Overview of Climate Change Science:Fundamentals and Recent Developments

Prepared for the Climate Change and Adaptation Symposium

John Merrill,URI GSO

November 18, 2011

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Talk organization

I’ll present observational evidence of global warming, startingwith cycling between glacial and interglacial conditions over thelast half million years.

This will lead in to the role and impact of greenhouse gases,their recent buildup and consequent rapid changes, ending withinformation on how climate models are used to assess thesechanges.

The figures are from the 2007 report of the IntergovernmentalPanel on Climate Change, a UN organization of scientists anddiplomats charged with bringing the best science to bear on theunderlying science and on policy issues.

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Glacial-Interglacial Cycles - observational evidence

Data on atmospheric gases, collected from polar ice cores. Shadedareas = Interglacial periods.

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(IPCC 2007)

I Gas concentrationsrise/fall over 1000s ofyears.

I Middle curves: CO2 - red,CH4 (methane) - blue.

I Oxygen isotope data =ice volume andtemperature. T scale,right.

I Brief warm periods withhigh gas concentrations =Interglacials.

I Most of last 450 ky spentin glacial conditions.

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Ice core and instrumental gas data - last 2000 years

CO2 and CH4 concentrations over last 2 ky, including recentinstrumental data. Grey bands indicate range over prior 450 ky.

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(IPCC 2007)

I Note CO2 and CH4 nowfar exceed paleo values.

I These are greenhousegases.

I Together with watervapor, H2O, they keep theearth warm.

I Emphasis on CO2 andCH4 - they are altered byhumans, oranthropogenically.

I Increased greenhouse gasconcentrations and othereffects shift the globalheat budget.

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Induced heating from anthropogenic effects - Summary

Heating by greenhouse gases, and cooling by aerosols and landuse changes, over last 255 years.

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(IPCC 2007)

I Top two bars are heatingby changes in primaryanthropogenicgreenhouse gases.

I Land use change example:forest clearing.

I Aerosols are particulates,smoke, dust, haze.

I Changes in solar heatingare smaller thanuncertainty.

I Estimated heating minuscooling shown in brownbar.

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Schematic of global thermal energy balance

Heating from sunlight in left half; cooling by emitted thermalenergy on right. Multiple sources of data required for eachquantity.

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(IPCC 2007)

I Quantitative summary ofheat balance of earth.

I Thick bands on rightindicate atmosphere emitsalmost as much as itabsorbs.

I Greenhouse gases absorbinfared energy, heatingthe atmosphere.

I Increased amounts ofthese gases, increasedtemperatures.

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Evidence of warming in air temperature records

Global average land-surface air temperature over 1850 - 2003.Bars are individual years; curves are smoothed estimates bydifferent groups. Continued warming in recent years.

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Climate model simulations background

Simulations used to check our understanding of processes, andto predict state of climate in times to come.

I Numerical models simulate climate.

I Most significant physical processes included.

I Multiple models, using different approaches.

I Simulations for prior years compared with observations.

I Available computer power enables multidecadal runs.

I Processes can be included/excluded to highlight effects.

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Climate model simulations of temperature trends

Black, observed temperature; color, model estimates. Bottom, onlynatural processes; top, including anthropogenic greenhouse gases.

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(IPCC 2007)

I Dozens of independentsimulations of a centuryof climate.

I In blue at bottom aHypothetical scenario:steady GHG amounts.

I In yellow/red at topRealistic scenario ofincreasing gases.

I Vertical lines indicatevolcanic eruptions -transient cooling events.

I Increased gases mustcause the observedwarming.

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Recently observed trends - impacts of warming

I Arctic summer sea ice melt has reached record extent.

I Coming exploitation of Arctic fossil fuel and fisheries resources- troubling implications.

I Rate of melting of Greenland and Arctic sea ice has increasedand is accelerating.

I Warming of tundra - release of huge amounts of GHGstrapped there.

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Additional observed trends

I The ocean’s heat content is increasing. Isaac’s talk.

I Droughts more frequent and more intense.

I Record high temperatures outnumber record lows by over3:1.

I Midlatitude storm tracks migrating towards the poles.

I Climate zones are shifting toward the poles, by hundreds ofmiles.

I Floods becoming more frequent and more intense.

I Ocean acidification impacting fish, corals and other biota.

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Future scenarios simulated

Surface warming increment vs. time in various scenarios.Numerals indicate number of model simulations aggregated.

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(IPCC 2007)

I The rate of temperaturerise is increasing.

I That is, the warming isaccelerating.

I 5◦C range = 9◦F change.

I Lleveling off only withdrastic changes, e.g. B1.

I Temperature increaseexpected even if allemissions stopped now.

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(Wickford, 2011 - J. Denton)

IPCC is a process as much asa group. Their next Report isbeing drafted now, and is tobe released in June, 2013.

Emissions of GHGs haveexceeded targets everywhereand always, even withcurtailment steps.

Cautious scientists seeimportant impacts as “virtuallycertain”.

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