When Did Global Warming Begin?

Image Credit: Marathon Nation – Soft Pixel

By WUWT regular “Just The Facts”

There have been a number of statements made recently about “Global Warming Deniers”, e.g. “White House: Global Warming Deniers Wrong to Reference Polar Vortex” US News, “Watch The Daily Show mock Trump and other global warming deniers” The Week and “Global warming denier Jim Inhofe: ‘Fewer and fewer’ senators believe in climate change ‘hoax’”. The Raw Story

Given the apparent prevalence of “Global Warming Deniers”, it seems prudent to take a look at the data so that everyone is clear when Global Warming began and what is undeniable. As such, from the following EPICA Dome C Ice Core record from Vostok, Antarctica, over the last 450,000 years Earth has experienced numerous Glacials, commonly referred to as Ice Ages, and Interglacials,  like the Holocene Interglacial we are experiencing today:

“The Last Glacial Maximum (LGM) refers to a period in the Earth’s climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period.” As such, one could argue that Global Warming began about “19,000–20,000 years ago”.

However, there was “the Late Glacial Maximum (ca. 13,000-10,000 years ago), or Tardiglacial (“Late Glacial”)” which was “defined primarily by climates in the northern hemisphere warming substantially, causing a process of accelerated deglaciation following the Last Glacial Maximum (ca. 25,000-13,000 years ago)”. “As such, one could also argue that Global Warming began about “13,000-10,000 years ago”.

Now looking at the GISP2 Ice Core record from Greenland, over the last 10,700 years, you can see the rapid warming that occurred at the end of last Glacial and that the current Holocene Interglacial reached it’s maximum peak between 8000 – 7500 years ago:

Since the peak of the Holocene Intreglacial, Earth has experienced several additional descending peaks, including the Minoan Warm Period between 3500 – 3000 years ago, the Roman Warm Period between 2250 – 1500 years ago and the Medieval Warm Period between 1250 – 750 years ago. The Medieval Warm Period and subsequent Little Ice Age can be seen clearly on the following temperature reconstruction based upon Alexandre, 1987 and Lamb, 1988, found Page 250, Figure 7.1 of IPCC Assessment Report 1:

The Little Ice Age “has been conventionally defined as a period extending from the sixteenth to the nineteenth centuries, or alternatively, from about 1350 to about 1850, though climatologists and historians working with local records no longer expect to agree on either the start or end dates of this period, which varied according to local conditions. NASA defines the term as a cold period between AD 1550 and 1850 and notes three particularly cold intervals: one beginning about 1650, another about 1770, and the last in 1850, each separated by intervals of slight warming.” As such, one could argue that Global Warming began in “about 1850”.

However, generally when referring to “Global Warming Deniers” there is an implication that the “Global Warming” that’s being denied is caused by anthropogenic carbon dioxide emissions. Anthropogenic CO2 emissions were de minimis in 1850. In fact, anthropogenic CO2 Emissions from Fossil-Fuels did not become potentially consequential until approximately 1950:

This is why the IPCC only claims to be;

“95% certain that humans are the “dominant cause” of global warming since the 1950s” BBC

As such, one could argue the Global Warming began in “the 1950s”.

However, if you look at the Met Office – Hadley Center  HadCRUT4 Global Surface Temperature record for the last 163 years you can see that temperatures didn’t warm during the 1950s, nor the 60s:

In fact it was not until approximately 1975 that temperatures began to rise. As such, one could argue that Global Warming began in approximately 1975.

However, in 2010 Phil Jones was asked by the BBC, “Do you agree that according to the global temperature record used by the IPCC, the rates of global warming from 1860-1880, 1910-1940 and 1975-1998 were identical?” Phil Jones responded that,”Temperature data for the period 1860-1880 are more uncertain, because of sparser coverage, than for later periods in the 20th Century. The 1860-1880 period is also only 21 years in length. As for the two periods 1910-40 and 1975-1998 the warming rates are not statistically significantly different. I have also included the trend over the period 1975 to 2009, which has a very similar trend to the period 1975-1998. So, in answer to the question, the warming rates for all 4 periods are similar and not statistically significantly different from each other.”

The warming during the periods of “1860-1880” and “1910-1940”, before anthropogenic carbon dioxide emissions became potentially consequential, is “not statistically significantly different” from the warming during the periods “1975-1998” and “1975 to 2009”. Thus there is no indication that the warming between “1975-1998” and “1975 to 2009” is unnatural, unusual and/or caused by anthropogenic carbon dioxide emissions. Global Warming may have started in 1975, but there is no observable evidence [of] anthropogenic CO2 emission based Global Warming began in 1975. As such, one could argue that anthropogenic CO2 emission based Global Warming began sometime [after] 1975.

However, if you look at following UAH Satellite Lower Atmosphere graph for the last 34 years;

and this NASA GISS Mean Monthly Surface Temperature Anomaly graph for the last 17 years;

you can see that Global Warming stopped in the late 1990s or early 2000s, which has been referred to as “The Pause” in Earth’s temperature. In fact, looking at the Werner Brozek’s recent article, the Pause in each major temperature data set is as follows:

For GISS, the slope is flat since July 2001 or 12 years, 6 months.

For Hadcrut3, the slope is flat since July 1997 or 16 years, 6 months.

For Hadcrut4, the slope is flat since December 2000 or 13 years, 1 month.

For Hadsst3, the slope is flat since December 2000 or 13 years, 1 month.

For UAH, the slope is flat since October 2004 or 9 years, 3 months. (goes to December using version 5.5)

For RSS, the slope is flat since September 1996 or 17 years, 4 months.”

Shown graphically, that looks like this:

As such, one could argue that for the last 17 – 9 years Global Warming hasn’t been occurring, and thus Global Warming began in 1975 and ended between 1996 and 2004.

However, this would not  resolve the question of when the “Global Warming” that’s being caused by anthropogenic Carbon Dioxide emissions began. If you look at Global CO2 Emissions from Fossil-Fuels and;

and Cumulative Anthropogenic CO2 Emissions from Fossil-Fuels, you can see that emissions have been growing rapidly in the last few decades:

In fact the Economist noted in 2013 that “The world added roughly 100 billion tonnes of carbon to the atmosphere between 2000 and 2010. That is about a quarter of all the CO₂ put there by humanity since 1750. And yet, as James Hansen, the head of NASA’s Goddard Institute for Space Studies, observes, ‘the five-year mean global temperature has been flat for a decade.'”

Thus, while anthropogenic CO2 emissions are the highest they’ve ever been, and growing rapidly, Earth’s temperature has been in a 9 – 17 year Pause. And the only period of warming that anthropogenic CO2 emissions could have had a significant influence on, 1975 – 1998, is “similar and not statistically significantly different from” the periods of 1860-1880 and 1910-1940 when there is no evidence of anthropogenic CO2 emission influence. As such one could argue that “Global Warming” due to anthropogenic Carbon Dioxide emissions may not have begun, that Earth’s sensitivity to CO2 may be low, that natural processes may be large enough to outweigh the effects of anthropogenic CO2 emissions, and/or that preparing for a period of rapid and catastrophic Global Warming, when there is no observational evidence that it is in fact occurring, may be a historic folly.

Anyway, what do you think, when did Global Warming begin?