Week 9 Flashcards
If your model is unperturbed by human influence etc. what would you expect it to do and what would be a sign something is wrong?
you would want it to maintain a constant temperature
if it is gradually getting warmer - there may be too much energy not escaping at the toa
tweak parameters in the model to do with sub-grid scale processes like clouds to alter the behaviour of the model
what is a constraint?
using observable relationship to constrain the simulations
eg. southern hemisphere jet move with clamte change, high speeds south does that move closer to the poles as we get warmer
What are some general reasons for differences in climate model projections?
differences in atmosphere and ocean parameterisations (huge uncertainty- no one answer)
these differences affect the climate feedbacks especially cloud feedbacks
to assess the range of possible future outcomes, we perform large model inter-comparisons
What does CMIP stand for?
coupled model intercomparison project
what does CMIP do?
international collaboration of climate science
coupled modles = coupled ocean atmosphere models plus other components
several doznes of climate modelling groups around the world perform the same experiments and make the results available for free
all models have a slight geographic bias
For CMIP6 what is the core experiement features?
it is called DECk
pre-industrial historical
1% Co2 increase every year
4x Co2 instantaneous quadruple to see how much warmer the model gets over time
historical changes - same boundary conditions and simluate the 20th cent - how well can this be done
What is part of the CMIP experiements?
there are lot of potential experiements simple or advanced - run to test for a range of factors like potential geoengineering
what does SSP stand for?
shared socioeconomic pathways
What are SSPs?
a set of socioeconomic pathways that can be modulated by different levels of mitigation action
gives a matrix for possible forcing scenarios
smaller forcing = smaller temperature change
these give you boundary conditions
they have storylines narratives describing how the associated emissions might come about - they enable fixed comparison between climate models
What is the difference between SSP and RCPs?
SSp = AR6 and RCp = AR5
rcp = representative concentration pathways
the two scenario groups were chosen to sample the whole range of plausible forcing outcomes
RCP 8.5 and SSP 8.5 = burn everything - 8.5 is the amount of the atmospheric forcing - for every sq meter of the atmosphere you have 8.5 what’s coming in - earth heats up
SSP 5 - ff development scanrio
What is the climate sensativity in CMIP6?
on average 3.9k as opposed to 3.5k in CMIP5
this is due to more positive cloud feedback and changes in cloud phase = reducing potential for negative feedback associated with clouds
How is internal variability a source of uncertainty?
tested using an ENSEMBLE = a set of identical simulations except for slightly different initial conditions
each ensemble member experiences different weather and internal variability
eg. the stage of the quasi-random oscilation of el-nino are you in - uncertainty at which stage you will be in thus uncertainty for global mean variation
what is the likelihood of each of the potential future cliamtes in an ensemble?
they are all equally likely
Does it matter what the starting temperature is in models?
not really- they will all end up with different temperatures - however it is trend in temperatures that is important
they can look different due to starting conditions and internal variability
but ensembles help quantify the internal variability of a projections
how is the scenario that we are in an uncertainty?
dont know which trajectory we are in
this uncertainty drives divergence in time and future feedbacks
uncertainty as to what feedbacks have been triggered in each scenario have huge implications for future projections of climate and resulting global mean temperature ranges
What are the three projection uncertainties?
- scenario - how much radiative forcing in the future
- model physics - uncertainty about the physical processes mainly the feedbacks
- internal variability - climate varies naturally from year to year - decades to decade
How much does a model vary for a given projection?
for a given scenario, projected global warming varies by a factor of ~2 among global climate models
How does spatial scale impact scenario projections?
location specific is more difficult to predict as regional difference become larger, more focused is difficult to predict
parameterization becomes increasingly important
the different scenarios give the same change but the uncertainty is huge - precipitation
How does uncertainty change over time?
the importance of different uncertainty sources become more or less over time
on long time scales the scenario uncertainty dominates for global mean temperature
range of internal variation are more important in the shorter term projections
model variability is somewhere in the middle - change depends on what we are looking at
how about signficance of different projection uncertainty for the mean surface air temperature in the UK?
internal variabilty remain signficant for a longer period of time due to the jet stream
model uncertainty is important in the mid timescales
what about the significance of model uncertainty for precipitation predictions?
scenario uncertainty has a relatively small effect
- uncertainty remains high for some time even with a perfect model
What is the relaitonship between global warming and total CO2?
it is basically linear
What is the carbon emissions budget>
if we are to meet a particular warming target we have a carbon emissions budget of the max allowable cumulative emissions
What is the transient climate response to cumulative Co2 emissions?
the amount of warming for a given amount of emissions
the gradient of the line between co2 and temperature increase
What unit of measurement is TCRE?
K/GtC
What is the change in atmospheric carbon unit of measurement?
GtC
What does the TCRE depend on?
- the amount of warming per unit Co2 increase - the climate sensitivity or TCR
- the amount of Co2 that remains in the atmsiphere for a given amount of emissions
what is the airborne fraction?
the amont of Co2 that remains in the atmosphere for a given amount of emissions
what is the TCRE equation?
TCRE = chagne in temp/cum. emissions
= the TCR x airborne fraction
how are the TCRE and the carbon budget related?
the TCRE determines how much Co2 can be emitted for a given global warming limit
therefore the carbon budget is dependent on the TCRE
higher TCRE = smaller carbon budget for a given limit
what is the likelihood of staying below 2oc on the current carbon budget of 3thousand Gtc
66% change of staying below 2oc
Why is there a large uncertainty range for the TCRE?
uncertainities in both cliamte sensativity and carbon airborne fraction
What is the usual range of certianty for a given carbon budget?
50-60% change of not exceeding a given limit
What is the idea of negative emissions?
in order to reach 2/1.5o most scenairos suggest emissions of Co2 would have to go below 0 - negative emissions
very challenging
would have to be removing carbon from the atmosphere
How can you make projections about future climate?
using a climate model, but you need to provide the boundary conditions - emissions and concentrations- given by a scenario
what are the uncertainites in cliamte projections?
uncertainties come in three main forms- internal variability, model errors and scenario uncertainty
with varying importance over time and region and variable too
what do climate projections tell us?
warming is proportional to cumulative carbon emissions, which gives us a carbon budget for a given climate target like 2oc warming
at what temp does CMIP6 predict sea ice will be lost?
2 degree
What has happened with sea ice and how does it compare with what we predicted?
happened a lot faster than we predicted
roughly proportional to surface termparure change
feedbacks and tipping points however, it has progressed much more rapidly than we anticipated
