Prior to 2019, July 2016 was Anchorage, Alaska's warmest month on record. Summer 2019 as a whole was even warmer than July 2016.
A warm synoptic pattern that occurred within the context of increased anthropogenic greenhouse gas forcing resulted in exceptional and persistent record-breaking warmth this summer. During June-August 2019, Anchorage experienced its warmest summer on record by 2.0°F (1.1°C). Its summer mean temperature (62.815°F/17.119°C) exceeded that of its warmest month on record prior to 2019 (62.694°F/17.052°C in July 2016).
Summer 2019 saw Anchorage record its warmest-ever June, July, and August. Anchorage tied its all-time record high minimum temperature on two consecutive days. Anchorage reached 90°F (32.2°C) for the first time on record. The duration of the excessive warmth and extreme temperatures recorded during the summer would have been very unlikely, if not improbable, without human-induced climate change.
PREDOMINANT SYNOPTIC PATTERN
Summer 2019 featured a remarkable coupling of atmosphere and ocean. A persistent upper air ridge that promoted warm and dry conditions was anchored over the waters with the highest sea surface temperature anomalies.
These conditions promoted a synoptic pattern where the East Pacific Oscillation (EPO) was negative. The negative EPO combined with a negative Arctic Oscillation (AO) to form a negative Arctic Oscillation-negative EPO pattern (AO-/EPO-) that predominated during the summer. During summer 2019, the AO was negative on 81/92 (88%) days. The EPO was negative on 60/92 (65%) of days.
An AO-/EPO- pattern is typically a warm one in Anchorage. For the current climate reference period (1981-2010), the average summer temperature in Anchorage was 56.9°F (13.8°C). During AO-/EPO- patterns, the average was 57.6°F (14.2°C). Climate change has led to summers becoming warmer and also warm synoptic patterns (AO-/EPO-) becoming warmer.
ROLE OF CLIMATE CHANGE
The observed global warming since the 1950s is unequivocal with anthropogenic greenhouse gas emissions being the dominant driver of that warming (IPCC Climate Change Synthesis Report 2014). The warming is a global phenomenon with 98% of the world having experienced its warmest 51 years during the current 2,000 years (Neukom, et al. 2019).
Since 1880, Arctic temperatures have been increasing at more than twice the rate of global temperatures (GISTEMP Data Set). In recent decades, the rate at which the Arctic has been warming relative to worldwide temperatures has increased. From 1980 through 2018, the Arctic has warmed at a decadal rate of 1.51°F (0.84°C), which is just over 3.5 times the global rate (GISTEMP Data Set). Multiple lines of evidence corroborate the rapid warming that is taking place in the Arctic. Increases in humidity, precipitation, river discharge, glacier equilibrium line altitude and land ice wastage; warming of near-surface permafrost; and, decreases in sea ice thickness and extent, and spring snow cover extent and duration are consistent with rising temperatures (Box, et al. 2019).
Consistent with the Arctic warming, Alaska has recently experienced temperatures that are warmer than they have been at any time in the past century (Thoman et al., 2019). As Alaska has warmed, Anchorage has also experienced rising temperatures. A disproportionate share of Anchorage's warmest months has occurred in 2000 or later.
During the 1961-1990 base period, Anchorage had a summer (June 1-August 31) mean temperature of 56.4°F (13.6°C). During the current climate reference period (1981-2010), Anchorage's average summer temperature had risen to 56.9°F (13.8°C). For the most recent 30-year period (1989-2018), Anchorage's average summer temperature had increased further to 57.6°F (14.2°C). The last time Anchorage had a cooler than normal summer (mean temperature below the 1981-2010 reference period) was 2012 when the average summer temperature was 56.0°F (13.3°C).
Without climate change, the extreme summer 2019 warmth would have been improbable. However, the combination of a rising average summer temperature and increasing variability (1961-1990: mean temperature 56.4°F/13.6°C; standard deviation: 1.4°F/0.8°C vs. 1989-2018: mean temperature: 57.6°F/14.2°C; standard deviation: 1.6°F/0.9°C) has made summers like 2019 approximately 190 times more likely than they had been.
The long duration of the AO-/EPO- synoptic pattern led to the relentless persistence of above to much above normal temperatures in Anchorage that allowed monthly warm temperature records to be set in June, July, and August. Rapid Arctic warming has contributed to an increasing frequency of long-duration upper air patterns (Francis, et al. 2018). Should the world warm 3.6°F (2.0°C) above its pre-industrial temperatures, the persistence of boreal summer weather will likely increase further (Pfleiderer, et al. 2019).
Based on the above evidence, human-driven climate change played a key role in bringing about Anchorage's historic summer warmth. Without anthropogenic warming, the combination of the exceptional heat and remarkable duration of the warmth in Anchorage would have been very unlikely, if not improbable.
DATA AND RECORDS
Summer 2019 Temperature Thresholds:
Lows 60°F (15.6°C) or above: 9 days (previous summer and annual record: 4, 2016)
Highs 70°F (21.1°C) or above: 49 days (previous summer record: 40, 2004; previous annual record: 42, 2013)
Highs: 80°F (26.7°C) or above: 8 days (previous summer and annual record: 4 days, 2015)
Highs: 90°F (32.2°C) or above: 1 day (none prior to 2019)
Daily Record High Minimum Temperatures:
June 8: 54°F (12.2°C) (old record: 53°F/11.7°C, 1978)
June 24: 58°F (14.4°C) (tied record set in 1984)
June 28: 57°F (13.9°C) (old record: 56°F/13.3°C, 2015 and 2016)
June 29: 60°F (15.6°C (old record: 58°F/14.4°C, 1984 and 1990)
June: 3 new records and 1 tied record
July 2: 57°F (13.9°C) (tied record set in 1970)
July 3: 58°F (14.4°C (tied record set in 1979 and tied in 1999 and 2014)
July 5: 61°F (16.1°C) (old record: 60°F/15.6°C, 1984)
July 6: 59°F (15.0°C) (tied record set in 2015)
July 8: 61°F (16.1°C) (old record: 59°F/15.0°C, 1968, 2003, and 2004)
July 9: 62°F (16.7°C) (old record: 59°F/15.0°C, 2003)
July 12: 60°F (15.6°C) (old record: 59°F/15.0°C, 1977)
July 13: 59°F (15.0°C) (tied record set in 1972 and tied in 2013)
July 20: 59°F (15.0°C) (old record: 58°F/14.4°C, 1973, 1983, 2003, 2004, and 2016)
July 22: 58°F (14.4°C) (tied record set in 1984 and tied in 1996, 2013, and 2016)
July 24: 59°F (15.0°C) (tied record set in 1984)
July: 5 new records and 6 tied records
August 7: 61°F (16.1°C) (old record: 58°F/14.4°C, 1979 and 1983)
August 13: 63°F (17.2°C) (old record: 57°F/13.9°C, 2003) ***tied all-time record***
August 14: 63°F (17.2°C) (old record: 58°F/14.4°C), 2001) ***tied all-time record***
August 16: 58°F (14.4°C) (tied record set in 1967)
August 17: 57°F (13.9°C) (old record: 56°F/13.3°C, 1984)
August: 4 new records and 1 tied record
Summer: 12 new records and 8 tied records
Daily Record High Maximum Temperatures:
June 23: 78°F (25.6°C) (old record: 75°F/23.9°C, 1974)
June 24: 75°F (23.9°C (old record: 74°F/23.3°C, 2015)
June 27: 79°F (26.1°C) (old record: 78°F/25.6°C, 1997)
June 28: 81°F (27.2°C) (old record: 80°F/26.7°C, 1997)
June 29: 82°F (27.8°C) (old record: 77°F/25.0°C 1968, 1989, and 1990)
June: 5 new records
July 3: 80°F (26.7°C) (tied record set in 2018)
July 4: 90°F (32.2°C) (old record: 77°F/25.0°C), 1999) ***all-time record***
July 5: 81°F (27.2°C) (old record: 77°F/25.0°C, 1999)
July 6: 81°F (27.2°C) (tied record set in 2015)
July 7: 85°F (29.4°C) (old record: 79°F/26.1°C, 2009)
July 8: 85°F (29.4°C) (old record: 84°/28.9°C, 2003)
July: 4 new records and 2 tied records
August 7: 77°F (25.0°C) (tied record set in 2015)
August 10: 77°F (25.0°C) (old record: 75°F/23.9°C, 1960, 1972, and 2004)
August 12: 77°F (25.0°C) (tied record set in 2005)
August 13: 77°F (25.0°C) (old record: 75°F/23.9°C, 1963, 1977, 2007)
August 14: 75°F (23.9°C) (old record: 74°F/23.3°C, 1990)
August 15: 77°F (25.0°C) (old record: 76°F/24.4°C, 1984)
August: 4 new records and 2 tied records
Summer: 13 new records and 4 tied records
CONCLUSION
Anchorage experienced a historically warm summer. The all-time record high temperature was established, the all-time record warm minimum temperature was tied on two consecutive days, and numerous daily record high maximum and minimum temperatures were set or tied. June 2019 was the warmest June on record. July 2019 was the warmest July and month on record. August 2019 was the warmest August on record.
Ocean-atmosphere coupling produced a persistent pattern associated with warmer than normal temperatures. Anthropogenic climate change, that has driven global and Arctic warming and led to increasing temperature variability in the Arctic region, has dramatically increased the probability of persistent warmth and extreme high temperatures. Absent the contribution of climate change, the kind of warmth seen during summer 2019 was extremely unlikely, if not improbable.
Going forward, the ongoing warming is likely to continue on account of a continuing rise in the atmospheric concentration of greenhouse gases. Although summer warmth equivalent to 2019 will likely remain rare over the next decade or two, the probability of such occurrences will very likely increase.
The matter of climate's being dynamic is not controversial. That notion has been misdirected to discount the reality of contemporary climate change.
The impact of human activity on the climate is a new phenomenon. That climate change predated humans and occurred before human activities reached a scale sufficient to influence the climate has also been used to discount the reality of contemporary cllimate change. Global temperatures have diverged from trends in natural forcings e.g., solar irradiance.
Contemporary temperatures have risen at a rate that once occurred at geological timeframes. Put another way, the warming that has been observed over a period of decades is what once occurred over hundreds of thousands of years.
The Stein paper did not attempt to assert that today's cryosphere is "healthier" than it was in the past. That was not its purpose. In part, his research (as noted in another paper he authored) was actually aimed at understanding why the decreasing extent and thickness in Arctic sea ice had been larger than anticipated by the climate models.
Two major takeaways from the paper that are relevant to understanding what happened and what currently is happening:
The increase in sea ice extent during the late Holocene seems to be a circum‐Arctic phenomenon, coinciding with major glacier advances on Franz Josef Land, Spitsbergen and Scandinavia...
The main factors controlling the millennial variability in sea ice and surface‐water productivity are probably changes in surface water and heat flow from the Pacific into the Arctic Ocean as well as the long‐term decrease in summer insolation, whereas short‐term centennial variability observed in the high‐resolution middle Holocene record was possibly triggered by solar forcing.
onlinelibrary.wiley.com/doi/full/10.1002/jqs.2929
Fast forward to today:
1. Summer solar insolation in the Arctic has continued to decline. However, there has been an abrupt reversal in the Arctic cooling that had been occurring at a millennial scale and dramatic reduction in average annual Arctic sea ice extent and summer Arctic sea ice extent despite the natural factor that should be leading to a persistence in such trends. This dramatic decoupling from a key natural driver is a major "red flag."
2. With some exceptions, global glaciers are now in dramtic retreat.
Again, one is dealing with rapid changes that once occurred at geological timeframes. A paper published in the January 25, 2019 edition of Nature revealed what is, in fact, an 'unhealthy' situation in the cryosphere:
Here we show that pre-Holocene radiocarbon dates on plants collected at the margins of 30 ice caps in Arctic Canada suggest those locations were continuously ice covered for > 40 kyr, but are now ice-free. We use in situ 14C inventories in rocks from nine locations to explore the possibility of brief exposure during the warm early Holocene. Modeling the evolution of in situ 14C confirms that Holocene exposure is unlikely at all but one of the sites. Viewed in the context of temperature records from Greenland ice cores, our results suggest that summer warmth of the past century exceeds now any century in ~115,000 years.
www.nature.com/articles/s41467-019-08307-w
Indeed, in another paper, Dr. Stein explained et al., explained:
Over the last about four decades, coinciding with global warming and atmospheric CO2increase, the extent and thickness of Arctic sea ice has decreased dramatically, a decrease much more rapid than predicted by climate models. The driving forces of this change are still not fully understood. In this context, detailed paleoclimatic records going back beyond the timescale of direct observations, i.e., high-resolution Holocene records but also records representing more distant warm periods, may help to to distinguish and quantify more precisely the natural and anthropogenic greenhouse gas forcing of global climate change and related sea ice decrease.
adsabs.harvard.edu/abs/2017AGUFMPP54A..01S
Over coming decades, this will likely result in Arctic sea ice extent minimum figures that match or exceed the prior Holocene minimum figures, especially if the Arctic continues to warm.
Finally, the role of the carbon cycle on climate is clear in an overwhelming body of scientific evidence. If one wants to refer to that role as a 'climate control knob' or in other terms is a matter of semantics. The science makes clear the link.
2019 had a mean temperature of 62.0°F (16.7°C). That surpassed the previous record warm figure of 60.0°F (15.6°), which was set in 2016.
The 10 warmest June 1-September 15 timeframes occurred as follows:
Prior to 1980: 2 cases
2000 or later: 7 cases
2010 or later: 5 cases
2015 or later: 4 cases
www.nature.com/articles/d41586-019-02653-x
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