Potential sea level rises were seriously underestimated by a failure to take account of land ice changes in Antarctica and Greenland while the range of potential temperature increases was restricted by a failure to adequately model the results of a scenario where curbs on future carbon dioxide emissions are ineffective – an increasingly likely outcome.
Both issues came to light with the publication of a special climate edition of the journal of UK national science academy, the Royal Society, yesterday (30 November) which was timed to coincide with the opening of the Cancun climate change conference in Mexico. The special issue of the Philosophical Transactions of the Royal Society subtitled “Four degrees and beyond: the potential for a global temperature increase of four degrees and its implications” contained a number of papers including one on potential future sea level increases and one on potential future temperature rises.
The paper on sea level rise projected a potential maximum increase in sea level of 2m by the end of the century compared with the IPCC's highest estimate which was for a sea level rise of 59cm. The reason for the difference is that the IPCC estimates “did not include significant ice mass loss from Greenland and Antarctica” whereas the Royal Society paper did, a well informed source explained to Reporting Climate Science .Com. The paper is called "Sea level rise and its possible impacts given a ‘beyond 4C world’ in the twenty-first century".
The reason the IPCC Fourth Assessment Report (AR4) report excluded polar land ice mass reductions was because of the uncertainties involved in estimating the land mass, commented a source. “This may be scientifically understandable but it gave them the wrong answer,” he added. Rapid changes in the continental ice sheets of Greenland and Antarctica had been spotted prior to the publication of AR4 but could not be reproduced in computer models of ice sheets. So the IPCC authors ignored the issue since the difference between observation and model results created an uncertainty. This in turn led to the exclusion of estimates for sea level rises resulting from significant mass loss from continental ice sheets.
A second paper focusing on the likely increases in future temperature projected a potential average global surface temperature rise of up to 7C by 2090. More significantly, in the increasingly likely scenario of continued high growth of carbon dioxide emissions, its best estimate was for a temperature rise by 2090 of 4.6C above the average for the period 1980-1999 compared with the IPCC AR4 best estimate of 4C for a high emission scenario.
This difference is because the IPCC results are based on using different computer modeling techniques for different emission scenarios. Specifically, the IPCC AR4 report did not include sophisticated general circulation model (GCM) projections based on the highest future emission (the so called A1FI) scenario whereas the new research does. "General circulation models have greater sensitivities to greenhouse gas emissions," one source explained to Reporting Climate Science .Com. "The IPCC's best guess on temeperature (in the A1FI scenario) was based on a simple climate model tuned to give what a complex model would have given."
The authors of the new Royal Society paper, called “When could global warming reach 4C?”, are clear on this point, stating: “The highest emissions scenario considered in the AR4 (scenario A1FI) was not examined with complex general circulation models (GCMs) in the AR4, and similarly the uncertainties in climate–carbon-cycle feedbacks were not included in the main set of GCMs. Consequently, the projections of warming for A1FI and/or with different strengths of carbon-cycle feedbacks are often not included in a wider discussion of the AR4 conclusions.”
"There's a lot of speculation that we are currently on a high emission scenario but its too early to say. We can't say we are and we can't say we're not," said lead author Richard Betts of the UK Meteorological Office.
The researchers, from the Met Office and from the University of Exeter, behind the Royal Society paper used the Met Office's sophisticated HadCM3 general circulation computer model to conduct multiple simulations. “The estimated AR4-projected warming for A1FI is lower than that projected by HadCM3-QUMP”, they state in their paper.
These two separate issues had the affect of restricting the range of possible climate change outcomes discussed within the AR4 report and of reducing the reported potential for sea level rise and reducing the potential maximum “best estimate” increase in average global surface temperatures. This does not mean that these outcomes are any more or any less likely but it does highlight the fact that the conclusions of the IPCC report essentially exclude the obvious possibility of melting ice caps and do not fully reflect the impact of the increasingly likely failure to curb emissions.
It is important to point out that these are flaws or shortcomings and not errors in the sense of previously publicised mistakes in the IPCC's AR4 report, regarding Himalayan glaciers and potential flooding in the Netherlands. However, they are nevertheless significant flaws in that they result in a potential underestimation of the impact of global warming that is indicated by the science that was available at the time. And it is not clear that the changes to IPCC structures and processes proposed by the Inter Academy Council this year, in the wake of the earlier errors, would necessarily identify such shortcomings in future reports since the reforms focus very much on the facts of what is said rather than what is not present or what has not been done.
The IPCC alludes to the ice cap issue in its publications but fails to be explicit as to the implications. The IPCC AR4 science report “Climate Change 2007: the Physical Science Basis”, published in February 2007, includes a table in the summary for policymakers that lists the potential range of sea level rises against each potential emission scenario. It uses the words “ Model-based range excluding future rapid dynamical changes in ice flow” in the column above the sea level rises. The report also includes the statement “current scientific understanding leaves poorly known uncertainties in the methods used to make projections for land ice”. But in both cases the text fails to explain the significance of what this means in terms of potential sea level rises.
The IPCC AR4 Synthesis Report, which was published in November 2007, after the publication of the main report, states on page 45: “Because understanding of some important effects driving sea level rise is too limited, this report does not assess the likelihood, nor provide a best estimate or an upper bound for sea level rise.” Again the text fails to explain the significance of what this means.
The IPCC AR4 Synthesis Report for Policymakers, published in 2008, includes the following statement regarding sea levels: “The projections do not include uncertainties in climate-carbon cycle feedbacks nor the full effects of changes in ice sheet flow, therefore the upper values of the ranges are not to be considered upper bounds for sea level rise. They include a contribution from increased Greenland and Antarctic ice flow at the rates observed for 1993-2003, but this could increase or decrease in the future.” Again the full significance of this statement is not explained.
With respect to temperatures, the IPCC AR4 report gives a “likely range” of potential temperature rises by the 2090s, above the average for 1980-1999, of between 1.1C and 6.4C; with its “best estimate” ranging being 1.8C and 4.0C depending on assumptions about emissions. These ranges span six possible carbon dioxide emission scenarios which are used to generate the forecasts that form the basis of the temperature range data.
The AR4 report used the most sophisticated climate models, the so called complex ocean atmosphere general circulation models, to examine only three of the six emission scenarios known as B1, A1B and A2. It used much less complex so-called simple climate models to examine the other three possibilities including the high emission A1F1 scenario. This approach generated a best estimate for the A1FI scenario of 4C a rise above 1980-1999 averages by 2090 with a range between 2.4C and 6.4C.
The authors of the paper on temperature in the Royal Society's journal used multiple computer model simulations, known as an ensemble, to generate their forecast. They point out: “The ensemble mean warming by the 2090s is 5.1C relative to 1861–1890, with the individual members projecting warming between 3.2C and 6.7C”. When this increase is measured against the 1980-1999 period used by the AR4 report this is equivalent to a projected temperature rise under the A1FI emission scenario of 4.6C compared with the 4C estimated by the IPCC AR4.
Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007
Click here for report.
Intergovernmental Panel on Climate Change (IPCC). 2007 Climate change 2007: synthesis report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva, Switzerland: IPCC.
Click here for the report.
Climate Change 2007: Synthesis Report Summary for Policymakers
Click here for report.
"When could global warming reach 4C?" by RICHARD A. BETTS, MATTHEW COLLINS, DEBORAH L. HEMMING, CHRIS D. JONES, JASON A. LOWE AND MICHAEL G. SANDERSON
Published in Philosophical Transactions of the Royal Society
Phil. Trans. R. Soc. A (2011) 369, 67–84 doi:10.1098/rsta.2010.0292
For the paper click here.
"Sea level rise and its possible impacts given a ‘beyond 4C world’ in the twenty-first century" BY ROBERT J. NICHOLLS, NATASHA MARINOVA, JASON A. LOWE, SALLY BROWN, PIER VELLINGA, DIOGO DE GUSMÃO, JOCHEN HINKEL, AND RICHARD S. J. TOL Published by Philosophical Transactions of The Royal Society
Phil. Trans. R. Soc. A (2011) 369, 161–181 doi:10.1098/rsta.2010.0291
Click here or the paper.
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