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Weighing change in Antarctica

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Guest commentary by Matt King, Michael Bentley and Pippa Whitehouse

Determining whether polar ice sheets are shrinking or growing, and what their contribution is to changes in sea level, has motivated polar scientists for decades. Genuine progress began in the early 1990s when satellite observations started to provide (nearly) spatially comprehensive sets of observations. Three very different, and hence complementary, approaches are now employed, although each has a particular limitation:

  • Satellite altimetry: measurements of ice sheet volume changes from laser or radar altimeters (e.g. IceSat) can be converted to mass changes through correction of spatially- and temporally-varying surface density together with spatial extrapolation to unsampled regions. The main limitation lies in the models used to correct for surface density changes.
  • Input-minus-output: calculating the difference between the mass of snow accumulated and that of the ice (and meltwater) being discharged gives the mass imbalance. The snow accumulation is normally estimated from numerical models and the discharge is computed using the multiple of measured velocity at the edge of the ice sheet with its measured or inferred ice thickness and density. Thus, uncertainty in accumulation models and sub-glacial topography at the grounding line propagate into mass balance uncertainties.
  • Satellite gravimetry: changes in Earth’s gravity field can be measured from satellite (e.g. from Gravity Recovery and Climate Experiment, GRACE) and used to determine changes in ice mass but only after accounting for mass-change effects that are not due to ice mass redistribution – in particular the glacial isostatic adjustment (GIA).

Our recently published Nature paper (King et al, 2012), used GRACE gravity data to infer the ice mass trends as in previous work, but with an updated estimate of the GIA correction.
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References

  1. M.A. King, R.J. Bingham, P. Moore, P.L. Whitehouse, M.J. Bentley, and G.A. Milne, "Lower satellite-gravimetry estimates of Antarctic sea-level contribution", Nature, vol. 491, pp. 586-589, 2012. http://dx.doi.org/10.1038/nature11621

Climate Change videos: Part I

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The US National Research Council has been doing a lot recently to expand background knowledge of the climate system and of climate change. In tandem with a new report discussing strategies for advancing climate modeling, they have put up a an introductory web site on climate models (including some interviews with some actual climate modelers).

More comprehensively, they have helped put together a series of videos discussing everything from the definition of climate to attribution of climate changes and future projections. The series is in seven parts, viewable here. There are additional resources here.

We thought it would be interesting to have a separate post on each of the seven videos so that discussions on the videos themselves, or the topics covered (or not) could be more focused. So, with no further ado, here is part I: “What is Climate?”

My oh Miocene!

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Guest commentary by Sarah Feakins

Our recent study in Nature Geoscience reconstructed conditions at the Antarctic coast during a warm period of Earth’s history. Today the Ross Sea has an ice shelf and the continent is ice covered; but we found the Antarctic coast was covered with tundra vegetation for some periods between 20 million and 15.5 million years ago. These findings are based on the isotopic composition of plant leaf waxes in marine sediments.

That temperatures were warm at that time was not a huge surprise; surprising, was how much warmer things were – up to 11ºC (20ºF) warmer at the Antarctic coast! We expected to see polar amplification, i.e. greater changes towards the poles as the planet warms. This study found those coastal temperatures to be as warm as 7ºC or 45ºF during the summer months. This is a surprise because conventional wisdom has tended to think of Antarctica being getting progressively colder since ice sheets first appeared on Antarctica 34 million years ago (but see Ruddiman (2010) for a good discussion of some of the puzzles).
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References

  1. S.J. Feakins, S. Warny, and J. Lee, "Hydrologic cycling over Antarctica during the middle Miocene warming", Nature Geoscience, vol. 5, pp. 557-560, 2012. http://dx.doi.org/10.1038/NGEO1498
  2. W.F. Ruddiman, "A Paleoclimatic Enigma?", Science, vol. 328, pp. 838-839, 2010. http://dx.doi.org/10.1126/science.1188292

Plugging the leaks

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Guest commentary by Beate Liepert, NWRA

Clouds and water vapor accounts for only a tiny fraction of all water on Earth, but in spite of it, this moisture in the atmosphere is crucially important to replenishing drinking water reservoirs, crop yields, distribution of vegetation zones, and so on. This is the case because in the atmosphere, clouds and water vapor, transports a vast amount of water from oceans to land, where it falls out as precipitation. Scientists generally agree that rising temperatures in the coming decades will affect this cycling of water. And most climate models successfully simulate a global intensification of rainfall. However, physical models often disagree with observations and amongst themselves on the amount of the intensification, and global distribution of moisture that defines dry and wet regions.
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