Unique and unnatural: modern warming from an historical viewpoint

A Roman altar with the Sun in its chariot on the left, and Vulcan, the god of fire and volcanoes on the right. The climate gods long favoured the Roman Empire, with wobbles in Earth's orbit credited for increasing the amount of solar energy falling on Earth at the time. Image copyright: Nick Thompson, used via Flickr Creative Commons License.

A Roman altar with the Sun in its chariot on the left, and Vulcan, the god of fire and volcanoes on the right. The climate gods long favoured the Roman Empire, with Earth’s orbital dance credited for increasing the amount of solar energy falling on Earth at the time. Image copyright: Nick Thompson, used via Flickr Creative Commons License.

Our climate has changed before. It’s something most of us realise and can agree on and, according to Skeptical Science, it’s currently the most used argument against human-caused warming. If such changes have happened naturally before, the argument goes, then surely today’s warming must also be natural. It’s an appealing idea, with an instinctively ‘right’ feel. Nature is so huge compared to us puny humans, how can we alter its course? The warming we’re measuring today must just be a natural fluctuation.

It’s such an appealing argument that at the beginning of the 20th century that’s just what many scientists thought – that humans couldn’t alter Earth’s climate. In the time since, our knowledge has come a long way. We’ve explored space, become able to build the electronics that are letting you read this, and climate science has likewise advanced and benefited from these advances.

So what do we know today that might convince the sceptical scientists of 115 years ago that we’re warming the planet? Recently, Richard Mallett, one of my Twitter friends who describes himself as sceptical about mainstream climate science, made a point that serves as an excellent test of our current knowledge:

Of the historical warmings he’s referring to, perhaps the least familiar is the Holocene, which is ironic, as the Holocene is now. It’s the current period of geological time that started at the end of the last ice age, 11,700 years ago. By 1900 scientists would have known the term, but they couldn’t explain why it wasn’t as icey as before.

Three variables of the Earth’s orbit—eccentricity, obliquity, and precession—affect global climate. Changes in eccentricity (the amount the orbit diverges from a perfect circle) vary the distance of Earth from the Sun. Changes in obliquity (tilt of Earth’s axis) vary the strength of the seasons. Precession (wobble in Earth’s axis) varies the timing of the seasons. For more complete descriptions, read Milutin Milankovitch: Orbital Variations Image credit: NASA/Robert Simmon.

Three variables of the Earth’s orbit—eccentricity, obliquity, and precession—affect global climate. Changes in eccentricity (the amount the orbit diverges from a perfect circle) vary the distance of Earth from the Sun. Changes in obliquity (tilt of Earth’s axis) vary the strength of the seasons. Precession (wobble in Earth’s axis) varies the timing of the seasons. For more complete descriptions, read Milutin Milankovitch: Orbital Variations. Image credit: NASA/Robert Simmon.

The explanation we have today comes thanks to the calculations Milutin Milanković worked out by hand between 1909 and 1941. Milutin showed that thanks to the gravitational pull of the Moon, Jupiter and Saturn, Earth’s orbit around the Sun varies in three ways. Over a cycle of roughly 96,000 years our path varies between more circular and more oval shapes. The other two ways come because Earth’s poles are slightly tilted relative to the Sun’s axis, which is why we have seasons. The angle of that tilt shifts over a roughly 41,000 year cycle. Earth also revolves around that tilted axis, like a spinning top does when it slows down, every 23,000 years.

Together these three cycles change how much of the Sun’s energy falls on and warms the Earth, in regular repeating patterns. Though that idea would be the subject of much controversy, by the 1960s data measured from cylinders of ancient ice and mud would resolve any doubt. The slow descent into ice ages and more abrupt warmings out of them – like the one that ushered in the Holocene – come from Earth’s shimmies in space.

Four horseman of the medieval climate

Since human civilisation has emerged during the relatively warm Holocene epoch, it’s been through several significant shifts in climate. In its latest report the UN Intergovernmental Panel on Climate Change (IPCC) highlights the PAGES 2K temperature reconstruction from the past 2000 years.

“Warm European summer conditions were prevalent during [the] 1st century, followed by cooler conditions from the 4th to the 7th century,” the IPCC report says. “Persistent warm conditions also occurred during the 8th–11th centuries, peaking throughout Europe during the 10th century. Prominent periods with cold summers occurred in the mid-15th and early 19th centuries.”

During the ‘Medieval Climate Anomaly’ (MCA) from 950-1250 there were periods of several decades that were in some regions as warm as the mid- or late 20th century. However the IPCC has ‘high confidence’ – equating to an 8 in 10 chance – that these regional warm periods didn’t happen all at once like modern warming. The IPCC explains the MCA and the ‘Little Ice Age’ (LIA), a cold spell that lasted from 1450-1850, through the results of comparisons between the temperature reconstructions and climate simulations.

Reconstructed (a) Northern Hemisphere and (b) Southern Hemisphere, and (c) global annual temperatures during the last 2000 years. Note the general cooling trend, which Milutin Milanković's cycle predicts, and the strong deviation from it in the past century. Individual reconstructions are shown as indicated in the legends, grouped by colour (red: land-only all latitudes; orange: land-only latitudes outside tropics; light blue: land and sea latitudes outside tropics; dark blue: land and sea all latitudes) and instrumental temperatures shown in black (Hadley Centre/ Climatic Research Unit (CRU) gridded surface temperature-4 data set (HadCRUT4) land and sea, and CRU Gridded Dataset of Global Historical Near-Surface Air TEMperature Anomalies Over Land version 4 (CRUTEM4) land-only). All series represent anomalies (°C) from the 1881–1980 mean (horizontal dashed line) and have been smoothed with a filter that reduces variations on time scales less than about 50 years. Image credit: IPCC

Reconstructed (a) Northern Hemisphere and (b) Southern Hemisphere, and (c) global annual temperatures during the last 2000 years. Note the general cooling trend, which Milutin Milanković’s cycle predicts, and the strong deviation from it in the past century. Individual reconstructions are shown as indicated in the legends, grouped by colour (red: land-only all latitudes; orange: land-only latitudes outside tropics; light blue: land and sea latitudes outside tropics; dark blue: land and sea all latitudes) and instrumental temperatures shown in black (Hadley Centre/ Climatic Research Unit (CRU) gridded surface temperature-4 data set (HadCRUT4) land and sea, and CRU Gridded Dataset of Global Historical Near-Surface Air TEMperature Anomalies Over Land version 4 (CRUTEM4) land-only). All series represent anomalies (°C) from the 1881–1980 mean (horizontal dashed line) and have been smoothed with a filter that reduces variations on time scales less than about 50 years. Image credit: IPCC

The comparisons call in three factors other than Earth’s orbit wiggles. The first is natural changes in the energy the Sun produces. For example, the LIA is often linked to an especially quiet period for the Sun, known as the Maunder Minimum. The second factor is volcanic eruptions, which play a role because the dust they throw into the air can reflect the Sun’s energy back into space. The third factor is the chaotic randomness we all experience in day-to-day weather, otherwise known as natural variability. The IPCC has high confidence that these factors together ‘contributed substantially’ to where and when the MCA and LIA happened.

Sol Non-Invictus

Today such researchers have linked such warm and cold periods to the rise and fall of historical civilisations. One of the most notable is the Roman Empire, which rose from 100BC to 200AD amid an exceptionally stable climate. Summer temperature reconstructions from parts of the Alps show several intervals during Roman times as warm or warmer than most of the 20th century.

In this case, Martin Grosjean’s team at the University of Bern, Switzerland, say Earth’s wobbles were more important than variations in how much energy the sun put out. Orbital changes meant 4-6W more energy fell per square metre of surface from 600BC-100AD than over the last 100 years, they point out. That’s a bigger ‘forcing’ than we’d expect from doubling CO2 levels in the air.

Not only can climate science explain past changes, it shows how exceptional modern warming is compared to them. The PAGES 2K reconstruction shows the long-term cooling trend we’d been following thanks mainly to Milutin Milanković’s cycles has been reversed by recent warming. That’s thanks to a wholly new factor – human-emitted greenhouse gases that trap more of the Sun’s energy in our atmosphere.

The IPCC’s latest report compares the ‘radiative forcing’ – the energy flows warming or cooling the planet – for various factors today against what they were in 1750. I include its summary graph below, but want to stress a key point. The human-caused, or anthropogenic, energy flow has increased by around 44 times as much as the flow from the Sun. For me, contrasting that with the past gives a striking perspective, showing just how unnatural current warming is. Thanks to our contribution, we are living in perhaps the most unique and historic moment for climate change in human history.

Radiative forcing (RF) estimates in watts per square metre (Wm<sup>-2</sup>) in 2011 relative to 1750 and totalled uncertainties for the main drivers of climate change. The best estimates of the net radiative forcing are shown as black diamonds with corresponding uncertainty intervals; the numerical values are provided on the right of the figure, together with the confidence level in the net forcing (VH – very high, H – high, M – medium, L – low, VL – very low). Increased sunlight absorption by black carbon on snow and ice is included in the black carbon aerosol bar. Small forcings due to aeroplane contrails (0.05 Wm<sup>-2</sup>) , including contrail induced cirrus), and hydrofluorocarbon, perfluorocarbon and sulphur hexafluoride greenhouse gases (total 0.03 Wm<sup>-2</sup>) ) are not shown. Concentration-based RFs for gases can be obtained by summing the like-coloured bars. Volcanic forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms. In the bottom box, total anthropogenic radiative forcing is provided for three different years relative to 1750. Image credit: IPCC

Radiative forcing (RF) estimates in watts per square metre (Wm-2) in 2011 relative to 1750 and totalled uncertainties for the main drivers of climate change. The best estimates of the net radiative forcing are shown as black diamonds with corresponding uncertainty intervals; the numerical values are provided on the right of the figure, together with the confidence level in the net forcing (VH – very high, H – high, M – medium, L – low, VL – very low). Increased sunlight absorption by black carbon on snow and ice is included in the black carbon aerosol bar. Small forcings due to aeroplane contrails (0.05 Wm-2), including contrail induced clouds), and hydrofluorocarbon, perfluorocarbon and sulphur hexafluoride greenhouse gases (total 0.03 Wm-2 ) are not shown. Concentration-based RFs for gases can be obtained by summing the like-coloured bars. Volcanic forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms. In the bottom box, total anthropogenic radiative forcing is provided for three different years relative to 1750. Image credit: IPCC

Further reading:

Read my brief profile of Milutin Milanković‘s fascinating life here.

The parts of the IPCC fifth assessment report I referred to for this blog entry are from working group I. The main discussion of recent climate comes from chapter 5, especially p409 (p27 in the PDF file for the chapter).

The last part, discussing radiative forcings, comes from the WGI Summary for Policymakers, pp11-12.

Stewart, M., Larocque-Tobler, I., & Grosjean, M. (2011). Quantitative inter-annual and decadal June-July-August temperature variability ca. 570 BC to AD 120 (Iron Age-Roman Period) reconstructed from the varved sediments of Lake Silvaplana, Switzerland Journal of Quaternary Science, 26 (5), 491-501 DOI: 10.1002/jqs.1480
Ahmed, M., Anchukaitis, K., Asrat, A., Borgaonkar, H., Braida, M., Buckley, B., Büntgen, U., Chase, B., Christie, D., Cook, E., Curran, M., Diaz, H., Esper, J., Fan, Z., Gaire, N., Ge, Q., Gergis, J., González-Rouco, J., Goosse, H., Grab, S., Graham, N., Graham, R., Grosjean, M., Hanhijärvi, S., Kaufman, D., Kiefer, T., Kimura, K., Korhola, A., Krusic, P., Lara, A., Lézine, A., Ljungqvist, F., Lorrey, A., Luterbacher, J., Masson-Delmotte, V., McCarroll, D., McConnell, J., McKay, N., Morales, M., Moy, A., Mulvaney, R., Mundo, I., Nakatsuka, T., Nash, D., Neukom, R., Nicholson, S., Oerter, H., Palmer, J., Phipps, S., Prieto, M., Rivera, A., Sano, M., Severi, M., Shanahan, T., Shao, X., Shi, F., Sigl, M., Smerdon, J., Solomina, O., Steig, E., Stenni, B., Thamban, M., Trouet, V., Turney, C., Umer, M., van Ommen, T., Verschuren, D., Viau, A., Villalba, R., Vinther, B., von Gunten, L., Wagner, S., Wahl, E., Wanner, H., Werner, J., White, J., Yasue, K., & Zorita, E. (2013). Continental-scale temperature variability during the past two millennia Nature Geoscience, 6 (5), 339-346 DOI: 10.1038/ngeo1797

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9 Responses to “Unique and unnatural: modern warming from an historical viewpoint”

  1. edaviesmeuk Says:

    “So what do we know today that might convince the sceptical scientists of 115 years ago that we’re warming the planet?”

    Perhaps the most important point known now which wasn’t known to, say, Arrhenius or Callendar was the buffering effect in the oceans such that only about half our emissions are absorbed. From what I understand Arrhenius himself didn’t take his own calculations of the effect of CO₂ that seriously from the point of view of human caused warming probably because of the assumption that most would be absorbed by the oceans. Similarly, Callendar’s results weren’t taken seriously by professional meteorologists, etc, at least in part because he couldn’t explain why human emissions weren’t being absorbed by the oceans – without that explanation he was only really pointing at an interesting statistical correlation.

    Also, the quantity of emissions we’re now putting out would have been difficult for them to believe. Even as late as LBJ’s speech to Congress on the subject it seems that there was an assumption of a linear, rather than exponential, increase in emissions from the mid-60s to the end of the 20th century. To somebody looking at the amount of effort involved in coal mining and the limited area it was carried out in (North America and Europe, mostly for large scale operations) in 1900 today’s output of CO₂ would be difficult to comprehend.

  2. Richard Mallett Says:

    Thank you for quoting me. I agree that Milutin Milankovic was a fascinating guy. He was the one who first got me interested in all this.

    It’s interesting that the IPCC starts its comparison in 1750. As you know, the Met Office Hadley Centre daily temperature records start in 1772, so pretty close. It can be found at http://www.metoffice.gov.uk/hadobs/hadcet/

    We basically have lots of warming and cooling periods with the amplitude gradually decreasing as we come out of the LIA.

    After about 1900-1910 (when we have more cars, ships, aircraft and factories) it becomes relatively smoother, until the ‘hockey stick’ of about 1986-2006, and we’re now in another cooling period.

    As you know, you can download monthly, seasonal and annual average temperatures from 1659 from http://www.metoffice.gov.uk/hadobs/hadcet/data/download.html

    Annual average temperature in 1750 = 9.69 C
    Annual average temperature in 2013 = 9.56 C

    Interesting.

    • andyextance Says:

      I know you like the CET because it’s accessible (and maybe because it’s long and local to us?) I also know you’ve heard its shortcomings pointed out before eg here: http://simpleclimate.wordpress.com/2014/05/10/how-lessons-from-space-put-the-greenhouse-effect-on-the-front-page/#comment-4984. I’ve also pointed out that the effect of natural variability makes it fairer to use a short-term temperature average such as decadal averages than individual years, which are ripe for cherry-picking.

      Even if we put all these points to one side for a moment, you’ll no doubt have noticed that the IPCC graph of reconstructed temperatures for 1750 in the blog entry has figures that are typically somewhat lower than today’s? I don’t know how many of these you can also get access to – perhaps you’d like to find out?

      • Richard Mallett Says:

        Judging by my lack of success in getting even the SPMs from the IPCC SAR, I have no chance of getting their (probably proxy) data.

        I am (slowly) plotting temperature records from CRUTEM4 station records that start before 1850. So far I have those that start between 1706 and 1836.

        10 year average 1740-1750 = 7.50 C.
        10 year average 2000-2010 = 9.16 C.

        Increase = 1.66 C over 260 years = 0.64 C per century. Same as the average of GISS / HadCRUT4 / NCDC from 1880 to 2014.

        Just to make clear my comment about the Holocene – I was referring to the Holocene maximum, as shown (e.g.) in the second graph (reproduced from the IPCC) at http://climateaudit.org/2005/06/25/ipcc-1990-an-extended-excerpt/

        The IPCC comment underneath the graphs (also reproduced at Climate Audit) is also very interesting.

      • And Then There's Physics Says:

        I have never specifically heard – or actually looked for – a good reason why the radiative forcings are relative to 1750, but had always assumed it was because in 1750 we would expect the planetary energy imbalance to be small (close to zero). If so, we can therefore be confident that a dominant reason for any warming since 1750 is because of the changes in radiative forcings, and not because of an imbalance that already existed in 1750. I could be wrong though.

      • Richard Mallett Says:

        Your link to comment 4984 in the other thread put me in the middle of Marco’s comment – is that the one you meant ? I know that many people on WUWT have asked for comments to be visibly labelled with their numbers.

        Anyway, I hope that my decade averages from CRUTEM4 long records have helped to make things clearer. Please let me know if you meant something else.

    • andyextance Says:

      Also, thanks for giving me such interesting questions to address!

  3. And Then There's Physics Says:

    I looked through the paper that you reference when you mention this

    Orbital changes meant 4-6W more energy fell per square metre of surface from 600BC-100AD than over the last 100 years, they point out.

    The paper does indeed say this (The warmer summer conditions during the Iron Age–Roman Period compared to the last 800 a (Medieval Warm Period, Little Ice Age and present) were attributed to orbital forcing (D4–6 W m2, July). ) but I wonder if this is correct or properly explained. An increased forcing of 4-6 Wm-2 would require an increase in solar luminosity (TSI) of 16-24 Wm-2 which seems rather high. I wonder if the paper doesn’t mean an increase in TSI of 4 – 6 Wm-2 and hence an increase in forcing of 1-1.5 Wm-2 (i.e., smaller than the change due to a doubling of CO2).


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