May 7, 2007
Sea Ice Melting at Higher Than Predicted Rate
Noreen Parks of ScienceNOW Daily News reported on May 2d on Stroeve's study of Arctic sea ice in Geophysical Research Letters. Stroeve's study suggests that sea ice is melting three times as fast as the IPCC climate models predict. So rather than disappearing by 2050, the ice may be gone much sooner:
With its wreath of sea ice shrinking ever smaller over the last half-century, the Arctic has served as global warming's canary in the coal mine. By 2050 to 2100, according to climate model predictions, Arctic summers will be ice-free for the first time in about a million years. But new research reveals the ice has been vanishing about 3 times faster than the models have predicted, shifting the inevitable meltdown about 30 years ahead of schedule.
Global climate forecasts from the Intergovernmental Panel on Climate Change (IPCC) are based on computer models that use mathematical equations to describe key aspects of the physical world such as greenhouse gas levels. To simulate past climates and project future trends, the models start at a given year in the past, say 1800, and run forward in time, allowing all the parts of the faux world to interact. Although the simulations include real-world observations, this information doesn’t necessarily capture fluctuations on small spatial-scales in factors such as ocean heat and ice thickness. Together, these changes may significantly diminish sea ice. That's why climatologist Julienne Stroeve of the National Snow and Ice Data Center at the University of Colorado, Boulder, and her colleagues wanted to double-check the IPCC projections on melting with records of what actually happened to the ice.
Stroeve's team compared results from the IPCC's 18 climate models with data from aircraft and ship reports and satellite measurements. The team found that, on average, the IPCC models simulated ice losses in September (when ice retreats to its annual minimum) at 2.5% per decade from 1953 to 2006. In contrast, the real-world observations show September ice actually diminished by about 7.8% per decade during that period. This suggests current model projections are overly conservative, and summer sea ice may disappear considerably earlier than thought, the authors conclude online 1 May in Geophysical Research Letters.
Arctic modeler Wieslaw Maslowski of the Naval Postgraduate School in Monterey, California, says that one reason global climate models lowball the rate of melting is that they underestimate the amount of heat transported into the Arctic from the Atlantic Ocean and Bering Sea, which affects the rate of sea ice melting.
Return from the Depths: Which are Getting Deeper Faster than We Thought
I have returned from the depths of grading furiously so that our 3rd years can graduate. This note does a great job of summarizing why the depths may get deeper than we expect -- the reasons -- with a note of humility about extrapolating long-term trends from this analysis.
Observations of the
climate system are crucial to establish actual climatic trends,
whereas climate models are used to project how quantities like global
mean air tem- perature and sea level may be expected to respond to
anthropogenic perturbations of the Earth's radiation budget. We
compiled the most recent observed climate trends for carbon dioxide
concentration, global mean air tempera- ture, and global sea level,
and we compare these trends to previous model projections as sum-
marized in the 2001 assessment report of the Intergovernmental Panel
on Climate Change (IPCC) (1). The IPCC scenarios and
projections start in the year 1990, which is also the base year of the
Kyoto protocol, in which almost all industrialized nations accepted a
binding commitment to reduce their greenhouse gas emissions. Although
published in 2001, these model projections are essentially independent
from the observed climate data since 1990: Climate models are
physics-based models developed over many years that are not
"tuned" to reproduce
the most recent temperatures, and global sea-level data were not yet
available at the time. The data now available raise concerns that the
climate system, in particular sea level, may be responding more
quickly than climate
models indicate. Carbon dioxide concentration follows the projections almost exactly, bearing in
mind that the measurements shown from Mauna Loa (Hawaii) have a slight positive offset due to the slightly higher CO2 concentration in the Northern Hemisphere compared with the global mean. The level of agreement is partly coincidental, a result of compensating errors in industrial emissions [based on the IS92a scenario (1)]and carbon sinks in the projections.
The global mean surface temperature increase (land and ocean combined) in both the
NASA GISS data set and the Hadley Centre/Climatic Research Unit data set is 0.33°C for the 16 years since 1990, which is in the upper part of the range projected by the IPCC. Given the relatively short 16-year time period considered, it will be difficult to establish the reasons for this relatively rapid warming, although there are only a few likely possibilities. The first candidate reason is intrinsic variability within the climate system. A second candidate is climate forcings other than CO2: Although the con-
centration of other greenhouse gases has risen more slowly than assumed in the IPCC scenarios, an aerosol cooling smaller than expected is a possible cause of the extra warming. A third candidate is an underestimation of the climate sensitivity to CO2 (i.e., model error). The dashed scenarios shown are for a medium climate sensi-
tivity of 3°C for a doubling of CO2 concentration, whereas the gray band surrounding the scenarios shows the effect of uncertainty in climate sensitivity spanning a range from 1.7° to 4.2°C.
Since 1990 the observed sea level has been rising faster than the rise projected by models, as shown both by a reconstruction using primarily tide gauge data (2) and, since 1993, by satellite altimeter data (3) (both series are correctedfor glacial isostatic adjustment). The satellite data show a linear trend of 3.3 ± 0.4 mm/year
level is 25% faster than the rate of rise in any 20-year period in the preceding 115 years. Again, we caution that the time interval of overlap is short, so that internal decadal climate variability could cause much of the discrepancy; it would
be premature to conclude that sea level will continue to follow this "upper limit" line in future.
The largest contributions to the rapid rise come from ocean thermal expansion (4) and the melting from nonpolar glaciers as a result of the warming mentioned above. Although the ice sheet contribution has been small, observations are indicating that it is rapidly increasing, with contributions both from Greenland and Antarctica [e.g., (5)].
Overall, these observational data underscore the concerns about global climate change. Previous projections, as summarized by IPCC, have not exaggerated but may in some respects even have underestimated the change, in particular for sea level.
References and Notes
1. IPCC, Climate Change 2001: The Scientific Basis
(Cambridge Univ. Press, Cambridge, 2001).
2. J. A. Church, N. J. White, Geophys. Res. Lett. 33, L01602
3. A. Cazenave, R. S. Nerem, Rev. Geophys. 42, 20 (2004).
4. J. K. Willis, D. Roemmich, B. Cornuelle, J. Geophys. Res.
109, C12036 10.1029/2003JC002260 (2004).
5. A. Cazenave, Science 314, 1250 (2006); published
online 18 October 2006 (10.1126/science.1133325).