Climate Change: Theory and Forecasts

David Gay

National Atmospheric Deposition Program (NADP)dgay@uiuc.edu, (217) 244-0462

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Two PartsTwo Parts

1.1.What exactly is climate change?What exactly is climate change?a.a.TheoryTheoryb.b.What we knowWhat we know

2.2.ForecastsForecasts

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So, what is “Climate Change”?

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First, what is climate?

• DefinitionAverage course or conditions of

weather at a place, usually over a period of years, as exhibited by •Temperature •Wind velocity•Precipitation (Webster’s)

•Prevailing set of weather conditions (temperature, humidity, etc.)

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So, can WE change climate?

• Thomas Jefferson guessed we could in early 1800’s• Trees in VA• Started making measurements

• Urban concrete, no trees• Chemical composition of the atmosphere• Sensible energy vs. latent heating• Albedo changes• Natural changes (ice ages, warm periods)• Changing evaporation/precipitation (trends)• Other patterns (Malenkovitch cycles, el Nino/la

Nina)• Cloud cover changes?

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Definition of Climate Change

IPCC usage:• Any change in climate over time, whether due

to natural variability or from human activity.

Alternate:• Change of climate, attributed directly or

indirectly to human activity, that • Alters composition of global atmosphere and

• Is in addition to natural climate variability observed over comparable time periods

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So?

More than just temperature

•Precipitation (amount and patterns)•Atmospheric pressure•Humidity•Circulation changes•Number of storms, droughts, freezes,

etc.•And more….

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Average Temperatures, in ◦C

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Type of Ecosystem, w T and P

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Water Cycle

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Radiation Balance“The Greenhouse”

• ExampleYour stove top

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Radiation Balance

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“Effective”GreenhouseGases

Atmospheric Window

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So, what do we So, what do we know?know?

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (World Meteorological Organization; UN Env. Program)

http://www.ipcc.ch/

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Carbon Dioxide Concentration Over Time

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Methanealso a Greenhouse Gas

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Quiz

•How much has the earth warmed in the last 150 years?

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Answer

≈ 0.95°C (1.62 °F) since 1850

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Historic Thermometer Records

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“Diurnal Temperature Range”

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Temperature Trends, ◦C

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Changes In…..

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Sea-Level Rise

Total Change = +170 mm or +6.7 inches

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Causes of Sea Level Rise

Sea-level rise during the 20th century (cm)

Ocean thermal expansion 4+

Mountain glaciers 5

Greenland & Antarctic ice 2

Total 11

Observed rise 17

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Arctic Sea IceSep 1979 Sep 2003

2007 = 4.3 x106 km2 , record low

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Glacial Retreat

Argentiere Glacier, French Alps

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Trends in Precipitation

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IPCC Summary of Observations

• Global mean surface temperatures have risen• By 0.74°C ± 0.18°C over the last 100 years (1906–

2005)• 2005 was one of two warmest years on record

• Land regions have warmed at a faster rate than the oceans.

• Changes in extremes of temperature are consistent with warming of climate • Widespread reduction in number of frost days in mid-

latitude regions,• Increasing number of warm extremes & reducing number

of daily cold extremes observed in 70 to 75% of land regions, and

• Most marked changes are for cold (lowest 10%, based on 1961–1990) nights, rarer over 1951 to 2003 period.

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IPCC Summary (cont.)• Sea-surface temperatures warming at all latitudes

over all oceans,

• Urban-heat island effects real but local, have not biased large-scale trends,

• Average arctic temperatures increased at almost twice global average in past 100 years,

• Lower-tropospheric temperatures increases slightly greater than those at surface between 1958 to 2005

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IPCC Summary (cont.)

• Lower stratospheric temperatures cooling since 1979,

• Precipitation has generally increased over land north of 30°N between 1900 to 2005, but downward trends dominate tropics since 1970s,

• Droughts more common, especially in tropics and subtropics, since 1970s,

• Changes in large-scale atmospheric circulation apparent

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Historical Perspective of Temperatures

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Human Contribution to Climate

Change (in one

slide) IPCC, 2007

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Causes of climate change

• Climate change is driven by five causes (forcings)• Internal components of the climate system respond by changing

and interacting in many ways

Changes in atmospheric composition (greenhouse)

Volcanic eruptions

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Natural Causes of Climate Change

( non anthropogenic)

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Earth orbital changes• Result from cyclic variations in Earth's orbit around the Sun• Alter the amount of solar radiation (insolation) received on Earth by

season and by latitude

today

Milanchovitch Cycles, Orbital Changes

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Precession of the equinoxes (23k years)

Oval/circular changes

Inclination Change

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Changes in Solar Energy Output

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• The smoothed sunspot curve correlate with temperature

• Some intervals almost entirely lack sunspots, such as the Maunder sunspot minimum from 1645-1715 AD

• These sunspot minima occurred during the Little Ice Age when the sun was 0.25% weaker

• Solar activity was generally high during the Medieval Warm Period

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Other Natural CausesMt Pinotubo

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The Modeled Future

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CO2 Should Increase

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Predicting TemperatureGlobal Climate Models (GCMs)

• Physical equations• Lots of computing• Divide the globe

into little boxes

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Forecast Temperatures

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Projections of Surface Temperatures

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Volcanoes

GH gases

Solar

With GH gases

W/out GH gases

IPCC, 2007

How Good Are the Models?

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Further Reading

• Intergovernmental Panel on Climate Change (IPCC)• UN organization (Weather Meteorological

Office)

•http://www.ipcc.ch/• Summary for the physical science basis• http://ipcc-wg1.ucar.edu/wg1/wg1-report.html

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Figures from W. Ruddiman, 2008; Earth’s Climate: Past and Future

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Climate Change: Theory and Forecasts

David Gay

National Atmospheric Deposition Program (NADP)dgay@uiuc.edu, (217) 244-0462