West Antarctic Change Indistinguishable From Natural Variability
For some reason a lot of people have become fixated on Antarctic ice—is it waxing or waning, accumulating or melting. Climate alarmists have striven mightily to show that ice at the poles in on the decline, melting in the face of rising global temperatures. Antarctica, with the largest store of glacial ice on the planet, is the primary focus of attention. If Antarctica’s ice sheets were to melt it would be a calamity for mankind. Unfortunately, Earth's climate system contains many cyclic trends, operating on decadal and longer periods of time. In the past, what some claim are clear trends have turned out to be only short term in nature. A new report, just published online, concludes that it is unclear if changes in atmospheric circulation over West Antarctica during the past few decades are part of a longer-term trend. In fact, ice cores reveal a significant increase in the oxygen isotopes from precipitation over the past 50 years, but the anomaly cannot be distinguished from natural climate variability.
The argument about melting polar ice rages on the pages of scientific journals and the evening TV news. People are fascinated by glaciers, those huge masses of frozen H2O that only 20,000 years ago covered much of the Northern Hemisphere. Scientists have recently observed melting in the West Antarctic Ice Sheet (WAIS) but a thickening of ice elsewhere on the southern most continent. The important question is not if there is melting in some areas or buildup in others, but what the net change in ice mass is overall. A paper published online in Nature Geoscience uses water-isotope (δ18O) data to gauge precipitation over Antarctica. We start with the paper's abstract:
Changes in atmospheric circulation over the past five decades have enhanced the wind-driven inflow of warm ocean water onto the Antarctic continental shelf, where it melts ice shelves from below. Atmospheric circulation changes have also caused rapid warming over the West Antarctic Ice Sheet, and contributed to declining sea-ice cover in the adjacent Amundsen–Bellingshausen seas. It is unknown whether these changes are part of a longer-term trend. Here, we use water-isotope (δ18O) data from an array of ice-core records to place recent West Antarctic climate changes in the context of the past two millennia. We find that the δ18O of West Antarctic precipitation has increased significantly in the past 50 years, in parallel with the trend in temperature, and was probably more elevated during the 1990s than at any other time during the past 200 years. However, δ18O anomalies comparable to those of recent decades occur about 1% of the time over the past 2,000 years. General circulation model simulations suggest that recent trends in δ18O and climate in West Antarctica cannot be distinguished from decadal variability that originates in the tropics. We conclude that the uncertain trajectory of tropical climate variability represents a significant source of uncertainty in projections of West Antarctic climate and ice-sheet change.
Occasionally an important scientific result is an inconclusive one. That is the case here. In “Recent climate and ice-sheet changes in West Antarctica compared with the past 2,000 years,” a large team of investigators, led by Eric J. Steig, use records of the oxygen isotopic composition of precipitation (denoted δ18O) from ice cores to assess the significance of recent West Antarctic climate trends, and attempt to place observed glaciological changes in a longer-term context. “Unlike temperature or other conventional climate variables, δ18O is relatively well sampled in West Antarctica and thus provides the best available data for this purpose.” they claim.
When some global warming fan boy authoritatively claims that the melting of polar ice is an established long-term trend he is speaking nonsense. The truth is that science has only been studying climate change in the Antarctic for a short period of time. All conscientious scientists know that we do not posses enough good data spanning a long enough time period to establish significant long-term trends. Our ignorance was part of the motivation for Steig et al.'s investigation.
It is unknown whether the climate and glaciological changes that have occurred in West Antarctica in recent decades are part of a longer-term trend associated with anthropogenic climate forcing. This question cannot be evaluated with direct observations. West Antarctic temperature and pressure observations begin only in 1957, and reliable satellite observations of Antarctic sea ice date to 1979. Comprehensive observations of glacier dynamics in the most rapidly changing areas were initiated in the 1990s. Borehole temperature data from the WAIS, although confirming the recent rapid rise in temperature, do not resolve decadal-scale variability in the past.
In an attempt to discover what has remained hidden, these scientists made another study, with a different methodology. Using isotopes of oxygen as a proxy, from samples extracted from ice cores, they built a history for the past 2,000 years. Naturally, they have included the output from a computer climate model—no climate paper is complete without some model results. What they found is captured in the figure below.
Decade-average δ18O from the WAIS Divide ice core for the past 2,000 years.
The grey shading shows 2 s.d. about the decadal mean, based on the upper 100 years of the multi-core δ18O composite, providing an estimate of the 95% confidence range. The dashed line shows the 97.5 percentile value relative to the average linear trend. The frequent excursions above the line are easily visible. Below is a comparison of results for the years 1880–2009.
Modeled versus observed West Antarctic δ18O and tropical SSTs.
In the illustration above: a, Difference in mean simulated decadal-mean δ18O in Antarctic precipitation between 1991 and 2000 and the three preceding decades; b, Model range (2 s.d., grey shading) of simulated annual mean δ18O in precipitation averaged over West Antarctica compared with observed δ18O anomalies (black line). In both a and b, the model simulations are from 10-member ensembles of ECHAM simulations forced by global tropical SST with a slab ocean in the extratropics; c, Sea surface temperature (SST) anomalies averaged over the central tropical Pacific Niño region (thin solid line) and over the entire tropics (thick line).
The bottom line here is that Steig et al. found 50 year δ18O trends as large as today's appearing ~2% of the time over the past 2,000 years. “Hence, the anomalous δ18O values of recent decades in West Antarctica do not seem to be unprecedented, but are near the upper limit of the range of natural variability,” they conclude. A similar conclusion was reached for the northern Antarctic Peninsula. The speculative cause of the variation was identified as “variability originating in the tropical Pacific.”
What about all that ice mass loss from Antarctica we keep hearing about? “Projections of the contribution of the WAIS to future sea-level rise that are based on present rates of ice-sheet mass loss should be treated with caution,” Steig et al. warn. Science speak for “that ice loss stuff is not proven.” In other words, there is nothing strange or scary here. As I have stressed before, the climate has warmed in recent years but it is neither unusual nor unprecedented. Dangerous climate change is only in the minds of researchers with no proper sense of history or the system they study.
Be safe, enjoy the interglacial and stay skeptical.