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:
@andyextance Climate science should be able to explain the Holocene, Roman, Medieval and current warmings and the cold periods in between.
— Richard Mallett (@richard_mallett) May 11, 2014
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.
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.
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.
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.
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
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