When Did Anthropogenic Global Warming Begin?

By WUWT Regular “Just The Facts”:

Note: This article builds upon a previous article, When Did Global Warming Begin?, which offers highly recommended background for this article.

There appears to be some confusion as to when humans might have begun to influence “Earth’s Temperature”. For example, “Global warming is the unusually rapid increase in Earth’s average surface temperature over the past century primarily due to the greenhouse gases released as people burn fossil fuels.” NASA Earth Observatory “Ninety-seven percent of climate scientists agree that climate-warming trends over the past century are very likely due to human activities, and most of the leading scientific organizations worldwide have issued public statements endorsing this position.”

NASA Climate Consensus page “‘Global warming started over 100 years ago‘: New temperature comparisons using ocean-going robots suggest climate change began much earlier than previously thought”. The Daily Mail “The temperature, they pointed out, had fallen for much longer periods twice in the past century or so, in 1880-1910 and again in 1945-75 (see chart), even though the general trend was up. Variability is part of the climate system and a 15-year hiatus, they suggested, was not worth getting excited about.” Economist “Our Earth is warming. Earth’s average temperature has risen by 1.4°F over the past century, and is projected to rise another 2 to 11.5°F over the next hundred years.” EPA

However, there is not compelling evidence that anthropogenic CO2 was sufficient to influence Earth’s temperatures prior to 1950, i.e. “Climate model simulations that consider only natural solar variability and volcanic aerosols since 1750—omitting observed increases in greenhouse gases—are able to fit the observations of global temperatures only up until about 1950.” NASA Earth Observatory “The observed global warming of the past century occurred primarily in two distinct 20 year periods, from 1925 to 1944 and from 1978 to the present. While the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear since this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing.” NASA Geophysical Fluid Dynamics Laboratory / Delworth et al., 2000 “Internal climate variability is primarily responsible for the early 20th century warming from 1904 to 1944 and the subsequent cooling from 1944 to 1976.” Scripps / Ring et al., 2012: “There exist reasonable explanations, which are consistent with natural forcing contributing significantly to the warming from 1850 to 1950”. EPA

So how to clear up this confusion? Let’s take a look at the data…

If you look at Global CO2 Emissions from Fossil-Fuels;

Global CO2 from Fossil-Fuel Emissions By Source;

and Cumulative Anthropogenic CO2 Emissions from Fossil-Fuels,

you can see that Anthropogenic CO2 Emissions from Fossil-Fuels did not become potentially consequential factor until approximately 1950, and then grew rapidly thereafter. Per the Economist, “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.’” The large increase in Anthropogenic CO2 Emissions from Fossil-Fuels since 1950 is quite clear in this Global Per Capita Carbon Emissions graph:

There have also been claims made that Land Use Changes measured as Annual Net Flux of Carbon to the Atmosphere were a significant source of Anthropogenic CO2  i.e.:

However, when you look at the Net Flux of Carbon to the Atmosphere from Land-Use Changes from 1850 to 1990;

it is apparent that the majority of the increase occurred after 1950, and the change between 1900 and 1950 was de minimis. Furthermore, the Houghton data these graphs are based upon is highly suspect, i.e. from IPCC AR4: “Although the two recent satellite-based estimates point to a smaller source than that of Houghton (2003a), it is premature to say that Houghton’s numbers are overestimated.” Houghton’s method of reconstructing Land-Use Based Net Flux of Carbon appears arbitrary and susceptible to bias; i.e. “Rates of land-use change, including clearing for agriculture and harvest of wood, were reconstructed from statistical and historic documents for 9 world regions and used, along with the per ha [hectare] changes in vegetation and soil that result from land management, to calculate the annual flux of carbon between land and atmosphere.” Furthermore Houghton’s findings have varied significantly over time, i.e. in Houghton & Hackler, 2001 they found that, “The estimated global total net flux of carbon from changes in land use increased from 397 Tg of carbon in 1850 to 2187 Tg or 2.2 Pg of carbon in 1989 and then decreased slightly to 2103 Tg or 2.1 Pg of carbon in 1990“. However, by Houghton, R.A. 2008 he found, “The estimated global total net flux of carbon from changes in land use increased from 500.6 Tg C in 1850 to a maximum of 1712.5 Tg C in 1991“.

Given Houghton’s overestimations, arbitrary reconstruction method and highly variable results, his Net Flux of Carbon to the Atmosphere from Land-Use Change data is not credible. However, even if it was, Net Flux of Carbon to the Atmosphere from Land-Use Change was inconsequential prior to 1950;

and it appears that Land and Ocean Sinks would have absorbed any increace, along with much of the minimal pre-1950 Anthropogenic CO2 Emissions from Fossil-Fuels. This is supported by the findings of Canadell et al., 2007 that, “Of the average 9.1 PgC y −1 of total anthropogenic emissions (F Foss + F LUC) from 2000 to 2006, the AF was 0.45; almost half of the anthropogenic emissions remained in the atmosphere, and the rest were absorbed by land and ocean sinks.” Furthermore, they found “increasing evidence (P = 0.89) for a long-term (50-year) increase in the airborne fraction (AF) of CO2 emissions, implying a decline in the efficiency of CO2 sinks on land and oceans in absorbing anthropogenic emissions.” Thus absorption rates of Land and Ocean Sinks were likely significantly higher prior to 1950.

As such, since there is not compelling evidence that Anthropogenic CO2 was sufficient to have ab influence Earth’s temperatures prior to 1950, Anthropogenic CO2 cannot be the cause of the warming that occurred before 1950. However, this doesn’t mean that CO2 based Anthropogenic Global Warming began in 1950, because 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. However, this doesn’t mean that CO2 based Anthropogenic Global Warming began in 1975, because as Phil Jones noted during a 2010 BBC interview, “As for the two periods 1910-40 and 1975-1998 the warming rates are not statistically significantly different.” As such, the warming from 1910 – 1940, before Anthropogenic CO2 became potentially consequential, is “not statistically significantly different” from the warming during the period from 1975 – 1998 when the IPCC AR5 claims to be ” extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century”. Given that “causes of the earlier warming are less clear“, our understanding of Earth’s climate system is rudimentary at best, and our historical record is laughably brief, it is confounding how the IPCC can be so “extremely” sure “that human influence has been the dominant cause of the observed warming since the mid-20th century”, which is “not statistically significantly different” from the natural warming that occurred between 1910 – 1940.

Regardless, claims that “Global warming is the unusually rapid increase in Earth’s average surface temperature over the past century” are erroneous and indicative of either ignorance or duplicity on the part of NASA’s Earth Observatory, NASA’s Climate Consensus page, The Daily Mail, the EPA and many others. So what do you think? When Did Anthropogenic Global Warming Begin?