Earth’s Climate Evolution –A New (Geological) Perspective.

Colin SummerhayesScott Polar Research Institute, Cambridge & Erskine Fellow, University of Canterbury, NZ

The Great Cooling – Did the Continents Move?

Tropical climate

Cold Climate

‘Continents therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages’.

Given Humboldt’s isothermal lines, Lyell deduced- if climate zones had not changed, then the continents had moved!!

“I will give you a receipt [i.e. recipe] for growing tree ferns at the pole, or if it suits me, pines at the equator; walruses under the line [the Equator], and crocodiles in the arctic circle.”

Charles Lyell (1797-1875) Lyell, 1875)

Alfred Wegener Drifts the Continents

Wegener fought against the entrenched ideas of Eduard Suess (1888) “The Face of the Earth” • southern continents linked by land that later sank.• the linked continents formed Gondwanaland.

• Not until plate tectonics came along were geologists convinced Wegener was right.

Wegener’s 1920 map of theCarboniferous coal period

at 300 Ma1880-1930

(1831-1914)

Earth’s Climate Evolution (2015)

First Palaeoclimate Maps (1924)

Wladimir Köppen and Alfred Wegener mapped coal (humid), aeolian sands (deserts), ice indicators (cold),salts (evaporites), and plant indicators (Glossopteris).

Deduced position of equator and south pole (Carboniferous) (without palaeomagnetic data).

(1846-1940)

Earth’s Climate Evolution (2015)

Pattern of indicators proved the persistenceof Earth’s climate system (e.g. Hadley Cell)

Modified from www.scotese.com

Closed seaway

Plate Tectonics Can Control Climate byChanging Oceanic Heat Path (1)

Pliocene Heat PathPliocene 3-5 Ma

Open seaway

Miocene Heat Path

Modified from www.scotese.com

Closed seaway

Plate Tectonics Can Control Climate byChanging Oceanic Heat Path (2)

Pliocene Heat PathPliocene 3-5 Ma

Open seaway

Miocene Heat Path

Warmth melts Ross Ice Shelf, parts of WAIS and EAIS.Globe warms 2-3°C; Sea level rise 15m.But, Pacific-Arctic current gradually cools Greenland making Arctic cold.

The Climate Clock: Celestial Mechanics Affect Climate

A = Eccentricity. Orbit now near circular; seasons are shorter and warmer at points close to sun. Max eccentricity = 23% cooler at aphelion.

B = Tilt of Earth’s axis: 21.5° to 24.5°. Currently 23° (medium seasonality). Most effective at high latitudes.

C = Precession (combines two signals)(i) Axial precession changes where the axis

points at a given time; (ii) Earth’s position at equinoxes and

solstices migrates around the orbit (Apsidal precession).

Earth now closest to sun in mid-winter.In 11,000 years it will be closest in

midsummer.V Masson Delmotte (2006).

Orbital Change Explains Ice Ages

(1879-1958)

1924 and 1941: Milutin Milankovitch

Insolation at 65°N in June over past 600Ka.Shading = coldsummers.Likely link to known glacial periods

Earth’s Climate Evolution (2015)

Günz Mindel Riss Würm

Köppen pointed out the links to field observations – invited Milankovitch to contribute to Köppen & Wegener 1924 “The Climates

of the Geological Past” (translated 2015).

The Orbital – Climate ConnectionAndré Berger refines Milankovitch calculations (1970s).

(1937-2006)

(1942-)

Nick Shackleton linksocean temperatureto orbital changes inmarine sediments(with Hays and Imbrie, Science 194, 1976).

Global Warming Art

The Orbital – Climate ConnectionAndré Berger refines Milankovitch calculations (1970s).

(1937-2006)

(1942-)

Spectral Analysis tiesocean temperature changeIn marine sediments to orbital changes (with Hays and Imbrie, Science 194, 1976). Global Warming Art

Logvariance

IPCC 2013 AR5 Fig 5.3

Solid lines = dataDashed = models

9°C range (polar); 4°C global.Last interglacial +2-3°C.

SL -130m at LGM= +4 to +9m

Ice Age Glacials and Interglacials

Change primarilydriven by orbitalchange

Cesare Emiliani (1955)uses oxygen isotopes in foraminifera to establish past ocean temperatures

Jim Zachos (2001)refines oxygen isotope curve (palaeotemperature)

Zachos et al, 2001

(1922-1995)

Lyell’s Great Cooling

DSDP collectsdeep sea samplesfrom 1968

JOIDESResolution

Deep Sea Drill Office

Confirming Lyell’s Great Cooling

(Francis and Poole, 2002)

Zachos et al, 2001

Jane Francis uses fossil leaves to establish temperatures

(1956-)

Alexander Is.2500 km fromS Pole, 100 Ma.

R NicholsINDEPENDENT MEASURES AGREEBUT: Solar radiationIncreased by 6% from600 Ma to present.So WHY THE COOLING?

14

The Role of Greenhouse Gases John Tyndall (1859)H2O, CO2, CH4, and O3 could have produced “all the mutations of climate which the researches of geologists reveal”

(1786-1830)

Joseph Fourier (1824)Air traps outgoinginfrared radiation.Warms the atmosphere. (1822- 1893)

Tyndall, 1959

CO2 and Ice Ages

Svante Arrhenius (stimulated by geologist Högbom) 1896: Decrease of 0.6 x CO2 lowers temp 5°C). Double CO2 raises temp 4°C. Names this: “Hothouse Theory”.

(1859-1927)

(1843-1928)

T.C. Chamberlin develops Theory of Climate Change (1899).

Volcanoes CO2; chemical weathering absorbs CO2

Cold glacial ocean dissolves CO2, keeps climate cool.Warm interglacial ocean releases CO2, keeps climate warm.

Carboniferous glaciation 300 Ma ago caused by coal extracting CO2 from atmosphere.

sourcesink

Understanding the Slow Carbon Cycle

Bob Berner (1935-2015) Models Earth’s Elemental Cycles

(Am.Jl.Sci. 1983): The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years

(2004) The Phanerozoic Carbon Cycle: CO2 and O2. Ox. Uni. Press

Berner, 1990

RCO2 = multiple of today’s CO2 value

The slow carbon cycle

Berner, 2004

Pre-Ice Core CO2 = Tectonic Balance of Volcanism vs Chemical Weathering

Beerling and Royer, Nature Geoscience 2011

Temperature of bottom water

Multiple sourcesfor past CO2.

Massive injection of1000+ Gt Carbon.

Temperature up 5-6°C

Slow recovery

Acidification ofbottom water dissolvesCaCO3; kills benthicorganisms.

Sea level up c.12m.

Climate Catastrophe at 55 Ma: Analog for Tomorrow?

IPCC WG-1, 2007, based on Zachos et al, 2008 56 55 54

14

8

ODP 2081267B

THE SLOW CARBON CYCLE AT WORK

Ice Age Glacials and InterglacialsCO2 causes 30-50% of temperature rise. Orbital change is

main driver.

400

Recent Sunspot Maxima same as 1780s & 1860s

Clette, Svalgaard, et al, Solar and Stellar Astrophysics (2014; figure provided by F Clette)

Based on 2014 revision of sunspot numbers since 1740.

Dalton Minimum Gleissberg Minimum

THE NATURAL ENVELOPE OF SOLAR CHANGE (250 years)

Little Ice Age(1250-1850)

Harvests better in warm periods (maximum sunspots)

Should be cooling now, but we are not

Changes In Solar Output Also Affect Climate

• When there are lots of sunspots, the solar wind (ejected plasma) is strong..

• The solar wind deflects cosmic rays.

• Cosmic Rays form 14C and 10Be isotopes.

• 14C and 10Be in ice cores/tree rings give us past sunspot variability.

http://25.media.tumblr.com/cc5e0188b54dac9a521693bbef51ca1e/tumblr_my4l616V7i1ri1icuo1_1280.jpg

Solar Variability Past 1000 Years

19501150

MWP

Little Ice Age

Steinhilber et al, PNAS 109(16) (2013)

Sunspot Min = Cosmic Ray Max = Strong 10Be and 14C = cold

Sunspot Max = Cosmic Ray Min = Weak 10Be and 14C = warm

Cold

Warm

Ups and downs = 208-year Suess Cycle, 88-year Gleissberg Cycle

THIS IS THE NATURAL ENVELOPE OF SOLAR CHANGE (1000 years)

GlobalPAGES 2ky(2013)

InsolationPAGES 2k(2013)

Insolation Control on Average Planetary Temperature Past 2000 Years

Earth’s Climate Evolution (2015)

Temperatures Moved Outside Natural Envelope After 1970.Plants emerging from under Baffin Island Ice last saw daylight 500 AD.

Natural Envelope of Temperature Combines Orbital Insolation (STRONG) with Solar Variability (WEAK)

MWP LIA

Wanner et al, JGSL 2015

How to account for rise in temperature since 1800?

David MacKay, 2009, Sustainable Energy Without the Hot Air

IPCC 2007 has very high confidence that humans are increasing the concentration of greenhouse gases, so making temperatures rise –

From ice cores

From atmospheric data

1769 Watt patents his steam engine;the start of the Industrial Revolution

1000 2000

260ppm

400ppm Present = 20ppm in 11 yrs

Glacial = 20ppm in 1000 yrs

450

500

550

-5 0 5 10 15 20Age / kyr

Jun

21 in

sola

tion

65N

/ W

m-2

150

200

250

300

350

400

CO

2 /

ppm

Our CO2 Emissions Cut Across The Cooling Trend Of Orbital Insolation

We should still be in the Little Ice Age.Greenhouse warming has pushed us outside

the natural envelope of climate change.

Source E Wolff, Cambridge; modified from http://www.geolsoc.org.uk/climaterecord (2013)

Pleistoceneglacial

Holocene interglacial

?Anthropocene?

Animation: Pollard and DeConto, 2009

Why Does This Matter?West Antarctica Melts 200,000 Years Ago(when next?)

Summary: the Sun is the major driver of climate, but -

CO2 can change Temperature, and Temperature can change CO2

Volcanoes provide major CO2 source (warming); chemical weathering provides major CO2 sink (cooling). Climate reflects the balance (source vs sink).

Orbital change plays a subsidiary role within a well-defined natural envelope.

Orbital warming releases oceanic CO2; a cooling ocean absorbs CO2.

Solar change modifies the effect of orbital change within its own, smaller, natural envelope.

CO2 is now above the natural envelope of the past 800 kyr.

Temperature is now above the orbital and solar envelopes of the past 1.6 kyr.

Geology says more CO2 will warm us more, raise our sea level, and acidify our ocean. The IPCC says the same. We should be concerned.

Thank you for your attention!

ANY QUESTIONS?

Global Temperature to end 2015

http://data.giss.nasa.gov/gistemp/graphs_v3/

Rapid Change

MIS: marine isotope stage; ACR: Antarctic cold reversal; DO: Dansgaard-Oeschger event; H: Heinrich event; AIM: Antarctic Isotope Maximum

Cores aligned by CH4 signal

EPICA, 2006

GreenlandTemp rise10°C in 10 yrs

Ocean See-Saw Causes Rapid Change

MIS: marine isotope stage; ACR: Antarctic cold reversal; DO: Dansgaard-Oeschger event; H: Heinrich event; AIM: Antarctic Isotope Maximum

Cores aligned by CH4 signal

EPICA, 2006

1. Northern sea ice forceswarm salty water south.2. Warming Gulf Stream forceswarm salty water north.3.Tipping point past which seaIce retreats within a decade.

CO2 in Fossil Air from Ice Cores

warm

cold

Dome C (3233m)CO2 – temperature link.

Same regardless of age down core (no CO2 migration or loss)

Hans Oeschger (1927-1988): First analyses of CO2 in bubbles of fossil air in ice cores (1978).

Siegenthaler et al, 2005

HOLOCENETemperatureChange

Waters et al,Science, Jan. 2016

Rising Human Influence – the Lead-Into the Anthropocene

Ruddiman et al., 2015

Neolithic

Cu Br Fe

Relation Between CO2 and Temperature Signals

Parrenin et al, Science 339 (2013)

EDC δ2H = temperature

Antarctic temperature stack

Forcing by CO2

CO2 content

NO DELAY - As one would expect from theory (warm water holds less gas; PV = kT)

IPCC 2013 AR5 Fig 5.3

Solid lines = dataDashed = models

400

280

9°C range (polar); 4°C global.Last interglacial +2-3°C

SL = -130m at LGM;= +4 to +9m

CO2 accelerates warming

Ice Age Glacials and Interglacials

CO2 nowoutside naturalenvelope

CO2 causes30-50% oftemperature rise.Orbital changeis main driver;

The Climate Clock: Predictable Astronomical Controls on Climate

Berger and Loutre, 2002, 2006

Milutin Milankovitch

Convertedastronomicalcalculations intovariations inclimate.

Long cold summers Initiated glaciation.

1879-1958

Eccentricity, Precession, Tilt

Pre-Ice Core CO2 = Tectonic Balance of Volcanism vs Chemical Weathering

Beerling and Royer, Nature Geoscience 2011

Temperature of bottom water

Multiple sourcesfor past CO2.

Independent confirmation(i) CO2 increase;CCD rise; High-MgCalcite (chalk); (ii) CO2 fall;CCD fall; Low-MgCalcite +Aragonite.

Magnitude and Pacing of Atmospheric pCO2 Change

Royer, Ann. Rev. Earth Planet. Sci. 2016. 44:277–93

Early to Late Eocene Cooling and CO2 Fall

Anagnostou et al, Nature, May 2016

Equilibrium Climate Sensitivity (ECS) = the global mean surface temperature change (°C) for2x CO2

18th-19th Century Climate Discoveries

1788 - The ‘Great Cooling’ (Cretaceous to now)

1800 - The role of sunspots

1830 - Changes of climate within individual continents

1830 - The role of orbital change

1837 - An Ice Age (Pleistocene) with ice sheets

1855 - Permo-Carboniferous glaciation

1859 - Greenhouse gases – CO2, H2O, CH4, O3

1896 - First climate model involving CO2

1899 - First geological model of climate change

Clark et al Nature Climate Change 2016

Sea Level - The Long Term View (10ka hence)Pg C Post- 2000Orbital forcing

T°C and CO2 stoprising

Sea level rises anextra 45m

Mass loss from ice sheets; Results of recent deglaciation; Both combined Assumes lowest

cumulative emissionsGlobal mean +21m

THE EQUILIBRIUM LAG

Dave Pollard & Rob DeConto

Müller et al, Geology, 2013; also see M Arthur, 1980

Flux of hydrothermal fluids from ocean crust= volume of mid-ocean ridge= volume of volcanic CO2 emitted= basalt alteration (source for Ca, sink for Mg)

CO2 low = COOL = Mg/Ca ratio >2 = High Mg-calcite + Aragonite reef builders (o)

CO2 high = WARM = Mg/Ca ratio <2 = Low Mg Calcite (forams & coccoliths form massive chalks).

Sediment Composition is a Climate ClueReverse engineering: Seafloor chemistry tells us about atmospheric

chemistry (Holland, 2003)

The calcite-aragonite metronome (Zalasiewicz & Williams, 2012). More dissolved CO2 (warm) = shallower CCD; less CO2 in air (cool) = deeper CCD [Palike et al. 2012, Nature 488].

CO2 and Global Temperature for 600 Ma

Royer et al., 2004

Oxygen isotope data from Veizer et al (1999) and Shaviv & Veizer (2003), unadjusted for pH effects.Suggests too many glacial periods (e.g.Jurassic).

pH adjusted oxygenIsotope data

Oxygen isotope fractionation depends on pH, hence on CO32- concentration, hence on CO2 abundance (Zeebe, 2007).

Modeled and ProxyCO2 data suggest thecorrect number ofglaciations

Glaciations

b. shows oxygen isotope fractionation is dependent on pH

Zeebe, Geochemistry, Geophysics, Geosystems V 8 (9) Q09002, 7 SEP 2007 DOI: 10.1029/2007GC001663http://onlinelibrary.wiley.com/doi/10.1029/2007GC001663/full#ggge1118-fig-0001

As Dissolved CO2 Increases, pH & Dissolved CO32- Decrease, & CCD Shoals

Can Geology Be Predictive?

Hansen & Sato, 2015

Exceptions to 1:1 relationshipEl Niños cause warmingBig volcanoes cause coolingSunspots cause both

Trenberth & Fasullo, 2013

§

Met Office 2013

Ice Modulates Climate Via Polar Amplification: poles warm, sea ice shrinks, albedo falls, sea absorbs heat,

poles warm more (+ve feedback) [or cool]

Global Sea Level from Satellite Altimeters

Met Office 2013

Oceans are warming, even down deep

Met Office 2013

Unprecedented deep ocean warming since 2000 – helps to explain why sea level is still rising

Trenberth and Fasullo, 2013

The International Council for Science and Climate Change: 60 years of facilitating climate change research and

informing policy (Autumn 2015)

• what tangible objectives will limit global warming to 2°C?• what pathways best combine climate change mitigation and adaptation?• how should we best use the remaining carbon budget to ensure well-being?• what is the cost of inaction? • how can we frame climate change as an opportunity rather than a burden?

To better understand the Earth System we need: Ø expanded long-term space observations, ground-based monitoring and

information-processing, data management capabilities, observing networks, high-performance computing, Earth-system models, theoretical frameworks, data-management systems and research infrastructure);

Ø new technologies (e.g. small satellites, constellations, unmanned aerial vehicles, drones, balloons, and even smart phones);

Ø to merge large volumes of diverse data (e.g. via new data sharing policies).

Geological Society Statement on Climate Change (www.geolsoc.org.uk/climaterecord)

• November 2010 – “Climate change - evidence from the geological record”

• December 2013 – “An addendum to the Geological Society Statement on Climate Change”

• C.Summerhayes, J.Cann, E.Wolff, R.Larter, I.McCave, J.Lowe, A.Cohen, J.Francis (P.Barrett, external review), plus P.Pearson, A.Haywood, E.Nickless, S.Day (2010), and P.Valdes, N.Bilham (2013)

• CONCLUSION: “In the light of the evidence presented here it is reasonable to conclude that emitting further large amounts of CO2 into the atmosphere over time is likely to be unwise, uncomfortable though that fact may be”.

Global Sea Level from Satellite Altimeters

Met Office 2013

Church et al., GRL 2011; www.skepticalscience.com/print.php?n=1202

Oceans are warming, even down deep

Black = surface temperature

Blue = heat content of top 700 m

Met Office 2013

Unprecedented deep ocean warming since 2000 –

helps to explain why sea level is still rising

Trenberth and Fasullo, 2013

Antarctic Ice Loss 1994-2012

Paolo et al, 2015

From satellite radar altimeter data.Note accelerating loss since 2000.

NAS/NSF 2015

Response Of Antarctic Peninsula Glaciers To Warming And Increased

Snowfall244 glaciers : 87% have retreated over last 50y

Cooke et al., 2005

Antarctic Sea Ice Differs From The Arctic

Arctic has no shielding wall of wind, and easy access by warm water and warm wind from the south

1978 2015 1978 2015

ALSO THINNING

NSIDC, 2015

Larsen B breakout 2002

Brabant Is

Summerhayes Inlet

Larsen A breakout 1995

Prince Gustav Channel breakout1995

JamesRoss Is

Next? Larsen B1

Predicted Southern Ocean Acidification through the 21st century % saturation in Aragonite

(CaCO3)

Orr et al. 2005

CO2-Temperature Link in Ice Cores

warm

cold

Dome C (3233m)

Vostok (3488m) is higher, more central and so colder due to ‘lapse rate’ (10°Ccooling for 1000mrise in height in dry air)

The natural envelope from ice cores is 180-280 ppm CO2

Siegenthaler et al 2005

Projected Change In Sea Level To 2100

–

Hansen et al., 2012

u Possible max = 1.4 mu i.e. Not a tsunami.u A creeping catastrophe.

Ø need coastal engineering solutions.

Greenland melt = 7mW.Antarctica = 5-6mUnlikely in short term 1990 2100

1993 2012

3.2 mm/yr

Climate Cycles

Cool conditions from Vaughan, 2007; palaeotemp curve from Royer et al., 2004

Al Fischer

1981-84:Greenhouse toIcehouse cyclesdriven by mantle convection, hence link to ocean chemistry.

[Cool = low CO2 = aragonitic; warm = high CO2 = calcitic; Holland, 2003]

Climate Cycles

Cool conditions from Vaughan, 2007; palaeotemp curve from Royer et al., 2004

Al Fischer (1921-)

1981-84:Greenhouse toIcehouse cyclesdriven by mantle convection, hencelink to ocean chemistry.

[Cool = low CO2 = aragonitic; warm = high CO2 = calcitic; Holland, 2003]

Orbital Change Explains Ice Ages1864: James Croll

(1821-1890)

Eccentricity of Earth’s orbit for 3 Ma before 1800 and 1 Ma after

(1879-1958)

1924 and 1941: Milutin Milankovitch

Insolation at 65°NIn June over past 600 Ka.Shading = coldsummers.Likely link to known glacial periods

Earth’s Climate Evolution (2015)

Gunz Mindel Riss Wurm

How Does CO2 Affect IR Radiation?US Air Force – How to Shoot Down Russian MIGs withheat-seeking missiles? Does CO2 affect their ability?Military funding advanced IR spectroscopy (early 1950s)

Gilbert Plass (1920-2004)1956: re-calculated effect of CO2 on atmosphere.

Reductionsin IR radiationto spaceat top ofatmosphere:CO2 = 20%; H2O = 50%; Cloud = 25%

Sun heats Earth.Warm Earthradiates to Space viaIR radiation.

www.giss.nasa.gov/research/briefs/schmidt_05/

Pre-Ice Core CO2 = Tectonic Balance of Volcanism vs Chemical Weathering

Beerling and Royer, Nature Geoscience 2011

500

1000

0

ppm CO2

60 0Ma

°C

12

0

Temperature of bottom water1. Tethyan Ocean carbonates are

subducted under Asia as India moves N.

2. Volcanoes spew out CO2 at Asian margin.

3. Temperatures rise.4. India collides with Asian margin. 5. Subduction halts; CO2 emission

stops.6. Himalayas begin to rise.7. Chemical weathering of mountains

begins to strip CO2 from air.

THE SLOW CARBON CYCLE AT WORK

GlobalPAGES 2ky(2013)

InsolationPAGES 2k(2013)

Insolation Control on Average Planetary Temperature Past 2000 Years

Earth’s Climate Evolution (2015)

Global Temperature Combines Orbital Insolation (STRONG) with Solar Variability (WEAK)

MWP LIA

Wanner et al, JGSL 2015

Why is today’s temperature what it was in the Roman Warm Period?

Neukom et al., 2014 (supplied by R Neukom)

Medieval Warm Period & Little Ice Age

IPCC FAR 1990From Lamb 1966CENTRAL ENGLAND DATAMISLABELLED GLOBAL(but still being used as Global by contrarians)

The Mystery of Erratic Blocks

Hutton –extended glacial action.

Bench for scale

The Pierre-a-Bot (Toad Rock) near Neuchatel.

Saussure – raging torrents.

Lyell - icebergs. Renamed ‘diluvium’ as ‘drift’.

(1740-1799)

(1726-1797) (1784-1856)

(1797-1875)

Buckland –Noah’s flood (diluvian geology)

Agassiz invented “The Ice Age”

A vast ice sheet (God’s Great Plough)deposited erratic blocks from the Baltic to the Mediterranean.

(1807-1883)

The Iceberg Controversy

Carlson and Winsor, 2012

40°

60°

Lyell was right: ice can carry rocks(but not over western Europe)

Erebus & Terror Gulf, Antarctica

Agassiz was right: Europe was partcovered by a great ice sheet

(Puzzle solved by James Geikie, 1874)

Both agreed mountains wereglaciated.

Even the Gods have feet of clay

The Iceberg Controversy

Carlson and Winsor, 2012

40°

60°

Erebus & Terror Gulf, Antarctica

Icebergs were an important feature of past glacial periods.Iceberg armadas dumped ice-rafted debris over the N

Atlantic (Heinrich Events)

Ice Shelves CollapseLarsen B (spring 2002)

Cavalieri et al, 2008, NSIDC, U Colorado

Glacier Retreat and Sea Level Rise

Map - Nature 526, 2015

Thwaites Glacier likely to retreat fast, contributing2.5 cm/century to sea level rise.

Cornford et al, 2015

Totten Glacier Ice Shelf cavity,may allow 3.5 m of sea level rise

Greenbaum et al, 2015; Fretwell, 2015

Ice Melt Progress: Reaches + 17 m at 5000 yrs

WAIS melts first (it’s thinner).

Pollard, De Conto & Alley, 2015

Due to hydrofracturing by surface water, plus ice cliff collapse

CO2 kept at 400 ppm; ocean temperature +2°C.