G. Comer Foundation

The Biggest Control KnobThe more we learn of the large natural changes of the past, the more confident we are that humans are driving today

Richard B. Alley, Penn State

Please note: I work for Penn State University, And help UN IPCC, NRC, etc., But I am not representing them, Just me. Meteorology, 2011

Paleoclimatology• What happened? Reconstruct past climate;• When did it happen? Date past events; • Why did it happen? Reconstruct possible

causes of climate change, such as drifting continents, changes in the sun’s output, shifts in Earth’s orbit, eruptions of sun-blocking dust or fall of meteorite dust, changes in greenhouse gases, etc.

CO2 Paleobarometers• Gold standard is ice-core record;• So far “only” 800,000 years, duplicated to 450,000 yrs, and

multiply duplicated younger; • How do we know it works?Youngest samples agree with instrumental recordAntarctic sites with different temperature, snowfall and

impurities give same recordSensible “failures”; e.g., in rare refrozen-meltwater layers

expect and find excess CO2 localized, so diffusion isn’t smearing record. In warm tropical-glacier ice with dead bugs, find anomalously high CO2 as expected.

CO2 Paleobarometers• Many others, based on influence of CO2

abundance on something else that is preserved in sedimentary record, plus data-driven biogeochemical modeling;

• Nothing as simple and easy as ice-core data; • Systematic errors of the different techniques

are largely independent, so look for agreement among multiple techniques;

CO2 Paleobarometers• 13C of alkenones, soil carbonates, or liverworts: faster

diffusion of lighter species allows preferential use in plants if CO2 abundant, but heavier used if CO2 scarce;

• 11B or B:Ca of foraminiferal shells: B(OH) 3 enriched in 11B vs. the B(OH)4

- incorporated in shells, and B residence time long, so 11B or B:Ca paleo-pH meters linked to CO2

• Plant leaves grow more stomata when CO2 lower;

• Offspring-of-BLAG modeling—track inputs and outputs of CO2 from independent data (e.g., more volcanoes release more CO2, and more fossil-fuel formation removes CO2 from air)

History of Climate• No instruments way back, so use proxies• Find sediment-climate relations• Past sand dunes, glaciers, or lakes easy to identify, tell

different things about climate• Some are mainly physics (Greenland’s ice is colder a

mile down than ice above or below because still warming from ice age, tells how cold ice age was)

• Some based on assuming little change in modern correlations (e.g., relation between temperature and who lives where)—for such, we look for agreement among multiple independent indicators

History of Climate• Find ages of sediments in many ways• Oldest tree ~5000 years, but overlapping pattern

of thick and thin rings in living and nearby dead wood to >12,000 years; we counted >100,000 years in Greenland ice (match historical volcanic fallout, etc. as far back as written history goes)

• Older, a host of damage-accumulation and radiometric-dating techniques

• Again, look for agreement among multiple ways

Big volcanoes cool (1-2oC for 2-3 years). But, big volcanoes don’t get really organized (a little; Huybers & Langmuir, 2009), so explosive volcanoes don’t control climate. (Note that flood-basalt eruption does seem to warm.)

(Stack of GISP2, Greenland 18O records from 7 VEI 6-7 eruptions; Stuiver et al. 1995.)

Volcanoerupts Acid falls on

Greenland

Cooling from volcanoes

Figure 6.14. Simulated temperatures during the last 1 kyr with and without anthropogenic forcing, and also with weak or strong solar irradiance variations. Global mean radiative forcing (W m–2) used to drive climate model simulations due to (a) volcanic activity, (b) strong (blue) and weak (brown) solar irradiance variations, and (c) all other forcings, including greenhouse gases and tropospheric sulphate aerosols (the thin flat line after 1765 indicates the fixed anthropogenic forcing used in the ‘Nat’ simulations). (d) Annual mean NH temperature (°C) simulated by three climate models under the forcings shown in (a) to (c), compared with the concentration of overlapping NH temperature reconstructions (shown by grey shading, modified from Figure 6.10c to account for the 1500 to 1899 reference period used here). ‘All’ (thick lines) used anthropogenic and natural forcings; ‘Nat’ (thin lines) used only natural forcings. All forcings and temperatures are expressed as anomalies from their 1500 to 1899 means; the temperatures were then smoothed with a Gaussian-weighted filter to remove fluctuations on time scales less than 30 years. Note the different vertical scale used for the volcanic forcing compared with the other forcings. The individual series are identified in Table 6.3.

From Muschler et al., 2005, QSR. 18O (proxy for temperature) from GRIP core (top), the concentration of 10Be (middle), and the flux of 10Be (bottom). The Laschamp event of near-zero magnetic field (red arrow) allowed increased cosmic-ray flux producing more 10Be, but with no apparent effect on climate.

Climate didn’t change

When more cosmic rays reached Earth

Climate didn’t change

Today

War

mer

Cosmic rays, magnetic field don’t matter much to climate.

Changes in space dust have been small, and haven’t affected climate much.Helium-3 is mostly from space dust.

If space dust changed a lot, that might affect climate some.

But there has been little change in space dust over last 30,000 years (ice-core data shown here) and beyond (other data not shown).

(Very rarely, a big meteorite does matter, such as the one that killed the dinosaurs 65 million years ago.)

Winckler & Fischer, 2006, Science

Vostok, Petit et al.

Vostok, Petit et al.

Vostok, Petit et al.

CO2 and temperature changed essentially together over ice-age cycles, as shown by Antarctic ice-core data.

But, the science shows that a bit of the warming happened over a few centuries before the CO2 rose.

The temperature never went far without the CO2, but the CO2 appears to have lagged temperature.

What does that mean for CO2 causing warming?

First, a modern analogy…

Overspending

Overspending

Going into debt

Overspending

Going into debt

Interest payments

Overspending

Going into debt

Interest payments

More debt

Interest lags debt. How do we know interest adds to debt?

Interest lags debt. How do we know interest adds to debt?

We can’t explain size of debt without interest (and, economics says so)

Climate of the past

Orbits

Orbits

Warming

Orbits

Warming

CO2 rise

Orbits

Warming

CO2 rise

More warming

CO2 lags warming. How do we know CO2 adds to warming?

CO2 lags warming. How do we know CO2 adds to warming?

We can explain changes if and only if we include physics of CO2 (places getting more sun cooled when CO2 fell, and places getting less sun warmed as CO2 rose)

e.g., Jansen et al., 2007

CO2 as part of ice-age cycling• Ice ages paced by orbit• Climate everywhere changed the same way, but orbits just

moved sunshine around on the planet• ~5-6oC globally averaged surface temperature change despite

almost zero change in total sun;• Less summer sun in Canadamore icehigher albedo (also more

dust, changing vegetation, etc.);• Set these to ice-age values in a climate model and get about half

the cooling (but this is cheating, because need help from CO2 to get these);

• Add greenhouse-gas changes and get the rest

(e.g., Jansen et al 2007 IPCC; Hansen et al 2008; Alley 2003; Cuffey & Brook 2000)

Figure 4.24 Atmospheric CO2 and continental glaciation 400 Ma to present. Vertical bluebars, timing and palaeolatitudinal extent of ice sheets (after Crowley, 1998). Plotted CO2 records represent five-point running averages from each of four major proxies (see Royer, 2006 for details of compilation). Also plotted are the plausible ranges of CO2derived from the geochemical carbon cycle model GEOCARB III (Berner and Kothavala,2001). All data adjusted to the Gradstein et al. (2004) time scale. Continental ice sheetsgrow extensively when CO2 is low. (after Jansen, 2007, that report’s Figure 6.1)

CO2 and climate over longer times• Strong correlation of past CO2 and temperature;

• Known physics of CO2 explain most of this;

• Other things did contribute at some times (drifting continents, volcanic eruptions, etc.)

• But others physically inadequate to explain most large changes and not strongly correlated

• Warming increases CO2 over “short” times but decreases CO2 over long times (>≈500,000 years).

CO2 (gas)

CaSiO3 (solid)

Volcano—eruption rate independent of climate

Rock Weathering—CaSiO3+3H2O+2CO2 Ca+2+H4SiO4+2HCO3

- Faster when warmer

Shell Growth— Ca+2+H4SiO4+2HCO3

- CaCO3+SiO2+3H2O+CO2

Shell subduction CaCO3+SiO2

Walker, J.C.G., P.B. Hays and J.F. Kasting, 1981

Rock-Weathering Thermostat—Too cold, and CO2 builds up to warm.

Many events show CO2-climate connection

• Faint young sun of 4 billion years ago;• Snowball Earth’s of a billion or so years ago• Ordovician glaciation• End-Permian extinction• “Saurian Sauna” of the Cretacteous• Ocean Anoxic Events and black-shale formation• Paleocene-Eocene Thermal Maximum• Cooling since the Eocene• Mid-Pliocene warmth• I won’t subject you to all of them here, but we go over them

in class

Climate sensitivity to CO2 from paleo-history

• Overall, tests of climate models against paleo-record show that models are pretty good;

• Various suggested values range from <1oC to ~12oC; history excludes low and high values with fairly high confidence;

• ~3oC warming for doubled CO2 over decades, and somewhat larger over centuries to millennia, may not be too bad (see Hansen et al., 2008);

• No evidence that warming lowers sensitivity (ice-albedo isn’t that big for modern and warmer);

• Suggestion that there is something missing in models in warm climates in polar winters.

So, where does that leave us?• If higher CO2 warms, climate history sensible, as CO2 caused or

amplified the main changes;• There is now no plausible alternative to this; • If higher CO2 does not warm, we must explain how radiation

physicists are so wrong, and how a lot of really inexplicable climate events happened;

• CO2 may be forcing or feedback—no matter how a CO2 molecule got into air, it affects radiation;

• Paleoclimatic data show climate sensitivity similar to values in modern models (~3oC for doubled CO2), perhaps with somewhat higher values over long times especially in polar regions.

So, where does that leave us?• Lots of knobs control the Earth’s climate system;• The “Sun” knob isn’t twiddled very much over short times,

and hasn’t done very much over long times because of CO2;

• If cosmic-ray, space-dust, magnetic-field, other “space” knobs matter, available evidence indicates that they do no more than fine-tune, and even that is not demonstrated;

• Lots of things on Earth matter regionally—moving a continent from equator to pole cools it—but evidence weak for major control of global climate, except through CO2