The Biggest Control Knob
The 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
G. Comer
Foundation
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 sunblocking 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 record
Antarctic sites with different temperature,
snowfall and impurities give same record
Sensible “failures”; e.g., in rare refrozenmeltwater 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 datadriven 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
• d13C 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;
• d11B 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 d11B 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
Volcano
erupts Acid falls on
Greenland
Cooling from
volcanoes
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 d18O records from 7 VEI 6-7 eruptions; Stuiver et
al. 1995.)
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.
Climate didn’t
change
Warmer
Today
Climate didn’t change
When more cosmic rays
reached Earth
Cosmic rays, magnetic field don’t matter much to climate.
From Musch ler et al., 2005, QSR. d18O (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.
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 iceage 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 Canadamore icehigher
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 blue
bars, 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 CO2
derived 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 sheets
grow 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+2HCO3Faster when warmer
Rock-Weathering
Thermostat—Too
cold, and CO2
builds up to warm.
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
Many events show CO2-climate connection
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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 paleorecord 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 (icealbedo 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