Transient versus Equilibrium warming

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1 Transient versus Equilibrium warming (Shallow ocean) Transient warming is smaller Transient warming is asymmetric across hemispheres Transient warming is modest in the northern North Atlantic

2 Ocean heat uptake - warming in the ocean from 2XCO2 (deg C) Antarctic Deep Heats up! Arctic Near Surface Heats up

3 Equilibrium warming estimates from 2XCO2 Basic Global Warming Forecast Equation DT s = l DF Table 1: Climate sensitivity D T(2xCO2 ) l Notes (K) (K/(W/m 2 )) Stefan-Boltzmann Law (no feedback case) IPCC low IPCC medium IPCC high

4 Equilibrium warming from 2XCO2 DT s ranges from C National Research Council, 1979: "We estimate the most probable global warming for a doubling of CO2 to be near 3 degrees C, with a probable error of plus or minus 1.5 degrees." IPCC, 2001: "Climate sensitivity [to CO2 doubling] is likely to be in the range 1.5 to 4.5C." The problem is, this range is awfully large (factor of three!) Is it even useful for policy-making purposes?

5 Question How long to heat surface by 1K given climate forcing of 4 W/m 2? - atmosphere - surface ocean (~100 m) - entire ocean (~4000 m deep) Note: 4 W/m 2 is forcing for doubled CO 2 (Ignore carbon uptake) Specific heat capacity, C How much energy per unit mass does it take to warm a substance by one degree Kelvin? C = Energy Absorbed Mass Temperature Change = E M DT E = C*M*DT = Energy Required to raise temperature by DT

6 Time (s) = Energy Required (J) Energy Flux Rate (W) Consider 1 m 2 of Earth surface Time (s) = E (J/m 2 ) 4 (W/m 2 ) = C*M*DT 4 (W/m 2 ) Reservoir Specific Heat Capacity, C (J/kg/K) Mass (kg/m 2 ) Time to heat by 1K for flux of 4 W/m 2 Atmosphere days Surface Ocean years Entire Ocean* * years *assuming instantaneous mixing - which is unrealistic

7 Question How long to heat surface by 1K given climate forcing of 4 W/m 2? Answer (according to this simple model) a. 30 days if we just have to heat the atmosphere b. 3 years if we just have to heat the ocean surface layer c. 130 years if we heat the entire ocean via turbulent mixing (silly) Conclude Surface ocean provides thermal inertia on time scale of several years Deep ocean provides thermal inertia on time scale of many centuries Oceans have a very strong stabilizing effect on climate

8 aerosols: direct effect: reflect sunlight back to space indirect effect: modify clouds (more droplets) causing increase in cloud albedo The addition of direct effects of aerosol forcing played a key role in the 1995 IPCC report Specifically, this allowed a much improved match between models and 20th century climate change. Problem: forcing estimates of aerosols is -3 to 0 W/m 2 over the last century!!! So is this match incidental? HAVE WE REALLY ATTRIBUTED 20TH CENTURY CLIMATE CHANGE TO HUMANS?

9 Change in temperature projections over last decade IPCC 1995 projections: IPCC 2001 projections: 1.0 to 3.5 K 1.5 to 5.8 K >> 1.4 K increase in mean and 2.3 K increase in max in six years! IPCC explanation... "The higher projected temperatures... are due primarily to the lower projected sulfur dioxide emissions." translation: Aerosols don t offset GHG as much in 2001 projection > human feedback: people won't tolerate deadly pollution Aerosols forcing remains uncertain in the future BUT the forcing from GHG eventually far exceeds even the uncertainty in aerosols.

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12 A1F1, A1B, B1: Scenarios from IPCC, see SPM A1F1 B1 A1B A1F1 A1B B1 A1F1 B1 A1B A1F1 A1B B1 Range due to two factors: (1) variety of emissions scenarios and (2) Model disagreement A1F1 A1B B1

13 Why do models disagree? Clouds, clouds, clouds - either positive or negative feedback depending on height and thickness Magnitude of Ice-albedo feedback Ocean heat uptake

14 Uncertainty in 20th century attribution: Uncertainty in total forcing (primarily due to aerosols) Uncertainty in 21th century projections: Uncertainty in total forcing AND disagreement among models STILL IPCC REPORTS FIND 1990: "generally consistent" 1995: "a discernable influence" 2001: "new and stronger evidence that most of the observed warming over the past 50 years is attributable to human activities."

15 Global warming impacts No one lives in the global mean climate Climate change in a specific region is what we really care about... but, regional changes are much harder to predict than global-mean changes

16 Sea-level projections sea-level is a global index but it has regional impact difficult to predict because it is a balance of sources and sinks thermal expansion causes rise Antarctica is expected to grow (more snowfall than melting) Greenland is expected to shrink (more melting than snowfall) mountain glaciers are expected to shrink dramatically, but this is a fairly small reservoir

17 The problem of West Antarctica Ice streams into shelves (floating fresh ice). Warming the ocean could cause shelves to destabilize Weddell Sea + ice shelf Trans-Antarctic Mountains West Antarctica Ross Sea + ice shelf

18 IPCC Fig Sea level rise on right Range is cm w/o land-ice Range is cm w/ land-ice but W. Antarctica stays intact

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