week 5 Flashcards
(28 cards)
what is the IPCC
the IPCC is the intergovernmental panel on climate change, the united nations body for assessing climate change science. its purpose is to provide scientific information to governments that they can use to develop climate policies
what is radiative forcing
- radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system
- positive RF results in a net positive radiative flux that will warm the earth
- negative RF results in a net negative radiative flux that will cool the earth
- factors with the same radiative forcing should produce the same change in global mean temperatures
what are the biggest radiative forcing agents
- CO2 has the largest warming effect
- other GHGs also important
- aerosols (pollution) have a large and uncertain cooling effect
- anthropogenic forcings much greater than natural
describe the radiative forcing timeseries
positive forcings:
- GHG forcing rising
- other anthro rising
- solar 11-year cycle
negative forcing:
- aerosols forcing peaked in the 1980s, stable-ish
- land-use (deforestation)
- periodic volcanic eruptions
describe how aerosols offset a substantial fraction of forcing
- net anthropogenic forcing is significant and positive
- however, very roughly one 3rd of the forcing from GHGs is offset by the cooling effect of anthropogenic aerosols
- cleaning up particulate (aerosol) pollution will remove this cooling effect and result in greater warming
describe climate feedbacks and temperature response
- as the planet warms in response to a radiative forcing there are internal changes in its radiative processes, altering its radiative flux
- assuming these internal changes in the top-of-atmosphere radiative flux (FTOA) are proportional to surface temperature change (Ts):
deltaFTOA = lambda x deltaTs - lambda is the climate feedback parameter with units of Wn^-2K^-1. this must be negative for the earth to stabilise at a new temperature
- in equilibrium the surface temperature response to an effective radiative forcing (ERF) is given by:
deltaTs = - ERF/lambda - the climate feedback parameter determines how sensitive the climate is to changes in forcing and is thus a critical climate parameter
- it is possible to break it down into linearly additive components
(see powerpoint slide 11)
describe planck or black-body feedback
- as the planet warms it emits more infrared radiation to space
- this is the strongest negative feedback in the climate system
describe water vapour feedback
- as air warms, it carries more water vapour (~7% degrees C^-1)
- as the planet warms, there is more water vapour in atmosphere
- water vapour is a powerful GHG gas, so this adds to warming effect producing a positive feedback.
describe lapse rate feedback
- warmer vapour has stronger warming effect at high altitudes
- this decreases the rate at which temperature decreases with altitude (lapse rate)
- warmer temperatures at high altitudes increase outgoing infrared radiation
- the lapse-rate feedback is therefore negative:
describe cloud feedbacks
- clouds scatter light producing a strong negative shortwave forcing (~50 Wm^-2). they also absorb longwave radiation producing a positive longwave forcing (~30 Wm^-2)
- climate change affects clouds in a variety of ways but they are believed to produce a net positive feedback, though it is very uncertain:
describe snow, ice and surface albedo feedback
- as temperature rises, snow and ice melts. snow and ice have much higher albedos than the surfaces they cover. so as they melt the surface albedo drops and more sunlight is absorbed, producing a positive feedback:
lambdaalb = +0.27 +/- 0.06 Wm^-2K^-1 - its effects are greatest at the poles and contribute to the greater warming seen there, known as ‘polar amplification’
describe climate sensitivity and the climate feedback parameter
- equilibrium climate sensitivity (ECS) - the equilibrium increase in global mean temperature in response to a doubling of CO2 concentrations
ECS (deltaT2xCO2) is inversely proportional to the climate feedback parameter (lambda)
deltaT2xCO2 = deltaTECS = -ERF2xCO2/lambda - if the black-body feedback were the only feedback, then:
deltaT2xCO2 = -ERF2xCO2/lambdaBB = -3.71 Wm^-2/-3.2 Wm^-2K^-1 = 1.16K - instead, due to other feedbacks, the intergovernmental panel on climate change estimates that climate sensitivity is likely between 1.5 degrees C and 4.5 degrees C, with a best guess of 3.0 degrees C.
what is the gregory plot
by plotting top-of-atmosphere (TOA) radiative imbalance against global mean temperature for each year in an instantaneous forcing experiment a linear relationship is evident
- the y-intercept gives an estimate to the radiative forcing. the x-intercept gives an estimate of the equilibrium temperature response (ECS is a 2xCO2 experiment)
- the Gregory plot allows climate modelers to estimate both these values in a relatively short model run
describe how the ocean determines the long-term temperature response to forcing
- to reach a new equilibrium the planet must warm sufficiently that its top-of -atmosphere radiative balance is restored
- 93% of the heat trapped by anthropogenic actions has ended up in the oceans
- ~40% of that has accumulated in the top 300m and ~60% in the top 700m
- the ocean remains a long way from equilibrium. it will take thousands of years for ocean to fully warm up in response to current anthropogenic forcings
describe the transient climate response
transient climate response (TCR) - the temperature at the point at which CO2 concentrations double in an experiment where CO2 rises at 1% per year (after 70 years, as 1.0170 = 2.007)
- TCR is a more relevant metric for estimating the magnitude of global warming this century than equilibrium climate sensitivity
- the transient climate response is estimated to be in the range of 1.0-2.5 degrees C, as compared to the1.5-4.5 degrees C of ECS
describe TCR vs ECS
- CO2 concentration increases at 1% a year until it reaches 2xCO2 or 4xCO2 where it stops
- temperature rises linearly while CO2 concentration increases exponentially. CO2 forcing rises logarithmically with CO2
- temperatures then slowly evolve towards their equilibrium value as the oceans warm
what are factors shaping the global temperature response to a change in forcing
- the less negative the feedback parameter is, the more the planet must warm to reach a new radiative equilibrium -> the higher the climate sensitivity
- ECS is always greater than TCR (temperature at point CO2 doubles in experiment where CO2 is 1% higher per year) ECS>TCR
- the less efficient the ocean is at transferring hear from the surface to the deep ocean, the greater the fraction of this warming realised this century -> the smaller the difference between ECS and TCR
describe the carbon cycle
- atompsber, surface ocean and land are strongly coupled
- over millennia, most CO2 will end up in deep ocean
- over millions of years, most CO2 will end up in rock
Historical emissions and where they’ve ended up
The primary source of anthropogenic CO2
is from the burning of fossil fuels, however
land-use change (e.g. deforestation) is also an important source
CO2 emissions have ended up:
* ~45% in the atmosphere
* ~30% in the land
* ~25% in the ocean
describe energy use
- Fossil fuels are burned to produce energy so energy production must be decarbonized to bring emissions down.
Despite rapid growth in wind and solar generation in recent years, fossil fuel generation continues to rise, driven by the
demand for more energy. - Other low-carbon energy sources, except nuclear, have also grown
Who is emitting the CO2?
- China is the world’s largest emitter of CO2 but its per capita emissions are less than half that of the USA.
- India is a rapidly growing source of CO2 emissions, but its per capita emissions are still less than half that of the global mean.
- Another perspective is cumulative CO2 emissions by which standard the UK has
made a world-leading contribution.
what is the kaya identity
CO2 emissions = population x GDP/population x energy/GDP x CO2 emissions/energy
- The Kaya identity relates a set of key drivers for CO2 emissions, allowing their
relative contributions to be assessed.
- Economic growth has contributed about twice what population growth has to emissions
- The economy is becoming more energy efficient, countering growth in economy and population
- CO2 emissions have risen in lockstep with energy use → There has been no progress in decarbonising energy supply!
Scenarios – terminology used by the IPCC
The Representative Concentration Pathways (RCPs) are a set of scenarios
or pathways including time series of concentrations and emissions of:
* greenhouse gases, aerosols and chemically active gases
* land use/land cover
The Shared Socioeconomic Pathways (SSPs) are a set of scenarios or pathways depicting potential future global societal changes including:
* policy
* (in)equality
* economic development
* energy use
describe future emissions and climate policy
- A wide range of emissions scenarios have been developed by economic modelers.
- These are used to simulate possible future climate states
- The Paris 2015 climate agreement set a goal of limiting warming to less than 2 or 1.5 C
- However, the commitments made by countries (if met) may lead to something closer to 3 C
