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

CHANGES IN SNOWMELT RUNOFF TIMINGIN WESTERN NORTH AMERICA UNDER A ‘BUSINESS AS USUAL’CLIMATE CHANGE SCENARIOIRIS T. STEWART1, DANIEL R. CAYAN1, 2 and MICHAEL D. DETTINGER

More Rain and/or Earlier Melting Snow

October December April June

100

50

0

River Flow

Is the Global Temperature Tied to Human Activity or “Natural”?

Global climate is primarily controlled by the earth’s radiation budget

Human Climate Forcing ParametersCarbon 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?

©IPCC AR4 and BCW

4. Global Climate Models

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 TogetherPutting it All Together

Climate models often Climate models often test their skill by test their skill by hind hind castingcasting observed observed climate from 1860 to climate from 1860 to the present.the present.

As the model As the model sophistication sophistication increases to include increases to include greenhouse gasses, greenhouse gasses, aerosols, and changes aerosols, and changes in solar radiation the in solar radiation the model has improved model has improved prediction.prediction.

What Do These Models Say About the Future?

Projecting Future Climate

Rapid riseModerate 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

Stratospheric Temperature

Lower Atmospheric 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?

ERA40

6

Grell

30km x 30km

Year runs

August 1995-Sept. 1996

MM5 Regional Modeling System

100 year trends from the UK Meteorological Office Model Regional Model Boundary Conditions

50403020100-10-20-30-40-50

0.10.080.060.040.020-0.02-0.04-0.06-0.08-0.1

100806040200-20-40-60-80-100

f : 1996-1901q2 % change

50403020100-10-20-30-40-50

0.5 ms-1

a : 1996-1901Tmax

b : No_Irrig-1901Tmax

d : 1996-1901Soil Moisture

e : 1996-1901Winds; SnowWater % change

c: 1996-1901LH

50403020100-10-20-30-40-50

0.10.080.060.040.020-0.02-0.04-0.06-0.08-0.1

100806040200-20-40-60-80-100

f : 1996-1901q2 % change

50403020100-10-20-30-40-50

0.5 ms-1

a : 1996-1901Tmax

b : No_Irrig-1901Tmax

d : 1996-1901Soil Moisture

e : 1996-1901Winds; SnowWater % change

c: 1996-1901LH

How Global Temperature Changes are Estimated

Local Observation at time t, T’=T-

Tmean

Average all “good” T’ s over a region

Average T’ s over all regions of the globe

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