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CLIMATE CHANGE:
What We Know and What We Don't

Presentation by Daniel L. Albritton, Director of NOAA Aeronomy Laboratory, during early 2001. Dr. Albritton provided additional testimony on climate change research before the US House of Representatives Committee on Science, March 14, 2001. A detailed account of the hearing was prepared by the NOAA Office of Legislative Affairs.

CONTEXT: The Climate System and Humankind

THREE MAJOR QUESTIONS:

1. How well do we understand the climate system and our role in changing it?
2. How well can we characterize the impacts of climate change?
3. What are out future options?

Climate Change Forcings create
Physical Processes that cause
Physical Responses that affect
Biological Processes that have
IMPACTS (crop yield, coastal habitation, forest migration)

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A SCIENTIFIC STATUS REPORT:

There is a natural greenhouse effect. It keeps the earth warmer than it would be otherwise   Confidence Index=10/10

POINTS:

• IN TERMS OF BASIC PHYSICS:
  • If an object is bathed in visible light...
  • It warms up and...
  • It emits infrared light
• IN TERMS OF OUR PLANET EARTH THE ATMOSPHERE IS COMPOSED OF::
  • 78% Nitrogen
  • 20% Oxygen
  • *2% Water Vapor (*greenhouse gas)
  • *0.03% Carbon Dioxide (*greenhouse gas)
• KEY ASPECTS:
  • Water Vapor and Carbon Dioxide have been part of our atmosphere for millions of years.
  • Their presence yields an average surface temperature of ~60 degF
  • Without them the average would be ~5 degF.

Q: So, what's the PROBLEM?

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Greenhouse gases are increasing in the atmosphere because of human activities, and they are increasingly trapping more heat.

POINTS:

• IMPECCABLE SCIENTIFIC MEASUREMENTS   Confidence Index=9/10
  • The abundance of Carbon Dioxide increased 31% over the industrial era between 1800 and 2000, as measured by ice cores and air samples
• OTHER GASES HAVE INCREASED TOO
  • Methane (one-third the effect of Carbon Dioxide)
  • Sulfur (a cooling tendency)
    
    
• THE SOURCES ARE HUMAN-CAUSED
  • Carbon Dioxide ~all (combustion)
  • Methane ~most

Q: But, what are the CONSEQUENCES?

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There is a collective picture of a warming world, and human activities have likely contributed.

POINTS:

• GLOBAL TEMPERATURES ARE UP 0.7 - 1.4 degF OVER PAST 100 YEARS (recorded by thermometers rather than tree rings, corals, etc.)
  Consistent with the warming:
  • Glacial retreat
  • Snow-cover decrease
  • Freeze-free periods lengthened
  • Sea-level increased 4-8 inches
    
    
• MOST OF THE WARMING OVER THE PAST 50 YEARS IS LIKELY TO BE DUE TO GREENHOUSE-GAS INCREASES   Confidence Index=8/10
  Reasons: Comparisons of simulated vs observed temperatures:
  • Simulations with natural and human factors match observations best
  • Correspondences increase with time
  • Probability is low that a "natural-only" earth would have such correspondences

Q: What could this mean for the FUTURE?

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A continued growth in greenhouse gases is projected to lead to very significant increases in global temperatures and sea level.

POINTS:

• CARBON DIOXIDE ABUNDANCE WILL LIKELY DOUBLE BEFORE 2100
  Indeed...to stabilize at doubled would require cuts in emissions
• PREDICTED CLIMATE RESPONSES    Confidence Index=6/10
  For a range of future emission scenarios (non-intervention, economics, technology, population,...)
  • Global temperature rise of 2.5 - 10 deg F by 2100
  • If so, this would exceed the natural changes over the past 10,000 years
  • Corresponding sea level rise of 4 - 35 inches by 2100
    
    
• A GREENHOUSE WARMING COULD BE REVERSED ONLY VERY SLOWLY
  Reason: The oceans are sluggish

Q: Beyond "GLOBAL AVERAGES"?

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It is a complex planet and we have imperfect knowledge; so, prediction of further details suffers.

POINTS:

• What will happen in particular places?    Confidence Index=4/10
  • Regional changes cannot yet be predicted reliably
  • Some projections are likely robust...
    • Land areas warm more than oceans (N. North America: 40% above average)
    • Increased mid-continental soil drying
• How about "extreme events"?
  • A warmer world means a more vigorous "Hydrological Cycle" (heavier rains, with large variance from region to region)
  • Hurricanes: more or less frequent? Much tougher to call
• Surprises?
  Currently unknown geophysical processes activated? Abrupt climate shifts? Both possible because
  • We are entering a new regime of climate perturbation
  • Climate is a non-linear system

   

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CONCLUDING COMMENTS

BOTTOM LINES - THE VAST MAJORITY SCIENTIFIC VIEWPOINT

• The issue is a real one
• The first signs of human-caused climate change have likely occurred
• Some degree of further changes appears inevitable
• Exactly where (regions), when (rate of change), how much (magnitude) is hard to predict
• Human-caused climate change would be slow to reverse

BASIS OF THIS INFORMATION:

• Intergovernmental Panel on Climate Change: Third major assessment
  • Prepared by the expert communities
  • Internationally based
  • Policy-relevant information, but no policy statements

   

THE LAST BOTTOM LINE: KEY INPUT TO POLICY FORMULATION

ILLUSTRATIONS