ESS15 Lecture 16. Past climates, Part 1

ESS15 Lecture 16 Past climates, Part 1

Thanks for your midterm evaluation! Based on the results I have decided to: Post solutions to practice exam questions & i-clicker questions. Slow down on especially important conceptual diagrams. Do more math problems during lecture. Continue rapping* * Form a band!?

Review.

Earth s climate as a black box F = S 0 (1 α) 4 240 W m -2 Climate System 15 C T S 15 C Absorbed Sunshine In climate feedbacks Surface Temperature Out

Climate forcing, response, and sensitivity. Essential for thinking about climate change. Forcing (change in absorbed sunshine) Response: (Change in Surface Temperature)

Climate sensitivity - an important definition. The climate sensitivity is a number that quantifies how many degrees of warming/cooling occur per W/ m 2 addition/removal of energy to the climate system. Last time we calculated that if the Earth were a bare rock its climate sensitivity would be: ΔT / ΔF ~ 0.27 K/W/m 2

Of course the Earth is not a bare rock. So the actual climate sensitivity is not 0.27 K/W/m2. It is determined by multiple competing feedbacks.

Climate feedback processes Positive Feedbacks (amplify changes) Water vapor Δ Lapse Δ high cloud Δ low cloud Ice-albedo High clouds ΔF ΔT S Δvapor Δ albedo Negative feedbacks (damp changes) Lapse rate Low clouds

Water vapor feedback. Radiative forcing warms surface Warmer surface evaporates more water Warmer air can hold more water Increased water vapor (GHG) absorbs more outgoing radiation, amplifying warming

i-clicker - ice albedo feedback Warming leads to mel0ng ice, exposing darker surfaces that absorb more solar radia0on. This is: A: A posi)ve feedback process, amplifying warming. B: A nega)ve feedback process, resis)ng warming through added cooling. C: Could be either, but depends on where the ice is. D: Not a feedback process.

Ice - albedo feedback. Radiative forcing melts snow and ice Darker surface absorbs more radiation Amplifies warming or cooling

Cloud feedbacks - depend on the cloud type. High clouds act like greenhouse gases - trap longwave radiation, heat the surface. Low clouds act like ice sheets, reflect solar radiation, cool the surface. Additional water vapor makes more clouds Low clouds cool, but high clouds warm Cloud feedbacks can be both positive and negative.

In the last decade climate scientists have discovered that high cloud feedbacks are very likely positive.

But the sign of the low cloud feedback is uncertain. (A major frontier of modern climate science)

"Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds IPCC AR5, Ch. 7

In other words, we happen to live next to one of the most climatically mysterious cloud systems on the planet.

Think about that next time you witness our coastal clouds. http://icons.wunderground.com/data/wximagenew/j/jrkerns/3.jpg

The marine layer clouds we see at the coast are just the edge of a massive brilliant white sheet of low clouds extending over 100,000 square kilometers! San Diego, CA http://icons.wunderground.com/data/wximagenew/n/nickgeorge/4-800.jpg

View out the airplane window on your way to Hawaii.

The Californian stratocumulus system is one of several massive low cloud decks in other regions of the Earth. low cloud cover R. Wood, Monthly Weather Rev., 2012

Summary - Low clouds and climate change If Earth s low cloud decks shrink it will amplify global warming in a positive feedback, like shrinking ice sheets. But if low cloud decks grow in size with climate change this could partially offset global warming, helping stabilize the planet! Climate science cannot yet predict what they will actually do - an active research frontier. Some geo-engineering enthusiasts want to try to artificially brigthen low clouds to offset global warming.

The trillion dollar question

A lot of feedback processes - but what s the total climate sensitivity if you add them up?

Estimating total climate sensitivity 1. Paleoclimate analogs: how much has climate changed in the past when forcing of known strength was applied? Advantage: all feedbacks included Disadvantage: hard to know exactly how much forcing & global temperature response 2. Calculation from physical principles including feedback processes (complex global climate models) Advantage: Physical insight Disadvantage: All models are wrong

Learning from the past 1. Geologic past (100 s of millions of years) 2. Deglaciation analog (18,000 years ago to preindustrial time) 3. Last Millennium analog (Medieval Warm Period to Little Ice Age) 4. Modern Climate Record (20 th Century changes) The further back we go, the less data we have to work with. Using modern data, we have only brief transients to study.

Climate changes in the recent past provides a way to estimate how sensitive climate is.

Ice age world Two main causes of fewer Watts: 1. High albedo 2. Low CO 2

CO2 and the ice ages Over the past 420,000 years atmospheric CO 2 has varied between 180 and 280 ppm, beating in time with the last four glacial cycles CO 2 ice ice ice ice Vostok (400k yr) Ice Core data (Petit et al, 1999)

Paleoclimate observations of the last glacial can tell us something about climate sensitivity. Recall climate sensitivity is the change in temperature per W / m2 change in forcing So, if we can estimate how much colder it was 20,000 years ago. And if we can estimate how many less Watts the Earth system was forced by compared to today. Then we can estimate a climate sensitivity from observations.

Reconstructed forcing of the last ice age. implies climate sensitivity of 0.75 K/ W / m2. This is three times higher than the bare rock model predicts Source: Hansen and Sato (2011)

i-clicker: The fact that the climate sensitivity of Earth is 3x stronger than the bare rock model predicted means our climate has: Stonger positive feedbacks than negative feedbacks. Stronger negative feedbacks than positive feedbacks. Equally strong positive and negative feedbacks.

But 20,000 years was a long time ago.

What can we learn from the more recent past?

What processes force changes in climate by changing the Watts absorbed by the system? Changing GHGs Changing sun

Our variable star. i-clicker: How big a change in incoming energy (Watts per m 2 ), happens from the crest to the trough of the 11-year solar cycle? A: < 0.25 W/m 2 B: 0.25-0.5 W/m 2 C: 1-3 W/m 2 D: 4-8 W/m 2 E: > 8 W/m 2

How big a change in absorbed Watts, peak vs. valley of 11-yr solar cycle? Climate forcing = ΔS(1-α)/4 ~ 2 W m -2 x 0.7 / 4 = 0.35 W m -2

What processes force changes in climate by changing the Watts absorbed by the system? Changing GHGs Changing sun Volcanic aerosols

BOOM! Mt. Pinatubo, 1991 Volcanoes release huge amounts of SO2 gas and heat SO2 oxidizes to SO4 particles ( aerosol ) and penetrates to stratosphere SO4 aerosol scatters solar radiation back to space

Volcanic aerosol amount over the past 50 years. After major eruptions, the world cools for a few years.

What processes force changes in climate by changing the Watts absorbed by the system? Changing GHGs Changing sun Volcanic aerosols Pollution aerosols

People make aerosol particles too from air pollution.

Human aerosol pollution can change cloud albedos in certain conditions. Ship tracks off west coast Cloud droplets condense on tiny particles Makes more/ smaller cloud drops Clouds are brighter (higher albedo )

i-clicker Which of these has been increasing Watts most significantly over the past 1000 years? Changing GHGs Changing (A) Changing sun (B) Changing volcanic aerosols pollution aerosols (D) (C)

Reconstructed climate forcing since 1000 AD 1816 1991 0.5 1.0 W m -2 Tambora year w/out a summer Pinatubo

i-clicker: Over the past 500 years, the solar Watts have been: A: generally decreasing B: generally increasing C: neither decreasing nor increasing D: Insufficient data to say.

Which has been adding more Watts recently sun or people? sun people Sun brightened by ~ 1 W/m 2 gradually over past 500 years GHGs have already added even more than this (1.5 W/m 2 ), more rapidly. During next 100 years people could add a heck of a lot more Watts!

Global climate during the past 2000 years

This 600-year period of global cooling provides another analogy for estimating climate sensitivity. 0.8 K

The Second millenium analog. Cooling from Medieval Warm Period to Little Ice Age ~ 0.8 K Solar forcing changed by ~ 1.0 W m-2 (somewhat complicated by volcanic forcing) Implied total climate sensitivity (including feedback) 0.8 K per (W m-2) 1826

The Volcanic analog.

The Volcanic analog. Cooling following eruption of Mount Pinatubo in 1991 ~ 0.5 K Volcanic forcing ~ 0.7 W m -2 Implied total climate sensitivity (including feedback) 0.7 K per (W m -2 )

Climate sensitivity Many lines of evidence from the past suggest about 0.8 C of warming per (W m -2 ) Remember each doubling of CO 2 adds 3.7 Watts per square meter So expect about (3.7 W m -2 ) x (0.8 C (W m -2 ) -1 ) = 3 C per doubling of CO 2

Climate sensitivity Distribu(ons and ranges for climate sensi(vity from different lines of evidence. The circle indicates the most likely value. The thin colored bars indicate very likely value (more than 90% probability). The thicker colored bars indicate likely values (more than 66% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5 C) and most likely value (3 C) are indicated by the ver(cal grey bar and black line, respec(vely (Source: KnuN & Hegerl, Nature, 2008)

Current and future climate change in the context of the past 20,000 years.

Greenhouse Gas Radiative Forcing Note different scales Modern changes comparable in magnitude to postglacial, but much faster!

Temperature history since the ice age. (when my son is old) Holocene Optimum Modern Record Deglaciation Little Ice Age

Next time: Climates of the deep past (geologic time)