Apparently Arctic Sea ice has grown dramatically over the past two years, contrary to the predictions of the more dire forecasts.
The most widely used measurements of Arctic ice extent are the daily satellite readings issued by the US National Snow and Ice Data Center, which is co-funded by Nasa. These reveal that – while the long-term trend still shows a decline – last Monday, August 25, the area of the Arctic Ocean with at least 15 per cent ice cover was 5.62 million square kilometres.
This was the highest level recorded on that date since 2006, ... and represents an increase of 1.71 million square kilometres over the past two years – an impressive 43 per cent.
Other figures from the Danish Meteorological Institute suggest that the growth has been even more dramatic. Using a different measure, the area with at least 30 per cent ice cover, these reveal a 63 per cent rise – from 2.7 million to 4.4 million square kilometres.
Who knows what the long-term trend really is and whether it has changed? Different people take different years as their starting point for measuring the long-term trend, and they obtain quite different trend lines as a result. As I wrote nearly two years ago, who knows?
...year-to-year fluctuations seriously mask the long-term trends, if there are any.
Is the growth in Arctic Sea ice over the past two years just a blip deviation from a downward trend? Or is it a sign that the downward trend doesn't exist, no longer exists, has shifted, or is much flatter than some have argued?
In trying to answer this question, remember that forecasting errors have confidence intervals that expand massively the farther into the future one tries to predict, looking like exponential horns. E.g. Google references many images showing this phenomenon.
Fit a trend line to global temperatures (keeping in mind that even that concept is contentious). Then calculate the variance around that trend line. Now use that variance to project confidence intervals for future global temperatures. Once that is done, you are led to the inexorable conclusion that it is very difficult to say with much confidence what will happen in the future to global temperatures.
Here is one attempt. This one uses just short time segments to make forecasts, and even these confidence intervals expand.
Other images there have different starting points and use different estimation techniques. One of my favourites is this one, indicating that the IPCC predicted much greater deviation from long term trend than seems to be occurring: