The Anthropocene – the concept that the Earth’s surface geological processes are now human-dominated – is often considered to be more or less synonymous with climate change.  Today, however, climate change is only one component – relatively minor, still – of the changes to the Earth system invoked to justify the Anthropocene.  However, the last couple of centuries have seen large and accelerating changes in some of the key factors that drive global climate, and it is widely agreed among scientists that climate change has begun.  In future decades, centuries and millennia, climate change seems likely to emerge as a central driver of changing geological process on Earth.

The Anthropocene concept has a long history, stretching back to the earliest days of organised geology.  However, the idea that humanity’s geologically brief impact could produce Earth system change on a par with the effects of, say, mountain building episodes and enormous volcanic outbursts, was long thought absurd by geologists.  Then, Nobel Prize-winning atmospheric chemist Paul Crutzen revived the concept and proposed the name ‘Anthropocene’ for it a little over a decade ago.  By that time, the realization was spreading that humans could indeed make a considerable impact on the Earth system, and so the term quickly began to be used, both in scientific circles and more widely.

The Anthropocene is currently informal.  However, it is currently being examined as a potential formal addition to the Geological Time Scale.

There is substantial evidence that the Anthropocene represents a real, and important geological phenomenon (explained in a Geological Society of America article).  The Earth, particularly since the Industrial Revolution, has undergone significant geological change, some of it without parallel in the Earth’s 4.6 billion year history.  Major changes include those to patterns of erosion and sedimentation on land, now amplified by about an order of magnitude over background levels.  As part of this the ‘urban stratum’ associated with our towns and cities makes up a quite novel addition to the Earth’s geology.  The Earth’s animals and plants (the fossils of the future) have changed markedly as a result of habitat change and human predation, with a ‘sixth mass extinction’ looming and species invasions already pervasive worldwide.  Chemical cycles such as those of carbon, nitrogen and phosphorus have been severely perturbed.

The definition of the Anthropocene (whether formal or informal) is still under discussion, particularly in regards to when it might be considered to have begun.  Suggestions have ranged as far back as several thousand years ago, with the origin and spread of agriculture; to the Industrial Revolution, with a steep rise in both population ( to exceed a billion globally) and energy use; to the social and economic ‘Great Acceleration’ of the mid-20th century, as changes intensified and became more global in extent.  These (and other) potential boundaries are still under debate, as is the hierarchical level of the Anthropocene (if it is to be formalised, should it be an age, an epoch, a period or even an era?).  Decisions over stratigraphic boundaries generally are not quickly arrived at, given the complexity of Earth history, and Anthropocene question includes some unique problems.  For instance, it is a geological phenomenon that is only in its early stages. The process of formalisation, therefore, will take some time yet.


The Anthropocene within geological time


The changes of the Anthropocene include important drivers of climate, particularly as regards levels of carbon dioxide and methane in the atmosphere.  Carbon dioxide levels, now having exceeded 400 parts per million, are at levels not experienced on Earth since the Pliocene Epoch, three million years ago, and the rate of increase of this gas is geologically striking: it is estimated to be an order of magnitude faster than even its rate of increase in dramatic ‘fossil global warming events’ of the geological past, such as that which took place at the boundary between the Paleocene and Eocene epochs, 55 million years ago.

The human-made changes to various factors that influence climate (including greenhouse gases, and also such factors as the way land, sea and atmosphere reflect or absorb heat and light) has been evaluated by the Intergovernmental Panel on Climate Change (IPCC) (AR5 WG1, Anthropogenic and Natural Radiative Forcing).  The sum effect is complex, with both short- and long-term effects influenced by a range of feedback mechanisms, and so climate change cannot be predicted in detail, although increasingly sophisticated attempts are made to produce projections of future climate by means of computer models.

The recent history of climate has seen a global temperature rise of a little under 1° C over the past century (more at the poles, less at the equator).  It is extremely likely (95-100% certainty, IPCC) that much of the recent rise in temperature is human-induced.  The pattern of rise is not smooth, but forms an irregular ‘staircase’, with rises in the early and late 20th century, and still-stands mid-20th century and in the past decade.  The causes of these changes in rate of warming are debated, but may involve changing patterns of particulates and aerosols in the atmosphere, and changing patterns of heat exchange between the ocean (that has a relatively high heat capacity) and atmosphere (with a relatively low heat capacity).

The sum of this century’s worth of climate change is significant in, for instance, affecting the distribution of climate-sensitive species and of Arctic sea ice.  However, the overall scale of this change does not approach the climate changes associated with the 50+ glacial-interglacial cycles (‘Ice Ages’) of the Quaternary Period, the last 2.6 million years – nor even of some of the changes within individual glacial or interglacial phases (for instance any of the 26 Dansgaard-Oeschger climate cycles, each about 1500 years long, of the last glacial phase).

So, while there is good evidence that the Anthropocene is a geologically significant phenomenon on the scale of ‘deep time’, the climate change component is thus far relatively minor on a geological timescale.  However, the scale and significance of Anthropocene climate effects will likely increase in the near future.  Geological history tells us that the Earth is prone to relatively large (and often rapid) climate changes from relatively small provocations.  The provocations driven by human action are now substantial (and growing). Thus, the Earth will likely continue to warm.

What is important, and novel, here is the direction of climate change.  For the past three million years, and more, the Earth has oscillated between glacial and interglacial states, and for the last million years that pattern has been intensifying.  The world’s many animal and plant species have adapted to these successive changes, and simply migrated as climate belts shifted: overall, the Ice Ages have not seen abnormal species losses.

Human footprint: Amazon deforestation.

Now, though, the climate looks set to warm significantly from what is already a starting point of peak interglacial warmth, to temperatures not seen since before the Ice Ages.  Moreover, this is happening on a world where habitats have been fragmented by agriculture and urbanization, making it harder for species to migrate, and hence increasing the likelihood of extinctions.  As climate change intensifies its effects on our planet, the Anthropocene looks set to evolve to become an increasingly fundamental transition in Earth history.

The more striking this phenomenon becomes, the worse it will be overall for human civilization.  It would be prudent to try to minimize the scale of the future Anthropocene.  Efforts to mitigate climate change are central to this.