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).
- 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
Most people know that the Antarctic Peninsula is one of the most rapidly warming places on earth. But like everywhere else in Antarctica, the length of available temperature data is short — most records begin in 1957 (when stations were put in place during the International Geophysical Year); a few start in the late 1940s. This makes the recent rapid warming difficult to evaluate; in general, what’s interesting is how the trend compares with the underlying variability. As anyone who’s been there can tell you, the weather on the Antarctic Peninsula is pretty wild, and this applies to the climate as well: year to year variability is very large. Put another way, the noise level is high, and discerning the signal requires more data than is available from the instrumental temperature record. This is where ice cores come in handy — they provide a much longer record, and allow us to evaluate the recent changes in a more complete context.
By popular demand, a thread devoted to the continuing decline of Arctic sea ice, and a potential new record minimum this year. As before, the figures are hot-linked and will update day-by-day.
JAXA Sea ice extent:
Estimated sea ice volume from UW PIOMAS (updated every month):
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).
- 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
- W.F. Ruddiman, "A Paleoclimatic Enigma?", Science, vol. 328, pp. 838-839, 2010. http://dx.doi.org/10.1126/science.1188292