AGU Fall Meeting, San Francisco, December 11-15, 2006

1. AGU Fall Meeting, San Francisco, December 11-15, 2006 Arctic Sea Ice Decline: Are Current Climate Models too Conservative? Julienne Stroeve, Marika Holland, Walt Meier, Ted Scambos, Mark Serreze http://nsidc.org IPCC AR4 Modeled Ice Extent vs. Observations Introduction Since 1979, when routine satellite monitoring began, Arctic sea ice extent at the end of the summer melt season has declined by more than 20%, a rate of -9.1% per decade or 100,000 km2/year. Extrapolation is risky – the satellite-based sea ice record is short and sensitive to imprints of natural climate variability – but if this trend continues, a seasonally ice-free Arctic Ocean is possible by 2060. Is this reasonable? To address this question, we compare the observed trajectory of September sea ice extent (the region with at least 15% surface ice cover) with output from the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC-AR4) simulations. September March Methodology Figure 1. Arctic September sea ice extent (x106km2) from observations and 13 IPCC AR4 climate models. The observational record spans 1953-2006 (thick red line). Models with more than one ensemble member are indicated with a *. The multi-model ensemble mean with standard deviation is shown in the inset. Figure 2. Arctic March sea ice extent (x106km2) from observations and 18 IPCC AR4 climate models. The observational record spans 1953-2006 (thick red line). Models with more than one ensemble member are indicated with a *. The multi-model ensemble mean with standard deviation is shown in the inset. Observed trends are compared over several time periods using a combination of satellite observations, aircraft, and ship reports. We focus on model output from the “business as usual” SRES A1B scenario, in which CO2 is projected to rise to 720 ppm by the year 2100. Only models with ice extents within 20% of the observed mean from 1953-1995 were included (13 out of 18 models). Some models had more than one ensemble member, resulting in a total of 29 individual runs. For each model, these were averaged into a single time series before deriving the multi-model ensemble mean. The AR4 models also appear conservative in their depiction of the winter trend. The model ensemble winter trend of -0.6%/decade over 1953-2006 compares with an observed value of -1.8%/decade. However, there are large discrepancies in winter trends among the models for this period, with some agreeing reasonably well with the observations and others simulating an increase in ice cover. From 1953-2006, the observed September trend is -7.8%/decade, compared to the multi-model mean trend of -2.6%/decade. For 1979-2006, the numbers are -9.2% (observed) and -4.3% (modeled). Only 5 of 29 individual ensemble runs showed trends comparable to observations. Even larger differences are found in the last 10 years. Table 1. List of models participating in IPCC AR4. Data available from PCMDI [http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php]. Conclusions • All models participating in the IPCC AR4 show declining Arctic ice cover over the period of reliable observations, 1953-2006, in agreement with observations. • The observed trend, particularly since the modern satellite record began in 1979, is greater. Current summer minima (2002-2006) are 30 years ahead of the mean model-predicted decline. • While observed trends have strong imprints of natural variability, the AR4 models as a group are too conservative regarding the rate of ice loss, and the date at which a seasonally ice free Arctic Ocean may be realized. Table 2. September ice extent trends (%/decade) and means (x106km2) from IPCC AR4 models and observations for 1953-2006, 1979-2006 and 1995-2006. The top section is for models with September ice extent within 20% of observations from 1953-1995. The bottom section is for models outside this range. When more than one ensemble member was available for a particular model, the range in trends and mean values are shown together with the ensemble mean. Observations are shown in bold.