GPHY 314 Quiz: Week 6

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29 Dec 2020
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WEEK 6: Predicting Future Climate Change
Overall increase of 1.2°C since pre-industrial period, associated with variety of indicators
showing evidence of warming
Particular focus on carbon dioxide emissions, contributing to greenhouse warming
Attribution and detection methods lead us to conclude that 100% of temperature changes
are due to human activity
How can we predict what to expect in the future?
o Empirical estimates and climate models
Climate sensitivity: expressed as global temperature response to change in forcing
o Presented most regularly as response to a doubling of GHG forcing (+3.5 W/m2)
o Doesn’t necessarily have a huge practical purpose but it was used at the time, so
we continue to use it now
o This area is where there is still some debate in the literature
Not that it will lead to warming, but how much warming
o Answers how sensitive the climate is to a change in radiative forcing
Transient climate response and equilibrium
Transient Climate Response
Transient climate response: measure of shorter-term response to
change in radiative forcing
Only includes faster climate feedbacks i.e. shallow oceans, sea ice
Originally calculated as change in global temperature for a 20-year
period at time of doubling of CO2
o Originally defined in terms of climate modelling so not very helpful
Recent studies calculate it as the change in global temperature divided by the change in
radiative forcing over a period
o IPCC AR5 gives likely TCR range of ~0.27°C to ~0.69°C per W/m2
Assumes a liner response between changes in temperature and radiative forcing
o Remember this method excludes slower feedbacks
o TCR = ∆T / ∆RF
TCR = transient climate response
∆T = change in global temperature
∆RF = change in radiative forcing
Whole host of climate forcings, can’t just consider GHG forcings for temperature (even
though they’re significant)
o Current estimate +2.58 W/m2
o TCR = 1.2 / 2.58 = 0.47°C W/m2
Estimate right in the middle of the IPCC’s range, so it gives us some
confidence it’s pretty straightforward and matches with the best estimates
Important to remember there is always a lag in the system due to energy imbalance
(approx. 0.6 W/m2 at earth’s surface)
o Maybe temperatures we’re currently seeing are reflective of changes minus the
imbalance amount
o Account for difference… TCR = 1.2 / 2.0 W/m2 = 0.6°C W/m2
Subtracted 0.6 W/m2 imbalance estimate from values in equation
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This estimate keeps us on the higher range of the IPCC threshold
We can pretty much rule out the lower end of that spectrum
Applying this method to a doubling of GHG preindustrial forcing i.e. +3.6 W/m2
o 0.47°C W/m2 x 3.6 W/m2 = ~1.7°C
o 0.60°C W/m2 x 3.6 W/m2 = ~2.2°C
o ^Changes we expect to see for doubling of GHG forcing
o When will Earth reach this doubling point?
Total radiative forcing to date is +2.6 W/m2
Human radiative forcing currenting increasing at 0.44 W/m2 per decade
By about 2043 Earth will reach +3.6 W/m2
Summary: status quo emissions scenario
o Warming since preindustrial between around +1.7°C to +2.2°C by 2043
Warming of 0.5°C to 1.0°C from now
Equilibrium Climate Sensitivity
Equilibrium climate sensitivity: change in global surface temperature that will occur
once short- and long-term feedback processes reach
equilibrium; measure of long-term climate response to
change in radiative forcing
o Even though we might see transient climate response, expect climate to keep
warming slowly along longer scales
IPCC AR5 gives likely ECS range of 1.5°C to 4.5°C for a doubling of CO2
o Much higher than TCR, showing we have better control over transient and we can
get substantial changes of really long time-periods
o Can further constrain ECS by looking at variety of different methods
Usually best estimate over time is approx. +3°C
o Review paper by Sherwood et al. further narrowed range from IPCC numbers
(2.3°C to 4.5°C) -> More likely to be used in next IPCC report
o Scratch out 1.5°C lower bound… At least 2°C
o Helps reconcile with transient climate response, since short-term changes
sufficiently large enough that we know a lot of feedbacks are occurring
If TCR is in upper 1 threshold, would expect ECS to be higher
Linear association between ECS and TCR (x1.5 range)
Climate Modelling
Discussed via attribution and detection, also considering past climate changes which can
be used as simulations, now discuss how they can be used in future
Whole host of different groups around the world that do climate modelling
o Used to have just a few main models (mostly based in US and UK ~30 years ago)
o More global representation now, but difficult to keep people producing
comparable results
Climate Model Intercomparison Project (CMIP)
o Goal to advance understanding of earth system
o Global coordination of climate modelling
o Define common experiment protocols, forcings and outputs
o Ensure inter-model comparability i.e. standardized scenarios
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