As a series of Arctic heatwaves baked large swaths of tundra to the point of wildfire eruption, the world entered its 341rst consecutive month of temperatures above the 20th Century average. According to reports from the National Climate Data Center, global average land and ocean temperatures were .61 degrees Celsius above 20th Century levels and about .81 degrees Celsius above the 1880s average. This temperature increase represents about 1/6th the difference between now and the last ice age, but on the side of hot.
(Image source: NCDC)
NCDC analysis showed a majority of land and ocean surfaces experiencing hotter than average to much hotter than average conditions with numerous locations experiencing record warmest conditions. Only small and isolated regions experienced cooler than average conditions, with no areas experiencing record cold.
The near-record global heat occurred despite ENSO conditions in the Pacific Ocean remaining on the cool side of neutral with near-average sea surface temperatures across the central and east-central equatorial Pacific and below-average sea surface temperatures in the eastern equatorial Pacific. The cool water upwelling that continues over the eastern equatorial Pacific tends to depress global atmospheric temperatures by transferring heat content from the atmosphere to the sub-surface ocean. Such trends tend to dominate during negative phases of the Pacific Decadal Oscillation (PDO). These negative PDO periods are punctuated by numerous La Nina or cooler surface water conditions in the eastern equatorial Pacific. The Earth climate system has been in a negative PDO phase since the early 2000s, a phase which continues to this day. Despite the relative atmospheric cooling effects of this natural ocean circulation and temperature change, the decade of the 2000s was the hottest on record. Natural variability, which in this case would push for atmospheric cooling, had been overwhelmed by human-caused warming.
The dark side of a negative PDO is that it keeps the atmosphere relatively cooler at the expense of transferring more surface heat into the middle and deep oceans. Recent research conducted by Kevin Trenberth and others found just such a rapid heat transfer. The most ominous finding of the Trenberth study was that the deep ocean warmed the fastest over the most recent period of negative PDO, which showed that the ocean had a very effect means of rapidly transferring heat. A rapidly warming deep ocean is one more vulnerable to becoming stratified and anoxic — events that, in the past, have been implicated as causes for mass extinctions both in the ocean and on land.
We don’t currently know exactly when PDO will switch from a negative to a positive state. However, past PDO fluxes would indicate a switch is in the offing sometime during the next 2 to 17 years. Normal oscillations switch after remaining for periods of 15-30 years and the current negative phase has lasted for about 13 years. However, increasing atmospheric heat content may, ironically, drive the Pacific into an increasingly negative phase as it uptakes more and more of the atmospheric heat imbalance. Eventually, though, PDO will switch, dumping some of the excess heat back into the atmosphere. At that point, atmospheric warming rates will spike as more frequent La Ninas switch to more frequent El Ninos.
You can see the current state of ocean surface heat anomaly distribution with relatively cooler than average waters still dominating the region of the eastern equatorial Pacific in the map below:
(Image source: NOAA)
ENSO has remained neutral since 2012 and is predicted to remain so as summer proceeds into fall. With human atmospheric GHG levels continuing to rise and to push atmospheric forcing levels higher and with Earth Systems starting to contribute larger volumes of CO2 and methane, especially from Arctic environments, it is possible that ENSO neutral years over the coming decade may experience record warmth. In any case, the next El Nino year will almost certainly break global records.
It is important to note that changes in PDO are not a negative or positive feedback human warming of the climate system. PDO is simply an element of natural variability. Unfortunately, most feedbacks, including albedo loss and Earth carbon feedbacks, under human warming will be positive. That said, a single powerful negative feedback may be in the offing over the medium to long term. And this involves large fresh water and iceberg releases from Greenland and West Antarctica. Called the iceberg cooling effect, this feedback response may hold some degree of human warming and Earth Systems feedback warming in check at the cost of a very rapid and destructive ice sheet destabilization and sea level rise occurring in conjunction with related extreme weather events (see Hansen paper below).
Overall, 2013 is the 6th hottest year on record for the January to July period, according to NCDC, despite somewhat cooler eastern Pacific waters continuing to uptake excess heat and dump it into the middle and deep oceans during this time-frame. A combined warmth at or near record ranges and ocean uptaking record volumes of atmospheric heat is not an indication of cooling or a global warming pause, as some climate change deniers have alleged. To the contrary, it is an indication of a planet warming at break-neck rates and beginning a very dangerous trend toward a number of damaging Earth changes. Mitigation, in the form of rapid GHG reduction, is sorely needed if we are to have much hope of blunting the force of these emerging changes.