BREVIA
Lee R. Kump1* and David Pollard2
dimethylsulfide is the major pathway for CCN
production. Andreae (4) concludes that biological
productivity determined the CCN concentrations
over prehuman unpolluted land and sea, ranging
from a few tens per cm3 in low-productivity regions to a few hundred per cm3 in high-productivity
regions, supporting the notion of a prominent
role for the biota in climate regulation on the prehuman Earth (5). If CO2-induced warming during
supergreenhouse intervals reduced global primary
productivity by temperature stress and enhanced
vertical stratification of the ocean, causing a reduction in CCN concentration, would lower cloud
amounts and albedo have caused further warming?
To explore this hypothesis, we used a global
climate model (GENESIS version 3.0) (GCM) to
simulate middle Cretaceous [~100 million years
ago (Ma)] climate with various atmospheric CO2
amounts. This GCM has a slab mixed-layer
ocean and prognostic cloud water amounts, and
version 3 uses the National Center for Atmospheric Research (NCAR) Community Climate
Model 3 (CCM3) radiation code with prescribed
cloud droplet radii re (3). Cloud droplet radii
mainly affect cloud optical depth, infrared emissivity, and precipitation efficiency, Pe, the rate at
uring supergreenhouse intervals of the geologic past, both tropical and polar temperatures were considerably warmer than
today, and the gradient between the two was reduced. To even approach these equable climate
states with climate models, atmospheric CO2 levels
must be specified that significantly exceed most
proxy estimates for the Cretaceous and the Eocene
(1).Thus,climatemodelershaveinvokedviablebut
hard-to-evaluate hypotheses of elevated atmospheric
methane levels, greater poleward oceanic heat transport, and enhanced polar stratospheric clouds (2).
An unexplored alternative involves planetary
albedo, the fraction of the incoming solar radiation that is reflected to space, which is largely
dependent on cloud cover and cloud albedo. A
major determinant of cloud properties is the
abundance of cloud condensation nuclei (CCN).
When CCN are abundant, many small cloud
droplets form, creating optically dense, high-albedo
clouds; when abundance is low, fewer and larger
droplets form, creating optically thinner, loweralbedo, and, importantly, shorter-lived clouds (3).
Today, pollution dominates continental CCN,
producing abundances of thousands per cm3.
In remote oceanic regions, biological release of
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Downloaded from www.sciencemag.org on September 20, 2014
Amplification of Cretaceous Warmth by
Biological Cloud Feedbacks
which cloud water is converted to precipitation.
Modern large-scale observations and theory suggest that for ~10- to 100-fold global reductions in
past aerosol and CCN amounts, ~30% (over ocean)
to ~100% (over land) increases in liquid droplet
radii are plausible (3). We simulate the Cretaceous climate with these increases in re and with
Pe increased for liquid clouds by a factor of 2.2,
reflecting the maximum likely effect of extreme
global warmth on marine and terrestrial biological productivity and CCN production rate.
Our Cretaceous model results are shown in
Fig. 1. In common with previous GCM studies,
increasing CO2 from 1× to 4× preindustrial
atmospheric level (PAL) (Fig. 1, A and B) fails to
produce the extreme high-latitude warmth implied
by temperature proxy data (Fig. 1D). We then
performed another 4× PAL simulation with the
increases in re and Pe described above (Fig. 1C).
Global cloud cover is reduced from 64 to 55%, and
the less extensive and optically thinner clouds
reduce planetary albedo from 0.30 to 0.24. The
ensuing warming is dramatic, both in the tropics
and in high latitudes, where it is augmented by
surface albedo feedback of almost vanishing snow
and sea-ice cover. (Other feedbacks due to changes
in cloud types and levels are minor.) High-latitude
continental temperatures remain above or very
close to freezing year round, in better accord with
proxy evidence (Fig. 1D).
Our results support the hypothesis that widespread increases in re can explain the drastic
warming and equable high latitudes during supergreenhouse intervals of the Cretaceous and early
Cenozoic. The increases in re could plausibly
have been caused by an order of magnitude decrease in CCN concentrations, which we suggest
was caused in turn by declines in biological productivity triggered by the climatic consequences
of high CO2 levels of ~ 4× PAL.
References and Notes
1. K. L. Bice et al., Paleoceanography 21, PA2002
10.1029/2005PA001203 (2006).
2. L. C. Sloan, D. Pollard, Geophys. Res. Lett. 25, 3517 (1998).
3. Materials and methods are available on Science Online.
4. M. O. Andreae, Science 315, 50 (2007).
5. R. J. Charlson, J. E. Lovelock, M. O. Andreae, S. G. Warren,
Nature 326, 655 (1987).
6. J. E. Francis, I. Poole, Palaeogeogr. Palaeoclim.
Palaeoecol. 182, 47 (2002).
7. R. A. Spicer, R. M. Corfield, Geol. Mag. 129, 169 (1992).
8. This work was supported in part by grants from NSF’s
Carbon and Water in the Earth System (to L.R.K.) and
Paleoclimate History (to D.P.) programs.
LAND ONLY
OCEAN ONLY
Supporting Online Material
www.sciencemag.org/cgi/content/full/320/5873/195/DC1
Materials and Methods
Fig. S1
References
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Fig. 1. Annual mean surface-air temperatures (°C) in GCM simulations of Middle Cretaceous (~100 Ma, low sealevel stand) and zonal averages (A) with CO2 concentration 1× PAL (280 parts per million by volume), (B) with 4×
PAL CO2, and (C) with 4× PAL CO2 and increased liquid-cloud re and Pe. (D) Zonal average temperatures for land
and ocean, land only, and ocean only, with ocean (1) and terrestrial (6, 7) proxy temperature data for the Middle
Cretaceous shown as solid rectangles. Dotted line indicates data from simulation with 1× PAL CO2; dashed, with
4× PAL CO2; and solid, with 4× PAL CO2 and increased liquid-cloud re and Pe.
www.sciencemag.org
SCIENCE
VOL 320
7 December 2007; accepted 19 February 2008
10.1126/science.1153883
1
Department of Geosciences and Earth System Science Center,
Pennsylvania State University, University Park, PA 16802, USA.
2
Earth and Environmental Systems Institute, Pennsylvania
State University, University Park, PA 16802, USA.
*To whom correspondence should be addressed. E-mail:
lkump@psu.edu
11 APRIL 2008
195