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Uncertainty in climate science:what it means for the current debate

Myles AllenDepartment of Physics, University of Oxford

myles.allen@physics.ox.ac.uk

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Three kinds of uncertainty

Big picture uncertainty: what is the chance that climate scientists have got it all wrong?

Large-scale projection uncertainty: how much global warming should we expect over the coming decades and centuries?

Small-scale prediction uncertainty: what does this mean for flood risk in OX1/OX2?

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Copenhagen Diagnosis

Big picture: global emissions continue to rise

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And the climate system continues to warm – on multi-decade if not sub-decade timescales

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January 2010 – the warmest on record:but not everywhere

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But can we trust the record?

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Yes. The impact of the “climategate” revelations on the instrumental temperature record

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Multiple indicators of “unequivocal” warming: Arctic sea ice in September 2005 & 2007

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Uncertainty in projected warming over the next few decades

Global temperature response to greenhouse gases and aerosolsSolid: climate model simulationDashed: recalibrated prediction using HadISST & CRUTEM data to August 1996(Allen et al, 2000)14 years

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There was a time when people took 14-year climate forecasts seriously

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The article in question

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Uncertainty in projected warming over the next few decades

Global temperature response to greenhouse gases and aerosolsSolid: climate model simulationDashed: recalibrated prediction using HadISST & CRUTEM data to August 1996(Allen et al, 2000)14 years

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Uncertainty in projected warming over the next few decades

Global temperature response to greenhouse gases and aerosolsSolid: climate model simulationDashed: recalibrated prediction using HadISST & CRUTEM data to August 1996(Allen et al, 2000)Forecast verification 01/01/00 to 31/12/09

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But what does this mean for flood risk in OX1?

South Oxford on January 5th, 2003

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Models predict increasing winter rainfall in North West Europe over the next 80 years

Figure 10.9

IPCC

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Are recent UK floods affected by climate change? Pall et al 2010

Aim: to quantify the role of increased greenhouse gases in precipitation responsible for 2000 floods.

Challenge: relatively unlikely event even given 2000 climate drivers and sea surface temperatures (SSTs).

Approach: large (multi-thousand-member) ensemble simulation of April 2000 – March 2001 using forecast-resolution global model (90km resolution near UK).

Identical “non-industrial” ensemble removing the influence of increased greenhouse gases, including attributable SST change, allowing for uncertainty.

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Performing simulations using distributed computing: climateprediction.net

>300,000 volunteers (50,000 active), 90M model-years

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Autumn 2000 in the ERA-40 reanalysis…

…and in one of the wetter members of our ensemble.

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Risk of floods in the year 2000, with and without the influence of increased greenhouse gases

2x increase in risk

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Uncertainties in the science: the current state of play

Uncertainty in basic causes is climate change is relatively low: IPCC (2007) concluded human influence was “very likely” the cause of most of the warming over the past 50 years – meaning a 10% chance that it wasn’t.

Uncertainty in large-scale trends still around a factor of two – meaning changes predicted for 2040 might occur in 2030 or 2050 – does this matter?

Uncertainty in the impacts of climate change, and the costs for nations, organisations and individuals, are still very high.