperature Changes Over the
past 250 Years
Global Temperature Changes Over the
past 1200 Years
November to March Surface Temperature Trends
Temperature Records
Have Problems: Is
There Other Evidence?
One of the most important aspects of
climate change for California is the
impact on Snow Water Storage
Less snow, more rain in winter means:
1. More flooding in winter
2. Less water in reservoirs in summer for agriculture and cities
During the past 50 years peak spring stream
flow has gotten earlier in the season
100
More Rain and/or
Earlier Melting Snow
River Flow
50
0
October
December
April
June
CHANGES IN SNOWMELT RUNOFF TIMING
IN WESTERN NORTH AMERICA UNDER A ‘BUSINESS AS USUAL’
CLIMATE CHANGE SCENARIO
IRIS T. STEWART1, DANIEL R. CAYAN1, 2 and MICHAEL D. DETTINGER
Is the Global
Temperature Tied to
Human Activity or
“Natural”?
Global climate is primarily controlled by the
earth’s radiation budget
Human Climate
Forcing Parameters
Carbon Dioxide- increases primarily
from fossil fuel burning, deforestation
Methane- increases primarily from
intensive agriculture, poor natural gas
production practices
Nitrous Oxide- increases primarily
from vehicles, agriculture
Sulfur Aerosols (dust)- changes
primarily from volcanoes, fossil fuel
(coal) burning
Human Contribution to Climate Forcing in the Past 100 Years
But it’s the Sun, isn’t it?
Predictions for the
Future Are Based on
Models: Why Can We
Trust Them?
4. Global Climate Models
©IPCC AR4
and BCW
How are these Model Verified?
Comparisons of Means with Observations: Surface Temperature
©IPCC AR4
Comparisons of
Means with
Observations:
Precipitation
©IPCC AR4
Putting it All Together
Climate models often
test their skill by hind
casting observed
climate from 1860 to
the present.
As the model
sophistication
increases to include
greenhouse gasses,
aerosols, and changes
in solar radiation the
model has improved
prediction.
What Do These Models
Say About the Future?
Projecting Future Climate
Rapid rise
Moderate
rise
Slow rise,
then decline
Multi-Model Prediction
Future projections are for earlier steam flows
Future projections indicate less snow in April
“The message for the general public should be that
this is a real problem, that climate change is
happening.
It's projected to occur at a rate in the future that's
unlike anything seen in the last 10,000 years, and
that rate is likely therefore to be disruptive.”
Kevin Trenberth, Director Climate Division
National Center for Atmospheric Research
Other Indicators of
Global Climate
Change
As lower
atmosphere
temperatures rise,
the temperatures in
the stratosphere
fall. This is in
agreement with
basic theories of
global warming
Lower Atmospheric Temperature
Stratospheric Temperature
Land Use Changes May Also be Influencing Climate
Urban and Irrigated Crops Have Often Replace Grasslands.
This influence can be estimated using a regional model.
Upward trends are strongest
for the minimum
temperatures in the Valley
Might this be related to
increased irrigation?
MM5 Regional Modeling System
Grell
ERA40
6
30km x 30km
Year runs
August 1995-Sept. 1996
100 year trends from the UK Meteorological Office Model
Regional Model Boundary Conditions
a : 1996-1901
Tmax
b : No_Irrig-1901
Tmax
50
40
30
20
10
0.1
0.08
0.06
0.04
0.02
50
40
30
20
10
0
-10
-20
-30
-40
-50
c: 1996-1901
LH
e : 1996-1901
Winds; Snow
Water % change
0.5 ms-1
100
80
60
40
20
0
-20
-40
-60
-80
-100
d : 1996-1901
Soil Moisture
f : 1996-1901
q2 % change
0.1
0.08
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
50
40
30
20
10
0
-10
-20
-30
-40
-50
How Global Temperature Changes are Estimated
Local Observation at
time t, T’=T-Tmean
Average T’ s over all
regions of the globe
Average all “good”
T’ s over a region
A point on the curve at t
Distribution of
Rainfall
Stations
Averages
over Parts of
the Northern
Hemisphere
Changes in Frequencies of Wet (top) and
Dry (bottom) Spells: 1901 to 1998
More Wet
Periods
Constant or Fewer
Dry Periods
positive
+
negative
+
-
+
Carbon Dioxide Emission Scenarios