IPCC Fourth Assessment Report: The Polar Perspective

12 Feb 2007 - Articles, Logistics, Water & Oceans, Ice & Snow, Arctic, Antarctic

IPCC meeting room, Paris

On the 2nd of February 2007, the Intergovernmental Panel on Climate Change (IPCC) Working Group 1 (out of three) released its contribution to the IPCC's Fourth Assessment Report. Entitled 'The Physical Science Basis, Summary for Policy Makers', the contribution makes predictions on future climate change and describes the progress made since IPCC's 2001 Third Assessment Report in understanding the human and natural drivers of climate change, observed climate change, and climate processes.

Overall, the report concludes that global average temperature will rise between 1.1°C to 6.4°C by 2100, and that it is 'very likely' (90% certainty) that human activities and emissions are causing global warming

As key components and contributors to global climate, the Polar Regions are central to these predictions, whether in the context of their response to warming, or as regards the consequences of this response for the Earth system as a whole.

Although the report clearly presents the Arctic as amongst the most affected and rapidly changing regions, it also suggests that most of Antarctica will be less affected by the global rise in temperatures. This has lead some prominent experts to criticize Working Group 1 for overlooking the potential magnitude and rate of the contribution of the West Antarctic Ice Sheet to sea level rise, as well as the fact that the greatest temperature rise on Earth over the last five decades has been measured on the Antarctic Peninsula.

The report, however, makes clear that there are still many uncertainties about the reactions of the Polar Regions to global warming and that these need to be resolved as a matter of urgency. This will be the challenge of the 4th International Polar Year (IPY) starting on 1st March 2007 and ending on 1st March 2009.

Below are some of the findings and predictions of change in the Arctic and Antarctic, as published in the "Summary for Policy Makers" (the full report will be available in May) of this first part of the IPCC Fourth Assessment Report (Source: IPCC WG1, 'Climate Change 2007: The Physical Science Basis, Summary for Policy Makers, 2 February 2007):

The Arctic

Average Arctic temperatures increased at almost twice the global average rate in the past 100 years. Arctic temperatures have high decadal variability, and a warm period was also observed from 1925 to 1945.

Increases in the amount of precipitation are very likely (90% certainty) in high-latitudes (including the Arctic).

The Arctic Ocean

Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.1 to 3.3% per decade, with larger decreases in summer of 5 to 9.8% per decade. These values are consistent with those reported in the Third Assessment Report.

Sea ice is projected to continue shrinking under all future scenarios. In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century.

Permafrost in the Arctic

Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic (by up to 3°C). The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900, with a decrease in spring of up to 15%.

Global warming is expected to be at its most pronounced over land across (most) high northern latitudes. Continued widespread increases in thaw depth are projected over most permafrost regions.

Greenland

New data since the Third Assesment Report show that losses from the Greenland ice sheet have very likely (more than 90% certainty) contributed to sea level rise over 1993 to 2003. Flow speed has increased for some Greenland outlet glaciers, which drain ice from the interior of the ice sheets. The corresponding increased ice sheet mass loss has often followed thinning, reduction or loss of ice shelves or loss of floating glacier tongues. Such dynamical ice loss is sufficient to explain approximately half of the Greenland net mass loss. The remainder of the ice loss from Greenland has occurred because losses due to melting have exceeded accumulation due to snowfall.

Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise up to and after 2100. Current models suggest ice mass losses increase with temperature more rapidly than gains due to precipitation, and that the surface mass balance becomes negative at a global average warming (relative to pre-industrial values) in excess of 1.9 to 4.6°C.

If a negative surface mass balance were sustained for millennia, this would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m.

Antarctic Ice

Editor's comment: there appear to be some contradictions in Working Group 1's findings on Antarctica and Antarctic ice caps.

It is likely (more than 60%) that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica.

Current global model studies project that the Antarctic ice caps will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance. This statement seems to be in contradiction with the next one.

New data since the Third Assessment Report now show that losses from the ice caps of Antarctica have very likely (90% certainty) contributed to sea level rise over 1993 to 2003. Flow speeds have increased for some Antarctic outlet glaciers, which drain ice from the interior of the ice sheets. The corresponding increased ice caps mass loss has often followed thinning, reduction or loss of ice shelves or loss of floating glacier tongues. Such dynamical ice loss is sufficient to explain most of the Antarctic net mass loss.

Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.

The Southern Ocean

Antarctic sea ice extent continues to show inter-annual variability and localized changes, but no statistically significant average trends, consistent with the lack of warming reflected in atmospheric temperatures averaged across the region.

Future global warming is predicted to be at its least pronounced in the Southern Ocean (and the North Atlantic).

Paleoclimate

The last time the Polar Regions were significantly warmer than present for an extended interglacial period (about 125,000 years ago). Global average sea level during that period was likely 4 to 6 m higher than during the 20th century, mainly due to the retreat of polar ice. Ice core data indicate that average polar temperatures at that time were 3 to 5°C higher than present, because of differences in the Earth's orbit.

Contribution of Polar Regions to Sea Level Rise

Models project a sea level rise of between 24cm and 48cm by the end of the 21st century. The ranges are narrower than in the Third Assessment Report, mainly because of improved information about some uncertainties in the projected contributions. The projections include a contribution due to increased ice flow from Greenland and Antarctica, at rates equivalent to those observed for 1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM-3 would increase by 0.1 m to 0.2 m. Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise.

By: Richard de Ferranti