Arguments
What ended the Little Ice Age?
What the science says...
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The sun was warming up then, but the sun hasn’t been warming since 1970. |
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Climate Myth...
We're coming out of the Little Ice Age
"The global temperature has been rising at a steady trend rate of 0.5°C per century since the end of the little ice age in the 1700s (when the Thames River would freeze over every winter; the last time it froze over was 1804) ...
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The IPCC blames human emissions of carbon dioxide for the last warming. But by general consensus human emissions of carbon dioxide have only been large enough to be significant since 1940—yet the warming trend was in place for well over a century before that." (David Evans)
Climate change sceptics suggest that because the climate has changed dramatically in the past – and without man’s intervention – it is possible that current changes to the Earth’s climate are also a natural event. You may be familiar with paintings depicting Londoners skating on the frozen River Thames, when winters, at least in the northern hemisphere, were more severe. The beginning and end of this period are subject to various interpretations, but the period is referred to as the Little Ice Age (LIA) and occurred between the 16th to 19th centuries.
Limited History
If we are to understand the LIA, we need to figure out what caused it. Scientists have examined several important strands of evidence about the LIA, including the activity of the sun, of volcanoes, and ocean heat circulation, principle drivers of natural climate change.
The activity of the sun can be assessed by looking at proxies – processes we know are affected by the sun’s activity. One of these is the formation of the radioactive isotope Carbon-14 in the atmosphere, which plants then absorb. By measuring carbon-14 in tree rings and other materials we know are from a certain period, we can estimate how active the sun was at the time. This graph shows the sun’s activity over the last millennium:
The carbon-14 data used in this graph go up to 1950. The graph below gives a fuller picture, showing that in the last three decades, the sun's normal cycle of activity has remained steady, while temperatures have shot up:
Yet while the dips in solar activity correlate well with the LIA, there are other factors that, in combination, may have contributed to the climate change:
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Volcanic activity was high during this period of history, and we know from modern studies of volcanism that eruptions can have strong cooling effects on the climate for several years after an eruption.
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The ‘ocean conveyor belt’ – thermohaline circulation – might have been slowed down by the introduction of large amounts of fresh water e.g. from the Greenland ice cap, the melting by the previous warm period (the Medieval Warm Period).
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Sudden population decreased caused by the Black Death may have resulted in a decrease of agriculture and reforestation of agricultural land.
Can We Draw a Conclusion?
In truth, not really. The Little Ice Age remains for the present the subject of speculation. The most likely influence during this period is variable output from the sun combined with pronounced volcanic activity. We know that from the end of the LIA to the 1950s the sun’s output increased. But since WW2 the sun has slowly grown quieter, yet the temperature on Earth has gone up.
The sceptical argument that current warming is a continuation of the same warming that ended the LIA is unlikely. There is a lack of evidence for a suitable forcing (e.g. the sun) and numerous correlations with known natural forcings that can account for the LIA itself, and the subsequent climate recovery. Taken in isolation, the LIA might cast doubt on the theory of climate change. Considered alongside the empirical evidence, model predictions and a century of scientific research into the climate, recovery from the LIA is not a plausible theory to explain the observed evidence and rate of global climate change.
Basic rebuttal written by GPWayne
Update July 2015:
Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial
Last updated on 5 July 2015 by skeptickev. View Archives
"The activity of the sun can be assessed by looking at proxies... One of these is the formation of the radioactive isotope Carbon-14 in the atmosphere... By measuring carbon-14 in tree rings... we can estimate how active the sun was at the time."
Can you please clarify if so-called skeptics do this? If they rely on proxy data to establish history of solar activity, how can they reject it (proxy data) for temperature?
There seems to be some very comprehensive collation of historical records here:
climate-and-human-civilization-over-the-last-18000-years
The large graphic image does expand to be very readable and informative. Hopefully the very interesting references will not be taken to be too off-topic (as they go back 18000 years).
[TD] It is pretty much off topic, for the reason you gave. And not especially interesting or informative, because it uses only a very few very small geographic regions' temperature proxies with rather low resolutions. How about posting your comment in one of these instead:
What evidence is there for the hockey stick?
Real Skepticism About the New Marcott 'Hockey Stick'
TD47 @63, the poster uses just three temperature poxies for the holocene. Two (Agassiz/Renland and GISP2, ie, Alley et al, 2000) are from the north Atlantic region. The former is a composite of four ice cores from the Agassize Ice Cap on Ellesmere Island (just west of the northern end of Greenland) and one ice core from Renland (on the south east coast of Greenland, more or less north of Iceland). They represent the regional signal, therefore, of just one region on Earth, and one of the most variable temperature wise. The author mis-cites the source of the Agassiz/Renland data as Vinther et al (2009), whereas it is in fact Vinther et al (2008).
The third core is the Vostock core from Petit et al (2001). That means all three cores are from polar regions, and exhibit polar amplification. They are therefore not representative of global temperatures. In addition, they represent just two regions, and consequently show the typically large regional fluctuations in temperature which cancel out when averaged across the globe. As a result, they significantly overstate temperature change when compared to global figures.
To compound this problem, there are two errors in the presentation of the proxies. First (unsurprisingly), the GISP2 data is plotted to end in 1905 (determined by pixel count). In fact it terminated in 1855, as discussed here. You should note that Richard Alley has confirmed that the that 1855 is the correct termination of the data. More troubling is the extended, uniform plateau at the end of the Vostok period. Checking the data, I find the last data point is for a gas age of 724 BP (=774 B2K), or 1226 AD. The extended plateau at the end of the data shown in the poster must be samples taken from the firn, ie, the upper region of the ice core where pressure has not yet sealed air gaps, allowing free exhange with the atmosphere. The consequence is that it represents an average temperature over the last few centuries rather than modern temperatures, and completely conceals all variation over that period. Coupling these facts with the fact that the final data point for the Agassiz/Renland composite core is 1960, and there are no proxy data points that actually show recent temperatures.
These flaws (regional, polar amplified proxies PLUS incorrect terminations of ice cores with no modern, regional comparisons) tend to reinforce Andy May's false claim that "...we have not seen unusual warming in the present warm period, relative to other warming events in the last 18,000 years...". In fact recent warming is unusual relative to the past 18,000 years, as is shown by Marcott et al (see second link by the moderator); and may be unprecedented in that period.
I also note that May has relied on the very obsolete, and obvsiously schematic temperature reconstruction by Scotese rather than an actual, modern reconstruction of temperatures over the Phanerozoic, such as this one by Dana Royer:
The preference May shows for obsolete data, inaccurately presented suggests the poster is of dubious value as an information source.
"The IPCC blames human emissions of carbon dioxide for the last warming. But by general consensus human emissions of carbon dioxide have only been large enough to be significant since 1940—yet the warming trend was in place for well over a century before that." (David Evans)"
Hello,
Is this 1940's thing in dispute? I read through the LIA stuff here, but it only talks about the LIA itself. It is in the "myth" section, but unless I'm missed something, I didn't see anything saying anything otherwise. I was curious when was human industry significant enough to impact climate (roughly)?
[PS] Please see "It cooled mid-century" section. Climate change is a response to net forcings not just one - aerosols are important as well.
greg84 @65:
It depends on what you mean by impact.
William Ruddiman believes (and I believe that he has shown) that absent anthropgenic emissions from land clearing and agriculture, CO2 concentrations would have fallen from a high aroun 8,000 years ago to at least 240 ppmv, or lower. That fall may well have been enough to kick us into a new iceage by now, and would certainly have resulted in LIA conditions being the norm rather than the exception. (Note, the linked paper is the earliest presentation of his hypothesis, which is supported by a number of more recent papers, and by more than just Ruddiman. His theory is, however, not universally accepted among climate scientists.)
From about 1650, coal burning in London was sufficient that there was a selective advantage for dark forms of the Peppered Moth due to soot coating trees. So from then there was an appreciable anthropogenic forcing from black carbon.
More directly related to your question, in 1940, the forcing from CO2 relative to 1832 was 0.47 W/m^2, or 29.4% of the 1.6 W/m^2 in forcing 2008. (The exact figure and percentage will change, depending on your base year. I have used 1832 for convenience as I have a spreadsheet with CO2 concentrations back to 1832.) That is clearly significant, but also not enough to account for the majority of the warming trend from 1900-1940. Deniers like David Evans, however, tend to talk as though it was completely irrelevant. In fact, that forcing is larger than the probable chang in solar forcing over that period. It is, however, likely smaller than the volcanic forcing over that period. The near complete absence of volcanoes from 1910 to 1940 is the probable primary cause of the warming in that period; supplemented by (in order of magnitude) anthropogenic forcing, and solar forcing; with those two combined being approximately of the same magnitude as the volcanic forcing.
The interesting thing is that since 1940, there has not been a pause in volcanism, so that the volcanic forcing has been negative. Likewise, the solar forcing, after peaking in 1950, has declined so that in 2008 it was almost as low as the 1910 minimum. Consequently, while anthropogenic forcings account for only about 25% of the early twentieth century warming (give or take), they account for nearly 100% of the warming from 1900 to 2013.
In a recent poll of climate scientists, 86.76% thought that 50% or more of the warming since 1850 was anthropogenic. Given a distinct, and clearly natural cooling trend from 1850 to 1900, that figure would rise significantly if the start year had been 1900, and to near unanimity for a start year of 1950. The modal (most preferred) value was 80%, with twice as many thinking it was above 80% as thought it was below 50%. Science is not settled by consensus, of course. But scientists hold their opinions for good reasons. Anybody challenging so strong a concensus must show very good reasons to do so. Unfortunately no such good reasons have been presented.
Replying to comment from here.
Donny, first point of call for any question like this has to be IPCC WG1. You want the chapter of paleoclimate, and eg Fig 5.7.
Do you accept the physics law of conservation of energy? Assuming you do, then warming of the surface is a change in the energy flow. Apart from ocean-atmosphere heat exchanges, then warming or cooling are due to changes in forcings. These can be natural or man-made. To accept the idea that "half the change" is natural, then you need evidence that there is a natural forcing, operating since LIA, of about the same magnitude as anthropogenic forcing. That evidence is tough to find.
Instead LIA (which is much more pronounced in NH) is adequately accounted for by changes in volcanic and solar forcings, certainly not operating today.
Scaddenp. ...
Are you suggesting that there are no natural forcings now?
How can you say there are no solar forcings today?
Donny @69, a forcing is the change in net energy flux at the top of the atmosphere. The Sun provides essentially all of the energy that warms the Earth, but is very constant. Over the solar cycle, it changes its output by only around 0.1%, so the change in forcing is small. Short term (decade to millenia) changes in solar output in addition to the solar cycle are not much larger. More importantly, the greatest solar output in this century was acheive around 1958, with a slight decline thereafter, becoming more rapid over the last decade. Therefore the change in solar ouput, ie, the solar forcing, since 1950 has been very close to zero, and perhaps slightly negative.
Donny, estimates of forcings operating from AR4 below:
and for the current best estimates with error bars of forcing now, see:
Solar and volcanic are clearly natural but not long term players. Again, see the WG1 for estimates of forcings at play during LIA and for how the model perform.
In an apparent flyby comment, arationofreason wrote here:
Not only do I doubt it, I doubt the comment even means anything. That is because the key word is "recovery", and it is meaningless to talk about a recovery unless you can identify a ground state to which you are recovering. However, people who discuss the "recovery from the LIA" never identify that groundstate. Indeed, they insist the recovery was ongoing to the end of the 20th century even though end 20th century temperatures were likely higher than those of the Medieval Warm Period, a period of noteworthy and unusual global warmth, at least according to climate pseudo-skeptics. I have commented more on the purely rhetorical nature of the word "recovery" in this context elsewhere.
Not only do I doubt the "recovery" rhetoric based on its emptyness, however. More importantly it does not match what we know about global temperature trends.
I realized this due to a recent discussion I had about mean global temperature around 1750. The upshot is that mean global temperatures over the twenty year period centered on 1750 (1741-1760) are statistically indistiguishable from those centered on 1900, ie, there was no strong trend in mean global temperatures representing any "recovery" from the LIA.
arationofreason specifies the last 160 years, thereby restricting the timeframe to that covered by the instrumental record. Unfortunately for his hypothesis, the first 50 years of that record show a negative trend in GMST. Their "recovery" is actually an ongoing decline in GMST (see table in next paragraph), so that if we were to merely continue it, we would have had declining temperatures over the whole of the twentieth century.
Indeed, extending beyond the instrumental record, we see that there has been no recovery going back to the most intense phase of the LIA in the seventeenth century. Using Mann08 EIV global, we find trends as:
1616-1750 -0.097 C/century (Mann08)
1751-1850 0.000 C/century (Mann08)
1851-1900 -0.020 C/century (HadCRUT4)
1901-2014 0.768 C/century (HadCRUT4)
Clearly the "recovery" is a fiction of the imagination. Global temperatures declined from a peak around the 10th century AD (see graph) to a minimum around 1500 AD, from where they bumped around about the same level until after 1900, well after the start of industrial emissions of CO2.
So not only is the rhetorical appeal to the "recovery from the LIA" meaningless, it does not even get the facts about global temperature trends right.
The focus of this article is the LIA, so none of the three rebuttals address the Thames Frost Fairs mentioned by David Evans. As Robin at 57 mentions, these fairs were not every winter. BBC article: "between 1309 and 1814, the Thames froze at least 23 times and on five of these occasions -1683-4, 1716, 1739-40, 1789 and 1814 - the ice was thick enough to hold a fair".
As a Londoner, I can say it now seems implausible the Thames would freeze at London Bridge because it has been embanked by Bazalgette, flows fast in both directions, and is navigable and tidal as far as Teddington Lock. However, when it was shallower and slower, flow could become blocked at the Old London Bridge, which was demolished in 1831. There may be research into how important the LIA was as a factor in Thames freezing, but I recall reading how it was mostly down to commercial and architectual changes.
Separately, research published Jan 2017 clarifying the 'pre-industrial' global temperature baseline as mid-eighteenth century rather than 1850-1900 makes a difference of about 0.1 °C, which actually has policy implications related to Paris targets, shaving off a few years of inaction. That seems to give an indication of the order of magnitude of any difference the LIA made at a global scale.
Suggested supplemental reading:
Scientists discover ‘surprising’ cause of Europe’s little ice age in late medieval era
Change in ocean currents – similar to phenomena seen today – likely cause behind substantial cooling, US scientists say.
by Harry Cockburn, Climate, The independent (UK), Dec 16, 2021
https://www.independent.co.uk/climate-change/news/little-ice-age-ocean-currents-b1976776.html
This question may be off topic, but I'm not used to posting anything here. I'm interested to know what is the prevailing thinking about the lack of any change or signal in global CO2 levels as a result of the decline in CO2 emissions in 2020.