11Climate Change 101March 12, 2007

Thomas C. Peterson

NOAA’s National Climatic Data CenterAsheville, North Carolina

Climate Change 101:

An Introduction to Climate Change Science

Climate Change 101:

An Introduction to Climate Change Science

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Outline of the talk:• The nature of science• The greenhouse effect• The physics of climate change• Global climate models• Climate change detection and

attribution• Common questions• Concluding comments

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The nature of science

• . . . science, which I define as a set of methods designed to describe and interpret observed or inferred phenomena, past or present, and aimed at building a testable body of knowledge open to rejection or confirmation. In other words, science is a specific way of analyzing information with the goal of testing claims.– Michael Shermer, director of Skeptics

Society, 1997

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Science is never 100% certain

• Science does not deal in certainty, so “fact” can only mean a proposition affirmed to such a high degree that it would be perverse to withhold one’s provisional assent.– Stephen Jay Gould, geologist, 1999

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Science is self-correcting

• In practice, contemporary scientists usually submit their research findings to the scrutiny of their peers, which includes disclosing the methods and data which they use, so that their results can be checked through replication by other scientists.– IPCC 4th Assessment Report, 2007

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Competing claims, information, and even misinformation can be

assessed• Testability

– Can it be proved false?• Fruitfulness

– Does it yield observable surprising predictions?• Scope

– How many different phenomena does it explain?• Simplicity

– How many assumptions does it make?• Conservatism

– Is it consistent with our well founded beliefs?• Theodore Schick, Jr. & Lewis Vaughn, philosophers,

2001

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We need the greenhouse effect

• The Earth’s surface temperature is ~60ºF

• Without the greenhouse effect it would be ~5ºF

• But humans are changing the radiative properties of the atmosphere and thereby the greenhouse effect

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Climate Forcing Summary

From Ravishankara (2006)

Warming versus cooling effects are like the tortoise versus the hare.

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Do you believe in global warming?

• I believe in quantum physics.

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Quantum physics tells us that• Infrared (IR) energy can only be absorbed

and radiated in very small particle-like packets of energy called quanta

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Quantum physics tells us that• Infrared (IR) energy can only be absorbed

and radiated in very small particle-like packets of energy called quanta

• Each molecule can absorb and radiate quanta at different wavelengths

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Quantum physics tells us that• Infrared (IR) energy can only be

absorbed and radiated in very small particle-like packets of energy called quanta

• Each molecule can absorb and radiate quanta at different wavelengths

• Two atom molecules can absorb very little IR energy– E.g., Nitrogen (N2) and Oxygen (O2)

• 98% of the atmosphere

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Quantum physics tells us that• Infrared (IR) energy can only be absorbed

and radiated in very small particle-like packets of energy called quanta

• Each molecule can absorb and radiate quanta at different wavelengths

• Two atom molecules can absorb very little IR energy– E.g., Nitrogen (N2) and Oxygen (O2)

• 98% of the atmosphere

• Three or more atom molecules do absorb and radiate in the IR– E.g., Carbon Dioxide (CO2), water vapor

(H2O), methane (CH4)• 2% of the atmosphere• CO2 only 0.04% of the atmosphere

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Global climate models• Computer

generated numerical simulations of the climate system

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Climate change detection and attribution

• Often linked together but are two separate processes

• Very mathematically intensive– Involves the temporal and spatial

patterns of climate change– So this description is quite simplified

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Climate change detection

• Examine the instrumental temperature record for the last 100 years

• Examine the paleoclimate record for the past 1000 or 2000 years

• Examine climate model control runs– No changes in forcing– Run for 10,000s of years

• Is the recent observed climate change outside the bounds of natural climate variability?

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Yes, the recent observed climate change is beyond the bounds of

natural variability

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Attribution: What is the cause of the detected climate

change?• Attribution is primarily model

based analysis• What mix of forcings is required to

create the detected climate change?

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Attribution example: Most of the warming over the past 50 years is

likely due to greenhouse gas increases

IPCC TAR

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Are CO2 and other greenhouse gasses really responsible for

changing the global temperature?

• Quantum physics says we should expect them to be

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• Climate models say they are

Are CO2 and other greenhouse gasses really responsible for

changing the global temperature?

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Are CO2 and other greenhouse gasses really responsible for

changing the global temperature?

• Historical observations indicate they are related

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Are CO2 and other greenhouse gasses really responsible for

changing the global temperature?• Ice cores can give us the long view

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Are CO2 and other greenhouse gasses really responsible for

changing the global temperature?• The long view says they are definitely related

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Common questions

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You can’t predict the weather 10 days in advance, how can you predict the

climate 100 years from now?• Weather forecasting

and climate projections are very different– Weather forecasting

is primarily based movements and interactions of weather parameters

• Predicting a storm 1 day late is an error

– Climate projections are primarily based on the physics of long-term changes in solar energy and infrared radiation

• The same climate physics that allow us to 100% accurately predict that next summer will be warmer than next winter

After Kiehl and Trenberth (1997)

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Don’t urban heat islands – hot local temperatures caused by buildings and concrete- make U.S. and

global temperatures unreliable?• No• The urban effect is

minor with land data

• Ocean data has no urban effect and shows warming

• Increasing temperatures supported by:– plant bloom dates– Lake/river

freeze/thaw dates– Glaciers melting– Etc.

Peterson and Owen (2005)

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Additional supporting evidence: the shrinking Arctic

sea-ice

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Don’t satellites show no warming?

• One satellite data set did several years ago

• As another group tried to reproduce it, an error in the data processing was discovered

• Both satellite and surface data currently show warming

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What are the climate projections for my area?

• Models aren’t accurate at city level• But can use projections for a large

region such as the Eastern US• Projections are not from the

Intergovernmental Panel on Climate Change (IPCC)

• But based on all the models that were run to contribute to the IPCC– Over 25 models– Three emission scenarios

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Precipitation

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Total precipitation:

From Peterson et al., 2007b

1σ = ~68%2σ = ~95%

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Precipitation projections:

• Total precipitation very uncertain

• However, models project heavy precipitation will increase

Created for a report due to be released in late 2007.

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Temperature

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Low CO2 scenario

From Peterson et al., 2007b

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Mid-range CO2 increases

From Peterson et al., 2007b

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Business as usual CO2

From Peterson et al., 2007b

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Temperature projections

• Projections show more change in the future than recently observed

• Even if we stopped emitting CO2 now there would still be warming for the next few decades

• How warm it will be 100 years from now is dependent on future emissions of greenhouse gases

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Does anthropogenic global warming pass the rating

criteria?• Testability– Can it be proved false?

• Yes, the last decade could have been cold, laboratory tests on CO2 could have proven theory wrong

• Fruitfulness– Does it yield observable surprising predictions?

• Yes, predicts increase in heavy precipitation which has been observed

• Scope– How many different phenomena does it explain?

• Changes in temperature, precipitation, atmospheric circulation, storms, mountain glacier melting, arctic sea-ice melting, etc.

• Simplicity– How many assumptions does it make?

• None, based on quantum physics• Conservatism

– Is it consistent with our well founded beliefs?• Yes, no previously unknown phenomena are required to explain it

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Ockham’s razor• 14th Century English Franciscan friar and

philosopher William of Ockham developed this principle:– All things being equal, the simplest solution tends

to be the best one.• Greenhouse gases warming the planet is simple• Alternate climate change explanations are not

– Require ignoring CO2’s radiative effect– Paying attention to unproven explanations

» It is just part of a natural cycle (that doesn’t show up in the paleoclimate record)

» It is all due to changes in solar geomagnetism» It is all due to urban contamination of data sets» A negative feedback like the cloud-iris effect will save

us» It is all due to cosmic rays» Etc.

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Final comment

• Stepping out into record hot weather, a friend who is an expert on climate change detection and attribution was asked if the high temperatures they were experiencing were due to global warming

• He responded:– You can’t attribute any one day’s

temperature to global warming– But unusually warm weather like that does

give us the privilege of experiencing the weather we are bequeathing our children and grandchildren

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Selected References

• Kiehl, J., and K. Trenberth, 1997: Earth’s annual global mean energy budget. Bull. Amer. Meteor. Soc., 78, 197-206.

• Peterson, Thomas C. and Timothy W. Owen, 2005: Urban Heat Island Assessment: Metadata are Important. Journal of Climate, 18, 2637-2646.

• Peterson, Thomas C., Xuebin Zhang, Manola Brunet India, Jorge Luis Vázquez Aguirre, 2007a: Changes in North American extremes derived from daily weather data. Proceedings of the National Academy of Science, in preparation.

• Peterson, Thomas C., Marjorie McGuirk, Tamara G. Houston, Andrew H. Horvitz and Michael F. Wehner, 2007b: Climate Variability and Change with Implications for Transportation, National Research Council, in press.

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