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There's been a lot of controversy over the years with regards to the baseline climatology used to determine how far we've advanced in the global warming of the planet. Various baselines are thrown around. Many scientific studies and even the Intergovernmental Panel on Climate Change use the early era of widespread use of thermometers as the "pre-industrial" climatology...typically 1850-1900, in order to determine how far we've advanced in global warming. This is because they use "guardrails" for significant warming which will cause devastating to catastrophic impacts (+1.5 C, +2 C, etc). They've also used 1986-2005 as a recent climatology to give people a comparison to the climate we are currently experiencing, which can cause additional confusion.
Hawkins et al. (2018) estimated that the pre-industrial period (defined as 1720-1800) is 0.6-0.8 degrees C/1.1-4 degrees F cooler globally than the 1986-2005 period. This is for global average land-air/sea surface temperatures. This estimate works well for the globe and the hemispheres. So the warmest year on record, 2016 was, at the high-end estimate nearly +1.4 C above pre-industrial levels globally. I cite this recent research frequently in my assessments of where we are in terms of global warming.
However, while the globe as a whole is warming, the temperature of the Arctic region has been warming extremely rapidly. And in fact, the warming is at a much faster rate than the globe as a whole, making its change far more pronounced and anomalous relative to its (once) very cold climate. When looking at past temperatures within the region (typically north of 60-66 N latitude), it becomes apparent very quickly that it is regionally far more intense warming within its own regional climatological record than the globe as a whole.
To demonstrate this, I studied the NASA GISS global temperature dataset. The "Arctic" in the dataset is defined as 64 N to 90 N latitude. The anomalies are based on the 1951-1980 climatology. If we take a look at the mean departure in the Arctic over the most recent standard climatological period used by US weather/climate agencies (1981-2010), we see that the Arctic is 0.85 C warmer than the 1951-1980 climate of the GISS. So an adjustment of a temperature anomaly value from one baseline to the other would be an adjustment of 0.85 C (add to go from 1981-2010 baseline to 1951-1980, subtract going the other way).
However, what if we want to know how much cooler the Arctic was early in the record-keeping compared to the 1951-1980 baseline? NASA records go back to 1880. A thirty year period would be 1880-1909.
When we calculate the mean departure for that time period, we find the Arctic was 0.85 C cooler than the 1951-1980 climatology. From this, you can deduce that the Arctic was 1.7 C cooler during the 1880-1909 period than 1981-2010!
To clarify...an anomaly is only a magnitude of temperature from a given baseline norm. So the actual temperature values used to determine temperature anomalies from normal do not change, but what's "normal" is changed, providing a picture of how far warming of a region or the planet has advanced relative to a certain climatology. And as global warming caused by the Industrial Revolution commenced during the 1700s, it is important to understand how far along we have come relative to that period. This is because everything we've depended on as a civilization to thrive on this planet for the past 10,000 years; plant/animal species, agricultural production, abundant resources, safe environmental conditions) are based on the temperature of this planet and stable long-term weather conditions. Given that the pre-industrial period was the 18th century, the 1981-2010 period for the Arctic may reasonably be near 1.8 C warmer than the 18th century.
A quick side note...one thing I hear from some who otherwise understand and accept the science of climate change is that significant climate warming did not begin until later in the 20th century when fossil fuel burning really exploded post-WWII. While it certainly began to accelerated by the later-1970s and 80s as significant releases of carbon dioxide from from the earlier two decades caught up to the climate (and there were also some reductions in aerosol dimming which can block a small amount of sunlight), this idea is simply not correct. Globally, the temperature warmed by 0.3-0.4 C/0.5-0.7 F from the 1880-1909 period and the 1951-1980 period. A seemingly small amount, but given this mean warming occurred over decades and not the more typical hundreds to thousands of years with natural climate change (negating year-to-year variability and the short-term effects of most volcanoes), this is a significant rise in global average temperature forced by early fossil fuel burning. And if you're skeptical of global change, take a look at the Arctic. As previously mentioned, it warmed by nearly 0.9 C between those two periods. But also look at all the annual anomalies of -1 degree C/-1.8 degree F or colder relative to 1951-1980 climatology. They are very common during the late 19th century, but after the turn of the 20th century, become extremely rare. The last year with a temperature anomaly of -1 C or colder is 1918. By 1920, the first years with positive anomalies appear and by the late 1930s, the first years with anomalies of +1 C or warmer appear in the records. A climatic glimpse of the future.
The black arrow marks the final year the Arctic was at least 1 C colder than normal in the 1951-1980 climatology...1918. The red box denotes the first two years on record the Arctic would be at least 1 C above normal...just before the beginning of WWII. Now such anomalies are but a distant memory. The last time the Arctic was under 1 degree C above normal annually was 2004.
Sea ice declines began to really become notable during the 20th century and it is certain that both global mean warming by increasing carbon dioxide as well as the regional sea ice feedback from sea ice retreating, reducing albedo and opening up dark blue ocean to increased warming were causing this regionally accelerated warming which was already moving quickly along compared to the globe as a whole. That was all before 1950 and long before anyone was thinking about any sort of human-forced climate change. But before 1950, abrupt climate change and the early stages of current climate destabilization were already underway.
From Zhang et al. (2018) showing two statistical reconstructions of sea ice extent in the Barents and Kara Seas beginning in the year 1289. Notable long-term decline in sea ice has been ongoing since the 1800s, becoming greatly anomalous compared to the past variability after 1900.
Historical Bering Sea Ice Extent in February for 1850-2019. Persistent decline commenced during the mid-1970s, with a more recent abrupt collapse during the mid to late-2010s (graphic by PhD student Zach Labe).
Reconstructed Arctic sea ice extent using data from Kinnard et al. (2011) from before 600 AD to 2008 AD (40-year smoothing applied to reduce noisy year-to-year variability). Abrupt long-term decline in sea ice commenced by ~1950. The situation has worsened greatly since 2008.
Reconstructed summertime temperatures in the Arctic (north of 60 N latitude) for the past two millennia ending in the early-2000s. By Kauffman et al. (2009). The long-term, abrupt rise in regional temperatures commenced during the late-19th Century, the early part of the observational records. The classic hockey stick phenomenon associated with abrupt climate change.
The collapse of sea ice in the Arctic because of the extremely rapid regional warming have implications globally. This includes the intensification of overall mean global warming, continued dynamic destabilization of the jet stream circulation over the Northern Hemisphere, rapidly increasing mid-latitude extreme weather; increasing heating of the Arctic leading to accelerating releases of carbon dioxide and methane from terrestrial and subsea permafrost, peatlands and thermokarst lakes, also accelerating global warming; increasing ocean storm activity over the Arctic Ocean, placing communities at risk of the impacts of more powerful storm surges from lack of sea ice, also accelerating coastal permafrost destruction; accelerating melting of the Greenland Ice Sheet, accelerating sea level rise and weakening the Atlantic Meridional Overturning Circulation controlling the flow of heat from the tropics to the mid-latitudes leading to further extreme weather events.
---Meteorologist Nick Humphrey
The following are recommended baseline adjustments to estimate pre-industrial temperature anomalies. This is specific for the Arctic.
1951-1980 (used by NASA): Add 0.9 degrees C
1979-2000 (used by Climate Reanalyzer): Add 1.5 degrees C
1981-2010 (used by US and other met/climo agencies): Add 1.8 degrees C