Talk Abstracts 2010 SI—Draft form to be finalized once all are collected
Ocean Heat Uptake
Gregory C. Johnson , John M. Lyman3,1, and Sarah G. Purkey2,1
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2
NOAA/Pacific Marine Environmental Laboratory, Seattle WA 981115, U.S.A.
School of Oceanography, University of Washington, Seattle WA 98195, U.S.A.
3
JIMAR, University of Hawaii, Honolulu, HI 96822 U.S.A.
In recent decades, more energy has entered the top of the Earth’s atmosphere than has
exited, very likely due to the influence of anthropogenic greenhouse gasses. The oceans appear
to have taken up a large fraction of this energy imbalance – easily over 80%, so accurately
quantifying ocean heat uptake is necessary to constrain the Earth’s net energy imbalance.
Furthermore, the very large heat capacity of the oceans and their long equilibration time-scale
means that commitments to atmospheric warming and sea level rise have centennial and
millennial time-scales, respectively. We present recent estimates of ocean heat uptake,
suggesting the ocean absorbed at least 0.74 (0.12) W m-2 in the 1990s and 2000s. This
number is expressed in terms of a uniform heat flux over the surface of the entire Earth, with
90% confidence intervals. The bulk, 0.64 (0.11) W m-2, of the total is found in the upper 750m of the ice-free Global Ocean, but 0.095 (0.052) W m-2 is found in the deep (1000 < z < 4000
m) Southern Ocean and abyssal (z > 4000 m) Global Ocean. These estimates are based on
currently available data. For more complete and accurate ocean heat uptake estimates, a truly
global ocean observing system should be implemented.
Ice-sheet feedbacks
Ed Waddington
Dept of Earth and Space Sciences, UW
edw@uw.edu
What is an Ice Sheet?
Ice sheets are large fresh ice bodies formed from snow (meteoric precipitation), for example
the Greenland Ice Sheet. Ice sheets can be hundreds to thousands of meters thick. (The sea-ice
cover (frozen seawater a few meters thick) in the Arctic Ocean is sometimes called an “ice
sheet”, but we won’t follow that usage here.). Ice sheets can exhibit several feedbacks with
climate, and those feedbacks can exhibit a wide range of time scales. Here are 3 feedbacks.
Ice-Albedo Feedback
Albedo is the ratio of reflected sunlight to incident sunlight. The ice-albedo feedback
operates on ice sheets, just as it does on sea ice and seasonal snow. Glaciers and ice sheets
typically have an accumulation area at higher elevations, where snow survives all year, and is
therefore snow-covered all year with an albedo in the range 0.5-0.8. In a steady state, ice flows
to lower elevations, where the snow can melt entirely in an ablation area,, exposing bare glacier
ice with an albedo of 0.3 to 0.4, which is still higher than the albedo of typical forest, tundra, or
open water. A growing ice sheet with reduced surface melting can cool its climate by
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converting ablation area into accumulation area, and by covering dark forest or tundra with
brighter ice or snow as it advances. The cooler climate can then contribute to further advances,
producing a positive feedback.
Isostatic-Depression Feedback
Earth’s asthenosphere (the relatively weak part of the upper mantle at around 100 to 200 km
depth) can act like a viscous fluid over millennial time scales. When an ice sheet forms on land,
the additional weight of the ice causes Earth’s rigid lithosphere (upper 50-100 km) to bend and
sink as it slowly displaces asthenosphere material. The asthenosphere material slowly flows
away from the ice sheet center, and can cause forebulges well beyond the ice-sheet margin. A
characteristic time for this isostatic depression is 8 ka; however, this is only a crude
approximation, because the mantle response to loading can display a range of time scales
depending on underlying Earth structure, temperature, and constitutive properties. The amount
of isostatic depression depends on the relative densities of ice and the asthenospheric rock that
it displaces; a 3 km thick ice sheet can depress the underlying land surface by as much as 1 km.
A growing ice sheet can slowly settle into a hole of its own creation, lowering its surface
elevation and raising the temperature of its upper surface. This slow negative feedback can
contribute to rapid decay of ice sheets in a warming climate following long cold glacial
intervals.
Marine Ice-sheet Instability Feedback
When a thick ice sheet (e.g. West Antarctica) flows off the land into the ocean, it can rest on
bedrock that is below sea-level, as long as the water depth is less than 9/10 of the ice thickness.
This is called a marine ice sheet. If the ice gets thinner, or the water gets deeper, the ice can
ultimately float, becoming an ice shelf, and its bottom surface becomes extremely vulnerable to
melting by warm ocean water. Even when the ice can still “touch bottom”, more of the ice
front is exposed to melting when the water is deeper. In many ice-covered areas, particularly in
West Antarctica, the depth to bedrock increases upstream and away from the ice-sheet margins
(i.e. a retrograde slope), as a result of glacial isostatic depression, glacial erosion, or tectonics.
In such an area, even a small eustatic sea-level rise (e.g. from ice melting at the other pole)
could initiate a retreat into deeper water, and the increased frontal melting in deeper water could
initiate further retreat into yet deeper water. This positive feedback can drive retreat until the
ice-sheet terminus stabilizes on a prograde bed slope, or disappears entirely. If the ice-sheet
melt itself contributes significantly to eustatic sealevel, the feedback can be even stronger. The
ice sheet must then wait to thicken substantially before it can advance back up the retrograde
slope; this produces a hysteresis in the response of the ice sheet to its environment.
The carbon-climate feedback: interactive effects of changing moisture and temperature
Dave Schimel
National Ecological Observatory, Inc
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Feedbacks from the terrestrial carbon cycle to global climate have emerged as a significant
uncertainty in both projections of future climate and of policy analyses of the relationship
between fossil emissions and the ensuing climate change mitigation. Much of this uncertainty
is triggered by differences between models of the carbon-climate feedback, but analyses of the
strength of this feedback based on observations are far from being unambiguous! Within the
overall uncertainty of the carbon cycle, unknowns about the climate sensitivity of terrestrial
ecosystem carbon storage are large, and result from both uncertainties about the biology and
about future land climates. Yet within this uncertainty is a glimmer of hope. Much intermodel
difference results from 1) differences in predicted future precipitation and 2) differences in the
sensitivity of simulated carbon fluxes to changes in water balance (precipitation adjusted for
evaporation). The emerging trove of long-term flux data provides fascinating clues into how
the effects of changing temperature and moisture play off each other quantitatively to produce
robust modes of behavior. Additionally, it is becoming clear that ecosystem change affects
climate directly through the surface energy balance and not just through the carbon cycle. I will
also review new observational evidence for the strength of these effects. I will synthesize
evidence from long-term flux studies, the atmospheric concentration record and new data
assimilation models to the likely outcome of these observed mechanisms under future climates.
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