Climate Sensitivity & Climate Feedback Instructor: Prof. Johnny Luo

T s = 15 0 C > C Considering the Greenhouse Effect

Part I: Fundamentals of Climate Science 1.Introduction to the climate system 2.The Earth’s energy balance 3.Atmospheric radiation and climate 4.Surface energy balance 5.Atmosphere general circulation 6.Ocean general circulation Part II: Climate Change 1.Climate sensitivity & climate feedback 2.Natural & anthropogenic climate change 3.IPCC assessment of past & future climate change Energy budget (global balance & local imbalance) Fluid movement (due to local energy imbalance) What will happen if energy imbalance occurs at a global level? EAS 488/B8800 Climate & Climate Change

Outlines 1.Basic concepts: climate forcing, response, sensitivity and feedbacks 2.Climate sensitivity w/o feedback 3.Water vapor feedback 4.Ice albedo feedback 5.Cloud feedback 6.Tropical SST regulatory mechanism 7.Daisy world

Global energy balance: the starting point This chapter deals with: 1)what may break this balance? 2)what will happen when this balance is violated? First, we will look at a few fundamental concepts: 1)climate forcing, 2)climate response, 3)climate sensitivity 4)climate feedback

Climate Forcing: change in external factors that breaks the aforementioned energy balance (usually measured in changes in energy flux density in W m -2 at TOA). Climate Response: adjustment of the climate system in response to the external forcings (usually measured as change in surface temperature, T s ). Forcing & Response

Example: Forcing: When CO 2 is doubled, OLR will change from 240 W m -2 to 236 W m -2 (is this a warming or cooling for the climate system?). Response: For planet A: T s increases by 1 K; for planet B: T s increases by 10 K. Sensitivity: λ(A) = 1K/(4 W m -2 ) = 0.25 K/(W m -2 ). λ(B) = 10K/(4 W m -2 ) = 2.5 K/(W m -2 ). Climate Sensitivity: climate response (T s ) over climate forcing (Q).

Outlines 1.Basic concepts: climate forcing, response, sensitivity and feedbacks 2.Climate sensitivity w/o feedback 3.Water vapor feedback 4.Ice albedo feedback 5.Cloud feedback 6.Tropical SST regulatory mechanism 7.Daisy world

Suppose a forcing dQ is imposed on R TOA. Let’s calculate the climate sensitivity dT s /dQ. =1 equilibrium New equilibrium: R TOA = 0 Sensitivity parameter Sensitivity of the Earth’s climate

Suppose a forcing dQ is imposed on R TOA. Let’s calculate the climate sensitivity dT s /dQ. equilibrium Sensitivity of the Earth’s climate dQ: forcing; dT s : response

Suppose a forcing dQ is imposed on R TOA. Let’s calculate the climate sensitivity dT s /dQ. = 1 (b/c instantaneous changes in R TOA & dQ are the same) equilibrium New equilibrium at the TOA Sensitivity of the Earth’s climate dQ: forcing; dT s : response

Suppose a forcing dQ is imposed on R TOA. Let’s calculate the climate sensitivity dT s /dQ. = 1 (b/c instantaneous changes in R TOA & dQ are the same) equilibrium New equilibrium at the TOA Sensitivity parameter Sensitivity of the Earth’s climate dQ: forcing; dT s : response

Now we calculate: Assuming: 1) solar constant is unchanging, and 2) T e and T s change at the same rate

Now we calculate: Estimating the sensitivity parameter (T e = 255 K for current climate) What this means is: for every 1 W m -2 of energy we add to or subtract from the climate system, change of effective temperature (or surface temperature) will be 0.26 K. This is dictated by the Stefan-Boltzmann relation. Note that other factors (e.g., albedo, water vapor) are held unchanged at this point. Assuming: 1) solar constant is unchanging, and 2) T e and T s change at the same rate

Think-Pair-Share Questions: 1)For this kind of climate system, i.e., λ=0.26 K (W m -2 ) -1, what dQ is needed to warm up the Earth’s surface by 1K (i.e., dT s =1K) ? 2)How many W m -2 does the Solar Constant (S) have to increase to achieve dT s =1 K? Assume the albedo is 0.3 This is the climate sensitivity that is built-in of the σT e 4 relationship.

1 W m -2 -> 0.26 K  about 4 W m -2 is needed for 1 K. To achieve 4 W m -2 thru changing the Solar Constant (S 0 ) Think-Pair-Share Questions: 1)For this kind of climate system, i.e., λ=0.26 K (W m -2 ) -1, what dQ is needed to warm up the Earth’s surface by 1K (i.e., dT s =1K) ? 2)How many W m -2 does the Solar Constant (S) have to increase to achieve dT s =1 K? Assume the albedo is 0.3

Observations show that S 0 varies in magnitude of 1 W m - 2 (historical data dated back to 1870 can also support this estimate; however, over a longer history such as millions of years, there are larger variations). So,  S 0 (1-0.3)/4 = W m - 2. With this climate forcing, the response will be × 0.26 = K. Conclusion: the σT e 4 type of climate system is a rather stable one because of the fundamental way energy balance is achieved.

Outlines 1.Basic concepts: climate forcing, response, sensitivity and feedbacks 2.Climate sensitivity w/o feedback 3.Water vapor feedback 4.Ice albedo feedback 5.Cloud feedback 6.Tropical SST regulatory mechanism 7.Daisy world

Feedback mechanism: Sensitivity = Output/Input. With feedback, the sensitivity parameter will be different. T-P-S: How will water vapor affect the intrinsic climate sensitivity parameter? In other words, given the same forcing, how will water vapor changes the T s response?

Temperature Feedback mechanism: H2OH2O Water vapor: a strong positive feedback in global warming scenario Increasing CO 2 dQ dT s

Much of the infrared absorption (greenhouse effect) comes from the contribution of H 2 O IR absorption spectra (0 means no absorption; 100 means total absorption)

Clausius-Clapeyron relationship (C-C): saturation vapor pressure increases with temperature For current terrestrial conditions, for every 1 K increase in temperature, e s increases by ~ 6%. Calculate OLR as a function of surface temperature (holding RH constant so vapor pressure increases with T s ). This will need a radiative transfer model. For each T s, we calculate I (  OLR), so we have dT s /d(OLR)

OLR increases with increasing T s, but at a SLOWER rate than what the stefan-Boltzmann relationship gives: σ(T s - 30) 4. Conclusion: because of the water vapor feedback, climate sensitivity is HIGHER than a sigma-T- to-the-4th relationship. T * is the surface temperature (T s ). T * - 10, T * - 20, …, T * - 50 are attempts to estimate the effective temperature (T e ) from the surface temperature. For global average, T * = 288 K, T e = 255 K, so T * - 30 is a good approximation for global average curve. Red: assume clear sky Green: average cloudiness

Climate sensitivity has doubled with water vapor feedback K (Wm -2 ) -1

Sensitivity = response / forcing. Climate sensitivity w/o feedback: Double CO 2 forcing: 4 W m -2 -> 4×0.26 ≈ 1 K Climate Forcing: change in external factors that breaks the energy balance of the climate system (usually measured in changes in energy flux density in W m -2 at TOA). Climate Response: adjustment of the climate system in response to the external forcings (usually measured as change in surface temperature, T s ). Summary

Temperature goes up Feedback mechanism: H 2 O goes up Water vapor: a strong positive feedback, doubling the climate sensitivity Increasing CO 2 (or whatever causes the warming) dQdT s dQ dT s Summary