Sea ice volumes in polar regions are a key indicator of global change. In the Arctic, sea ice extent in summer has fallen dramatically in recent years, particularly in 2007 and 2008. The media do not exaggerate when they say that this is an alarming rate of decline. You can expect such media stories to be an annual event each september, when the sea ice melt peaks.
Sea ice extent has been monitored by satellites in polar orbit since 1978. The remote sensing technique employed (passive microwaves) can discriminate between open water and sea ice even during the dark polar winter or cloudy conditions. Microwave emissions are an important tool in monitoring water in all forms – atmospheric water vapour, sea temperatures, soil moisture, snow cover …
The plot below shows Arctic sea ice extent (SIE) derived from the AMSR-E instrument (Advanced Microwave Scanning Radiometer). AMSR-E was built by the Japanese Aerospace Exploration Agency (JAXA). The plot below is available from the International Arctic Research Center (IARC)-JAXA information site.
Sea ice extent time-series
Sea ice has been monitored using microwaves since 1978. AMSR-E is the newest instrument with the highest resolution (6km, 89GHz). It is therefore viewed as the most reliable, but it has only been in operation since 2002. Comiso and Nishio show that AMSR-E and earlier passive microwave data are compatible after minor re-calibration. Differences in derived SIE are accounted for by lower spatial resolution of the earlier data.
The plot on the left shows SIE in the 2002-2008 overlap period of AMSR-E and older data (“SMM/I“) obtained from National Snow and Ice Data Center (NSIDC). Plotting AMSR-E against SMM/I derived SIE (plot on right) shows a close linear relation. An AMSR-E compatible time-series can therefore be generated back to 1978. The attached R script also interpolates missing data to produce a continuous daily time-series (called final.ts).
The daily time-series final.ts looks like:
What is the trend in this time-series and how unusual are recent years? A standard method is used to remove the seasonal signal from final.ts, leaving the “trend” and “volatility”. Here is the short R code required:
seasonal_analysis <- stl(final.ts,s.window="periodic",robust=TRUE)$time.series
anomaly <- seasonal_analysis[,2]+seasonal_analysis[,3];
anom.fit <- lm(anomaly~time(final.ts));
coefs <- coefficients(anom.fit);
slope <- as.numeric(round(100*coefs*10/(coefs+coefs*1985),1)); # slope of fit
plot(anomaly,lwd=2,main=paste("Arctic Sea Ice Extent Anomaly to",end_date),ylim=c(0,13),ylab="10^6 km2",xlab="",font.lab=2,font.axis=2);
legend("bottomleft",legend=c("Arctic Sea Ice Extent (seasonal signal removed)",paste("trend=",as.expression(slope),"% per decade")),col=1:2,lty=1,lwd=2:1);
text(1995,4, "data from www.ijis.iarc.uaf.edu & www.nsidc.org");
The result is:
Arctic sea ice extent has been shrinking by nearly 4% per decade since 1978. Volatility about this trend has clearly picked up in the past few years. The past two summers (2007 and 2008) have been unusual, with deep below trend summer minima.
Ice has recovered somewhat so far this year, and it will be very interesting to see the extent of the melt next September.
 The “E” in AMSR-E indicates that the instrument is carried on board one of Nasa’s Earth Observing System satellites (EOS), Aqua.