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 TIMING IN WESTERN NORTH AMERICA UNDER A ‘BUSINESS AS USUAL’ CLIMATE CHANGE SCENARIO IRIS T. STEWART1, DANIEL R. CAYAN1, 2 and MICHAEL D. DETTINGER More Rain and/or Earlier Melting Snow October December AprilJune 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 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?

©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 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 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 MM5 Regional Modeling System

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

How Global Temperature Changes are Estimated Local Observation at time t, T’=T-T mean 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