Inaugural UCL Lecture on Climate and Human History by Bill Ruddiman - a critical review

Bill Ruddiman delivered this lecture, entitled "Top-down and Bottom-up Evidence of Large Early Anthropogenic Climate Change", on 21st October, 2014.

Ruddiman focussed on those areas where the early anthropogenic warming hypothesis have been challenged. These areas have also been addressed in his new afterword, published in the 2010 edition of his book "Plows, Plagues and Petroleum".

The original hypothesis compared the variations in atmospheric CO2 and CH4 during the Holocene with those of the preceding three interglacials. That comparison now stretches back 800,000 years over seven interglacials to MIS19, made possible by the Antarctic ice-coring carried out by EPICA. This has two advantages; firstly there are more interglacials with which to compare the Holocene and secondly the orbital forcing that occurred during MIS 19 is similar to that of the Holocene, enabling a like-for-like comparison.

I feel there is a degree of sleight of hand when comparing the trends from the various interglacials.  Ruddiman uses the first insolation minimum in each interglacial as a common point for aligning the trends; is that a valid point of alignment? Each interglacial lasts for a different length of time; over what time-period should those trends be aligned? Should the interglacials be matched by stretching and compressing them to fit a common scale? He omits MIS3, MIS13 and MIS15 from the comparisons, reasonably, because of the significantly different orbital forcing characteristics of those periods.

Ruddiman's hypothesis concentrates on what he characterises as anomalous increases in CO2 and CH4 towards the first insolation minimum of the current interglacial. Is it possible that the extended periods of low temperature during the preceding MIS3 and MIS5 (Jouzel et al, 2007, p.794) created a build-up of CO2 and CH4 which is now being released? This possibility is something I would like to see addressed.


Antarctic temperature variations from MIS5 to MIS1 (Jouzel et al, 2007, p.794)

Ruddiman's comparison with MIS19 was elegant, especially when the insolation similarities were highlighted. He also noted a feedback from his proposed anthropogenic effects, that a warm ocean cannot hold as much CO2 as a cold one, which usefully corrects for shortfalls seen in the results of his macro-scale analysis.

The bottom-up evidence was drawn mainly from archaeological sources and, as promised, I shall discuss those in another post.

This was an enjoyable lecture. Ruddiman has taken on board the criticisms and answered them coherently while expanding the evidence on which he draws to support his hypothesis.

Later, Ruddiman also commented on the efforts to define the Anthropocene (which I explored in the previous post). He felt that the term "Anthropocene", with a capital A, was fine if the geologists felt the need for it but the term "anthropocene", with a lower-case a, was an important mechanism for bringing together and co-ordinating the archaeological and palaeoclimatological research into how humans are affecting our environment.

References cited not available online:
Ruddiman, W., 2010. Plows, Plagues and Petroleum: How Humans Took Control of Climate. Oxford: Princeton University Press.

The Guardian: Anthropocene: is this the new epoch of humans?

This article in today's Guardian newspaper caught my eye.

The story concerns the conference of the Working Group on the Anthropocene which is meeting in Berlin this week to consider the definition and the time-frame of the Anthropocene geological unit. Geological units have a boundary which is visible in the geological record called a Global Stratigraphic Section and Point (GSSP) or "Golden Spike". For the Anthropocene to be recognised, it too should manifest a global stratigraphic marker. Commentators from within the group argue for various start dates for the Anthropocene, the start of industrialisation still being favourite but the start of atomic era is also suggested. These dates are usefully supported by stratigraphic markers.

Have humans truly displaced volcanoes and plate tectonics as the key agent of geological change on our world? Probably not; all signs of our civilisation will one day be erased as they slide under an adjacent plate in the earth's crust. But this conference, it seems, is not considering that argument; it appears to be considering when the impact of human behaviour started to affect the planet as a whole and how the start of that impact may be detected in the stratigraphic record. It is debating which start date to adopt.

This comment from Mike Ellis of the British Geological Survey is unequivocal: "The principal process of change on the planet is us, so the name of our epoch should reflect that. It’s as simple as that."

Might the Working Group consider the emergence of farming as another option for the start of the Anthropocene? What is the geological marker from farming that would provide the Golden Spike?

I await the outcome with interest.

What does Ruddiman’s hypothesis state?

Introduction

This post summarises the evidence and the hypothesis set out in Ruddiman's 2003 paper which considers two atmospheric gasses, methane (CH₄) and carbon dioxide (CO₂), and compares the currently observed concentrations with the concentrations that would be expected under natural climate change driven by orbital forces.

The Methane anomaly

For most of the last 350,000 years, levels of atmospheric methane have tracked the axial precession of the orbit of the earth through the rising and falling of summer insolation. An increase in the quantity of solar energy falling on land masses causes air to rise which sucks in air from over the oceans. As this moist air rises over land it heats and deposits the moisture as monsoon rains, causing methane to be released from soils. The Greenland Ice Core Project (Blunier et al, 1994) evidence showed an anomalous increase in methane levels since 5000 BP when a decrease was predicted. The actual increase in methane was ~100 parts per billion. Given the predicted reduction, the likely excess methane is ~250 pbb.

The anomalous increase in atmospheric methane since 5000 BP (modified from Ruddiman, 2003)

The carbon dioxide anomaly

Atmospheric carbon dioxide concentrations also track orbital forces with varying degrees of latency. The forcing is more complex as it is affected by the axial precession cycle of 23,000 years, the axial tilt variation cycle of 41,000 years and the orbital eccentricity cycle of 100,000 years. Hays et al (1976) showed that these cycles are responsible for a substantial amount of climate variation including glaciation events and atmospheric carbon dioxide concentrations. The carbon dioxide levels predicted by those cycles and matched to the three of the most recent interglacials are not matched by those observed for the current interglacial. The predicted contemporary concentration is 240-245 parts per million but the observed concentration is 280-285 ppm, an anomalous increase of 40 ppm; this divergence commenced ~8000 BP.

Explaining the anomalies

Loss of terrestrial vegetation would reduce the uptake of carbon dioxide by plants. Various scenarios were modelled (Foley, 1995) to assess whether natural forces, such as desert expansion caused by reduced monsoon precipitation, could explain the anomaly. An alternative theory (Broeker et al, 1999) proposed that the expansion of forests after the Last Glacial Maximum had reduced atmospheric carbon dioxide; when forest expansion ceased the ocean's acidity increased dissolving calcium carbonate and releasing carbon dioxide into the atmosphere; Ruddiman chooses to dismiss these explanations because the underlying theories should have manifested the same outcomes in previous interglacials, those outcomes were not observed.

The alternative explanation offered is that biomass was reduced by anthropogenic action, the clearance of Eurasian forests for agriculture. There is considerable and persuasive archaeological evidence that such deforestation has taken place since 8000 BP, complemented by historical evidence from Greek and Roman writers. Carbon dioxide concentrations also track human population variations caused by plagues.

Commentary

This is a long post but I wanted to establish the arguments for the hypothesis and the complexity of the natural forces that affect climate. What I learned by writing it is that:

• the effects of astronomical forcing on climate vary as the different cycles harmonise and compete with one another
• those astronomical forces continue to affect climate as anthropogenic effects also manifest themselves
• both the methane and carbon dioxide anomalies must be explained in a complementary manner

Next, I will explore the archaeological evidence.

Why does the Anthropocene matter?

Hello and welcome to my blog: "What started the Anthropocene, Farming or Factories?"

I am writing this blog as part of my Masters in Environmental Archaeology at University College London. Environmental Archaeology studies past human behaviours and societies through analysis of prehistoric plant and animal artefacts and analysis of pre-historic soils and landscapes. Analysis of these things allows us to describe models of past climates; why would we want to do this?

We live in an age where the impact of human behaviours on the planet has become a matter of global political and fiscal concern. It is no longer the domain of scientists or environmentalists who, in the past, have been sidelined as eccentric or misguided. Fundamental decisions about how we feed and power all our societies are now being taken with consideration about sustainability and environmental impact. These decisions must be informed by well-grounded and robust evidence for the likely consequences.

We need to look to the past to understand the history of environmental change and to determine how and why those environmental changes happened; we need to understand what we can control, what we cannot control and what we should control. Analysing past climatic and environmental change in the context of the emergence of modern civilisation informs us about the positive and negative impacts, and the risks and rewards of our behaviours. Environmental Archaeology is one tool we can deploy to achieve that understanding.

It is recognised that the development of industrialisation powered by fossil fuels has had a measurable impact on our environment. In the last thirty years some scientists have started to link those impacts with climate change. This has led to the development of the concept of the "Anthropocene" which is a 200-year geological period during which human behaviours have had a greater long-term impact on the environment than the natural forces acting on our planet.

The recent and short-term nature of contribution of humans to climate change is now being challenged by archaeologists, climatologists and geographers, amongst other disciplines, who have posited the theory that our pre-historic behaviours such as the adoption of sedentism, the development of agriculture, the clearance of great forests, the enrichment of soils, etc. is when the human impact started to outweigh the effects of natural forces and that the Anthropocene may have started as long as 8000 years ago. William Ruddiman's 2003 paper "The Anthropogenic Greenhouse Era Began Thousands of Years Ago" lays out the arguments for this hypothesis.

This blog will explore critically the evidence for that earlier emergence of the Anthropocene and consider the question: What started the Anthropocene, Farming or Factories?

I look forward to your comments, criticisms and contributions to this debate over the next few months.