Sunday, February 9, 2014

2013 climate review

2013 was notable in several ways, from the record warmth in Australia to the polar vortex that gripped the Eastern US in November and December due to a weakened Arctic polar jet stream and record drought in the US state of California, along with numerous extreme weather events around the globe.


First, the one everyone waits for: Global average temperature.  Using the new Cowtan and Way dataset, which corrects for the coverage bias in HadCRUT4 as well as the sea surface temperature bias in GISS, the Earth experienced its 5th warmest year on record.  The annual average temperature in 2013 was 0.544ºC above the 1961-1990 baseline, trailing only 2010, 2005, 2007, and 2009, in that order.


The 30-year trend in global average temperature stands at +0.1918 ± 0.0496ºC per decade (trend ± 95% confidence interval).  The trend since January 1998 (the denier's favorite start point) and December 2013 is nearing statistical significance even when accounting for autocorrelation, now clocking in at +0.09283 ± 0.10047ºC per decade (p = 0.0718), with much of the decrease in the rate since 1998 due to ENSO, aerosols, and the solar cycle as I showed previously.

The average CO2 concentration continued its rapid rise in 2013, with an annual average of 396.48 ppmv, a 2.66 ppmv rise from the 2012 average of 393.82 ppmv.


That puts us on track for at least 1.51ºC of warming above pre-industrial levels using a 100-year climate sensitivity value of 0.809ºC/W/m2.  Since we've already experienced ~0.9ºC of warming since the start of the Industrialized Age in AD 1850, we still have another 0.61ºC of warming to go just from today's CO2 levels.  Taking the longer (1,000-year) view, we have successfully locked in a total warming of 3.01ºC of warming and have another 2.11ºC of warming to go just from today's levels.  If today's rate of rise continues until AD 2100, we'll have 670.90 ppmv in the atmosphere by AD 2100.

Arctic average temperature in 2013 was 1.11ºC above the 1951-1980 baseline, the lowest annual average since 2004 and well below both 2011 (2.00ºC above the baseline) and 2012 (1.74ºC above the baseline).

GISS data, 64ºN-90ºN zonal mean
Given the drop in Arctic temperature, Arctic sea ice rebounded slightly from its September 2012 low, although in this case the "rebound" was still the 6th lowest September ice extent on record.  The smooth trend is still negative, indicating that the "rebound" is at this point, a minor wiggle and does not change the overall trend or its acceleration downward.  The overall rate of decline since 1979 is -54,567 km2 per year, with that loss accelerating by -1,377.8 km2/year.


September records the lowest extent of Arctic sea ice each year.  The pattern in September looks very similar to the overall trend.


At the current rate of decline, the Arctic will be ice-free in September 2032.

On the other side of the planet, Antarctic sea ice is increasing.  The annual average in 2013 was 12,448,330 km2, an increase of 467,500 km2 over the 2012 average and an increase of 235,830 km2 over the previous high in 2008.  The linear rate of increase since 1979 is +18,074 km2 per year, with a slow acceleration of +427.4 km2/year.  Both are roughly 1/3 as large as the Arctic rates.


Research suggests that one of the main reasons Antarctic ice has expanded is due to melting Antarctic land ice.  Antarctic land ice has lost an average of -71 billion metric tons of ice per year between 1992 and 2012 (Shepherd et al. 2012).  Runoff from the land combined with increased rain/snow is freshening the surface of the Southern Ocean, making it easier to freeze (Zhang 2007, Bintanja et al. 2013).  The other main reason is an increase in wind speed around the Antarctic continent due to the hole in the ozone layer, which create more openings between ice floes and leave more of the surface available to freeze (Turner et al. 2009, Zhang 2014).

Sea level rose by 2.82 mm above the average 2012 value in 2013.  The overall rate since 1992 continued at 3.2 mm/year.  Thanks to Shepherd et al. (2012) and Gardner et al. (2013), we know that about 1.3 mm/year of that rise came from glaciers and polar ice caps.

The main message from all this data?  Despite what science deniers claim, the data shows that the Earth is still warming.

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