The past calendar year proved to be an interesting one from the standpoint of the planet’s climate.

Twenty-thirteen was not the hottest year on record, though it ranked in the top 10 on most datasets. (4th in NOAA’s NCDC’s record, 7th in NASA GISS, 8th in the UK’s Hadley Centre HadCRUT4, and 5th in the new Cowtan and Way record.)

It was a year in which the hiatus in warming rose to public awareness and a year in which many possible explanations were published, with new datasets maintaining that claims of a warming “pause” were overblown.

IPCC in the fall released the first part of its Fifth Assessment, in which it concluded that the human fingerprint in warming is more certain than ever. IPCC in that report also raised sea-level rise estimates, but it slightly decreased the lower-end estimate of climate sensitivity. Arctic sea ice extent was lower than average, though not nearly as low as the record lows of 2012, and Antarctic sea ice extent hit record highs.

Global Temperatures

The figure above shows global temperature from 1880 through present for four global land temperature records — the UK’s Hadley Centre’s HadCRUT4, NOAA’s NCDC, NASA Goddard Institute for Space Studies (GISTemp), the new Cowtan and Way dataset, and the two satellite-based records (RSS and UAH). While all records largely agree with each other, the satellites tend to have slightly lower trends than surface measurements, a discrepancy yet to be adequately resolved.

Over the past 30 years some variations are evident. Satellite records show an enhanced response to volcanoes and El Niño/La Niña events, such as the 1991* eruption of Pinatubo or the 1998 “super” El Niño. Over the past decade most records have been somewhat flat, with the new Cowtan and Way dataset (which attempts to better measure Arctic temperatures) showing more warming and the RSS satellite dataset showing noticeably less. Interestingly, and as a result, those who want to minimize modern warming now are inclined to start using the RSS dataset rather than the UAH dataset, the latter having previously shown less warming.

While temperatures over the past decade have been somewhat flat, they are not necessarily inconsistent with the long-term trend in the data, in part because temperatures rose so much in 1998 and 2001 and stayed high thereafter.

Likewise, while temperatures are running low relative to what climate models had projected over the past decade, they are still within the bounds of uncertainty representing the range of model runs; more years of stagnant temperatures likely will have to occur before scientists can conclude anything regarding the climate sensitivity of models based on observations.

Sea Ice in the Arctic and Antarctic

Sea ice extent in 2013 in the Arctic was well below the long-term (1978-2000) average, but nowhere near as low as the record-breaking low of 2012. Antarctic sea ice was unusually high this year, pushing overall global sea ice close to the long-term average.

The figure above shows average sea ice extent for each day of the year for the Arctic (in red) and Antarctic (in blue) compared to the long-term averages for each (the dotted lines and grey error ranges). The long-term averages are based on the full satellite record (starting in 1978), when scientists first were able to accurately measure sea ice extent. From 1978 through 2013, Arctic sea ice has dropped fairly dramatically, especially during summer melting seasons (it tends to recover to roughly the same level in the winter, so winter Arctic sea ice extent has not declined nearly as much). Although there was less summer melt in 2013 than 2012, the overall trend is distinctly negative, and new record lows are considered likely some time in the next few years.

The extent of Antarctic sea ice, however, has been something of an enigma. It has risen notably in the past year, a pattern not predicted by climate models. Models generally predict moderate declines in the extent of Antarctic sea ice, though nothing close to what is seen in the Arctic given the slower warming in the Antarctic region.

Oceans

Sea-level rise has continued apace in the oceans, with 2013 reversing the slight slowdown that occurred in 2012, and again rising in line with the overall trend over the past decade. Sea-level measurements are now primarily taken with satellite altimeters, which have much greater spatial coverage than more limited tide gauges.

The big news in sea-level rise in 2013 came with IPCC’s upward revision of projected sea-level rise by 2100, up to about three feet (1 meter) in the worst-case scenario. This new estimate had been widely expected, as many had criticized the prior 2007 IPCC report for being overly conservative on sea-level rise projections. A number of researchers say they remain concerned that the new projections are still too conservative, arguing that rises of up to five feet (1.5 meters) are possible.

The other big news involving the planet’s oceans in 2013 involves El Niño, the periodic cycle in ocean temperatures that drives much year-to-year variability in global temperatures. El Niño years tend to be hotter than normal, while La Niña years tend to be colder. With 2013 neither an El Niño nor La Niña year, it ended up in the middle of the pack of recent years.

The figure above, from the International Research Institute for Climate and Society at Columbia University, shows the most recent set of forecasts for El Niño in 2014. A moderate El Niño forecast for the latter part of 2014, with some suggesting that the year could end up setting a new record for the hottest calendar year yet.

While it’s far too early in the year to tell, the underlying warming trend, coupled with short-term variability factors like El Niño, makes it a virtual certainty that 2010 sooner or later will be displaced as the planet’s hottest year.  There is virtually no doubt that 2014 will be warmer globally than average.  The question is just how warm it will be.

*Typographical error corrected 2/3/2014.