The IPCC Assessment Process: Future Projections of Climate

Change

Ronald J StoufferGeophysical Fluid Dynamics

LaboratoryNOAA

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?What is the IPCC?http://http://www.ipcc.chwww.ipcc.ch

• 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?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 FindingsIPCC 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 warming and other associated climate impacts (precipitation, sea level, etc.) over the next century.

IPCC- How is it organized?

• Three Working GroupsI. 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 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

• 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 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 consensus

> Can I live with statement?– Is statement wrong?– In plenary this question can be very important

Uncertainty Different 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- 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

• 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

Valencia, Spain November, 2007

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)

• 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 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 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

• 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.

IPCC WGI SPM

CarbonDioxide

Methane

NitrousOxide

Over the last 100 years the:Surface temperature is increasing

Sea level is rising

Snow cover is decreasing

IPCC WGI SPM

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).

IPCC WGI SPM

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

IPCC WGI SPM

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 2100IPCC WGI SPM

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

IPCC WGI SPM

Projection of future changes in climate – sea level

IPCC WGI SPM

Note:• No upper bound• No likelihood• No best estimate• Model based estimate only, no expert judgmentmeters

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 UncertaintyThe 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• 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

IPCC WGI SPM

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

and tropical regions.

21st Century Water Availability (Runoff) Changes(Annually averaged)

WetterDrier

IPCC AR4 Synthesis

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

• 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.

Response time scalesRole of Oceans

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.IPCC WGI SPM

Response time scalesRole of Oceans

What would 1 meter do?What would 1 meter do?

Variability

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.

IPCC WGI SPM

– 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

Summary• 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.

Thank you

More systematic understanding

of the timing and magnitude of

impacts related to differing

amounts and rates of climate

change.

WaterThere is high confidence that hundreds of millions of

people will be exposed to increased water stress

EcosystemsThere is high confidence that many species are at

increasing risk of extinction with increasing temperature.

FoodGlobally food production is projected to increase at

local temperature increases of 1-3 oC; decreases projected above (medium confidence).

CoastsThere 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.

HealthThe 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

• 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

• Projections very good – so far.

• Lots of issues: - No aerosols in FAR - No volcanoes in all - Natural variability large

IPCC AR4 WGI Chapter 1

• 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

Special Report on Emission Scenarios (SRES, 2000) and Post-SRES scenarios

Figure from AR4 Synthesis report