Despite the strong consensus that human beings are dangerously altering the climate, there are many important scientific questions about climate change that remain unanswered, or where additional research would be valuable. Improved scientific understanding of these questions can help guide appropriate policy-making. This series of posts identifies what some of these questions are and provides information on the scientific work that has been done on them so far.
For a number of reasons, increasing global temperatures cause sea levels to rise. Ocean water expands when heated. Also, melting icecaps and glaciers contribute to the rise.
Significant increases in sea level could have major consequences for humanity. Some countries are highly vulnerable to sea level increase, such as Bangladesh and the Netherlands. In addition, many of the world’s cities are built near sea level and responding to a significant increase could be exceptionally expensive or even impossible.
Velicogna, I. 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, doi:10.1029/2009GL040222.) See also related article: R.A. Kerr. Both of the world’s ice sheets may be shrinking faster and faster. Science, Vol 326, p. 217. Pritchard, H.D., R.J. Arthern, D.G. Vaughan and L.A. Edwards. 2009. Extensive dynamic thinning of the margins of the Greenland and Antarctic ice sheets. Nature, doi:10.1038/nature08471.
Two studies using satellite data reveal accelerated rates of ice mass-loss over the current decade for the Greenland and Antarctic ice sheets with the greatest rates along coastal margins.
Two recent studies employing different satellite data indicate that the rate of ice loss of the Greenland and Antarctic ice sheets is accelerating. Velicogna (2009) used satellite-based gravity measurements to estimate monthly ice mass changes of the Greenland and Antarctic ice sheets over the period April 2002 to February 2009. The results show that, over the short 7 year period, the rate of ice mass-losses accelerated significantly for both the Greenland and Antarctic ice sheets (the annual rate doubled for Greenland and more than doubled for Antarctica. The combined contribution of ice mass-loss from the two ice sheets over the study period corresponded to an equivalent acceleration in sea level rise of 0.17 ± 0.05 mm/yr2. In a separate study recently published in Nature, Pritchard et al (2009) use satellite based laser altimetry data to evaluate dynamic thinning (accelerated flow) along the entire coastal margins of the Greenland and Antarctic ice sheets over the period February 2003 to November 2007. These data are available at a much higher temporal and spatial resolution than previous studies using radar altimeters which enabled elevation changes related to fast flowing ice to be distinguished from those related to other causes. The results of the study reveal that the greatest changes in the ice sheets are currently occurring at coastal margins. In Greenland, dynamic thinning is evident at all latitudes and 81 of the 111 glaciers surveyed thinned dynamically at double the rate of slower flowing ice at the same altitude. Dynamic thinning is found to penetrate far into the interior of both the Greenland and Antarctic ice sheet and is spreading as ice sheets thin due to ocean-driven melt (i.e., it is more extensive and important than previously thought). These studies suggest that continued monitoring of ice sheet loss, particularly related to dynamic thinning which is poorly understood, is essential for constraining ice sheet models thus providing better estimates of the future contributions of ice sheet losses to sea level changes.
Summary courtesy of Environment Canada
Jevrejeva, S., A. Grinsted and J. C. Moore, 2009, Anthropogenic forcing dominates sea level rise since 1850, Geophysical Research Letters, vol. 36, L20706, doi:10.1029/2009GL040216.
More than 70% of the total 18 cm sea level rise during the 20th century is attributed to increases in greenhouse gases.
A team of scientists has reconstructed the history of sea level during the past 1000 years using a statistical model driven by four forcing time series reflecting major natural and anthropogenic forcings. Their approach assumes that 1) sea level rise is caused primarily by changes in global ice volume and global ocean heat content, both of which react to changes in forcing with some response time and; 2) there is an equilibrium sea level for a given mean global radiative forcing. They found that sea level has remained within about 20 cm of its present level over the past millennium and that until the 1800s, changes in sea level were driven mostly by natural forcings (solar and volcanic). They note the long lasting effects of volcanic eruptions and found that if no volcanic eruptions had occurred since 1880, then 20th century global sea level would have been 7 cm higher. They conclude that anthropogenic forcing has been the main contributor to SLR since 1900, accounting for more than 70% of the observed SLR. Over the longer period since 1850, anthropogenic forcing was found to account for between 50-70% of SLR. These findings are robust in all their experiments. Of the 18 cm of observed SLR during the 20th century, only 4 cm (± 3 cm) can be attributed to natural climate variability, with the remainder, 14 cm (± 3 cm), being due to the rapid increase of anthropogenic CO2 and other GHGs.
Summary courtesy of Environment Canada
Science 22 July 2011:
Vol. 333 no. 6041 p. 401
DOI: 10.1126/science.333.6041.401
Antarctic Ice’s Future Still Mired in Its Murky Past
Richard A. Kerr
A new reanalysis by two NASA scientists of the three standard ice-monitoring techniques slashes the estimated loss from East Antarctica, challenging the large, headline-grabbing losses reported lately for the continent as a whole. Although not the final word, the new study shows that researchers still have a lot to learn about the vast East Antarctic Ice Sheet. Understanding the role of East Antarctica is one key to figuring out what the ice sheets, and thus sea level, will be doing by century’s end.
During the previous interglacial period, approximately 127,000 to about 116,000 years ago, Earth’s climate was warmer than it is currently. Global average sea level was also 4 to 6 meters higher, but it is unclear how much additional ocean volume resulted from the melting of the Greenland Ice Sheet versus the Antarctic Ice Sheet. Colville et al. (p. 620) examined the Sr-Nd-Pb isotope ratios of silt-size sediment discharged from southern Greenland over the penultimate warm period in order to infer what terrane in Greenland was covered in ice. The results were compared with model outputs of the Greenland Ice Sheet in order to estimate the volume of ice and to calculate how much the ice sheet contributed to sea level. The findings indicate that the Greenland Ice Sheet supplied between 1.6 and 2.2 meters of the excess sea-level rise, which suggests that the Antarctic Ice Sheet also made a major contribution to the sea level.
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Science 29 July 2011:
Vol. 333 no. 6042 pp. 620-623
DOI: 10.1126/science.1204673
Sr-Nd-Pb Isotope Evidence for Ice-Sheet Presence on Southern Greenland During the Last Interglacial
Elizabeth J. Colville, Anders E. Carlson, Brian L. Beard, Robert G. Hatfield, Joseph S. Stoner, Alberto V. Reyes, David J. Ullman
To ascertain the response of the southern Greenland Ice Sheet (GIS) to a boreal summer climate warmer than at present, we explored whether southern Greenland was deglaciated during the Last Interglacial (LIG), using the Sr-Nd-Pb isotope ratios of silt-sized sediment discharged from southern Greenland. Our isotope data indicate that no single southern Greenland geologic terrane was completely deglaciated during the LIG, similar to the Holocene. Differences in sediment sources during the LIG relative to the early Holocene denote, however, greater southern GIS retreat during the LIG. These results allow the evaluation of a suite of GIS models and are consistent with a GIS contribution of 1.6 to 2.2 meters to the ≥4-meter LIG sea-level highstand, requiring a significant sea-level contribution from the Antarctic Ice Sheet.
S. Jevrejeva, J.C. Moore, A. Grinsted. Sea level projections to AD2500 with a new generation of climate change scenarios. Global and Planetary Change, 2011; DOI: 10.1016/j.gloplacha.2011.09.006
http://www.sciencedaily.com/releases/2011/10/111017102601.htm
Sea Levels to Continue to Rise for 500 Years? Long-Term Climate Calculations Suggest So
From London to Shanghai, world’s sinking cities face devastating floods
Threat to major population centres is increasing as planners fail to prepare for impacts of global warming, report says
President donald trump doubts that man-made climate change is a thing. According to his administration it is a clear, present and future danger to the United States. That conclusion—unsurprising to anyone who has been paying attention to climate scientists—emerges from the Fourth National Climate Assessment, compiled by 13 federal agencies and released on November 23rd. If the White House had hoped to bury the 1,600-page tome, part of a four-yearly exercise which it is obliged to prepare and make public by a law from 1990, under turkey-laden tables, the absence of news over the Thanksgiving weekend promoted it to the front pages.
The findings made for grim reading. The report details how climate change is already affecting America—through more frequent floods and droughts—and what to expect from falling crop yields, the spread of disease-carrying bugs, fiercer hurricanes and much else besides. Sea levels, up by 16-21cm in the past 120 years, may rise by another metre over the next 80, threatening $1trn in coastal property. Rising temperatures could force Texans to work fewer hours (and pay 10-20% more for energy). Wildfires should cause less devastation by 2090 than they do today—but only because many forests most prone to them will have burned to a crisp. All told, unchecked greenhouse-gas emissions could lop a tenth off American gdp this century.