Some Developments in Climate Science Since IPCC AR4 Prepared for the Climate Change Science Symposium
Some Developments in Climate Science Since IPCC AR4Prepared for the Climate Change Science Symposium John Merrill, URI GSO May 6, 2011
Talk organizationI’ll zoom over several topics from jet stream altitude, and thencover one ﬁnding in some detail.There’s little rationale for this inhomogeneity.
Multiple topics Since 1979, the Arctic sea ice extent has been declining at a rate of 11% per decade. Not only did the ice extent reduce but so did the average thickness of the sea ice, which decreased strongly; this increases the vulnerability to further decreases. Winter ice has decreased in thickness, and the February, 2011 the Arctic sea ice extent was a record low. Periods of rapid Arctic sea ice loss lead to faster warming over land in polar regions. The exposure of low-albedo ocean waters results in warming; the release of this heat into the atmosphere and its transport onshore allow an extended season of microbial decomposition of thawing peat and other organic matter.
Multiple topics In AR4 the IPCC concluded that Antarctica was the only continent where no global warming had been observed. Recent work indicates that there has been a positive surface temperature trend from 1957-2006 in West and East Antarctica. The airborne fraction of CO2 may have increased recently, the expected decrease in buﬀering capacity. Conﬂicting results from estimates over diﬀering time periods complicate the picture.
Multiple topics The Ocean Heat Content Anomaly, OHCA, has shown a gradual increase over the past 50 years, but analyses included substantial decadal scale ﬂuctuations that are larger than those seen in model most simulations. Recent work detecting systematic errors in the treatment of the data have led to revised estimates, with reduced interdecadal variability. Also, parallel analyses by diﬀerent groups yield similar results, providing corroboration. The linear trends in the Ocean Heat Content of the upper 700 m are 0.27± 0.04 ×1022 J/yr for the period 1955 - 2008 and 0.40± 0.05 ×1022 J/yr for the period 1969 - 2008.
Acceleration of mass loss from ice sheets Rignot et al. (GRL, 2011), present mass balance observations for Greenland and Antarctica ice sheets over two decades. The observations are corroborated by comparison of two techniques over the most recent 8 years. The ice data are Surface Mass Balance estimates (snowfall minus ablation), combined with perimeter loss (ice discharge) estimates. Satellite data are a time series of monthly time-resolved gravity data from the Gravity Recovery and Climate Experiment (GRACE).
Acceleration of mass loss from ice sheets Data are dM/dt estimates using Mass Budget Method (MBM) (solid black circle) and GRACE time-variable gravity (solid red triangle). Total ice sheet mass balance, dM/dt, in Gt/yr. 1992 - 2009 for (a) Greenland; (b) Antarctica; (c) the sum of Greenland and Antarctica. Note expanded y-axis range in (Rignot et al. 2011, Figure 2.) (c).
Acceleration of mass loss from ice sheets The agreement of the MBM and GRACE estimates is especially good in Antarctica.
The acceleration rate in ice sheet mass balance, in Gt/yr2 , isdetermined from a linear ﬁt of MBM over 18 yr (black line) andGRACE over 8 yr (red line).The combined acceleration of ice sheet mass loss totals over 36Gt/yr2 . This acceleration is over 3 times that for mountain glaciersand ice caps.The estimated contribution to sea level rise over the next 50 yearswould be 15±2 cm, exceeding the impact of melting of individualglaciers and ice caps, 8±4 cm, and thermal expansion of theocean, 9±3 cm.If continued, this trend would make ice sheet loss the dominantsource of sea level rise in the upcoming century.
Final notesRecall that ﬁndings published after March, 2006 are not cited orused substantively in AR4.Numerical models of sea ice extent signiﬁcantly underestimate theobserved downward trend. I have concluded that these models willremain unreliable until signiﬁcant advances can be made.
Some sourcesPeter Good, et al., An updated review of developments in climatescience research since the IPCC Fourth Assessment Report. [Anextended (177 page) analysis by British Met Oﬃce and aﬃliatedanalysts, up to date to November, 2010.]Kelly Levin and D. Tirpak, Climate Science 2008 - Major NewDiscoveries. World Resource Institute “Issue Brief”, 28 pp, July,2009.R. vanDorland et al.,News in Climate Science since IPCC 2007:Topics of interest in the scientiﬁc basis of climate change, KNMI,de Bilt, November, 2009.