The international scientific community’s new assessment of the estimated sea level rise caused by global warming is a significant development, but experts say the projections for higher sea levels in the Intergovernmental Panel on Climate (IPCC) assessment report (AR5) are still on the low side. The projections are of immediate concern to low-lying countries and small island states.
The new estimates are between 28cm and 98 cm by 2100, depending on the amount of carbon dioxide emitted and efforts to mitigate its effects.
“This is more than 50 percent higher than the old projections (18-59 cm) [in its last assessment in 2007- AR4] when comparing the same emission scenarios and time periods,” notes Stefan Rahmstorf, Head of Earth System Analysis at Potsdam University, Germany, and a leading authority on sea level rise.
The lower estimates of sea level rise resulting from climate change was seen as a major failing of the 2007 report. However, glacier science had not advanced enough at that time for IPCC to take into account ice melt from Greenland and Antarctica.
Rahmstorf told IRIN, “It is remarkable that IPCC has now come to its much higher sea level rise projections with their preferred method, independently of the semi-empirical models. The IPCC's own approach now confirms that its last report understated the risk of sea-level rise, and that was my main concern at the time.”
If there are no restrictions on emissions by the year 2300, sea levels will be from 1 metre to more than 3 metres higher, which would submerge many low -ying countries and most island states. According to IPCC’s new estimates, countries like Bangladesh, on the Bay of Bengal, could be facing a 20cm hike in sea levels in another three decades.
M. Monirul Qader Mirza, a leading author of the IPCC reports, says studies by the Institute for Water Modelling, based in Dhaka, capital of Bangladesh, show that a 32cm rise in the sea level could submerge 84 percent of the Sundarbans - the world's largest mangrove forest and a UNESCO Heritage Site - by 2050. The entire Sundarbans, which act as a coastal bioshield against cyclones, may be lost in the event of about a one-metre rise.
New studies differ
With several studies on polar ice melt now at hand, the “IPCC decided not to include estimates from at least five published studies that had higher numbers, including two studies with rises of 2 metres (6.6 feet),” notes Jeff Masters, a meteorologist and founder of Weather Underground, a weather information website.
Even the US government National Oceanic and Atmospheric Administration’s 2012 US National Assessment Report had a higher estimate of 2 meters as its “worst-case sea level rise scenario for 2100,” Masters says.
But that number might still be “too low”. He points out that Jason Box, an eminent glaciologist with the Geological Survey of Denmark and Greenland (GEUS), revealed in a recent presentation that Greenland’s contribution to sea level rise had doubled over the past decade, and could add as much as 1.4 meters to sea level rise by 2100 if that doubling were to continue on a 10 year interval.
The new projections are likely to spur low-lying countries at the UN Framework Convention on Climate Change (UNFCCC) talks to intensify their efforts for a new climate deal, and funds and policies to prepare for what could lie ahead.
Rahmstorf told IRIN that the models used to make projections of the possible impact of climate change on sea-level rise “are now much better than at the time of the last report”. But “These models cannot explain the high sea levels in the Pliocene period [from 5.3 million to 2.5 million years ago, which began warm and with high sea levels, but cooled towards the end], for example, where data point at a less stable Antarctic ice sheet than in the current models.”
However, “Some new developments in this area…came too late to be included by IPCC, and I wouldn't be surprised if the gap between process and semi-empirical models has narrowed further by the time of the next IPCC report,” Rahmstorf says.
Another eminent glaciologist, Aslak Grinsted, of the Centre for Ice and Climate at the University of Copenhagen, says the IPCC did not provide an “explicitly” higher estimate of sea-level rise over the next century “because we do not have models that reliably can predict how probable a collapse of the Antarctic ice sheet is. So what they have chosen to do is to present the sea-level projections [while] assuming this does not happen. The problem is that there are lines of evidence which point to an Antarctic collapse scenario being plausible.” He notes that ice sheet experts say we must consider a collapse of the Antarctic sheet as a significant probability by the end of the century.
Glaciologist Box told IRIN in an email, “There was controversy after AR4 that sea level rise estimates were too low. Now, we have the same problem for AR5 [that they are still too low].”
Should the IPCC issue reports on the latest scientific developments more frequently - perhaps annually?
“IPCC is far from alarmist, on the contrary, it is a highly conservative organisation,” says Rahmstorf, one of the lead authors of the AR4. “That is not a problem, as long as the users of the IPCC reports are well aware of this. The conservatism is built into its consensus structure, which tends to produce a lowest common denominator on which a large number of scientists can agree.”
Saleemul Huq, a contributing author of AR4 and AR5, agrees with Rahmstorf, but says more frequent reports covering current extreme events would be useful. Talks are taking place in the scientific community to see if this will be possible.
“In the current political climate [where climate change is often denied],” said Sivan Kartha, a senior scientist with the Stockholm Environment Institute (SEI), “even a vague and demonstrably incorrect perception of the IPCC as biased toward the ‘alarmist’ direction is quite distracting and unhelpful.”
Climate sceptics will distort the findings, “and work quite hard to make the alleged [“alarmist”] bias the news story, rather than the incontrovertible science, which is only getting more and more alarming without any need for ‘bias’”. In such… [an environment], it seems that scientists are being even more fastidious and conservative than their natural inclination already makes them.”
Richard Klein, a senior scientist with SEI and an author of the AR4 and AR5, says, “The IPCC wasn't set up to motivate action but to inform policymakers. The way in which the IPCC has communicated its findings is therefore fully appropriate.”
Box believes the IPCC will come under pressure to report more frequently on sea level rise. “How often? Annually sounds nice, but I would not hold my breath for that. The science community will be reporting on this issue for sure.”
Grinsted says he is “undecided” on how the process could be improved. “But perhaps smaller assessments such as SREX [IPCC’s Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation] are a good idea".