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The IPCC Assessment Process: Future Projections of Climate Change . Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA. The views described here are solely those of the presenter and not of GFDL/NOAA/DOC or any other agency or institution. What is the IPCC?

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the ipcc assessment process future projections of climate change

The IPCC Assessment Process: Future Projections of Climate Change

Ronald J Stouffer

Geophysical Fluid Dynamics Laboratory


The views described here are solely those of the presenter and not of GFDL/NOAA/DOC or any other agency or institution.


What is the IPCC?

  • Established by:
    • World Meteorological Organization (WMO)
    • United Nations Environment Programme (UNEP)
  • Open to all member governments of the UN and WMO
  • Started in 1988 - Full reports in 1990, 1995, 2001, 2007
  • From the IPCC web pages:
  • “The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation.”

What is the IPCC?

Every 5-6 years, over 1000 scientists from more than 100 nations assess the published scientific literature documenting the state of scientific knowledge related to climate change issues. The IPCC reports are ratified by the ~180 member nations.

NOAA GFDL has been a prime player in the 4 major assessment reports, including the IPCC 4th Assessment Report (AR4) published in early 2007.

ipcc wgi 2007 findings
IPCC WGI 2007 Findings
  • The planet is warming.The warming is not uniform. In fact, some small areas are cooling. Other climate and biophysical changes support the idea that the planet is warming. Sea ice and snow edges retreating; increased precipitation; more water vapor in the atmosphere; earlier river thaws; earlier spring migrations; plant blooms; etc.
  • Humans are the cause of increasing greenhouse gases in the atmosphere (CO2, methane, etc.).Humans also cause emissions of items that tend to cool the planet (sulfate aerosols for example).
  • Climate models using estimates of past forcings (GHG, aerosols, solar, volcanoes) can simulate much of the past climate variations at the global scale and many regional scales.
  • Using estimates of future emissions,climate models project relatively large increases in warmingand other associated climate impacts (precipitation, sea level, etc.) over the next century.
ipcc how is it organized
IPCC- How is it organized?
  • Three Working Groups

I. Physical climate changes

II. Impacts of physical climate changes on human and natural systems

III. Mitigation (cost/benefits) of future climate changes

+ Special Reports

ipcc how is it organized6
IPCC- How is it organized?
  • Role of Scientists
    • Assess peer-reviewed literature
    • Find consensus
      • Role of consensus
    • Express uncertainty – calibrated language
    • Write underlying report
    • Write draft of Summary for PolicyMakers
ipcc role of consensus
IPCC- Role of consensus
  • Finding consensus is a messy business
    • Scientists seem much better at finding points of disagreement than points of agreement
  • Does a given statement reflect the scientific literature, uncertainties and a wide range of opinions?
ipcc role of consensus8
IPCC- Role of consensus
  • Wording is a big issue
  • 2001 WGI bottom line as an example:

“There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities.”

    • Plenary turned “much” into “most”
    • Previous disagreement over

“substantial” resulted in “much”

  • What does substantial mean?
      • Majority?
      • Plurality?
ipcc role of consensus9
IPCC- Role of consensus

> Can I live with statement?

  • Is statement wrong?
  • In plenary this question can be very important
uncertainty different ways to express uncertainty
UncertaintyDifferent ways to express uncertainty
  • Probability (pdf) – likelihood – WG1
    • Virtually certain, Very likely, likely, etc.
  • Confidence – high confidence – WG2
    • High, medium, low confidence
  • Agreement – high agreement – WG3
    • High, medium, low agreement
  • Evidence – much evidence – WG3
    • Much, medium, limited evidence
ipcc uncertainty calibrated language
IPCC- UncertaintyCalibrated Language
  • Working Group I definitions:
    • Virtually certain: >99% probability (1:100)
    • Extremely likely: >95% (1:20)
    • Very likely: >90% (1:10)
    • Likely: > 66% (1:3)
    • More likely than not: >50%
    • Unlikely: <33%
    • Very unlikely: <10%
ipcc role of governments
IPCC- Role of governments
  • Must approve SPM (Summary for PolicyMakers) line by line (or word by word)
  • Scientists must also agree to wording changes
    • Must be consistent with underlying report
    • “Can I live with wording?” question
  • Possible to have footnotes saying that a given country or countries did not approve of a part of the text … occasionally used
ipcc plenary
IPCC- Plenary

Valencia, Spain November, 2007

ipcc plenary14
IPCC- Plenary
  • Typically go very slowly through text in the beginning
  • When progress stops on a wording/science/political issue => breakout groups
  • Breakout groups meet before/after meeting
    • Focus on a subset of the text
  • Last day(s) goes well into night
    • Rush to get things done
ipcc process author s viewpoint
IPCC- Process (Author’s viewpoint)
  • Organizational meeting
  • Literature search and community input
  • Write 1st draft
  • Limited expert review
  • Second meeting
    • Cross cutting issues between chapters
    • Plan next draft
  • Write 2nd draft
    • Publication deadline for referenced papers
ipcc process author s viewpoint16
IPCC- Process (Author’s viewpoint)
  • Deal with review comments (experts and national review)
    • Each must be answered
    • Review and Comments made public
    • More than 1000 comments per chapter
  • 3rd authors meeting
    • Cross cutting issues
    • Deal with review/comments
  • Write last draft
ipcc process author s viewpoint17
IPCC- Process (Author’s viewpoint)
  • Deal with review comments (national and NGO review)
    • Each must be answered
    • Review and Comments made public
    • More than 1000 comments per chapter
  • Write final version
    • Cross chapter references
    • Consistency between chapter and SPM
projections of future climate change
Projections of future climate change
  • Summary of WGI
  • Time scales of response
  • Variability
  • Abrupt climate change

Important greenhouse gases are increasing.

The largest increases are in the last 100 years or so.

Humans are the cause of the increases.








Over the last 100 years the:

Surface temperature is increasing

Sea level is rising

Snow cover is decreasing



Human activities are very likely the cause of the warming of last 100 years.

Black line: temperature observation from thermometers.

Pink shade: Climate model simulations using all past radiative forcings.

Blue shade: Climate model simulation using only natural forcings (solar, volcanoes).



Human activities are likely to be the cause of the warming over last 100 years on each continent.


projection of future changes in climate
Projection of future changes in climate

Range of projections is broadly consistent with the TAR.

Stronger climate-carbon cycle feedbacks.

Projection - scenario independent over next several decades.

Warming this century much larger than last century.

Best estimate and likely uncertainty range at 2100


Warming greatest over land and at most high northern latitudes and least over Southern Ocean and parts of the North Atlantic Ocean

Continuing recent observed trends in contraction of snow covered area, increases in thaw depth over most permafrost regions, and decrease in sea ice extent

In some projections using SRES scenarios, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century

Surface Warming Pattern

A1B, 2090-2099 relative to 1980-1999


projection of future changes in climate sea level
Projection of future changes in climate – sea level
  • Note:
  • No upper bound
  • No likelihood
  • No best estimate
  • Model based estimate only, no expert judgment



sea level rise uncertainty why so large
Sea Level Rise UncertaintyWhy so large?
  • Understanding of some important effects that determine sea level rise is too limited
  • Published literature lacking
    • Climate-carbon cycle feedbacks
    • Changes in ice sheet flow

IPCC AR4 Synthesis Report wording

sea level rise uncertainty
Sea Level Rise Uncertainty

The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future. Therefore the upper values of the ranges given are not to be considered upper bounds for sea level rise.

If this contribution (the observed rates) were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios would increase by 0.1 m to 0.2 m. {WGI 10.6, SPM}

IPCC AR4 Synthesis Report wording

other examples of regional changes
Other examples of regional changes

Very likely increase in frequency of hot extremes, heat waves, and heavy precipitation

Likely increase in tropical cyclone intensity; less confidence in global decrease of tropical cyclone numbers

Poleward shift of extra-tropical storm tracks with consequent changes in wind, precipitation, and temperature patterns

Very likely precipitation increases in high latitudes and likely decreases in most subtropical land regions, continuing observed recent trends



21st Century Water Availability (Runoff) Changes

(Annually averaged)



  • Very likely runoff will increase in high latitudes.
  • Likely runoff will decrease over some subtropical and tropical regions.

IPCC AR4 Synthesis

time scales of response
Time scales of Response
  • Human and natural systems
    • Even if mandated, it would take a while to replace the current fleet of inefficient cars
  • Physical climate system
    • Greenhouse gas lifetimes in atmosphere
    • Ocean
    • Ice sheets
response time scales
Response time scales
  • Note response in 2020’s very similar in spite of very different emissions.
  • Note response in 2090’s much more scenario dependent.
  • Actions taken today only have large impacts in climate response in the future.

Anthropogenic warming and sea level rise would continue for centuries, even if GHG concentrations were to be stabilized at or above today’s levels.AR4 estimates 0.2 to 0.6m sea level rise per oC at equilibrium due only to thermal expansion of sea water.

Response time scalesRole of Oceans



How “smooth” is the future temperature increase?


January 2008 is a cold month

  • Drop as large as December to January fairly common
  • Cause?
    • La Nina
    • Variability over NH continents

Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.

Partial loss of ice sheets on polar land could imply meters of sea level rise, major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands.

Such changes are projected to occur over millennial time scales, but more rapid sea level rise on century time scales cannot be excluded.


There is medium confidence that approximately 20-30% of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5-2.5oC (relative to 1980-1999).

As global average temperature increase exceeds about 3.5oC, model projections suggest significant extinctions (40-70% of species assessed) around the globe.

Anthropogenic warming could lead to some impacts that are abrupt or irreversible

Based on current model simulations, the Meridional Overturning Circulation (MOC) of the Atlantic Ocean will very likely slow down during the 21st century; nevertheless temperatures over the Atlantic and Europe are projected to increase.

The MOC is very unlikely to undergo a large abrupt transition during the 21st century.

Longer-term MOC changes cannot be assessed with confidence.

Anthropogenic warming could lead to some impacts that are abrupt or irreversible

  • The IPCC is a successful mechanism communicating climate change science.
  • The IPCC can influence policy through government actions
    • Importance of gov’t approval of SPMs
  • The projections of climate change for this century are larger than what has occurred in the past century.
  • Response time scales, natural variability complicate discussion and hinder understanding.
  • We know a lot about future climate changes, but some surprises are expected.

More systematic understanding of the timing and magnitude of impacts related to differing amounts and rates of climate change.



There is high confidence that hundreds of millions of people will be exposed to increased water stress



There is high confidence that many species are at increasing risk of extinction with increasing temperature.



Globally food production is projected to increase at local temperature increases of 1-3 oC; decreases projected above (medium confidence).



There is high confidence that millions of people could experience more coastal flooding if global temperature increases more than 2C in this century.

Sea level has very long times and will continue to rise for centuries after stabilization of GHG.



  • The health status of millions of people is projected to be affected through, for example:
  • Increases in malnutrition
  • Increased deaths, diseases and injury due to extreme weather events
  • Increased burden of diarrhoeal diseases
  • Increased frequency of cardio-respiratory diseases due to changes in air quality
  • Altered spatial distribution of some infectious diseases.
some regions are likely to be especially affected
Some regions are likely to be especially affected
  • The Arctic, because of the impacts of high rates of projected warming on natural systems and human communities
  • Africa, because of low adaptive capacity and projected climate change impacts
  • Small islands, where there is high exposure of population and infrastructure to projected climate change impacts
  • Asian and African megadeltas, due to large populations and high exposure to sea level rise, storm surges and river flooding.
past ipcc projections vs observations
Past IPCC Projections vs. Observations
  • Projections very good – so far.
  • Lots of issues:
  • - No aerosols in FAR
  • - No volcanoes in all
  • - Natural variability large

IPCC AR4 WGI Chapter 1

special report on emission scenarios sres 2000 and post sres scenarios
Special Report on Emission Scenarios (SRES, 2000) and Post-SRES scenarios
  • Emissions depend on:
    • Population
    • Technological development
    • Society’s “choices”
  • No mitigation assumed
  • Emissions differ in beginning of this century
  • Emissions very different by end of century

Figure from AR4 Synthesis report